diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index 4b2c1c1db..6ae909ef1 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -37,9 +37,664 @@ tags:
   - name: Devices
   - name: Export
   - name: Sync
+  - name: Cycle
   - name: Admin
   - name: Meta
 paths:
+  /api/cycle/day-logs:
+    post:
+      tags:
+        - Cycle
+      summary: Capture a single cycle day-log (v1.15.0)
+      description: "Upserts on `(userId, source, externalId)` when externalId present, else `(userId, date)`. `note` encrypts
+        at rest. 201 on insert, 200 on update. Gated: `cycle.disabled` 403 when the feature is off."
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              type: object
+              properties:
+                date:
+                  type: string
+                  pattern: ^\d{4}-\d{2}-\d{2}$
+                flow:
+                  type: string
+                  enum:
+                    - NONE
+                    - SPOTTING
+                    - LIGHT
+                    - MEDIUM
+                    - HEAVY
+                intermenstrualBleeding:
+                  type: boolean
+                basalBodyTempC:
+                  type: number
+                  minimum: 30
+                  maximum: 45
+                ovulationTest:
+                  type: string
+                  enum:
+                    - NEGATIVE
+                    - POSITIVE_LH_SURGE
+                    - ESTROGEN_SURGE
+                    - INDETERMINATE
+                cervicalMucus:
+                  type: string
+                  enum:
+                    - DRY
+                    - STICKY
+                    - CREAMY
+                    - WATERY
+                    - EGG_WHITE
+                sexualActivity:
+                  type: boolean
+                protectedSex:
+                  anyOf:
+                    - type: boolean
+                    - type: "null"
+                pregnancyTest:
+                  type: string
+                  enum:
+                    - NEGATIVE
+                    - POSITIVE
+                    - INDETERMINATE
+                progesteroneTest:
+                  type: string
+                  enum:
+                    - NEGATIVE
+                    - POSITIVE
+                    - INDETERMINATE
+                contraceptive:
+                  type: string
+                  enum:
+                    - NONE
+                    - UNSPECIFIED
+                    - IMPLANT
+                    - INJECTION
+                    - IUD
+                    - INTRAVAGINAL_RING
+                    - ORAL
+                    - PATCH
+                    - EMERGENCY
+                symptoms:
+                  maxItems: 40
+                  type: array
+                  items:
+                    type: object
+                    properties:
+                      key:
+                        type: string
+                        minLength: 1
+                        maxLength: 80
+                      severity:
+                        type: integer
+                        minimum: 1
+                        maximum: 4
+                    required:
+                      - key
+                note:
+                  type: string
+                  maxLength: 500
+                loggedAt:
+                  type: string
+                  format: date-time
+                  pattern: ^(?:(?:\d\d[2468][048]|\d\d[13579][26]|\d\d0[48]|[02468][048]00|[13579][26]00)-02-29|\d{4}-(?:(?:0[13578]|1[02])-(?:0[1-9]|[12]\d|3[01])|(?:0[469]|11)-(?:0[1-9]|[12]\d|30)|(?:02)-(?:0[1-9]|1\d|2[0-8])))T(?:(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d(?:\.\d+)?)?(?:Z|([+-](?:[01]\d|2[0-3]):[0-5]\d)))$
+                source:
+                  default: MANUAL
+                  type: string
+                  enum:
+                    - MANUAL
+                    - APPLE_HEALTH
+                externalId:
+                  type: string
+                  minLength: 1
+                  maxLength: 120
+              required:
+                - date
+                - loggedAt
+      responses:
+        "200":
+          description: Existing day-log updated.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/CycleDayLogEnvelope"
+        "201":
+          description: New day-log created.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/CycleDayLogCreatedEnvelope"
+        "401": &a1
+          description: Authentication required or invalid credentials.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
+        "403": &a2
+          description: Cycle tracking is not enabled for this account (errorCode `cycle.disabled`).
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
+        "422": &a3
+          description: Request validation failed.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
+        "429": &a4
+          description: Rate limit exceeded.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
+  /api/cycle/day-logs/{id}:
+    patch:
+      tags:
+        - Cycle
+      summary: Edit a single cycle day-log (v1.15.0)
+      description: Partial edit; an omitted field is left untouched. Owner-scoped (a cross-user id 404s). Gated.
+      parameters:
+        - in: path
+          name: id
+          schema:
+            type: string
+          required: true
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              type: object
+              properties:
+                flow:
+                  anyOf:
+                    - type: string
+                      enum:
+                        - NONE
+                        - SPOTTING
+                        - LIGHT
+                        - MEDIUM
+                        - HEAVY
+                    - type: "null"
+                intermenstrualBleeding:
+                  type: boolean
+                basalBodyTempC:
+                  anyOf:
+                    - type: number
+                      minimum: 30
+                      maximum: 45
+                    - type: "null"
+                ovulationTest:
+                  anyOf:
+                    - type: string
+                      enum:
+                        - NEGATIVE
+                        - POSITIVE_LH_SURGE
+                        - ESTROGEN_SURGE
+                        - INDETERMINATE
+                    - type: "null"
+                cervicalMucus:
+                  anyOf:
+                    - type: string
+                      enum:
+                        - DRY
+                        - STICKY
+                        - CREAMY
+                        - WATERY
+                        - EGG_WHITE
+                    - type: "null"
+                sexualActivity:
+                  type: boolean
+                protectedSex:
+                  anyOf:
+                    - type: boolean
+                    - type: "null"
+                pregnancyTest:
+                  anyOf:
+                    - type: string
+                      enum:
+                        - NEGATIVE
+                        - POSITIVE
+                        - INDETERMINATE
+                    - type: "null"
+                progesteroneTest:
+                  anyOf:
+                    - type: string
+                      enum:
+                        - NEGATIVE
+                        - POSITIVE
+                        - INDETERMINATE
+                    - type: "null"
+                contraceptive:
+                  anyOf:
+                    - type: string
+                      enum:
+                        - NONE
+                        - UNSPECIFIED
+                        - IMPLANT
+                        - INJECTION
+                        - IUD
+                        - INTRAVAGINAL_RING
+                        - ORAL
+                        - PATCH
+                        - EMERGENCY
+                    - type: "null"
+                symptoms:
+                  maxItems: 40
+                  type: array
+                  items:
+                    type: object
+                    properties:
+                      key:
+                        type: string
+                        minLength: 1
+                        maxLength: 80
+                      severity:
+                        type: integer
+                        minimum: 1
+                        maximum: 4
+                    required:
+                      - key
+                note:
+                  anyOf:
+                    - type: string
+                      maxLength: 500
+                    - type: "null"
+                loggedAt:
+                  type: string
+                  format: date-time
+                  pattern: ^(?:(?:\d\d[2468][048]|\d\d[13579][26]|\d\d0[48]|[02468][048]00|[13579][26]00)-02-29|\d{4}-(?:(?:0[13578]|1[02])-(?:0[1-9]|[12]\d|3[01])|(?:0[469]|11)-(?:0[1-9]|[12]\d|30)|(?:02)-(?:0[1-9]|1\d|2[0-8])))T(?:(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d(?:\.\d+)?)?(?:Z|([+-](?:[01]\d|2[0-3]):[0-5]\d)))$
+      responses:
+        "200":
+          description: Day-log updated.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/CycleDayLogPatchEnvelope"
+        "401": *a1
+        "403": *a2
+        "404":
+          description: Day-log not found / not owned.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
+        "422": *a3
+        "429": *a4
+    delete:
+      tags:
+        - Cycle
+      summary: Soft-delete a cycle day-log (v1.15.0)
+      description: Sets `deletedAt` + bumps `syncVersion`; surfaces as a tombstone on the next `/api/sync/changes` page. 204.
+        Idempotent.
+      parameters:
+        - in: path
+          name: id
+          schema:
+            type: string
+          required: true
+      responses:
+        "204":
+          description: Soft-deleted (no body).
+        "401": *a1
+        "403": *a2
+        "404":
+          description: Day-log not found / not owned.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
+        "422": *a3
+        "429": *a4
+  /api/cycle/day-logs/bulk:
+    post:
+      tags:
+        - Cycle
+      summary: Bulk drain cycle day-logs (Outbox / HealthKit) (v1.15.0)
+      description: "Up to 500 entries; `withIdempotency`; rate-limited `cycle:day-logs:bulk:<userId>` 60/min. Per-entry
+        status: inserted | duplicate | updated | skipped. Always 200."
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              type: object
+              properties:
+                entries:
+                  minItems: 1
+                  maxItems: 500
+                  type: array
+                  items:
+                    type: object
+                    properties:
+                      date:
+                        type: string
+                        pattern: ^\d{4}-\d{2}-\d{2}$
+                      flow:
+                        type: string
+                        enum:
+                          - NONE
+                          - SPOTTING
+                          - LIGHT
+                          - MEDIUM
+                          - HEAVY
+                      intermenstrualBleeding:
+                        type: boolean
+                      basalBodyTempC:
+                        type: number
+                        minimum: 30
+                        maximum: 45
+                      ovulationTest:
+                        type: string
+                        enum:
+                          - NEGATIVE
+                          - POSITIVE_LH_SURGE
+                          - ESTROGEN_SURGE
+                          - INDETERMINATE
+                      cervicalMucus:
+                        type: string
+                        enum:
+                          - DRY
+                          - STICKY
+                          - CREAMY
+                          - WATERY
+                          - EGG_WHITE
+                      sexualActivity:
+                        type: boolean
+                      protectedSex:
+                        anyOf:
+                          - type: boolean
+                          - type: "null"
+                      pregnancyTest:
+                        type: string
+                        enum:
+                          - NEGATIVE
+                          - POSITIVE
+                          - INDETERMINATE
+                      progesteroneTest:
+                        type: string
+                        enum:
+                          - NEGATIVE
+                          - POSITIVE
+                          - INDETERMINATE
+                      contraceptive:
+                        type: string
+                        enum:
+                          - NONE
+                          - UNSPECIFIED
+                          - IMPLANT
+                          - INJECTION
+                          - IUD
+                          - INTRAVAGINAL_RING
+                          - ORAL
+                          - PATCH
+                          - EMERGENCY
+                      symptoms:
+                        maxItems: 40
+                        type: array
+                        items:
+                          type: object
+                          properties:
+                            key:
+                              type: string
+                              minLength: 1
+                              maxLength: 80
+                            severity:
+                              type: integer
+                              minimum: 1
+                              maximum: 4
+                          required:
+                            - key
+                      note:
+                        type: string
+                        maxLength: 500
+                      loggedAt:
+                        type: string
+                        format: date-time
+                        pattern: ^(?:(?:\d\d[2468][048]|\d\d[13579][26]|\d\d0[48]|[02468][048]00|[13579][26]00)-02-29|\d{4}-(?:(?:0[13578]|1[02])-(?:0[1-9]|[12]\d|3[01])|(?:0[469]|11)-(?:0[1-9]|[12]\d|30)|(?:02)-(?:0[1-9]|1\d|2[0-8])))T(?:(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d(?:\.\d+)?)?(?:Z|([+-](?:[01]\d|2[0-3]):[0-5]\d)))$
+                      source:
+                        default: MANUAL
+                        type: string
+                        enum:
+                          - MANUAL
+                          - APPLE_HEALTH
+                      externalId:
+                        type: string
+                        minLength: 1
+                        maxLength: 120
+                    required:
+                      - date
+                      - loggedAt
+              required:
+                - entries
+      responses:
+        "200":
+          description: Batch processed (per-entry results).
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/CycleBulkEnvelope"
+        "401": *a1
+        "403": *a2
+        "422": *a3
+        "429": *a4
+  /api/cycle/period:
+    post:
+      tags:
+        - Cycle
+      summary: Period-boundary shortcut (v1.15.0)
+      description: One-tap started/ended period. `start` opens a new cycle (closing the prior); `end` stamps periodEndDate.
+        Writes the boundary day-log. Gated.
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              type: object
+              properties:
+                action:
+                  type: string
+                  enum:
+                    - start
+                    - end
+                date:
+                  type: string
+                  pattern: ^\d{4}-\d{2}-\d{2}$
+                externalId:
+                  type: string
+                  minLength: 1
+                  maxLength: 120
+                loggedAt:
+                  type: string
+                  format: date-time
+                  pattern: ^(?:(?:\d\d[2468][048]|\d\d[13579][26]|\d\d0[48]|[02468][048]00|[13579][26]00)-02-29|\d{4}-(?:(?:0[13578]|1[02])-(?:0[1-9]|[12]\d|3[01])|(?:0[469]|11)-(?:0[1-9]|[12]\d|30)|(?:02)-(?:0[1-9]|1\d|2[0-8])))T(?:(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d(?:\.\d+)?)?(?:Z|([+-](?:[01]\d|2[0-3]):[0-5]\d)))$
+              required:
+                - action
+                - date
+                - loggedAt
+      responses:
+        "200":
+          description: Cycle + boundary day-log.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/CyclePeriodEnvelope"
+        "401": *a1
+        "403": *a2
+        "422": *a3
+        "429": *a4
+  /api/cycle/calendar:
+    get:
+      tags:
+        - Cycle
+      summary: Predicted cycle calendar (v1.15.0)
+      description: "Runs the deterministic engine to build `{ profile, prediction, days }`. Fertile-window fields are
+        server-suppressed unless goal is TRYING_TO_CONCEIVE. Default range: today − 90d … +180d. Gated."
+      parameters:
+        - in: query
+          name: from
+          schema:
+            type: string
+        - in: query
+          name: to
+          schema:
+            type: string
+      responses:
+        "200":
+          description: Calendar grid + forecast.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/CycleCalendarEnvelope"
+        "401": *a1
+        "403": *a2
+        "422": *a3
+        "429": *a4
+  /api/cycle/cycles:
+    get:
+      tags:
+        - Cycle
+      summary: Cycle history + stats (v1.15.0)
+      description: Most-recent cycles (newest first) + `{ avgLengthDays, lengthVariabilityDays (MAD), avgPeriodLengthDays,
+        regularity }`. `limit` default 24. Gated.
+      parameters:
+        - in: query
+          name: limit
+          schema:
+            type: integer
+            minimum: 1
+            maximum: 60
+      responses:
+        "200":
+          description: Cycle history.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/CycleHistoryEnvelope"
+        "401": *a1
+        "403": *a2
+        "422": *a3
+        "429": *a4
+  /api/cycle/cycles/{id}:
+    delete:
+      tags:
+        - Cycle
+      summary: Soft-delete a menstrual cycle (v1.15.0)
+      description: Sets `deletedAt` + bumps `syncVersion`; tombstones on the next sync page. 204. Idempotent.
+      parameters:
+        - in: path
+          name: id
+          schema:
+            type: string
+          required: true
+      responses:
+        "204":
+          description: Soft-deleted (no body).
+        "401": *a1
+        "403": *a2
+        "404":
+          description: Cycle not found / not owned.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
+        "422": *a3
+        "429": *a4
+  /api/cycle/profile:
+    get:
+      tags:
+        - Cycle
+      summary: Read the full cycle profile (v1.15.0)
+      description: Returns the resolved CycleProfileDTO. Gated.
+      responses:
+        "200":
+          description: Cycle profile.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/CycleProfileEnvelope"
+        "401": *a1
+        "403": *a2
+        "422": *a3
+        "429": *a4
+  /api/auth/me/cycle-prefs:
+    get:
+      tags:
+        - Cycle
+      summary: Read cycle preferences (v1.15.0)
+      description: Returns the resolved CycleProfileDTO. NOT gated — this is the surface that flips the gate.
+      responses:
+        "200":
+          description: Cycle preferences.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/CyclePrefsGetEnvelope"
+        "401": *a1
+        "422": *a3
+        "429": *a4
+    patch:
+      tags:
+        - Cycle
+      summary: Update cycle preferences (v1.15.0)
+      description: Deep-merges the supplied fields. `enabled` flips `cycleTrackingEnabled`. Returns the merged
+        CycleProfileDTO. NOT gated.
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              type: object
+              properties:
+                enabled:
+                  type: boolean
+                goal:
+                  type: string
+                  enum:
+                    - GENERAL_HEALTH
+                    - AVOID_PREGNANCY
+                    - TRYING_TO_CONCEIVE
+                    - PERIMENOPAUSE
+                    - OFF
+                rawChartMode:
+                  type: boolean
+                predictionEnabled:
+                  type: boolean
+                discreetNotifications:
+                  type: boolean
+                sensitiveCategoryEncryption:
+                  type: boolean
+                typicalCycleLength:
+                  anyOf:
+                    - type: integer
+                      minimum: 15
+                      maximum: 60
+                    - type: "null"
+                typicalPeriodLength:
+                  anyOf:
+                    - type: integer
+                      minimum: 1
+                      maximum: 15
+                    - type: "null"
+                lutealPhaseLength:
+                  anyOf:
+                    - type: integer
+                      minimum: 8
+                      maximum: 20
+                    - type: "null"
+      responses:
+        "200":
+          description: Merged cycle preferences.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/CyclePrefsPatchEnvelope"
+        "401": *a1
+        "422": *a3
+        "429": *a4
   /api/meta/capabilities:
     get:
       tags:
@@ -57,24 +712,9 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/CapabilitiesEnvelope"
-        "401": &a1
-          description: Authentication required or invalid credentials.
-          content:
-            application/json:
-              schema:
-                $ref: "#/components/schemas/ErrorEnvelope"
-        "422": &a2
-          description: Request validation failed.
-          content:
-            application/json:
-              schema:
-                $ref: "#/components/schemas/ErrorEnvelope"
-        "429": &a3
-          description: Rate limit exceeded.
-          content:
-            application/json:
-              schema:
-                $ref: "#/components/schemas/ErrorEnvelope"
+        "401": *a1
+        "422": *a3
+        "429": *a4
   /api/export/health-record:
     post:
       tags:
@@ -207,8 +847,8 @@ paths:
                 type: string
                 format: binary
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/share-links:
     post:
       tags:
@@ -323,8 +963,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/ShareLinkCreated"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
     get:
       tags:
         - Export
@@ -339,8 +979,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/ShareLinkList"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/share-links/{id}:
     delete:
       tags:
@@ -368,8 +1008,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/fhir/metadata:
     get:
       tags:
@@ -387,8 +1027,8 @@ paths:
                 type: string
                 format: binary
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/fhir/Patient:
     get:
       tags:
@@ -414,8 +1054,8 @@ paths:
                 type: string
                 format: binary
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/fhir/Observation:
     get:
       tags:
@@ -441,8 +1081,8 @@ paths:
                 type: string
                 format: binary
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/fhir/MedicationStatement:
     get:
       tags:
@@ -468,8 +1108,8 @@ paths:
                 type: string
                 format: binary
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/fhir/MedicationAdministration:
     get:
       tags:
@@ -495,8 +1135,8 @@ paths:
                 type: string
                 format: binary
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/fhir/$everything:
     get:
       tags:
@@ -523,8 +1163,8 @@ paths:
                 type: string
                 format: binary
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/auth/login:
     post:
       tags:
@@ -556,8 +1196,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/LoginResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/auth/passkey/login-verify:
     post:
       tags:
@@ -577,8 +1217,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/PasskeyLoginVerifyResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/auth/refresh:
     post:
       tags:
@@ -608,8 +1248,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/RefreshResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/sync/state:
     get:
       tags:
@@ -626,8 +1266,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/SyncStateEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/sync/changes:
     get:
       tags:
@@ -663,8 +1303,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/SyncChangesEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/measurements:
     get:
       tags:
@@ -819,8 +1459,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/ListMeasurementsResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
     post:
       tags:
         - Measurements
@@ -940,8 +1580,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/CreateMeasurementResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/measurements/batch:
     post:
       tags:
@@ -965,8 +1605,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/BatchMeasurementsResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/measurements/by-external-ids:
     delete:
       tags:
@@ -988,8 +1628,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/MeasurementsDeleteByExternalIdsResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/sleep/night:
     get:
       tags:
@@ -1017,8 +1657,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/SleepNightResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/workouts:
     get:
       tags:
@@ -1065,8 +1705,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/ListWorkoutsResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/workouts/{id}:
     get:
       tags:
@@ -1095,8 +1735,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/workouts/batch:
     post:
       tags:
@@ -1310,8 +1950,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/devices:
     post:
       tags:
@@ -1339,8 +1979,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/medications:
     get:
       tags:
@@ -1358,8 +1998,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/ListMedicationsResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
     post:
       tags:
         - Medications
@@ -1381,8 +2021,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/CreateMedicationResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/medications/{id}:
     get:
       tags:
@@ -1410,8 +2050,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
     put:
       tags:
         - Medications
@@ -1446,8 +2086,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
     delete:
       tags:
         - Medications
@@ -1474,8 +2114,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/medications/{id}/intake:
     post:
       tags:
@@ -1517,8 +2157,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/medications/extract:
     post:
       tags:
@@ -1543,8 +2183,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/MedicationExtractResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
         "502":
           description: Upstream provider returned an empty, unparseable, or off-schema reply.
           content:
@@ -1594,8 +2234,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/medications/{id}/compliance:
     get:
       tags:
@@ -1626,8 +2266,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/auth/me/coach-prefs:
     get:
       tags:
@@ -1643,8 +2283,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/GetCoachPrefsResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
     put:
       tags:
         - Auth
@@ -1725,8 +2365,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/PutCoachPrefsResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/auth/me/disable-coach:
     get:
       tags:
@@ -1742,8 +2382,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/GetDisableCoachResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
     patch:
       tags:
         - Auth
@@ -1764,8 +2404,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/PatchDisableCoachResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/auth/me/source-priority:
     get:
       tags:
@@ -1781,8 +2421,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/GetSourcePriorityResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
     put:
       tags:
         - Auth
@@ -2278,8 +2918,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/PutSourcePriorityResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/user/avatar:
     post:
       tags:
@@ -2322,8 +2962,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
     delete:
       tags:
         - Auth
@@ -2334,8 +2974,8 @@ paths:
         "204":
           description: Avatar cleared (or already empty).
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/user/avatar/{id}:
     get:
       tags:
@@ -2381,8 +3021,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/user/profile:
     get:
       tags:
@@ -2398,8 +3038,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/GetProfileResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
     patch:
       tags:
         - Auth
@@ -2421,8 +3061,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/PatchProfileResponse"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/layout:
     get:
       tags:
@@ -2438,8 +3078,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/InsightsLayout"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
     put:
       tags:
         - Insights
@@ -2460,8 +3100,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/InsightsLayoutSaved"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
     delete:
       tags:
         - Insights
@@ -2475,8 +3115,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/InsightsLayoutReset"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/coach/facts:
     get:
       tags:
@@ -2494,8 +3134,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/CoachFactsList"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
     delete:
       tags:
         - Insights
@@ -2510,8 +3150,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/CoachFactsCleared"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/coach/facts/{id}:
     delete:
       tags:
@@ -2528,8 +3168,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/CoachFactDeleted"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/chat/messages/{id}/feedback:
     post:
       tags:
@@ -2564,8 +3204,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/admin/notifications/diagnostic:
     get:
       tags:
@@ -2588,8 +3228,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/dashboard/snapshot:
     get:
       tags:
@@ -2606,8 +3246,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/DashboardSnapshotResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/comprehensive:
     get:
       tags:
@@ -2623,8 +3263,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/InsightsComprehensiveResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/pregenerate:
     post:
       tags:
@@ -2650,8 +3290,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/InsightsPregenerateResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/analytics/range:
     get:
       tags:
@@ -2756,8 +3396,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/AnalyticsRangeResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/blood-pressure-status:
     get:
       tags:
@@ -2784,8 +3424,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/BloodPressureStatusResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/pulse-status:
     get:
       tags:
@@ -2812,8 +3452,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/PulseStatusResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/weight-status:
     get:
       tags:
@@ -2840,8 +3480,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/WeightStatusResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/bmi-status:
     get:
       tags:
@@ -2868,8 +3508,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/BmiStatusResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/mood-status:
     get:
       tags:
@@ -2896,8 +3536,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/MoodStatusResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/medication-compliance-status:
     get:
       tags:
@@ -2924,8 +3564,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/MedicationComplianceStatusResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/metric-status:
     get:
       tags:
@@ -3003,8 +3643,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/MetricStatusResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/derived:
     get:
       tags:
@@ -3075,8 +3715,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/DerivedMetricResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/derived/batch:
     get:
       tags:
@@ -3109,8 +3749,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/DerivedBatchResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
   /api/insights/correlations:
     get:
       tags:
@@ -3129,8 +3769,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/CorrelationDiscoveryResponseEnvelope"
         "401": *a1
-        "422": *a2
-        "429": *a3
+        "422": *a3
+        "429": *a4
 components:
   schemas:
     PasskeyLoginVerifyRequest:
@@ -4063,6 +4703,699 @@ components:
         primitives are mutually exclusive — enforced by the Zod refine on writes, the route layer, and a DB CHECK
         constraint (`medication_schedules_rrule_xor_rolling`). v1.7.0 adds `scheduleType` (SCHEDULED / PRN / CYCLIC) and
         the cyclic on/off-week fields.
+    CycleDayLogEnvelope:
+      type: object
+      properties:
+        data:
+          $ref: "#/components/schemas/CycleDayLogDTO"
+        error:
+          type: "null"
+        meta:
+          type: object
+          properties:
+            requestId:
+              type: string
+          additionalProperties: false
+      required:
+        - data
+        - error
+      additionalProperties: false
+    CycleDayLogDTO:
+      type: object
+      properties:
+        id:
+          type: string
+        date:
+          type: string
+        cycleId:
+          anyOf:
+            - type: string
+            - type: "null"
+        flow:
+          anyOf:
+            - type: string
+              enum:
+                - NONE
+                - SPOTTING
+                - LIGHT
+                - MEDIUM
+                - HEAVY
+            - type: "null"
+        intermenstrualBleeding:
+          type: boolean
+        basalBodyTempC:
+          anyOf:
+            - type: number
+            - type: "null"
+        ovulationTest:
+          anyOf:
+            - type: string
+              enum:
+                - NEGATIVE
+                - POSITIVE_LH_SURGE
+                - ESTROGEN_SURGE
+                - INDETERMINATE
+            - type: "null"
+        cervicalMucus:
+          anyOf:
+            - type: string
+              enum:
+                - DRY
+                - STICKY
+                - CREAMY
+                - WATERY
+                - EGG_WHITE
+            - type: "null"
+        sexualActivity:
+          type: boolean
+        protectedSex:
+          anyOf:
+            - type: boolean
+            - type: "null"
+        pregnancyTest:
+          anyOf:
+            - type: string
+              enum:
+                - NEGATIVE
+                - POSITIVE
+                - INDETERMINATE
+            - type: "null"
+        progesteroneTest:
+          anyOf:
+            - type: string
+              enum:
+                - NEGATIVE
+                - POSITIVE
+                - INDETERMINATE
+            - type: "null"
+        contraceptive:
+          anyOf:
+            - type: string
+            - type: "null"
+        symptoms:
+          type: array
+          items:
+            type: object
+            properties:
+              key:
+                type: string
+              severity:
+                anyOf:
+                  - type: integer
+                    minimum: 1
+                    maximum: 4
+                  - type: "null"
+            required:
+              - key
+              - severity
+            additionalProperties: false
+        note:
+          anyOf:
+            - type: string
+            - type: "null"
+        source:
+          type: string
+        externalId:
+          anyOf:
+            - type: string
+            - type: "null"
+        syncVersion:
+          type: integer
+          minimum: -9007199254740991
+          maximum: 9007199254740991
+        updatedAt:
+          type: string
+          format: date-time
+          pattern: ^(?:(?:\d\d[2468][048]|\d\d[13579][26]|\d\d0[48]|[02468][048]00|[13579][26]00)-02-29|\d{4}-(?:(?:0[13578]|1[02])-(?:0[1-9]|[12]\d|3[01])|(?:0[469]|11)-(?:0[1-9]|[12]\d|30)|(?:02)-(?:0[1-9]|1\d|2[0-8])))T(?:(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d(?:\.\d+)?)?(?:Z|([+-](?:[01]\d|2[0-3]):[0-5]\d)))$
+        deletedAt:
+          anyOf:
+            - type: string
+              format: date-time
+              pattern: ^(?:(?:\d\d[2468][048]|\d\d[13579][26]|\d\d0[48]|[02468][048]00|[13579][26]00)-02-29|\d{4}-(?:(?:0[13578]|1[02])-(?:0[1-9]|[12]\d|3[01])|(?:0[469]|11)-(?:0[1-9]|[12]\d|30)|(?:02)-(?:0[1-9]|1\d|2[0-8])))T(?:(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d(?:\.\d+)?)?(?:Z|([+-](?:[01]\d|2[0-3]):[0-5]\d)))$
+            - type: "null"
+      required:
+        - id
+        - date
+        - cycleId
+        - flow
+        - intermenstrualBleeding
+        - basalBodyTempC
+        - ovulationTest
+        - cervicalMucus
+        - sexualActivity
+        - protectedSex
+        - pregnancyTest
+        - progesteroneTest
+        - contraceptive
+        - symptoms
+        - note
+        - source
+        - externalId
+        - syncVersion
+        - updatedAt
+        - deletedAt
+      additionalProperties: false
+      description: The canonical day-log row iOS mirrors. `note` is decrypted on read. Soft-deleted rows ride
+        `/api/sync/changes` as tombstones.
+    CycleDayLogCreatedEnvelope:
+      type: object
+      properties:
+        data:
+          $ref: "#/components/schemas/CycleDayLogDTO"
+        error:
+          type: "null"
+        meta:
+          type: object
+          properties:
+            requestId:
+              type: string
+          additionalProperties: false
+      required:
+        - data
+        - error
+      additionalProperties: false
+    ErrorEnvelope:
+      type: object
+      properties:
+        data:
+          type: "null"
+        error:
+          type: string
+        meta:
+          type: object
+          properties:
+            requestId:
+              type: string
+            errorCode:
+              type: string
+          additionalProperties: false
+      required:
+        - data
+        - error
+      additionalProperties: false
+      description: "Standard error response: data is null, error is human prose."
+    CycleDayLogPatchEnvelope:
+      type: object
+      properties:
+        data:
+          $ref: "#/components/schemas/CycleDayLogDTO"
+        error:
+          type: "null"
+        meta:
+          type: object
+          properties:
+            requestId:
+              type: string
+          additionalProperties: false
+      required:
+        - data
+        - error
+      additionalProperties: false
+    CycleBulkEnvelope:
+      type: object
+      properties:
+        data:
+          type: object
+          properties:
+            processed:
+              type: integer
+              minimum: -9007199254740991
+              maximum: 9007199254740991
+            inserted:
+              type: integer
+              minimum: -9007199254740991
+              maximum: 9007199254740991
+            updated:
+              type: integer
+              minimum: -9007199254740991
+              maximum: 9007199254740991
+            duplicates:
+              type: integer
+              minimum: -9007199254740991
+              maximum: 9007199254740991
+            skipped:
+              type: integer
+              minimum: -9007199254740991
+              maximum: 9007199254740991
+            entries:
+              type: array
+              items:
+                type: object
+                properties:
+                  index:
+                    type: integer
+                    minimum: -9007199254740991
+                    maximum: 9007199254740991
+                  status:
+                    type: string
+                    enum:
+                      - inserted
+                      - duplicate
+                      - updated
+                      - skipped
+                  id:
+                    type: string
+                  externalId:
+                    type: string
+                  reason:
+                    type: string
+                required:
+                  - index
+                  - status
+                additionalProperties: false
+          required:
+            - processed
+            - inserted
+            - updated
+            - duplicates
+            - skipped
+            - entries
+          additionalProperties: false
+        error:
+          type: "null"
+        meta:
+          type: object
+          properties:
+            requestId:
+              type: string
+          additionalProperties: false
+      required:
+        - data
+        - error
+      additionalProperties: false
+    CyclePeriodEnvelope:
+      type: object
+      properties:
+        data:
+          type: object
+          properties:
+            cycle:
+              anyOf:
+                - $ref: "#/components/schemas/MenstrualCycleDTO"
+                - type: "null"
+            dayLog:
+              anyOf:
+                - $ref: "#/components/schemas/CycleDayLogDTO"
+                - type: "null"
+          required:
+            - cycle
+            - dayLog
+          additionalProperties: false
+        error:
+          type: "null"
+        meta:
+          type: object
+          properties:
+            requestId:
+              type: string
+          additionalProperties: false
+      required:
+        - data
+        - error
+      additionalProperties: false
+    MenstrualCycleDTO:
+      type: object
+      properties:
+        id:
+          type: string
+        startDate:
+          type: string
+        endDate:
+          anyOf:
+            - type: string
+            - type: "null"
+        periodEndDate:
+          anyOf:
+            - type: string
+            - type: "null"
+        lengthDays:
+          anyOf:
+            - type: integer
+              minimum: -9007199254740991
+              maximum: 9007199254740991
+            - type: "null"
+        ovulationDate:
+          anyOf:
+            - type: string
+            - type: "null"
+        ovulationConfirmed:
+          type: boolean
+        isPredicted:
+          type: boolean
+        syncVersion:
+          type: integer
+          minimum: -9007199254740991
+          maximum: 9007199254740991
+        updatedAt:
+          type: string
+          format: date-time
+          pattern: ^(?:(?:\d\d[2468][048]|\d\d[13579][26]|\d\d0[48]|[02468][048]00|[13579][26]00)-02-29|\d{4}-(?:(?:0[13578]|1[02])-(?:0[1-9]|[12]\d|3[01])|(?:0[469]|11)-(?:0[1-9]|[12]\d|30)|(?:02)-(?:0[1-9]|1\d|2[0-8])))T(?:(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d(?:\.\d+)?)?(?:Z|([+-](?:[01]\d|2[0-3]):[0-5]\d)))$
+      required:
+        - id
+        - startDate
+        - endDate
+        - periodEndDate
+        - lengthDays
+        - ovulationDate
+        - ovulationConfirmed
+        - isPredicted
+        - syncVersion
+        - updatedAt
+      additionalProperties: false
+      description: One menstrual cycle (observed or forward-predicted).
+    CycleCalendarEnvelope:
+      type: object
+      properties:
+        data:
+          type: object
+          properties:
+            profile:
+              type: object
+              properties:
+                goal:
+                  type: string
+                  enum:
+                    - GENERAL_HEALTH
+                    - AVOID_PREGNANCY
+                    - TRYING_TO_CONCEIVE
+                    - PERIMENOPAUSE
+                    - OFF
+                rawChartMode:
+                  type: boolean
+                predictionEnabled:
+                  type: boolean
+                cyclesObserved:
+                  type: integer
+                  minimum: -9007199254740991
+                  maximum: 9007199254740991
+              required:
+                - goal
+                - rawChartMode
+                - predictionEnabled
+                - cyclesObserved
+              additionalProperties: false
+            prediction:
+              anyOf:
+                - $ref: "#/components/schemas/CyclePredictionDTO"
+                - type: "null"
+            days:
+              type: array
+              items:
+                type: object
+                properties:
+                  date:
+                    type: string
+                  phase:
+                    anyOf:
+                      - $ref: "#/components/schemas/CyclePhase"
+                      - type: "null"
+                  isPredictedPeriod:
+                    type: boolean
+                  isFertileWindow:
+                    type: boolean
+                  isPredictedOvulation:
+                    type: boolean
+                  isPeriodLogged:
+                    type: boolean
+                  flow:
+                    anyOf:
+                      - type: string
+                        enum:
+                          - NONE
+                          - SPOTTING
+                          - LIGHT
+                          - MEDIUM
+                          - HEAVY
+                      - type: "null"
+                  hasSymptoms:
+                    type: boolean
+                  confidence:
+                    type: number
+                required:
+                  - date
+                  - phase
+                  - isPredictedPeriod
+                  - isFertileWindow
+                  - isPredictedOvulation
+                  - isPeriodLogged
+                  - flow
+                  - hasSymptoms
+                  - confidence
+                additionalProperties: false
+            meta:
+              type: object
+              properties:
+                generatedAt:
+                  type: string
+                  format: date-time
+                  pattern: ^(?:(?:\d\d[2468][048]|\d\d[13579][26]|\d\d0[48]|[02468][048]00|[13579][26]00)-02-29|\d{4}-(?:(?:0[13578]|1[02])-(?:0[1-9]|[12]\d|3[01])|(?:0[469]|11)-(?:0[1-9]|[12]\d|30)|(?:02)-(?:0[1-9]|1\d|2[0-8])))T(?:(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d(?:\.\d+)?)?(?:Z|([+-](?:[01]\d|2[0-3]):[0-5]\d)))$
+              required:
+                - generatedAt
+              additionalProperties: false
+          required:
+            - profile
+            - prediction
+            - days
+            - meta
+          additionalProperties: false
+        error:
+          type: "null"
+        meta:
+          type: object
+          properties:
+            requestId:
+              type: string
+          additionalProperties: false
+      required:
+        - data
+        - error
+      additionalProperties: false
+    CyclePredictionDTO:
+      type: object
+      properties:
+        method:
+          $ref: "#/components/schemas/CyclePredictionMethod"
+        nextPeriodStart:
+          type: string
+        nextPeriodStartLow:
+          type: string
+        nextPeriodStartHigh:
+          type: string
+        fertileWindowStart:
+          anyOf:
+            - type: string
+            - type: "null"
+        fertileWindowEnd:
+          anyOf:
+            - type: string
+            - type: "null"
+        predictedOvulation:
+          anyOf:
+            - type: string
+            - type: "null"
+        confidence:
+          type: number
+        cyclesObserved:
+          type: integer
+          minimum: -9007199254740991
+          maximum: 9007199254740991
+        stillLearning:
+          type: boolean
+        disclaimer:
+          type: string
+      required:
+        - method
+        - nextPeriodStart
+        - nextPeriodStartLow
+        - nextPeriodStartHigh
+        - fertileWindowStart
+        - fertileWindowEnd
+        - predictedOvulation
+        - confidence
+        - cyclesObserved
+        - stillLearning
+        - disclaimer
+      additionalProperties: false
+      description: The materialised forecast. Fertile-window fields are server-suppressed (null) unless the goal is
+        TRYING_TO_CONCEIVE.
+    CyclePredictionMethod:
+      type: string
+      enum:
+        - CALENDAR
+        - SYMPTOTHERMAL
+        - TEMPERATURE_TREND
+        - BLENDED
+    CyclePhase:
+      type: string
+      enum:
+        - MENSTRUAL
+        - FOLLICULAR
+        - OVULATORY
+        - LUTEAL
+    CycleHistoryEnvelope:
+      type: object
+      properties:
+        data:
+          type: object
+          properties:
+            cycles:
+              type: array
+              items:
+                $ref: "#/components/schemas/MenstrualCycleDTO"
+            stats:
+              type: object
+              properties:
+                avgLengthDays:
+                  anyOf:
+                    - type: integer
+                      minimum: -9007199254740991
+                      maximum: 9007199254740991
+                    - type: "null"
+                lengthVariabilityDays:
+                  anyOf:
+                    - type: number
+                    - type: "null"
+                avgPeriodLengthDays:
+                  anyOf:
+                    - type: integer
+                      minimum: -9007199254740991
+                      maximum: 9007199254740991
+                    - type: "null"
+                regularity:
+                  type: string
+                  enum:
+                    - REGULAR
+                    - IRREGULAR
+                    - LEARNING
+              required:
+                - avgLengthDays
+                - lengthVariabilityDays
+                - avgPeriodLengthDays
+                - regularity
+              additionalProperties: false
+          required:
+            - cycles
+            - stats
+          additionalProperties: false
+        error:
+          type: "null"
+        meta:
+          type: object
+          properties:
+            requestId:
+              type: string
+          additionalProperties: false
+      required:
+        - data
+        - error
+      additionalProperties: false
+    CycleProfileEnvelope:
+      type: object
+      properties:
+        data:
+          $ref: "#/components/schemas/CycleProfileDTO"
+        error:
+          type: "null"
+        meta:
+          type: object
+          properties:
+            requestId:
+              type: string
+          additionalProperties: false
+      required:
+        - data
+        - error
+      additionalProperties: false
+    CycleProfileDTO:
+      type: object
+      properties:
+        goal:
+          type: string
+          enum:
+            - GENERAL_HEALTH
+            - AVOID_PREGNANCY
+            - TRYING_TO_CONCEIVE
+            - PERIMENOPAUSE
+            - OFF
+        cycleTrackingEnabled:
+          type: boolean
+        rawChartMode:
+          type: boolean
+        predictionEnabled:
+          type: boolean
+        discreetNotifications:
+          type: boolean
+        sensitiveCategoryEncryption:
+          type: boolean
+        typicalCycleLength:
+          anyOf:
+            - type: integer
+              minimum: -9007199254740991
+              maximum: 9007199254740991
+            - type: "null"
+        typicalPeriodLength:
+          anyOf:
+            - type: integer
+              minimum: -9007199254740991
+              maximum: 9007199254740991
+            - type: "null"
+        lutealPhaseLength:
+          anyOf:
+            - type: integer
+              minimum: -9007199254740991
+              maximum: 9007199254740991
+            - type: "null"
+        updatedAt:
+          type: string
+          format: date-time
+          pattern: ^(?:(?:\d\d[2468][048]|\d\d[13579][26]|\d\d0[48]|[02468][048]00|[13579][26]00)-02-29|\d{4}-(?:(?:0[13578]|1[02])-(?:0[1-9]|[12]\d|3[01])|(?:0[469]|11)-(?:0[1-9]|[12]\d|30)|(?:02)-(?:0[1-9]|1\d|2[0-8])))T(?:(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d(?:\.\d+)?)?(?:Z|([+-](?:[01]\d|2[0-3]):[0-5]\d)))$
+      required:
+        - goal
+        - cycleTrackingEnabled
+        - rawChartMode
+        - predictionEnabled
+        - discreetNotifications
+        - sensitiveCategoryEncryption
+        - typicalCycleLength
+        - typicalPeriodLength
+        - lutealPhaseLength
+        - updatedAt
+      additionalProperties: false
+      description: The full per-user cycle settings row.
+    CyclePrefsGetEnvelope:
+      type: object
+      properties:
+        data:
+          $ref: "#/components/schemas/CycleProfileDTO"
+        error:
+          type: "null"
+        meta:
+          type: object
+          properties:
+            requestId:
+              type: string
+          additionalProperties: false
+      required:
+        - data
+        - error
+      additionalProperties: false
+    CyclePrefsPatchEnvelope:
+      type: object
+      properties:
+        data:
+          $ref: "#/components/schemas/CycleProfileDTO"
+        error:
+          type: "null"
+        meta:
+          type: object
+          properties:
+            requestId:
+              type: string
+          additionalProperties: false
+      required:
+        - data
+        - error
+      additionalProperties: false
     CapabilitiesEnvelope:
       type: object
       properties:
@@ -4237,26 +5570,6 @@ components:
       additionalProperties: false
       description: Live id vocabularies + contract version. Every list is derived server-side from the canonical registry it
         documents, so it cannot drift from the values the routes actually accept/emit.
-    ErrorEnvelope:
-      type: object
-      properties:
-        data:
-          type: "null"
-        error:
-          type: string
-        meta:
-          type: object
-          properties:
-            requestId:
-              type: string
-            errorCode:
-              type: string
-          additionalProperties: false
-      required:
-        - data
-        - error
-      additionalProperties: false
-      description: "Standard error response: data is null, error is human prose."
     ShareLinkCreated:
       type: object
       properties:
diff --git a/messages/de.json b/messages/de.json
index 29aa90b09..d0a4d488b 100644
--- a/messages/de.json
+++ b/messages/de.json
@@ -4890,6 +4890,9 @@
       "nausea": "Übelkeit",
       "diarrhea": "Durchfall",
       "constipation": "Verstopfung"
+    },
+    "prediction": {
+      "disclaimer": "Vorhersagen sind Schätzungen aus deinen erfassten Daten, keine medizinische Beratung und keine Verhütung. Die Genauigkeit steigt, je mehr Zyklen du erfasst."
     }
   }
 }
diff --git a/messages/en.json b/messages/en.json
index 0d19a389f..c46256bba 100644
--- a/messages/en.json
+++ b/messages/en.json
@@ -4890,6 +4890,9 @@
       "nausea": "Nausea",
       "diarrhea": "Diarrhea",
       "constipation": "Constipation"
+    },
+    "prediction": {
+      "disclaimer": "Predictions are estimates from your logged data, not medical advice or contraception. Accuracy improves as you log more cycles."
     }
   }
 }
diff --git a/messages/es.json b/messages/es.json
index 9eb7616a7..3df96040b 100644
--- a/messages/es.json
+++ b/messages/es.json
@@ -4890,6 +4890,9 @@
       "nausea": "Náuseas",
       "diarrhea": "Diarrea",
       "constipation": "Estreñimiento"
+    },
+    "prediction": {
+      "disclaimer": "Las predicciones son estimaciones basadas en tus datos registrados, no son consejo médico ni un método anticonceptivo. La precisión mejora a medida que registras más ciclos."
     }
   }
 }
diff --git a/messages/fr.json b/messages/fr.json
index 4274933f8..85653fb21 100644
--- a/messages/fr.json
+++ b/messages/fr.json
@@ -4890,6 +4890,9 @@
       "nausea": "Nausées",
       "diarrhea": "Diarrhée",
       "constipation": "Constipation"
+    },
+    "prediction": {
+      "disclaimer": "Les prévisions sont des estimations issues de tes données enregistrées, et non un avis médical ou un moyen de contraception. La précision s'améliore à mesure que tu enregistres des cycles."
     }
   }
 }
diff --git a/messages/it.json b/messages/it.json
index 420309e26..ebdcd177d 100644
--- a/messages/it.json
+++ b/messages/it.json
@@ -4890,6 +4890,9 @@
       "nausea": "Nausea",
       "diarrhea": "Diarrea",
       "constipation": "Stitichezza"
+    },
+    "prediction": {
+      "disclaimer": "Le previsioni sono stime basate sui dati registrati, non sono un consiglio medico né un metodo contraccettivo. La precisione migliora man mano che registri più cicli."
     }
   }
 }
diff --git a/messages/pl.json b/messages/pl.json
index a4475a321..2b1eea8b6 100644
--- a/messages/pl.json
+++ b/messages/pl.json
@@ -4890,6 +4890,9 @@
       "nausea": "Nudności",
       "diarrhea": "Biegunka",
       "constipation": "Zaparcia"
+    },
+    "prediction": {
+      "disclaimer": "Prognozy to szacunki na podstawie zapisanych danych, a nie porada medyczna ani metoda antykoncepcji. Dokładność rośnie wraz z liczbą zapisanych cykli."
     }
   }
 }
diff --git a/prisma/migrations/0132_v1150_cycle_enable_flag/migration.sql b/prisma/migrations/0132_v1150_cycle_enable_flag/migration.sql
new file mode 100644
index 000000000..2d8c53bc2
--- /dev/null
+++ b/prisma/migrations/0132_v1150_cycle_enable_flag/migration.sql
@@ -0,0 +1,17 @@
+-- v1.15.0 — cycle-tracking feature enable flag.
+--
+-- Adds the per-user `cycle_tracking_enabled` toggle to `cycle_profiles`.
+-- The column is NULLABLE on purpose: NULL means "derive from gender"
+-- (`FEMALE` → enabled, everything else → disabled), so no row backfill
+-- is required — existing profiles read NULL and resolve through the
+-- `isCycleEnabled()` gate helper. An explicit `true` opts a non-FEMALE
+-- account in (Settings toggle); an explicit `false` opts a FEMALE
+-- account out. Every `/api/cycle/*` route returns
+-- `403 { error, meta:{ errorCode:"cycle.disabled" } }` when the resolved
+-- value is false.
+--
+-- Reversibility (down):
+--   ALTER TABLE "cycle_profiles" DROP COLUMN IF EXISTS "cycle_tracking_enabled";
+
+-- AlterTable
+ALTER TABLE "cycle_profiles" ADD COLUMN "cycle_tracking_enabled" BOOLEAN;
diff --git a/prisma/schema.prisma b/prisma/schema.prisma
index 49ed7ec50..e49884e87 100644
--- a/prisma/schema.prisma
+++ b/prisma/schema.prisma
@@ -3308,6 +3308,14 @@ model CycleProfile {
   /// Drives all cycle copy + fertile-window gating (see CycleTrackingGoal).
   goal CycleTrackingGoal @default(GENERAL_HEALTH)
 
+  /// Per-user feature enable. NULL = derive from gender (`FEMALE` → on,
+  /// everything else → off) so the column never needs a backfill; an
+  /// explicit `true` opts a non-FEMALE account in, an explicit `false`
+  /// opts a FEMALE account out. Resolved through `isCycleEnabled()` —
+  /// every `/api/cycle/*` route 403s with `errorCode:"cycle.disabled"`
+  /// when the resolved value is false.
+  cycleTrackingEnabled Boolean? @map("cycle_tracking_enabled")
+
   /// Optional user-supplied priors. The engine applies a luteal default
   /// of 14 days in code (NOT a DB default) when `lutealPhaseLength` is
   /// NULL, so a future constant change never strands historical rows.
diff --git a/src/app/api/auth/me/cycle-prefs/route.ts b/src/app/api/auth/me/cycle-prefs/route.ts
new file mode 100644
index 000000000..3de329d1c
--- /dev/null
+++ b/src/app/api/auth/me/cycle-prefs/route.ts
@@ -0,0 +1,131 @@
+/**
+ * v1.15.0 — per-user cycle-tracking preferences.
+ *
+ *  GET   /api/auth/me/cycle-prefs   — current resolved CycleProfileDTO.
+ *  PATCH /api/auth/me/cycle-prefs   — body `{ enabled?, goal?, rawChartMode?,
+ *                                      predictionEnabled?, discreetNotifications?,
+ *                                      sensitiveCategoryEncryption?,
+ *                                      typicalCycleLength?, typicalPeriodLength?,
+ *                                      lutealPhaseLength? }`. Deep-merges the
+ *                                      supplied fields over the persisted row
+ *                                      (the notification-prefs precedent) and
+ *                                      returns the merged CycleProfileDTO.
+ *
+ * This route does NOT enforce the cycle gate — it is the surface that
+ * FLIPS the gate (`enabled` → `cycleTrackingEnabled`). `enabled` is
+ * idempotent last-writer-wins; the route is otherwise idempotent.
+ */
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import {
+  apiError,
+  apiSuccess,
+  getClientIp,
+  returnAllZodIssues,
+} from "@/lib/api-response";
+import { annotate } from "@/lib/logging/context";
+import { auditLog } from "@/lib/auth/audit";
+import { prisma } from "@/lib/db";
+import { checkRateLimit, rateLimitHeaders } from "@/lib/rate-limit";
+import { getOrCreateCycleProfile } from "@/lib/cycle/profile";
+import { isCycleEnabled } from "@/lib/cycle/gate";
+import { toCycleProfileDTO } from "@/lib/cycle/dto";
+import { cyclePrefsSchema } from "@/lib/validations/cycle";
+
+export const dynamic = "force-dynamic";
+
+const PATCH_RATE_LIMIT = 60;
+const PATCH_WINDOW_MS = 60_000;
+
+export const GET = apiHandler(async () => {
+  const { user } = await requireAuth();
+  annotate({ action: { name: "auth.me.cycle-prefs.get" } });
+
+  const profile = await getOrCreateCycleProfile(user.id);
+  const resolved = isCycleEnabled(user.gender, profile);
+  return apiSuccess(toCycleProfileDTO(profile, resolved));
+});
+
+export const PATCH = apiHandler(async (req: Request) => {
+  const { user } = await requireAuth();
+
+  const rl = await checkRateLimit(
+    `cycle-prefs:patch:${user.id}`,
+    PATCH_RATE_LIMIT,
+    PATCH_WINDOW_MS,
+  );
+  if (!rl.allowed) {
+    const response = apiError("Too many requests", 429);
+    for (const [k, v] of Object.entries(rateLimitHeaders(rl))) {
+      response.headers.set(k, v);
+    }
+    return response;
+  }
+
+  let body: unknown;
+  try {
+    body = await req.json();
+  } catch {
+    return apiError("Invalid JSON body", 422, {
+      errorCode: "cycle-prefs.body.invalid_json",
+    });
+  }
+
+  const parsed = cyclePrefsSchema.safeParse(body ?? {});
+  if (!parsed.success) {
+    annotate({
+      action: { name: "auth.me.cycle-prefs.patch.invalid_shape" },
+      meta: { issues: parsed.error.issues.length },
+    });
+    return returnAllZodIssues(parsed.error, 422, {
+      errorCode: "cycle-prefs.invalid",
+    });
+  }
+
+  // Ensure the row exists, then deep-merge: only the supplied fields are
+  // touched; siblings are left untouched (field-by-field, no mass
+  // assignment). `enabled` maps onto the `cycleTrackingEnabled` column.
+  await getOrCreateCycleProfile(user.id);
+  const p = parsed.data;
+
+  const updated = await prisma.cycleProfile.update({
+    where: { userId: user.id },
+    data: {
+      ...(p.enabled !== undefined && { cycleTrackingEnabled: p.enabled }),
+      ...(p.goal !== undefined && { goal: p.goal }),
+      ...(p.rawChartMode !== undefined && { rawChartMode: p.rawChartMode }),
+      ...(p.predictionEnabled !== undefined && {
+        predictionEnabled: p.predictionEnabled,
+      }),
+      ...(p.discreetNotifications !== undefined && {
+        discreetNotifications: p.discreetNotifications,
+      }),
+      ...(p.sensitiveCategoryEncryption !== undefined && {
+        sensitiveCategoryEncryption: p.sensitiveCategoryEncryption,
+      }),
+      ...(p.typicalCycleLength !== undefined && {
+        typicalCycleLength: p.typicalCycleLength,
+      }),
+      ...(p.typicalPeriodLength !== undefined && {
+        typicalPeriodLength: p.typicalPeriodLength,
+      }),
+      ...(p.lutealPhaseLength !== undefined && {
+        lutealPhaseLength: p.lutealPhaseLength,
+      }),
+    },
+  });
+
+  const resolved = isCycleEnabled(user.gender, updated);
+
+  await auditLog("user.cycle-prefs.update", {
+    userId: user.id,
+    ipAddress: getClientIp(req),
+    details: { changed: Object.keys(p) },
+  });
+
+  annotate({
+    action: { name: "user.cycle_prefs.update" },
+    meta: { changed: Object.keys(p), enabled: resolved },
+  });
+
+  return apiSuccess(toCycleProfileDTO(updated, resolved));
+});
diff --git a/src/app/api/auth/me/route.ts b/src/app/api/auth/me/route.ts
index 49f74a96a..22b43748f 100644
--- a/src/app/api/auth/me/route.ts
+++ b/src/app/api/auth/me/route.ts
@@ -4,6 +4,8 @@ import { annotate } from "@/lib/logging/context";
 import { setOnboardingPendingCookie } from "@/lib/auth/session";
 import { buildAvatarUrl } from "@/lib/avatar";
 import { decrypt } from "@/lib/crypto";
+import { prisma } from "@/lib/db";
+import { isCycleEnabled } from "@/lib/cycle/gate";
 
 export const dynamic = "force-dynamic";
 
@@ -19,6 +21,15 @@ export const GET = apiHandler(async () => {
 
   annotate({ action: { name: "auth.me" } });
 
+  // v1.15.0 — resolved cycle-tracking gate. Read the profile without
+  // forcing a row (a NULL toggle derives from gender); the resolver
+  // collapses both gates into the single boolean iOS hides the tab on.
+  const cycleProfile = await prisma.cycleProfile.findUnique({
+    where: { userId: user.id },
+    select: { cycleTrackingEnabled: true },
+  });
+  const cycleTrackingEnabled = isCycleEnabled(user.gender, cycleProfile);
+
   // v1.7.0 — patient-identity fields for the health-record export. The
   // KVNR is stored encrypted; decrypt fail-soft so a key-rotation gap on
   // one row never 500s the whole profile fetch (the field just reads
@@ -68,5 +79,8 @@ export const GET = apiHandler(async () => {
     insurerName: user.insurerName ?? null,
     insurerIkNumber: user.insurerIkNumber ?? null,
     insuranceNumber,
+    // v1.15.0 — cycle-tracking feature gate, resolved server-side. iOS
+    // hides the whole cycle tab when this is false.
+    cycleTrackingEnabled,
   });
 });
diff --git a/src/app/api/cycle/calendar/route.ts b/src/app/api/cycle/calendar/route.ts
new file mode 100644
index 000000000..ee72d97c5
--- /dev/null
+++ b/src/app/api/cycle/calendar/route.ts
@@ -0,0 +1,171 @@
+/**
+ * `GET /api/cycle/calendar?from&to` — predicted calendar read
+ * (ios-contract §2.D).
+ *
+ * Calls the pure engine (`predictCycle` + per-day phase) to build
+ * `{ profile, prediction, days }`. The synchronous engine call is cheap
+ * (pure stats); the heavy part is the cache persist, which is debounced
+ * stale-while-revalidate and fire-and-forget so it never blocks the GET.
+ * Fertile-window fields are goal-gated (null unless TRYING_TO_CONCEIVE),
+ * suppressed server-side.
+ *
+ * Default range: current cycle − 90d … +180d forward.
+ */
+import { NextRequest } from "next/server";
+
+import { prisma } from "@/lib/db";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { annotate } from "@/lib/logging/context";
+import { apiError, apiSuccess } from "@/lib/api-response";
+import { requireCycleEnabled } from "@/lib/cycle/gate";
+import { cycleCalendarQuerySchema } from "@/lib/validations/cycle";
+import {
+  buildCalendar,
+  type CalendarDayLogRow,
+} from "@/lib/cycle/engine-adapter";
+import {
+  toCyclePredictionDTO,
+  goalAllowsFertileWindow,
+} from "@/lib/cycle/dto";
+import { persistPredictionCache } from "@/lib/cycle/prediction-cache";
+import { addDays } from "@/lib/cycle/day-math";
+import { DEFAULT_TIMEZONE, moodDateKey } from "@/lib/mood/date-key";
+import { getServerTranslator } from "@/lib/i18n/server-translator";
+import { resolveServerLocale } from "@/lib/i18n/server-locale";
+
+const DEFAULT_PAST_DAYS = 90;
+const DEFAULT_FORWARD_DAYS = 180;
+/** Hard cap on the rendered span (days) to bound the day-grid build. */
+const MAX_SPAN_DAYS = 400;
+
+export const GET = apiHandler(async (request: NextRequest) => {
+  const { user } = await requireAuth();
+
+  const gate = await requireCycleEnabled(user.id, user.gender);
+  if (!gate.enabled) return gate.response;
+  const profile = gate.profile;
+
+  const url = new URL(request.url);
+  const parsed = cycleCalendarQuerySchema.safeParse({
+    from: url.searchParams.get("from") ?? undefined,
+    to: url.searchParams.get("to") ?? undefined,
+  });
+  if (!parsed.success) {
+    return apiError("Invalid calendar query", 422, {
+      errorCode: "cycle.calendar.invalid",
+    });
+  }
+
+  const tz = user.timezone ?? DEFAULT_TIMEZONE;
+  const today = moodDateKey(new Date(), tz);
+  const from = parsed.data.from ?? addDays(today, -DEFAULT_PAST_DAYS);
+  const to = parsed.data.to ?? addDays(today, DEFAULT_FORWARD_DAYS);
+
+  // Reject an inverted or absurdly wide range up front.
+  const fromMs = Date.parse(`${from}T12:00:00Z`);
+  const toMs = Date.parse(`${to}T12:00:00Z`);
+  if (toMs < fromMs || (toMs - fromMs) / 86_400_000 > MAX_SPAN_DAYS) {
+    return apiError("Calendar range too wide or inverted", 422, {
+      errorCode: "cycle.calendar.range",
+    });
+  }
+
+  const [cycles, dayLogRows, nightlyTemps] = await Promise.all([
+    prisma.menstrualCycle.findMany({
+      where: { userId: user.id, deletedAt: null },
+      orderBy: { startDate: "asc" },
+    }),
+    prisma.cycleDayLog.findMany({
+      where: { userId: user.id, deletedAt: null },
+      orderBy: { date: "asc" },
+      select: {
+        date: true,
+        flow: true,
+        basalBodyTempC: true,
+        ovulationTest: true,
+        cervicalMucus: true,
+        _count: { select: { symptomLinks: true } },
+      },
+    }),
+    // Apple Watch wrist/skin temperature feeds the temperature-trend
+    // ovulation layer. Read the WRIST_TEMPERATURE measurements as nightly
+    // values; the engine derives the trailing-mean deviation itself.
+    prisma.measurement.findMany({
+      where: {
+        userId: user.id,
+        deletedAt: null,
+        type: "WRIST_TEMPERATURE",
+        measuredAt: {
+          gte: new Date(Date.parse(`${addDays(today, -90)}T00:00:00Z`)),
+        },
+      },
+      orderBy: { measuredAt: "asc" },
+      select: { measuredAt: true, value: true },
+    }),
+  ]);
+
+  const dayLogs: CalendarDayLogRow[] = dayLogRows.map((l) => ({
+    date: l.date,
+    flow: l.flow,
+    basalBodyTempC: l.basalBodyTempC,
+    ovulationTest: l.ovulationTest,
+    cervicalMucus: l.cervicalMucus,
+    hasSymptoms: l._count.symptomLinks > 0,
+  }));
+
+  const nights = nightlyTemps.map((m) => ({
+    date: moodDateKey(m.measuredAt, tz),
+    valueC: m.value,
+  }));
+
+  const goalAllowsFertile = goalAllowsFertileWindow(profile.goal);
+
+  const { prediction, days } = buildCalendar(
+    profile,
+    cycles,
+    dayLogs,
+    nights,
+    from,
+    to,
+    today,
+    goalAllowsFertile,
+  );
+
+  // Persist the cache fire-and-forget (debounced) — never block the read.
+  let generatedAt = new Date().toISOString();
+  if (prediction) {
+    const now = new Date();
+    generatedAt = now.toISOString();
+    void persistPredictionCache(user.id, prediction, now);
+  }
+
+  const locale = await resolveServerLocale({
+    request,
+    userLocale: user.locale ?? undefined,
+  });
+  const t = getServerTranslator(locale);
+  const disclaimer = t.t("cycle.prediction.disclaimer");
+
+  annotate({
+    action: { name: "cycle.calendar.read" },
+    meta: {
+      days: days.length,
+      cycles_observed: prediction?.cyclesObserved ?? 0,
+      has_prediction: prediction !== null,
+    },
+  });
+
+  return apiSuccess({
+    profile: {
+      goal: profile.goal,
+      rawChartMode: profile.rawChartMode,
+      predictionEnabled: profile.predictionEnabled,
+      cyclesObserved: prediction?.cyclesObserved ?? cycles.length,
+    },
+    prediction: prediction
+      ? toCyclePredictionDTO(prediction, goalAllowsFertile, disclaimer)
+      : null,
+    days,
+    meta: { generatedAt },
+  });
+});
diff --git a/src/app/api/cycle/cycles/[id]/route.ts b/src/app/api/cycle/cycles/[id]/route.ts
new file mode 100644
index 000000000..2810bc90e
--- /dev/null
+++ b/src/app/api/cycle/cycles/[id]/route.ts
@@ -0,0 +1,55 @@
+/**
+ * `DELETE /api/cycle/cycles/{id}` — soft-delete a cycle
+ * (ios-contract §2.F): set `deletedAt` + bump `syncVersion`, emit a
+ * tombstone on the next sync page. 204. Idempotent. Owner-scoped + gated.
+ */
+import { NextRequest } from "next/server";
+
+import { prisma } from "@/lib/db";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { annotate } from "@/lib/logging/context";
+import { auditLog } from "@/lib/auth/audit";
+import { apiError, getClientIp } from "@/lib/api-response";
+import { requireCycleEnabled } from "@/lib/cycle/gate";
+
+type RouteParams = { params: Promise<{ id: string }> };
+
+export const DELETE = apiHandler(
+  async (request: NextRequest, { params }: RouteParams) => {
+    const { user } = await requireAuth();
+
+    const gate = await requireCycleEnabled(user.id, user.gender);
+    if (!gate.enabled) return gate.response;
+
+    const { id } = await params;
+
+    const existing = await prisma.menstrualCycle.findUnique({
+      where: { id },
+      select: { id: true, userId: true },
+    });
+    if (!existing || existing.userId !== user.id) {
+      return apiError("Cycle not found", 404);
+    }
+
+    await prisma.menstrualCycle.update({
+      where: { id },
+      data: { deletedAt: new Date(), syncVersion: { increment: 1 } },
+    });
+
+    await auditLog("cycle.cycle.delete", {
+      userId: user.id,
+      ipAddress: getClientIp(request),
+      details: { cycleId: id },
+    });
+
+    annotate({
+      action: {
+        name: "cycle.cycle.delete",
+        entity_type: "menstrual_cycle",
+        entity_id: id,
+      },
+    });
+
+    return new Response(null, { status: 204 });
+  },
+);
diff --git a/src/app/api/cycle/cycles/route.ts b/src/app/api/cycle/cycles/route.ts
new file mode 100644
index 000000000..d714c010b
--- /dev/null
+++ b/src/app/api/cycle/cycles/route.ts
@@ -0,0 +1,99 @@
+/**
+ * `GET /api/cycle/cycles?limit=24` — cycle history + summary stats
+ * (ios-contract §2.E).
+ *
+ * Returns the most recent observed cycles (newest first) plus
+ * `{ avgLengthDays, lengthVariabilityDays (MAD), avgPeriodLengthDays,
+ * regularity }`. The median / MAD reuse the engine helpers so the stats
+ * match the prediction's own variability math.
+ */
+import { NextRequest } from "next/server";
+
+import { prisma } from "@/lib/db";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { annotate } from "@/lib/logging/context";
+import { apiError, apiSuccess } from "@/lib/api-response";
+import { requireCycleEnabled } from "@/lib/cycle/gate";
+import { cycleHistoryQuerySchema } from "@/lib/validations/cycle";
+import { toMenstrualCycleDTO } from "@/lib/cycle/dto";
+import { median, mad } from "@/lib/cycle";
+import { dayDiff } from "@/lib/cycle/day-math";
+import { MIN_CYCLES_TO_PREDICT } from "@/lib/cycle/types";
+
+const DEFAULT_LIMIT = 24;
+
+/**
+ * Regularity gate. < MIN_CYCLES → LEARNING; a length MAD at/under 2 days
+ * (the clinical "regular cycle" rule of thumb) → REGULAR, else IRREGULAR.
+ */
+const REGULAR_MAD_THRESHOLD = 2;
+
+export const GET = apiHandler(async (request: NextRequest) => {
+  const { user } = await requireAuth();
+
+  const gate = await requireCycleEnabled(user.id, user.gender);
+  if (!gate.enabled) return gate.response;
+
+  const url = new URL(request.url);
+  const parsed = cycleHistoryQuerySchema.safeParse({
+    limit: url.searchParams.get("limit") ?? undefined,
+  });
+  if (!parsed.success) {
+    return apiError("Invalid history query", 422, {
+      errorCode: "cycle.cycles.invalid",
+    });
+  }
+  const limit = parsed.data.limit ?? DEFAULT_LIMIT;
+
+  const rows = await prisma.menstrualCycle.findMany({
+    where: { userId: user.id, deletedAt: null },
+    orderBy: { startDate: "desc" },
+    take: limit,
+  });
+
+  // Completed lengths (non-null lengthDays) drive the length stats.
+  const lengths = rows
+    .map((r) => r.lengthDays)
+    .filter((l): l is number => l != null && l > 0);
+
+  // Period lengths from periodEndDate − startDate + 1.
+  const periodLengths = rows
+    .filter((r) => r.periodEndDate != null)
+    .map((r) => dayDiff(r.periodEndDate as string, r.startDate) + 1)
+    .filter((l) => l > 0);
+
+  let avgLengthDays: number | null = null;
+  let lengthVariabilityDays: number | null = null;
+  let avgPeriodLengthDays: number | null = null;
+  let regularity: "REGULAR" | "IRREGULAR" | "LEARNING" = "LEARNING";
+
+  if (lengths.length > 0) {
+    const med = median(lengths);
+    avgLengthDays = Math.round(med);
+    lengthVariabilityDays = Math.round(mad(lengths, med) * 10) / 10;
+    if (lengths.length < MIN_CYCLES_TO_PREDICT) {
+      regularity = "LEARNING";
+    } else {
+      regularity =
+        lengthVariabilityDays <= REGULAR_MAD_THRESHOLD ? "REGULAR" : "IRREGULAR";
+    }
+  }
+  if (periodLengths.length > 0) {
+    avgPeriodLengthDays = Math.round(median(periodLengths));
+  }
+
+  annotate({
+    action: { name: "cycle.cycles.read" },
+    meta: { returned: rows.length, regularity },
+  });
+
+  return apiSuccess({
+    cycles: rows.map(toMenstrualCycleDTO),
+    stats: {
+      avgLengthDays,
+      lengthVariabilityDays,
+      avgPeriodLengthDays,
+      regularity,
+    },
+  });
+});
diff --git a/src/app/api/cycle/day-logs/[id]/route.ts b/src/app/api/cycle/day-logs/[id]/route.ts
new file mode 100644
index 000000000..362d039b0
--- /dev/null
+++ b/src/app/api/cycle/day-logs/[id]/route.ts
@@ -0,0 +1,192 @@
+/**
+ * `PATCH /api/cycle/day-logs/{id}` — edit a single day-log
+ * (ios-contract §2.A).
+ * `DELETE /api/cycle/day-logs/{id}` — soft-delete (ios-contract §2.F):
+ *   set `deletedAt` + bump `syncVersion`, emit a tombstone on the next
+ *   sync page. 204. Idempotent.
+ *
+ * Both are gated (`cycle.disabled` 403) and owner-scoped (a row owned by
+ * another user 404s).
+ */
+import { NextRequest } from "next/server";
+
+import { prisma } from "@/lib/db";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { annotate } from "@/lib/logging/context";
+import { auditLog } from "@/lib/auth/audit";
+import {
+  apiSuccess,
+  apiError,
+  getClientIp,
+  returnAllZodIssues,
+  safeJson,
+} from "@/lib/api-response";
+import { requireCycleEnabled } from "@/lib/cycle/gate";
+import { encrypt } from "@/lib/crypto";
+import { cycleDayLogPatchSchema } from "@/lib/validations/cycle";
+import { toCycleDayLogDTO, dayLogSymptomInclude } from "@/lib/cycle/dto";
+
+type RouteParams = { params: Promise<{ id: string }> };
+
+export const PATCH = apiHandler(
+  async (request: NextRequest, { params }: RouteParams) => {
+    const { user } = await requireAuth();
+
+    const gate = await requireCycleEnabled(user.id, user.gender);
+    if (!gate.enabled) return gate.response;
+
+    const { id } = await params;
+
+    const existing = await prisma.cycleDayLog.findFirst({
+      where: { id, deletedAt: null },
+      select: { id: true, userId: true },
+    });
+    if (!existing || existing.userId !== user.id) {
+      return apiError("Day-log not found", 404);
+    }
+
+    const { data: rawBody, error: jsonError } = await safeJson(request);
+    if (jsonError) return jsonError;
+
+    const parsed = cycleDayLogPatchSchema.safeParse(rawBody);
+    if (!parsed.success) {
+      annotate({
+        action: { name: "cycle.day-log.patch.validation-failed" },
+        meta: { issue_count: parsed.error.issues.length },
+      });
+      return returnAllZodIssues(parsed.error, 422, {
+        errorCode: "cycle.day-log.invalid",
+      });
+    }
+
+    const body = parsed.data;
+
+    // Field-by-field update (no mass assignment). Every field is
+    // optional; an omitted field is left untouched. `note` re-encrypts;
+    // an explicit null clears it.
+    await prisma.cycleDayLog.update({
+      where: { id },
+      data: {
+        ...(body.flow !== undefined && { flow: body.flow }),
+        ...(body.intermenstrualBleeding !== undefined && {
+          intermenstrualBleeding: body.intermenstrualBleeding,
+        }),
+        ...(body.basalBodyTempC !== undefined && {
+          basalBodyTempC: body.basalBodyTempC,
+        }),
+        ...(body.ovulationTest !== undefined && {
+          ovulationTest: body.ovulationTest,
+        }),
+        ...(body.cervicalMucus !== undefined && {
+          cervicalMucus: body.cervicalMucus,
+        }),
+        ...(body.sexualActivity !== undefined && {
+          sexualActivity: body.sexualActivity,
+        }),
+        ...(body.protectedSex !== undefined && {
+          protectedSex: body.protectedSex,
+        }),
+        ...(body.pregnancyTest !== undefined && {
+          pregnancyTest: body.pregnancyTest,
+        }),
+        ...(body.progesteroneTest !== undefined && {
+          progesteroneTest: body.progesteroneTest,
+        }),
+        ...(body.contraceptive !== undefined && {
+          contraceptive: body.contraceptive,
+        }),
+        ...(body.note !== undefined && {
+          notesEncrypted: body.note ? encrypt(body.note) : null,
+        }),
+        syncVersion: { increment: 1 },
+      },
+    });
+
+    // Replace symptom links only when `symptoms` was supplied.
+    if (body.symptoms !== undefined) {
+      const keys = Array.from(new Set(body.symptoms.map((s) => s.key)));
+      const symptoms =
+        keys.length > 0
+          ? await prisma.cycleSymptom.findMany({
+              where: {
+                key: { in: keys },
+                isActive: true,
+                OR: [{ userId: null }, { userId: user.id }],
+              },
+              select: { id: true },
+            })
+          : [];
+      await prisma.cycleSymptomLink.deleteMany({ where: { dayLogId: id } });
+      if (symptoms.length > 0) {
+        await prisma.cycleSymptomLink.createMany({
+          data: symptoms.map((s) => ({ dayLogId: id, symptomId: s.id })),
+          skipDuplicates: true,
+        });
+      }
+    }
+
+    const row = await prisma.cycleDayLog.findUniqueOrThrow({
+      where: { id },
+      include: dayLogSymptomInclude,
+    });
+
+    await auditLog("cycle.day-log.update", {
+      userId: user.id,
+      ipAddress: getClientIp(request),
+      details: { dayLogId: id },
+    });
+
+    annotate({
+      action: {
+        name: "cycle.day-log.update",
+        entity_type: "cycle_day_log",
+        entity_id: id,
+      },
+    });
+
+    return apiSuccess(toCycleDayLogDTO(row));
+  },
+);
+
+export const DELETE = apiHandler(
+  async (request: NextRequest, { params }: RouteParams) => {
+    const { user } = await requireAuth();
+
+    const gate = await requireCycleEnabled(user.id, user.gender);
+    if (!gate.enabled) return gate.response;
+
+    const { id } = await params;
+
+    const existing = await prisma.cycleDayLog.findUnique({
+      where: { id },
+      select: { id: true, userId: true },
+    });
+    if (!existing || existing.userId !== user.id) {
+      return apiError("Day-log not found", 404);
+    }
+
+    // Soft-delete: leave the row in place so the `/api/sync/changes`
+    // delta feed surfaces it as a tombstone. A re-delete re-bumps
+    // `syncVersion` harmlessly (idempotent).
+    await prisma.cycleDayLog.update({
+      where: { id },
+      data: { deletedAt: new Date(), syncVersion: { increment: 1 } },
+    });
+
+    await auditLog("cycle.day-log.delete", {
+      userId: user.id,
+      ipAddress: getClientIp(request),
+      details: { dayLogId: id },
+    });
+
+    annotate({
+      action: {
+        name: "cycle.day-log.delete",
+        entity_type: "cycle_day_log",
+        entity_id: id,
+      },
+    });
+
+    return new Response(null, { status: 204 });
+  },
+);
diff --git a/src/app/api/cycle/day-logs/bulk/route.ts b/src/app/api/cycle/day-logs/bulk/route.ts
new file mode 100644
index 000000000..27272e80c
--- /dev/null
+++ b/src/app/api/cycle/day-logs/bulk/route.ts
@@ -0,0 +1,169 @@
+/**
+ * `POST /api/cycle/day-logs/bulk` — Outbox / HealthKit drain
+ * (ios-contract §2.B / §3).
+ *
+ * The route iOS's Outbox flushes to. Per-entry UPSERT keyed on the
+ * NULL-distinct `(userId, source, externalId)` (or `(userId, date)`
+ * when no externalId). Capped at 500, wrapped in `withIdempotency`,
+ * rate-limited `cycle:day-logs:bulk:{userId}` 60/min. Always 200; each
+ * entry carries `{ index, status, id?, externalId?, reason? }` where
+ * status is `inserted` | `duplicate` | `updated` | `skipped`. `updated`
+ * = an externalId re-post that changed a field; `duplicate` = an
+ * unchanged re-post.
+ */
+import { NextRequest } from "next/server";
+
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { annotate } from "@/lib/logging/context";
+import { auditLog } from "@/lib/auth/audit";
+import {
+  apiSuccess,
+  apiError,
+  getClientIp,
+  returnAllZodIssues,
+  safeJson,
+  sanitiseZodIssues,
+} from "@/lib/api-response";
+import { withIdempotency } from "@/lib/idempotency";
+import { checkRateLimit } from "@/lib/rate-limit";
+import { requireCycleEnabled } from "@/lib/cycle/gate";
+import { cycleBulkSchema, MAX_CYCLE_BULK_ENTRIES } from "@/lib/validations/cycle";
+import { upsertCycleDayLog } from "@/lib/cycle/day-log-write";
+import { findOwningCycleId } from "@/lib/cycle/cycle-attribution";
+import { DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+
+const BATCH_RATE_LIMIT_MAX = 60;
+const BATCH_RATE_LIMIT_WINDOW_MS = 60 * 1000;
+
+type EntryStatus = "inserted" | "duplicate" | "updated" | "skipped";
+interface EntryResult {
+  index: number;
+  status: EntryStatus;
+  id?: string;
+  externalId?: string;
+  reason?: string;
+}
+
+export const POST = apiHandler(withIdempotency<[NextRequest]>(postBulk));
+
+async function postBulk(request: NextRequest): Promise<Response> {
+  const { user } = await requireAuth();
+
+  const gate = await requireCycleEnabled(user.id, user.gender);
+  if (!gate.enabled) return gate.response;
+
+  const rl = await checkRateLimit(
+    `cycle:day-logs:bulk:${user.id}`,
+    BATCH_RATE_LIMIT_MAX,
+    BATCH_RATE_LIMIT_WINDOW_MS,
+  );
+  if (!rl.allowed) {
+    return apiError("Too many bulk submissions, try again later", 429);
+  }
+
+  const { data: rawBody, error: jsonError } = await safeJson(request);
+  if (jsonError) return jsonError;
+
+  if (
+    typeof rawBody === "object" &&
+    rawBody !== null &&
+    "entries" in rawBody &&
+    Array.isArray((rawBody as { entries: unknown }).entries) &&
+    (rawBody as { entries: unknown[] }).entries.length > MAX_CYCLE_BULK_ENTRIES
+  ) {
+    return apiError(
+      `Batch exceeds the ${MAX_CYCLE_BULK_ENTRIES}-entry limit`,
+      422,
+      { errorCode: "cycle.bulk.too_large" },
+    );
+  }
+
+  const parsed = cycleBulkSchema.safeParse(rawBody);
+  if (!parsed.success) {
+    annotate({
+      action: { name: "cycle.bulk.validation-failed" },
+      meta: { issue_count: sanitiseZodIssues(parsed.error.issues).length },
+    });
+    return returnAllZodIssues(parsed.error, 422, {
+      errorCode: "cycle.bulk.invalid",
+    });
+  }
+
+  const { entries } = parsed.data;
+  const tz = user.timezone ?? DEFAULT_TIMEZONE;
+
+  const results: EntryResult[] = [];
+  let inserted = 0;
+  let updated = 0;
+  let duplicates = 0;
+  let skipped = 0;
+
+  for (let i = 0; i < entries.length; i++) {
+    const entry = entries[i];
+    try {
+      const cycleId = await findOwningCycleId(user.id, entry.date);
+      const r = await upsertCycleDayLog(user.id, entry, tz, cycleId);
+
+      let status: EntryStatus;
+      if (!r.existed) {
+        status = "inserted";
+        inserted += 1;
+      } else if (r.changed) {
+        status = "updated";
+        updated += 1;
+      } else {
+        status = "duplicate";
+        duplicates += 1;
+      }
+
+      results.push({
+        index: i,
+        status,
+        id: r.id,
+        ...(entry.externalId ? { externalId: entry.externalId } : {}),
+      });
+    } catch (err: unknown) {
+      const reason =
+        err instanceof Error ? err.message.slice(0, 120) : "upsert_failed";
+      skipped += 1;
+      results.push({
+        index: i,
+        status: "skipped",
+        reason,
+        ...(entry.externalId ? { externalId: entry.externalId } : {}),
+      });
+    }
+  }
+
+  await auditLog("cycle.bulk.ingest", {
+    userId: user.id,
+    ipAddress: getClientIp(request),
+    details: {
+      processed: entries.length,
+      inserted,
+      updated,
+      duplicates,
+      skipped,
+    },
+  });
+
+  annotate({
+    action: { name: "cycle.bulk.ingest" },
+    meta: {
+      processed: entries.length,
+      inserted,
+      updated,
+      duplicates,
+      skipped,
+    },
+  });
+
+  return apiSuccess({
+    processed: entries.length,
+    inserted,
+    updated,
+    duplicates,
+    skipped,
+    entries: results,
+  });
+}
diff --git a/src/app/api/cycle/day-logs/route.ts b/src/app/api/cycle/day-logs/route.ts
new file mode 100644
index 000000000..1490559ac
--- /dev/null
+++ b/src/app/api/cycle/day-logs/route.ts
@@ -0,0 +1,83 @@
+/**
+ * `POST /api/cycle/day-logs` — single cycle day-log capture
+ * (ios-contract §2.A).
+ *
+ * Upserts on `(userId, source, externalId)` when an `externalId` is
+ * present (the cross-device dedup key), else the canonical `(userId,
+ * date)` key. `note` is encrypted at rest (`notesEncrypted`); the intent
+ * fields stay queryable plaintext (they feed the rollup / correlation
+ * tier). Returns the full `CycleDayLogDTO`: 201 on insert, 200 on update.
+ *
+ * Gated: a disabled / non-FEMALE-without-opt-in account 403s with
+ * `errorCode:"cycle.disabled"` even with a valid Bearer token.
+ */
+import { NextRequest } from "next/server";
+
+import { prisma } from "@/lib/db";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { annotate } from "@/lib/logging/context";
+import { auditLog } from "@/lib/auth/audit";
+import {
+  apiSuccess,
+  getClientIp,
+  returnAllZodIssues,
+  safeJson,
+} from "@/lib/api-response";
+import { withIdempotency } from "@/lib/idempotency";
+import { requireCycleEnabled } from "@/lib/cycle/gate";
+import { cycleDayLogInputSchema } from "@/lib/validations/cycle";
+import { upsertCycleDayLog } from "@/lib/cycle/day-log-write";
+import { findOwningCycleId } from "@/lib/cycle/cycle-attribution";
+import { toCycleDayLogDTO, dayLogSymptomInclude } from "@/lib/cycle/dto";
+import { DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+
+export const POST = apiHandler(withIdempotency<[NextRequest]>(postDayLog));
+
+async function postDayLog(request: NextRequest): Promise<Response> {
+  const { user } = await requireAuth();
+
+  const gate = await requireCycleEnabled(user.id, user.gender);
+  if (!gate.enabled) return gate.response;
+
+  const { data: rawBody, error: jsonError } = await safeJson(request);
+  if (jsonError) return jsonError;
+
+  const parsed = cycleDayLogInputSchema.safeParse(rawBody);
+  if (!parsed.success) {
+    annotate({
+      action: { name: "cycle.day-log.validation-failed" },
+      meta: { issue_count: parsed.error.issues.length },
+    });
+    return returnAllZodIssues(parsed.error, 422, {
+      errorCode: "cycle.day-log.invalid",
+    });
+  }
+
+  const entry = parsed.data;
+  const tz = user.timezone ?? DEFAULT_TIMEZONE;
+
+  const cycleId = await findOwningCycleId(user.id, entry.date);
+  const result = await upsertCycleDayLog(user.id, entry, tz, cycleId);
+
+  const row = await prisma.cycleDayLog.findUniqueOrThrow({
+    where: { id: result.id },
+    include: dayLogSymptomInclude,
+  });
+
+  await auditLog("cycle.day-log.upsert", {
+    userId: user.id,
+    ipAddress: getClientIp(request),
+    details: { dayLogId: result.id, existed: result.existed },
+  });
+
+  annotate({
+    action: {
+      name: "cycle.day-log.upsert",
+      entity_type: "cycle_day_log",
+      entity_id: result.id,
+    },
+    meta: { existed: result.existed, source: entry.source },
+  });
+
+  return apiSuccess(toCycleDayLogDTO(row), result.existed ? 200 : 201);
+}
diff --git a/src/app/api/cycle/period/route.ts b/src/app/api/cycle/period/route.ts
new file mode 100644
index 000000000..cc866aa62
--- /dev/null
+++ b/src/app/api/cycle/period/route.ts
@@ -0,0 +1,151 @@
+/**
+ * `POST /api/cycle/period` — one-tap period-boundary shortcut
+ * (ios-contract §2.C).
+ *
+ * `action:"start"` opens a new `MenstrualCycle` anchored at `date`,
+ * closes the prior open cycle (endDate = day before, lengthDays set),
+ * and writes the boundary `CycleDayLog(flow=MEDIUM)`. `action:"end"`
+ * stamps the current cycle's `periodEndDate` and writes the boundary
+ * day-log. Convenience over §2.A; returns
+ * `{ cycle: MenstrualCycleDTO, dayLog: CycleDayLogDTO }`.
+ */
+import { NextRequest } from "next/server";
+
+import { prisma } from "@/lib/db";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { annotate } from "@/lib/logging/context";
+import { auditLog } from "@/lib/auth/audit";
+import {
+  apiSuccess,
+  getClientIp,
+  returnAllZodIssues,
+  safeJson,
+} from "@/lib/api-response";
+import { requireCycleEnabled } from "@/lib/cycle/gate";
+import { cyclePeriodSchema } from "@/lib/validations/cycle";
+import { upsertCycleDayLog } from "@/lib/cycle/day-log-write";
+import {
+  toCycleDayLogDTO,
+  toMenstrualCycleDTO,
+  dayLogSymptomInclude,
+} from "@/lib/cycle/dto";
+import { DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { addDays, dayDiff } from "@/lib/cycle/day-math";
+
+export const POST = apiHandler(async (request: NextRequest) => {
+  const { user } = await requireAuth();
+
+  const gate = await requireCycleEnabled(user.id, user.gender);
+  if (!gate.enabled) return gate.response;
+
+  const { data: rawBody, error: jsonError } = await safeJson(request);
+  if (jsonError) return jsonError;
+
+  const parsed = cyclePeriodSchema.safeParse(rawBody);
+  if (!parsed.success) {
+    annotate({
+      action: { name: "cycle.period.validation-failed" },
+      meta: { issue_count: parsed.error.issues.length },
+    });
+    return returnAllZodIssues(parsed.error, 422, {
+      errorCode: "cycle.period.invalid",
+    });
+  }
+
+  const { action, date, externalId, loggedAt } = parsed.data;
+  const tz = user.timezone ?? DEFAULT_TIMEZONE;
+
+  let cycleId: string;
+
+  if (action === "start") {
+    // Close the prior open cycle (its end is the day before this start)
+    // and record its observed length.
+    const prior = await prisma.menstrualCycle.findFirst({
+      where: { userId: user.id, deletedAt: null, startDate: { lt: date } },
+      orderBy: { startDate: "desc" },
+      select: { id: true, startDate: true },
+    });
+    if (prior) {
+      await prisma.menstrualCycle.update({
+        where: { id: prior.id },
+        data: {
+          endDate: addDays(date, -1),
+          lengthDays: dayDiff(date, prior.startDate),
+          syncVersion: { increment: 1 },
+        },
+      });
+    }
+
+    // Upsert the new cycle on the `(userId, startDate)` unique so a
+    // re-tap on the same day is idempotent.
+    const cycle = await prisma.menstrualCycle.upsert({
+      where: { userId_startDate: { userId: user.id, startDate: date } },
+      create: { userId: user.id, startDate: date, tz, isPredicted: false },
+      update: { deletedAt: null, isPredicted: false, syncVersion: { increment: 1 } },
+    });
+    cycleId = cycle.id;
+  } else {
+    // `end`: stamp the current cycle's periodEndDate.
+    const current = await prisma.menstrualCycle.findFirst({
+      where: { userId: user.id, deletedAt: null, startDate: { lte: date } },
+      orderBy: { startDate: "desc" },
+      select: { id: true },
+    });
+    if (!current) {
+      return apiSuccessNoCycle();
+    }
+    await prisma.menstrualCycle.update({
+      where: { id: current.id },
+      data: { periodEndDate: date, syncVersion: { increment: 1 } },
+    });
+    cycleId = current.id;
+  }
+
+  // Boundary day-log. Start → MEDIUM flow opens the bleed; end → SPOTTING
+  // tail (the last logged bleeding day).
+  await upsertCycleDayLog(
+    user.id,
+    {
+      date,
+      flow: action === "start" ? "MEDIUM" : "SPOTTING",
+      loggedAt,
+      source: "MANUAL",
+      ...(externalId ? { externalId } : {}),
+    },
+    tz,
+    cycleId,
+  );
+
+  const [cycleRow, dayLogRow] = await Promise.all([
+    prisma.menstrualCycle.findUniqueOrThrow({ where: { id: cycleId } }),
+    prisma.cycleDayLog.findFirstOrThrow({
+      where: { userId: user.id, date, deletedAt: null },
+      include: dayLogSymptomInclude,
+    }),
+  ]);
+
+  await auditLog("cycle.period.boundary", {
+    userId: user.id,
+    ipAddress: getClientIp(request),
+    details: { action, date, cycleId },
+  });
+
+  annotate({
+    action: {
+      name: "cycle.period.boundary",
+      entity_type: "menstrual_cycle",
+      entity_id: cycleId,
+    },
+    meta: { boundary: action },
+  });
+
+  return apiSuccess({
+    cycle: toMenstrualCycleDTO(cycleRow),
+    dayLog: toCycleDayLogDTO(dayLogRow),
+  });
+});
+
+/** `end` with no preceding cycle is a no-op the client should not hit. */
+function apiSuccessNoCycle(): Response {
+  return apiSuccess({ cycle: null, dayLog: null });
+}
diff --git a/src/app/api/cycle/profile/route.ts b/src/app/api/cycle/profile/route.ts
new file mode 100644
index 000000000..051d916d0
--- /dev/null
+++ b/src/app/api/cycle/profile/route.ts
@@ -0,0 +1,27 @@
+/**
+ * `GET /api/cycle/profile` — full CycleProfileDTO (ios-contract §2.G).
+ *
+ * Gated like every cycle route; returns the lazily-upserted profile with
+ * the resolved `cycleTrackingEnabled`.
+ */
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { annotate } from "@/lib/logging/context";
+import { apiSuccess } from "@/lib/api-response";
+import { requireCycleEnabled } from "@/lib/cycle/gate";
+import { isCycleEnabled } from "@/lib/cycle/gate";
+import { toCycleProfileDTO } from "@/lib/cycle/dto";
+
+export const dynamic = "force-dynamic";
+
+export const GET = apiHandler(async () => {
+  const { user } = await requireAuth();
+
+  const gate = await requireCycleEnabled(user.id, user.gender);
+  if (!gate.enabled) return gate.response;
+
+  const resolved = isCycleEnabled(user.gender, gate.profile);
+
+  annotate({ action: { name: "cycle.profile.read" } });
+
+  return apiSuccess(toCycleProfileDTO(gate.profile, resolved));
+});
diff --git a/src/app/api/sync/changes/route.ts b/src/app/api/sync/changes/route.ts
index 952b8ab56..6a6d62e1b 100644
--- a/src/app/api/sync/changes/route.ts
+++ b/src/app/api/sync/changes/route.ts
@@ -8,7 +8,9 @@
  * domain now soft-delete (set `deletedAt` + bump `syncVersion`) rather
  * than hard-delete.
  *
- * Domains served: `measurements`, `mood`, `intakes`. The iOS consumer
+ * Domains served: `measurements`, `mood`, `intakes`, `cycleDays`,
+ * `cycles` (the last two added v1.15.0; `cycleDays` tombstones key on
+ * `externalId` when present else `id`, `cycles` on `id`). The iOS consumer
  * is measurements-only this cycle (iOS-coord
  * `v1.7.0-ios-offline-sync-answers.md` §7.1); mood + intakes are
  * forward-prep so the contract is complete + tested before the
@@ -57,6 +59,13 @@ import {
   type SyncCursor,
   type SyncDomain,
 } from "@/lib/sync/cursor";
+import {
+  toCycleDayLogDTO,
+  toMenstrualCycleDTO,
+  dayLogSymptomInclude,
+  type CycleDayLogDTO,
+  type MenstrualCycleDTO,
+} from "@/lib/cycle/dto";
 
 const DEFAULT_LIMIT = 200;
 const MAX_LIMIT = 500;
@@ -126,6 +135,25 @@ interface IntakeTombstone {
   updatedAt: string;
 }
 
+// v1.15.0 — cycle domains. cycleDays key tombstones on `externalId` when
+// present (HealthKit-origin rows are externalId-keyed cross-device, like
+// measurements) else the server `id`; cycles key on the server `id`
+// (computed rows carry no externalId).
+interface CycleDayTombstone {
+  id: string;
+  externalId: string | null;
+  syncVersion: number;
+  deletedAt: string;
+  updatedAt: string;
+}
+
+interface CycleTombstone {
+  id: string;
+  syncVersion: number;
+  deletedAt: string;
+  updatedAt: string;
+}
+
 /**
  * Build the `(updatedAt, id)` keyset predicate for a domain's watermark.
  * Empty object (full scan) when the client has no watermark for it yet.
@@ -196,6 +224,8 @@ export const GET = apiHandler(async (request: NextRequest) => {
         measurements: { upserts: [], tombstones: [] },
         mood: { upserts: [], tombstones: [] },
         intakes: { upserts: [], tombstones: [] },
+        cycleDays: { upserts: [], tombstones: [] },
+        cycles: { upserts: [], tombstones: [] },
       },
     });
   }
@@ -204,7 +234,8 @@ export const GET = apiHandler(async (request: NextRequest) => {
   // same scan — a soft-delete bumps `updatedAt`, so a tombstone is just a
   // row whose `deletedAt` is non-null. Fetch limit+1 to detect `hasMore`
   // without a count query.
-  const [measurementRows, moodRows, intakeRows] = await Promise.all([
+  const [measurementRows, moodRows, intakeRows, cycleDayRows, cycleRows] =
+    await Promise.all([
     prisma.measurement.findMany({
       where: { userId: user.id, ...keysetFilter(cursor.measurements) },
       orderBy: [{ updatedAt: "asc" }, { id: "asc" }],
@@ -257,6 +288,17 @@ export const GET = apiHandler(async (request: NextRequest) => {
         updatedAt: true,
       },
     }),
+    prisma.cycleDayLog.findMany({
+      where: { userId: user.id, ...keysetFilter(cursor.cycleDays) },
+      orderBy: [{ updatedAt: "asc" }, { id: "asc" }],
+      take: limit + 1,
+      include: dayLogSymptomInclude,
+    }),
+    prisma.menstrualCycle.findMany({
+      where: { userId: user.id, ...keysetFilter(cursor.cycles) },
+      orderBy: [{ updatedAt: "asc" }, { id: "asc" }],
+      take: limit + 1,
+    }),
   ]);
 
   const nextCursor: SyncCursor = { ...cursor };
@@ -354,6 +396,49 @@ export const GET = apiHandler(async (request: NextRequest) => {
   advanceCursor(nextCursor, "intakes", intakePage);
   hasMore = hasMore || intakeHasMore;
 
+  // ── cycleDays ─────────────────────────────────────────────
+  const cycleDayHasMore = cycleDayRows.length > limit;
+  const cycleDayPage = cycleDayHasMore
+    ? cycleDayRows.slice(0, limit)
+    : cycleDayRows;
+  const cycleDayUpserts: CycleDayLogDTO[] = [];
+  const cycleDayTombstones: CycleDayTombstone[] = [];
+  for (const row of cycleDayPage) {
+    if (row.deletedAt) {
+      cycleDayTombstones.push({
+        id: row.id,
+        externalId: row.externalId,
+        syncVersion: row.syncVersion,
+        deletedAt: row.deletedAt.toISOString(),
+        updatedAt: row.updatedAt.toISOString(),
+      });
+    } else {
+      cycleDayUpserts.push(toCycleDayLogDTO(row));
+    }
+  }
+  advanceCursor(nextCursor, "cycleDays", cycleDayPage);
+  hasMore = hasMore || cycleDayHasMore;
+
+  // ── cycles ────────────────────────────────────────────────
+  const cycleHasMore = cycleRows.length > limit;
+  const cyclePage = cycleHasMore ? cycleRows.slice(0, limit) : cycleRows;
+  const cycleUpserts: MenstrualCycleDTO[] = [];
+  const cycleTombstones: CycleTombstone[] = [];
+  for (const row of cyclePage) {
+    if (row.deletedAt) {
+      cycleTombstones.push({
+        id: row.id,
+        syncVersion: row.syncVersion,
+        deletedAt: row.deletedAt.toISOString(),
+        updatedAt: row.updatedAt.toISOString(),
+      });
+    } else {
+      cycleUpserts.push(toMenstrualCycleDTO(row));
+    }
+  }
+  advanceCursor(nextCursor, "cycles", cyclePage);
+  hasMore = hasMore || cycleHasMore;
+
   annotate({
     action: { name: "sync.changes.pull" },
     meta: {
@@ -363,6 +448,10 @@ export const GET = apiHandler(async (request: NextRequest) => {
       mood_tombstones: moodTombstones.length,
       intake_upserts: intakeUpserts.length,
       intake_tombstones: intakeTombstones.length,
+      cycle_day_upserts: cycleDayUpserts.length,
+      cycle_day_tombstones: cycleDayTombstones.length,
+      cycle_upserts: cycleUpserts.length,
+      cycle_tombstones: cycleTombstones.length,
       has_more: hasMore,
       cursor_present: Boolean(parsed.data.cursor),
     },
@@ -380,6 +469,11 @@ export const GET = apiHandler(async (request: NextRequest) => {
       },
       mood: { upserts: moodUpserts, tombstones: moodTombstones },
       intakes: { upserts: intakeUpserts, tombstones: intakeTombstones },
+      cycleDays: {
+        upserts: cycleDayUpserts,
+        tombstones: cycleDayTombstones,
+      },
+      cycles: { upserts: cycleUpserts, tombstones: cycleTombstones },
     },
   });
 });
diff --git a/src/lib/cycle/__tests__/engine-adapter.test.ts b/src/lib/cycle/__tests__/engine-adapter.test.ts
new file mode 100644
index 000000000..900a3031d
--- /dev/null
+++ b/src/lib/cycle/__tests__/engine-adapter.test.ts
@@ -0,0 +1,126 @@
+/**
+ * v1.15.0 — calendar composition over the deterministic engine.
+ */
+import { describe, it, expect } from "vitest";
+
+import { buildCalendar, type CalendarDayLogRow } from "../engine-adapter";
+import type { CycleProfile, MenstrualCycle } from "@/generated/prisma/client";
+
+function profile(overrides: Partial<CycleProfile> = {}): CycleProfile {
+  return {
+    id: "p1",
+    userId: "u1",
+    goal: "GENERAL_HEALTH",
+    cycleTrackingEnabled: true,
+    typicalCycleLength: null,
+    typicalPeriodLength: null,
+    lutealPhaseLength: null,
+    predictionEnabled: true,
+    rawChartMode: false,
+    discreetNotifications: false,
+    sensitiveCategoryEncryption: true,
+    createdAt: new Date(),
+    updatedAt: new Date(),
+    ...overrides,
+  } as CycleProfile;
+}
+
+function cycle(startDate: string, overrides: Partial<MenstrualCycle> = {}): MenstrualCycle {
+  return {
+    id: `c-${startDate}`,
+    userId: "u1",
+    startDate,
+    endDate: null,
+    periodEndDate: null,
+    lengthDays: null,
+    ovulationDate: null,
+    ovulationConfirmed: false,
+    isPredicted: false,
+    tz: null,
+    syncVersion: 0,
+    deletedAt: null,
+    createdAt: new Date(),
+    updatedAt: new Date(),
+    ...overrides,
+  } as MenstrualCycle;
+}
+
+describe("buildCalendar", () => {
+  // Three ~28-day cycles → a real prediction.
+  const cycles = [
+    cycle("2026-01-01", { endDate: "2026-01-28", lengthDays: 28, periodEndDate: "2026-01-05" }),
+    cycle("2026-01-29", { endDate: "2026-02-25", lengthDays: 28, periodEndDate: "2026-02-02" }),
+    cycle("2026-02-26", { periodEndDate: "2026-03-02" }),
+  ];
+
+  it("labels logged-flow days and emits a forecast for ≥2 cycles", () => {
+    const dayLogs: CalendarDayLogRow[] = [
+      { date: "2026-02-26", flow: "MEDIUM", basalBodyTempC: null, ovulationTest: null, cervicalMucus: null, hasSymptoms: true },
+    ];
+    const { prediction, days } = buildCalendar(
+      profile(),
+      cycles,
+      dayLogs,
+      [],
+      "2026-02-20",
+      "2026-04-15",
+      "2026-03-10",
+      false,
+    );
+    expect(prediction).not.toBeNull();
+    expect(prediction!.cyclesObserved).toBeGreaterThanOrEqual(2);
+
+    const logged = days.find((d) => d.date === "2026-02-26")!;
+    expect(logged.isPeriodLogged).toBe(true);
+    expect(logged.flow).toBe("MEDIUM");
+    expect(logged.hasSymptoms).toBe(true);
+
+    // The forecast next-period start falls inside a predicted-period day.
+    const predStart = prediction!.nextPeriodStart;
+    const predDay = days.find((d) => d.date === predStart);
+    expect(predDay?.isPredictedPeriod).toBe(true);
+  });
+
+  it("suppresses the fertile window at the grid when the goal disallows it", () => {
+    const { days } = buildCalendar(
+      profile({ goal: "GENERAL_HEALTH" }),
+      cycles,
+      [],
+      [],
+      "2026-02-20",
+      "2026-04-15",
+      "2026-03-10",
+      false,
+    );
+    expect(days.every((d) => !d.isFertileWindow)).toBe(true);
+    expect(days.every((d) => !d.isPredictedOvulation)).toBe(true);
+  });
+
+  it("surfaces the fertile window for the conception goal", () => {
+    const { days } = buildCalendar(
+      profile({ goal: "TRYING_TO_CONCEIVE" }),
+      cycles,
+      [],
+      [],
+      "2026-02-20",
+      "2026-04-15",
+      "2026-03-10",
+      true,
+    );
+    expect(days.some((d) => d.isFertileWindow)).toBe(true);
+  });
+
+  it("emits no prediction in raw-chart mode", () => {
+    const { prediction } = buildCalendar(
+      profile({ rawChartMode: true }),
+      cycles,
+      [],
+      [],
+      "2026-02-20",
+      "2026-04-15",
+      "2026-03-10",
+      false,
+    );
+    expect(prediction).toBeNull();
+  });
+});
diff --git a/src/lib/cycle/__tests__/gate.test.ts b/src/lib/cycle/__tests__/gate.test.ts
new file mode 100644
index 000000000..df6dd2266
--- /dev/null
+++ b/src/lib/cycle/__tests__/gate.test.ts
@@ -0,0 +1,41 @@
+/**
+ * v1.15.0 — cycle-tracking feature gate resolver.
+ */
+import { describe, it, expect } from "vitest";
+
+import { isCycleEnabled, CYCLE_DISABLED_ERROR_CODE } from "../gate";
+
+describe("isCycleEnabled", () => {
+  it("derives from gender when the toggle is null", () => {
+    expect(isCycleEnabled("FEMALE", { cycleTrackingEnabled: null })).toBe(true);
+    expect(isCycleEnabled("MALE", { cycleTrackingEnabled: null })).toBe(false);
+    expect(isCycleEnabled(null, { cycleTrackingEnabled: null })).toBe(false);
+  });
+
+  it("derives from gender when no profile row exists", () => {
+    expect(isCycleEnabled("FEMALE", null)).toBe(true);
+    expect(isCycleEnabled("MALE", undefined)).toBe(false);
+    expect(isCycleEnabled(null, null)).toBe(false);
+  });
+
+  it("an explicit true opts a non-FEMALE account in", () => {
+    expect(isCycleEnabled("MALE", { cycleTrackingEnabled: true })).toBe(true);
+    expect(isCycleEnabled(null, { cycleTrackingEnabled: true })).toBe(true);
+  });
+
+  it("an explicit false opts a FEMALE account out", () => {
+    expect(isCycleEnabled("FEMALE", { cycleTrackingEnabled: false })).toBe(
+      false,
+    );
+  });
+
+  it("gates on UPPERCASE FEMALE only (server enum spelling)", () => {
+    // Lowercase / other spellings never enable by derivation.
+    expect(isCycleEnabled("female", { cycleTrackingEnabled: null })).toBe(false);
+    expect(isCycleEnabled("OTHER", { cycleTrackingEnabled: null })).toBe(false);
+  });
+
+  it("exports the wire errorCode the iOS retry classifier branches on", () => {
+    expect(CYCLE_DISABLED_ERROR_CODE).toBe("cycle.disabled");
+  });
+});
diff --git a/src/lib/cycle/cycle-attribution.ts b/src/lib/cycle/cycle-attribution.ts
new file mode 100644
index 000000000..17f91763b
--- /dev/null
+++ b/src/lib/cycle/cycle-attribution.ts
@@ -0,0 +1,24 @@
+/**
+ * Attribute a day-log date to its owning `MenstrualCycle` span.
+ *
+ * A day-log belongs to the cycle whose `startDate <= date` and which is
+ * the latest such start (the cycle is open-ended until the next cycle's
+ * start). Returns the cycle id or null when no cycle precedes the date.
+ */
+import { prisma } from "@/lib/db";
+
+export async function findOwningCycleId(
+  userId: string,
+  date: string,
+): Promise<string | null> {
+  const cycle = await prisma.menstrualCycle.findFirst({
+    where: {
+      userId,
+      deletedAt: null,
+      startDate: { lte: date },
+    },
+    orderBy: { startDate: "desc" },
+    select: { id: true },
+  });
+  return cycle?.id ?? null;
+}
diff --git a/src/lib/cycle/day-log-write.ts b/src/lib/cycle/day-log-write.ts
new file mode 100644
index 000000000..c02c1c01c
--- /dev/null
+++ b/src/lib/cycle/day-log-write.ts
@@ -0,0 +1,171 @@
+/**
+ * Cycle day-log write helper — the shared upsert behind the single POST,
+ * the PATCH, and the bulk drain (ios-contract §2.A / §2.B).
+ *
+ * UPSERT key: `(userId, source, externalId)` when `externalId` is present
+ * (the NULL-distinct cross-device dedup key, like MoodEntry), else the
+ * canonical `(userId, date)` key. `note` → `notesEncrypted` (AES-256-GCM);
+ * every other field is queryable plaintext. Symptom keys resolve against
+ * the seeded catalog (unknown keys dropped silently) and write the
+ * `CycleSymptomLink` join.
+ *
+ * Returns the upserted row id + whether a row already existed (so the
+ * caller can map `inserted`/`updated`/`duplicate`) plus whether any field
+ * actually changed on a re-post (drives the bulk `updated` vs `duplicate`
+ * distinction).
+ */
+import { prisma } from "@/lib/db";
+import { encrypt } from "@/lib/crypto";
+import type { CycleDayLogInput } from "@/lib/validations/cycle";
+
+export interface DayLogWriteResult {
+  id: string;
+  existed: boolean;
+  /** A pre-existing row whose persisted fields differ from the re-post. */
+  changed: boolean;
+}
+
+/** Resolve catalog symptom keys → ids, dropping unknown keys silently. */
+async function resolveSymptomIds(
+  userId: string,
+  keys: readonly string[],
+): Promise<string[]> {
+  if (keys.length === 0) return [];
+  const unique = Array.from(new Set(keys));
+  // Catalog rows (userId null) plus this user's own custom symptoms.
+  const rows = await prisma.cycleSymptom.findMany({
+    where: {
+      key: { in: unique },
+      isActive: true,
+      OR: [{ userId: null }, { userId }],
+    },
+    select: { id: true },
+  });
+  return rows.map((r) => r.id);
+}
+
+/**
+ * Upsert one cycle day-log. `tz` anchors the date string. When
+ * `cycleId` is supplied the row is attributed to that cycle span.
+ */
+export async function upsertCycleDayLog(
+  userId: string,
+  entry: CycleDayLogInput,
+  tz: string | null,
+  cycleId: string | null = null,
+): Promise<DayLogWriteResult> {
+  const source = entry.source;
+  const notesEncrypted = entry.note ? encrypt(entry.note) : null;
+
+  const where = entry.externalId
+    ? {
+        userId_source_externalId: {
+          userId,
+          source,
+          externalId: entry.externalId,
+        },
+      }
+    : { userId_date: { userId, date: entry.date } };
+
+  // Probe so the caller can distinguish inserted / updated / duplicate
+  // and (for an externalId re-post) detect whether a field changed.
+  const existing = await prisma.cycleDayLog.findUnique({
+    where,
+    select: {
+      id: true,
+      date: true,
+      flow: true,
+      intermenstrualBleeding: true,
+      basalBodyTempC: true,
+      ovulationTest: true,
+      cervicalMucus: true,
+      sexualActivity: true,
+      protectedSex: true,
+      pregnancyTest: true,
+      progesteroneTest: true,
+      contraceptive: true,
+      notesEncrypted: true,
+      deletedAt: true,
+    },
+  });
+
+  // Field-by-field create data (no mass assignment).
+  const baseData = {
+    flow: entry.flow ?? null,
+    intermenstrualBleeding: entry.intermenstrualBleeding ?? false,
+    basalBodyTempC: entry.basalBodyTempC ?? null,
+    ovulationTest: entry.ovulationTest ?? null,
+    cervicalMucus: entry.cervicalMucus ?? null,
+    sexualActivity: entry.sexualActivity ?? false,
+    protectedSex: entry.protectedSex ?? null,
+    pregnancyTest: entry.pregnancyTest ?? null,
+    progesteroneTest: entry.progesteroneTest ?? null,
+    contraceptive: entry.contraceptive ?? null,
+    notesEncrypted,
+  };
+
+  // Last-writer-wins on the field set. An externalId re-post also refreshes
+  // `date` (a re-zoned re-import lands the corrected wall-clock). A
+  // soft-deleted row is resurrected on a fresh write (deletedAt cleared).
+  const updateData = {
+    ...baseData,
+    ...(entry.externalId ? { date: entry.date } : {}),
+    ...(cycleId !== null ? { cycleId } : {}),
+    deletedAt: null,
+    syncVersion: { increment: 1 },
+  };
+
+  const row = await prisma.cycleDayLog.upsert({
+    where,
+    create: {
+      userId,
+      date: entry.date,
+      tz,
+      source,
+      externalId: entry.externalId ?? null,
+      cycleId,
+      ...baseData,
+    },
+    update: updateData,
+    select: { id: true },
+  });
+
+  // Replace the symptom-link set when symptoms were supplied. Absent
+  // `symptoms` leaves existing links untouched (a partial day-log edit
+  // that doesn't mention symptoms must not wipe them).
+  if (entry.symptoms !== undefined) {
+    const ids = await resolveSymptomIds(
+      userId,
+      entry.symptoms.map((s) => s.key),
+    );
+    await prisma.cycleSymptomLink.deleteMany({ where: { dayLogId: row.id } });
+    if (ids.length > 0) {
+      await prisma.cycleSymptomLink.createMany({
+        data: ids.map((symptomId) => ({ dayLogId: row.id, symptomId })),
+        skipDuplicates: true,
+      });
+    }
+  }
+
+  // `changed` only matters for the bulk `updated` vs `duplicate` split on
+  // an existing row: did any persisted field differ from the re-post?
+  let changed = false;
+  if (existing) {
+    changed =
+      existing.flow !== baseData.flow ||
+      existing.intermenstrualBleeding !== baseData.intermenstrualBleeding ||
+      existing.basalBodyTempC !== baseData.basalBodyTempC ||
+      existing.ovulationTest !== baseData.ovulationTest ||
+      existing.cervicalMucus !== baseData.cervicalMucus ||
+      existing.sexualActivity !== baseData.sexualActivity ||
+      existing.protectedSex !== baseData.protectedSex ||
+      existing.pregnancyTest !== baseData.pregnancyTest ||
+      existing.progesteroneTest !== baseData.progesteroneTest ||
+      existing.contraceptive !== baseData.contraceptive ||
+      existing.notesEncrypted !== baseData.notesEncrypted ||
+      existing.deletedAt !== null ||
+      (Boolean(entry.externalId) && existing.date !== entry.date);
+  }
+
+  return { id: row.id, existed: existing !== null, changed };
+}
diff --git a/src/lib/cycle/dto.ts b/src/lib/cycle/dto.ts
new file mode 100644
index 000000000..a5c651695
--- /dev/null
+++ b/src/lib/cycle/dto.ts
@@ -0,0 +1,217 @@
+/**
+ * Cycle row → wire-DTO mappers (ios-contract §2 + §3).
+ *
+ * The canonical `CycleDayLogDTO` and `MenstrualCycleDTO` shapes the iOS
+ * client mirrors. `notesEncrypted` is decrypted fail-soft on read (a
+ * key-rotation gap on one row reads `null`, never 500s the whole page).
+ * Symptom links are flattened to `[{ key, severity }]` using the catalog
+ * key (severity is not yet persisted per-link in v1.15.0 — the join is a
+ * presence link; severity rides the input but the catalog link carries
+ * presence only, so it reads back `null`).
+ */
+import { decrypt } from "@/lib/crypto";
+import type { CyclePredictionResult } from "@/lib/cycle/types";
+import type {
+  CycleDayLog,
+  CycleSymptomLink,
+  CycleSymptom,
+  MenstrualCycle,
+} from "@/generated/prisma/client";
+
+export interface CycleSymptomDTO {
+  key: string;
+  severity: number | null;
+}
+
+export interface CycleDayLogDTO {
+  id: string;
+  date: string;
+  cycleId: string | null;
+  flow: string | null;
+  intermenstrualBleeding: boolean;
+  basalBodyTempC: number | null;
+  ovulationTest: string | null;
+  cervicalMucus: string | null;
+  sexualActivity: boolean;
+  protectedSex: boolean | null;
+  pregnancyTest: string | null;
+  progesteroneTest: string | null;
+  contraceptive: string | null;
+  symptoms: CycleSymptomDTO[];
+  note: string | null;
+  source: string;
+  externalId: string | null;
+  syncVersion: number;
+  updatedAt: string;
+  deletedAt: string | null;
+}
+
+export interface MenstrualCycleDTO {
+  id: string;
+  startDate: string;
+  endDate: string | null;
+  periodEndDate: string | null;
+  lengthDays: number | null;
+  ovulationDate: string | null;
+  ovulationConfirmed: boolean;
+  isPredicted: boolean;
+  syncVersion: number;
+  updatedAt: string;
+}
+
+type DayLogWithLinks = CycleDayLog & {
+  symptomLinks?: (CycleSymptomLink & { symptom: Pick<CycleSymptom, "key"> })[];
+};
+
+/** Decrypt a day-log note fail-soft (null on missing / undecryptable). */
+function decryptNote(notesEncrypted: string | null): string | null {
+  if (!notesEncrypted) return null;
+  try {
+    return decrypt(notesEncrypted);
+  } catch {
+    return null;
+  }
+}
+
+export function toCycleDayLogDTO(row: DayLogWithLinks): CycleDayLogDTO {
+  return {
+    id: row.id,
+    date: row.date,
+    cycleId: row.cycleId,
+    flow: row.flow,
+    intermenstrualBleeding: row.intermenstrualBleeding,
+    basalBodyTempC: row.basalBodyTempC,
+    ovulationTest: row.ovulationTest,
+    cervicalMucus: row.cervicalMucus,
+    sexualActivity: row.sexualActivity,
+    protectedSex: row.protectedSex,
+    pregnancyTest: row.pregnancyTest,
+    progesteroneTest: row.progesteroneTest,
+    contraceptive: row.contraceptive,
+    symptoms: (row.symptomLinks ?? []).map((l) => ({
+      key: l.symptom.key,
+      severity: null,
+    })),
+    note: decryptNote(row.notesEncrypted),
+    source: row.source,
+    externalId: row.externalId,
+    syncVersion: row.syncVersion,
+    updatedAt: row.updatedAt.toISOString(),
+    deletedAt: row.deletedAt ? row.deletedAt.toISOString() : null,
+  };
+}
+
+export function toMenstrualCycleDTO(row: MenstrualCycle): MenstrualCycleDTO {
+  return {
+    id: row.id,
+    startDate: row.startDate,
+    endDate: row.endDate,
+    periodEndDate: row.periodEndDate,
+    lengthDays: row.lengthDays,
+    ovulationDate: row.ovulationDate,
+    ovulationConfirmed: row.ovulationConfirmed,
+    isPredicted: row.isPredicted,
+    syncVersion: row.syncVersion,
+    updatedAt: row.updatedAt.toISOString(),
+  };
+}
+
+/** The Prisma `include` that hydrates symptom links → DTO keys. */
+export const dayLogSymptomInclude = {
+  symptomLinks: { include: { symptom: { select: { key: true } } } },
+} as const;
+
+export interface CyclePredictionDTO {
+  method: string;
+  nextPeriodStart: string;
+  nextPeriodStartLow: string;
+  nextPeriodStartHigh: string;
+  fertileWindowStart: string | null;
+  fertileWindowEnd: string | null;
+  predictedOvulation: string | null;
+  confidence: number;
+  cyclesObserved: number;
+  stillLearning: boolean;
+  disclaimer: string;
+}
+
+/**
+ * Map the engine result to the wire `CyclePredictionDTO`. Fertile-window
+ * fields (and the predicted ovulation) are suppressed unless the goal
+ * permits them (ios-contract §2.D — GENERAL_HEALTH / PERIMENOPAUSE / OFF
+ * never surface a fertile window; the suppression is server-side, never
+ * iOS).
+ */
+export function toCyclePredictionDTO(
+  result: CyclePredictionResult,
+  goalAllowsFertile: boolean,
+  disclaimer: string,
+): CyclePredictionDTO {
+  return {
+    method: result.method,
+    nextPeriodStart: result.nextPeriodStart,
+    nextPeriodStartLow: result.nextPeriodStartLow,
+    nextPeriodStartHigh: result.nextPeriodStartHigh,
+    fertileWindowStart: goalAllowsFertile ? result.fertileWindowStart : null,
+    fertileWindowEnd: goalAllowsFertile ? result.fertileWindowEnd : null,
+    predictedOvulation: goalAllowsFertile ? result.predictedOvulation : null,
+    confidence: result.confidence,
+    cyclesObserved: result.cyclesObserved,
+    stillLearning: result.stillLearning,
+    disclaimer,
+  };
+}
+
+/** The full profile read DTO (ios-contract §2.G). */
+export interface CycleProfileDTO {
+  goal: string;
+  cycleTrackingEnabled: boolean;
+  rawChartMode: boolean;
+  predictionEnabled: boolean;
+  discreetNotifications: boolean;
+  sensitiveCategoryEncryption: boolean;
+  typicalCycleLength: number | null;
+  typicalPeriodLength: number | null;
+  lutealPhaseLength: number | null;
+  updatedAt: string;
+}
+
+export function toCycleProfileDTO(
+  row: MenstrualCycleProfileRow,
+  cycleTrackingEnabled: boolean,
+): CycleProfileDTO {
+  return {
+    goal: row.goal,
+    cycleTrackingEnabled,
+    rawChartMode: row.rawChartMode,
+    predictionEnabled: row.predictionEnabled,
+    discreetNotifications: row.discreetNotifications,
+    sensitiveCategoryEncryption: row.sensitiveCategoryEncryption,
+    typicalCycleLength: row.typicalCycleLength,
+    typicalPeriodLength: row.typicalPeriodLength,
+    lutealPhaseLength: row.lutealPhaseLength,
+    updatedAt: row.updatedAt.toISOString(),
+  };
+}
+
+type MenstrualCycleProfileRow = {
+  goal: string;
+  rawChartMode: boolean;
+  predictionEnabled: boolean;
+  discreetNotifications: boolean;
+  sensitiveCategoryEncryption: boolean;
+  typicalCycleLength: number | null;
+  typicalPeriodLength: number | null;
+  lutealPhaseLength: number | null;
+  updatedAt: Date;
+};
+
+/**
+ * Whether a goal surfaces the fertile window. Per the v1.15 contract the
+ * window is gated to the conception goal only — GENERAL_HEALTH /
+ * AVOID_PREGNANCY / PERIMENOPAUSE / OFF suppress it (the suppression is
+ * always server-side, never iOS).
+ */
+export function goalAllowsFertileWindow(goal: string): boolean {
+  return goal === "TRYING_TO_CONCEIVE";
+}
diff --git a/src/lib/cycle/engine-adapter.ts b/src/lib/cycle/engine-adapter.ts
new file mode 100644
index 000000000..94b471388
--- /dev/null
+++ b/src/lib/cycle/engine-adapter.ts
@@ -0,0 +1,233 @@
+/**
+ * Prisma-row → engine-input adapters + calendar composition
+ * (ios-contract §2.D). The engine itself (`predictCycle`, `phaseSeries`)
+ * is pure + DB-free; this module bridges the persisted rows to the plain
+ * input shapes and assembles the `CalendarDayDTO` grid the read returns.
+ */
+import {
+  predictCycle,
+  phaseForDay,
+  addDays,
+  dayDiff,
+  isWithin,
+  type CycleInput,
+  type DayLogInput,
+  type NightlyTempInput,
+  type CycleProfileInput,
+  type CyclePredictionResult,
+  type CyclePhase,
+  type PhaseCycle,
+  LUTEAL_DEFAULT,
+} from "@/lib/cycle";
+import type {
+  CycleProfile,
+  CycleDayLog,
+  MenstrualCycle,
+} from "@/generated/prisma/client";
+
+export interface CalendarDayDTO {
+  date: string;
+  phase: CyclePhase | null;
+  isPredictedPeriod: boolean;
+  isFertileWindow: boolean;
+  isPredictedOvulation: boolean;
+  isPeriodLogged: boolean;
+  flow: string | null;
+  hasSymptoms: boolean;
+  confidence: number;
+}
+
+/** Rows the calendar needs from each day-log. */
+export type CalendarDayLogRow = Pick<
+  CycleDayLog,
+  "date" | "flow" | "basalBodyTempC" | "ovulationTest" | "cervicalMucus"
+> & { hasSymptoms: boolean };
+
+/** Map MenstrualCycle rows (oldest→newest) to engine `CycleInput`. */
+export function toCycleInputs(
+  cycles: readonly Pick<
+    MenstrualCycle,
+    "startDate" | "endDate" | "periodEndDate" | "ovulationDate" | "ovulationConfirmed"
+  >[],
+): CycleInput[] {
+  return cycles.map((c) => ({
+    startDate: c.startDate,
+    endDate: c.endDate,
+    periodEndDate: c.periodEndDate,
+    ovulationDate: c.ovulationDate,
+    ovulationConfirmed: c.ovulationConfirmed,
+  }));
+}
+
+/** Map CycleDayLog rows to engine `DayLogInput`. */
+export function toDayLogInputs(
+  logs: readonly Pick<
+    CycleDayLog,
+    "date" | "flow" | "basalBodyTempC" | "ovulationTest" | "cervicalMucus"
+  >[],
+): DayLogInput[] {
+  return logs.map((l) => ({
+    date: l.date,
+    flow: l.flow,
+    basalBodyTempC: l.basalBodyTempC,
+    ovulationTest: l.ovulationTest,
+    cervicalMucus: l.cervicalMucus,
+  }));
+}
+
+/** Map a CycleProfile row to the engine's `CycleProfileInput`. */
+export function toProfileInput(profile: CycleProfile): CycleProfileInput {
+  return {
+    goal: profile.goal,
+    typicalCycleLength: profile.typicalCycleLength,
+    typicalPeriodLength: profile.typicalPeriodLength,
+    lutealPhaseLength: profile.lutealPhaseLength,
+    predictionEnabled: profile.predictionEnabled,
+    rawChartMode: profile.rawChartMode,
+  };
+}
+
+/** Period-length of an observed cycle: periodEnd − start + 1, or default. */
+function periodLengthOf(
+  cycle: Pick<MenstrualCycle, "startDate" | "periodEndDate">,
+): number | null {
+  if (!cycle.periodEndDate) return null;
+  return dayDiff(cycle.periodEndDate, cycle.startDate) + 1;
+}
+
+/**
+ * Build a contiguous `PhaseCycle[]` from the observed cycles (each cycle's
+ * span ends where the next begins; the latest open cycle runs to the
+ * predicted next-period start). Used to label every calendar day's phase.
+ */
+function buildPhaseCycles(
+  cycles: readonly Pick<
+    MenstrualCycle,
+    "startDate" | "endDate" | "periodEndDate" | "ovulationDate"
+  >[],
+  predictedNextStart: string | null,
+  lutealLength: number,
+): PhaseCycle[] {
+  const sorted = [...cycles].sort((a, b) => dayDiff(a.startDate, b.startDate));
+  const out: PhaseCycle[] = [];
+  for (let i = 0; i < sorted.length; i++) {
+    const c = sorted[i];
+    const next = sorted[i + 1];
+    const nextStart =
+      next?.startDate ?? predictedNextStart ?? addDays(c.startDate, 28);
+    out.push({
+      startDate: c.startDate,
+      nextStart,
+      ovulationDate: c.ovulationDate,
+      periodLength: periodLengthOf(c),
+      lutealLength,
+    });
+  }
+  return out;
+}
+
+/** Phase of a single date across the contiguous cycle list (or null). */
+function phaseAcross(date: string, phaseCycles: readonly PhaseCycle[]): CyclePhase | null {
+  for (const pc of phaseCycles) {
+    const r = phaseForDay(date, pc);
+    if (r.phase !== null) return r.phase;
+  }
+  return null;
+}
+
+export interface CalendarBuildResult {
+  prediction: CyclePredictionResult | null;
+  days: CalendarDayDTO[];
+}
+
+/**
+ * Compose the calendar grid for `[from, to]` inclusive. Runs the engine
+ * once for the forward forecast, then labels each day's phase, predicted
+ * period bar, fertile window, and logged-flow overlay.
+ *
+ * Fertile-window fields are caller-gated: pass `goalAllowsFertile=false`
+ * (GENERAL_HEALTH / PERIMENOPAUSE) to suppress `isFertileWindow` +
+ * `isPredictedOvulation` at the grid level (the prediction's own window
+ * fields are nulled upstream by the engine for those goals).
+ */
+export function buildCalendar(
+  profile: CycleProfile,
+  cycles: readonly MenstrualCycle[],
+  dayLogs: readonly CalendarDayLogRow[],
+  nights: readonly NightlyTempInput[],
+  from: string,
+  to: string,
+  today: string,
+  goalAllowsFertile: boolean,
+): CalendarBuildResult {
+  const lutealLength = profile.lutealPhaseLength ?? LUTEAL_DEFAULT;
+
+  // Forecast (only when prediction is on + not raw-chart mode).
+  let prediction: CyclePredictionResult | null = null;
+  if (profile.predictionEnabled && !profile.rawChartMode) {
+    prediction = predictCycle(
+      toCycleInputs(cycles),
+      toDayLogInputs(dayLogs),
+      toProfileInput(profile),
+      today,
+      nights,
+    );
+  }
+
+  const phaseCycles = buildPhaseCycles(
+    cycles,
+    prediction?.nextPeriodStart ?? null,
+    lutealLength,
+  );
+
+  // Forward predicted period bar: [nextPeriodStart, +predictedPeriodLength).
+  const predictedPeriodStart = prediction?.nextPeriodStart ?? null;
+  const predictedPeriodEnd =
+    prediction && predictedPeriodStart
+      ? addDays(
+          predictedPeriodStart,
+          Math.max(0, prediction.predictedPeriodLength - 1),
+        )
+      : null;
+
+  const logByDate = new Map<string, CalendarDayLogRow>();
+  for (const l of dayLogs) logByDate.set(l.date, l);
+
+  const days: CalendarDayDTO[] = [];
+  const span = dayDiff(to, from);
+  for (let i = 0; i <= span; i++) {
+    const date = addDays(from, i);
+    const log = logByDate.get(date);
+
+    const isPredictedPeriod =
+      predictedPeriodStart !== null &&
+      predictedPeriodEnd !== null &&
+      isWithin(date, predictedPeriodStart, predictedPeriodEnd);
+
+    const isFertileWindow =
+      goalAllowsFertile &&
+      prediction?.fertileWindowStart != null &&
+      prediction.fertileWindowEnd != null &&
+      isWithin(date, prediction.fertileWindowStart, prediction.fertileWindowEnd);
+
+    const isPredictedOvulation =
+      goalAllowsFertile &&
+      prediction?.predictedOvulation != null &&
+      prediction.predictedOvulation === date;
+
+    days.push({
+      date,
+      phase: phaseAcross(date, phaseCycles),
+      isPredictedPeriod,
+      isFertileWindow,
+      isPredictedOvulation,
+      isPeriodLogged:
+        log?.flow != null && log.flow !== "NONE",
+      flow: log?.flow ?? null,
+      hasSymptoms: log?.hasSymptoms ?? false,
+      confidence: prediction?.confidence ?? 0,
+    });
+  }
+
+  return { prediction, days };
+}
diff --git a/src/lib/cycle/gate.ts b/src/lib/cycle/gate.ts
new file mode 100644
index 000000000..8f4ce1062
--- /dev/null
+++ b/src/lib/cycle/gate.ts
@@ -0,0 +1,72 @@
+/**
+ * Cycle-tracking feature gate.
+ *
+ * Two gates collapse into one resolved boolean (ios-contract §1):
+ *   (a) the per-user `CycleProfile.cycleTrackingEnabled` toggle, and
+ *   (b) a gender-derived default when that toggle is NULL.
+ *
+ * `isCycleEnabled` is the single source of truth. The server gender enum
+ * ships UPPERCASE (`"MALE" | "FEMALE" | null`); the gate keys on
+ * `"FEMALE"`. A NULL toggle derives `gender === "FEMALE"`; an explicit
+ * `true`/`false` overrides the derivation (so a non-FEMALE account can
+ * opt in and a FEMALE account can opt out).
+ *
+ * NOTE: the gender enum normalisation (adding `OTHER` / lowercasing) is a
+ * separate, iOS-blocked decision (migration 0131, reserved). Until that
+ * lands, `OTHER` / non-binary accounts gate purely on the explicit
+ * `cycleTrackingEnabled = true` opt-in via the Settings toggle.
+ */
+import { apiError } from "@/lib/api-response";
+import { getOrCreateCycleProfile } from "@/lib/cycle/profile";
+import type { CycleProfile } from "@/generated/prisma/client";
+
+/** Wire `errorCode` the iOS retry classifier branches on. */
+export const CYCLE_DISABLED_ERROR_CODE = "cycle.disabled";
+
+/**
+ * Resolve whether cycle tracking is enabled for an account.
+ *
+ * @param gender the user's `gender` column (`"MALE" | "FEMALE" | null`).
+ * @param profile the user's `CycleProfile`, or just its
+ *   `cycleTrackingEnabled` field (NULL ⇒ derive from gender).
+ */
+export function isCycleEnabled(
+  gender: string | null | undefined,
+  profile: { cycleTrackingEnabled: boolean | null } | null | undefined,
+): boolean {
+  const toggle = profile?.cycleTrackingEnabled;
+  if (toggle === true) return true;
+  if (toggle === false) return false;
+  // NULL toggle → derive from gender.
+  return gender === "FEMALE";
+}
+
+/**
+ * Route-guard outcome: either the resolved profile (enabled) or a ready
+ * 403 envelope (disabled) to return verbatim.
+ */
+export type CycleGateResult =
+  | { enabled: true; profile: CycleProfile }
+  | { enabled: false; response: Response };
+
+/**
+ * Enforce the gate for a `/api/cycle/*` route. Lazily upserts the
+ * `CycleProfile` (so the toggle row exists), resolves `isCycleEnabled`,
+ * and returns a 403 `cycle.disabled` envelope when the account may not
+ * write/read cycle data — even with a valid Bearer token.
+ */
+export async function requireCycleEnabled(
+  userId: string,
+  gender: string | null | undefined,
+): Promise<CycleGateResult> {
+  const profile = await getOrCreateCycleProfile(userId);
+  if (!isCycleEnabled(gender, profile)) {
+    return {
+      enabled: false,
+      response: apiError("Cycle tracking is not enabled", 403, {
+        errorCode: CYCLE_DISABLED_ERROR_CODE,
+      }),
+    };
+  }
+  return { enabled: true, profile };
+}
diff --git a/src/lib/cycle/prediction-cache.ts b/src/lib/cycle/prediction-cache.ts
new file mode 100644
index 000000000..4ad546003
--- /dev/null
+++ b/src/lib/cycle/prediction-cache.ts
@@ -0,0 +1,80 @@
+/**
+ * Cycle prediction cache — debounced stale-while-revalidate (the v1.8.7
+ * insight-assessment cache pattern, ios-contract §2.D).
+ *
+ * `GET /api/cycle/calendar` runs the pure engine synchronously (it is
+ * cheap) for the live forecast it returns, AND persists that forecast to
+ * the `CyclePrediction` cache row so the Coach snapshot + notifications
+ * read a stable materialised value without re-running the engine. The
+ * persist is debounced: a re-write only lands when the prior row is older
+ * than the debounce window OR the forecast actually changed, so a tight
+ * read loop never thrashes the row.
+ */
+import { prisma } from "@/lib/db";
+import type { CyclePredictionResult } from "@/lib/cycle/types";
+
+/** Don't re-persist an unchanged forecast more often than this. */
+const PREDICTION_DEBOUNCE_MS = 6 * 60 * 60 * 1000; // 6h
+
+/**
+ * Persist (or refresh) the cached forecast for a user, debounced. A
+ * no-op when a fresh-enough identical row already exists. Best-effort:
+ * never throws into the read path.
+ */
+export async function persistPredictionCache(
+  userId: string,
+  result: CyclePredictionResult,
+  now: Date = new Date(),
+): Promise<void> {
+  try {
+    const existing = await prisma.cyclePrediction.findUnique({
+      where: { userId },
+      select: {
+        method: true,
+        nextPeriodStart: true,
+        nextPeriodStartLow: true,
+        nextPeriodStartHigh: true,
+        confidence: true,
+        cyclesObserved: true,
+        generatedAt: true,
+      },
+    });
+
+    const unchanged =
+      existing !== null &&
+      existing.method === result.method &&
+      existing.nextPeriodStart === result.nextPeriodStart &&
+      existing.nextPeriodStartLow === result.nextPeriodStartLow &&
+      existing.nextPeriodStartHigh === result.nextPeriodStartHigh &&
+      existing.confidence === result.confidence &&
+      existing.cyclesObserved === result.cyclesObserved;
+
+    const fresh =
+      existing !== null &&
+      now.getTime() - existing.generatedAt.getTime() < PREDICTION_DEBOUNCE_MS;
+
+    // Skip the write only when the row is BOTH unchanged AND fresh.
+    if (unchanged && fresh) return;
+
+    const data = {
+      method: result.method,
+      nextPeriodStart: result.nextPeriodStart,
+      nextPeriodStartLow: result.nextPeriodStartLow,
+      nextPeriodStartHigh: result.nextPeriodStartHigh,
+      fertileWindowStart: result.fertileWindowStart,
+      fertileWindowEnd: result.fertileWindowEnd,
+      predictedOvulation: result.predictedOvulation,
+      confidence: result.confidence,
+      cyclesObserved: result.cyclesObserved,
+      generatedAt: now,
+    };
+
+    await prisma.cyclePrediction.upsert({
+      where: { userId },
+      create: { userId, ...data },
+      update: data,
+    });
+  } catch {
+    /* best-effort cache write — the live engine result is authoritative */
+  }
+}
diff --git a/src/lib/openapi/registry.ts b/src/lib/openapi/registry.ts
index 3ce0fab6a..77cebceb3 100644
--- a/src/lib/openapi/registry.ts
+++ b/src/lib/openapi/registry.ts
@@ -67,6 +67,7 @@ const openApiBase: Pick<
     { name: "Devices" },
     { name: "Export" },
     { name: "Sync" },
+    { name: "Cycle" },
     { name: "Admin" },
     { name: "Meta" },
   ],
diff --git a/src/lib/openapi/routes.ts b/src/lib/openapi/routes.ts
index ac8bd49a2..4914a9c18 100644
--- a/src/lib/openapi/routes.ts
+++ b/src/lib/openapi/routes.ts
@@ -54,6 +54,18 @@ import {
   VITALS_BASELINE_TYPES,
 } from "@/lib/insights/derived/registry";
 import { ANALYTICS_RANGES } from "@/lib/analytics/range-delta";
+import {
+  flowLevelEnum,
+  ovulationTestEnum,
+  cervicalMucusEnum,
+  homeTestResultEnum,
+  cycleTrackingGoalEnum,
+  cycleDayLogInputSchema,
+  cycleDayLogPatchSchema,
+  cycleBulkSchema,
+  cyclePeriodSchema,
+  cyclePrefsSchema,
+} from "@/lib/validations/cycle";
 
 /**
  * Common envelopes — every HealthLog API response wraps payload in
@@ -2148,9 +2160,477 @@ const coachFactDeletedResponse = z.object({
     ),
 });
 
+// ── Cycle tracking (v1.15.0) ─────────────────────────────────────────
+// The `/api/cycle/*` capture / calendar / history / settings surface +
+// the cycle-prefs PATCH. Request bodies come from the Zod validation
+// module so the spec stays single-source; response DTOs are declared
+// here mirroring `src/lib/cycle/dto.ts`. Every `/api/cycle/*` route also
+// 403s `{ errorCode:"cycle.disabled" }` when the feature gate is off.
+
+const predictionMethodEnumOpenapi = z
+  .enum(["CALENDAR", "SYMPTOTHERMAL", "TEMPERATURE_TREND", "BLENDED"])
+  .meta({ id: "CyclePredictionMethod" });
+
+const cyclePhaseEnumOpenapi = z
+  .enum(["MENSTRUAL", "FOLLICULAR", "OVULATORY", "LUTEAL"])
+  .meta({ id: "CyclePhase" });
+
+flowLevelEnum.meta({ id: "FlowLevel", description: "Menstrual-flow intensity." });
+ovulationTestEnum.meta({
+  id: "OvulationTest",
+  description: "Ovulation predictor-kit (OPK) reading.",
+});
+cervicalMucusEnum.meta({ id: "CervicalMucus", description: "Cervical-mucus quality." });
+homeTestResultEnum.meta({
+  id: "HomeTestResult",
+  description: "At-home test result (pregnancy / progesterone).",
+});
+cycleTrackingGoalEnum.meta({
+  id: "CycleTrackingGoal",
+  description: "Drives cycle copy + fertile-window gating.",
+});
+
+cycleDayLogInputSchema.meta({
+  id: "CycleDayLogInput",
+  description:
+    "One day's cycle capture. `note` is encrypted at rest; every other field is queryable plaintext. UPSERT key: `(userId, source, externalId)` when externalId present, else `(userId, date)`. Shared by the single POST, the bulk drain, and the period shortcut.",
+});
+
+cycleBulkSchema.meta({
+  id: "CycleDayLogBulkRequest",
+  description:
+    "Outbox / HealthKit drain. Up to 500 entries per call; wrapped in `withIdempotency`; rate-limited 60/min. Each entry upserts per the day-log key.",
+});
+
+cyclePeriodSchema.meta({
+  id: "CyclePeriodRequest",
+  description:
+    "One-tap period boundary. `start` opens a new cycle (closing the prior), `end` stamps the current cycle's periodEndDate; both write a boundary day-log.",
+});
+
+cyclePrefsSchema.meta({
+  id: "CyclePrefsRequest",
+  description:
+    "Partial cycle-preferences deep-merge. `enabled` flips the feature gate (`cycleTrackingEnabled`). Omitted fields are left untouched.",
+});
+
+cycleDayLogPatchSchema.meta({
+  id: "CycleDayLogPatchRequest",
+  description:
+    "Partial day-log edit. Every field optional; `note` re-encrypts (explicit null clears it). `date` / `source` / `externalId` are immutable on update.",
+});
+
+const cycleSymptomDto = z.object({
+  key: z.string(),
+  severity: z.number().int().min(1).max(4).nullable(),
+});
+
+const cycleDayLogDto = z
+  .object({
+    id: z.string(),
+    date: z.string(),
+    cycleId: z.string().nullable(),
+    flow: flowLevelEnum.nullable(),
+    intermenstrualBleeding: z.boolean(),
+    basalBodyTempC: z.number().nullable(),
+    ovulationTest: ovulationTestEnum.nullable(),
+    cervicalMucus: cervicalMucusEnum.nullable(),
+    sexualActivity: z.boolean(),
+    protectedSex: z.boolean().nullable(),
+    pregnancyTest: homeTestResultEnum.nullable(),
+    progesteroneTest: homeTestResultEnum.nullable(),
+    contraceptive: z.string().nullable(),
+    symptoms: z.array(cycleSymptomDto),
+    note: z.string().nullable(),
+    source: z.string(),
+    externalId: z.string().nullable(),
+    syncVersion: z.number().int(),
+    updatedAt: z.iso.datetime({ offset: true }),
+    deletedAt: z.iso.datetime({ offset: true }).nullable(),
+  })
+  .meta({
+    id: "CycleDayLogDTO",
+    description:
+      "The canonical day-log row iOS mirrors. `note` is decrypted on read. Soft-deleted rows ride `/api/sync/changes` as tombstones.",
+  });
+
+const menstrualCycleDto = z
+  .object({
+    id: z.string(),
+    startDate: z.string(),
+    endDate: z.string().nullable(),
+    periodEndDate: z.string().nullable(),
+    lengthDays: z.number().int().nullable(),
+    ovulationDate: z.string().nullable(),
+    ovulationConfirmed: z.boolean(),
+    isPredicted: z.boolean(),
+    syncVersion: z.number().int(),
+    updatedAt: z.iso.datetime({ offset: true }),
+  })
+  .meta({
+    id: "MenstrualCycleDTO",
+    description: "One menstrual cycle (observed or forward-predicted).",
+  });
+
+const cyclePredictionDto = z
+  .object({
+    method: predictionMethodEnumOpenapi,
+    nextPeriodStart: z.string(),
+    nextPeriodStartLow: z.string(),
+    nextPeriodStartHigh: z.string(),
+    fertileWindowStart: z.string().nullable(),
+    fertileWindowEnd: z.string().nullable(),
+    predictedOvulation: z.string().nullable(),
+    confidence: z.number(),
+    cyclesObserved: z.number().int(),
+    stillLearning: z.boolean(),
+    disclaimer: z.string(),
+  })
+  .meta({
+    id: "CyclePredictionDTO",
+    description:
+      "The materialised forecast. Fertile-window fields are server-suppressed (null) unless the goal is TRYING_TO_CONCEIVE.",
+  });
+
+const cycleCalendarDayDto = z.object({
+  date: z.string(),
+  phase: cyclePhaseEnumOpenapi.nullable(),
+  isPredictedPeriod: z.boolean(),
+  isFertileWindow: z.boolean(),
+  isPredictedOvulation: z.boolean(),
+  isPeriodLogged: z.boolean(),
+  flow: flowLevelEnum.nullable(),
+  hasSymptoms: z.boolean(),
+  confidence: z.number(),
+});
+
+const cycleProfileDto = z
+  .object({
+    goal: cycleTrackingGoalEnum,
+    cycleTrackingEnabled: z.boolean(),
+    rawChartMode: z.boolean(),
+    predictionEnabled: z.boolean(),
+    discreetNotifications: z.boolean(),
+    sensitiveCategoryEncryption: z.boolean(),
+    typicalCycleLength: z.number().int().nullable(),
+    typicalPeriodLength: z.number().int().nullable(),
+    lutealPhaseLength: z.number().int().nullable(),
+    updatedAt: z.iso.datetime({ offset: true }),
+  })
+  .meta({
+    id: "CycleProfileDTO",
+    description: "The full per-user cycle settings row.",
+  });
+
+const cycleCalendarResponse = z.object({
+  profile: z.object({
+    goal: cycleTrackingGoalEnum,
+    rawChartMode: z.boolean(),
+    predictionEnabled: z.boolean(),
+    cyclesObserved: z.number().int(),
+  }),
+  prediction: cyclePredictionDto.nullable(),
+  days: z.array(cycleCalendarDayDto),
+  meta: z.object({ generatedAt: z.iso.datetime({ offset: true }) }),
+});
+
+const cycleHistoryResponse = z.object({
+  cycles: z.array(menstrualCycleDto),
+  stats: z.object({
+    avgLengthDays: z.number().int().nullable(),
+    lengthVariabilityDays: z.number().nullable(),
+    avgPeriodLengthDays: z.number().int().nullable(),
+    regularity: z.enum(["REGULAR", "IRREGULAR", "LEARNING"]),
+  }),
+});
+
+const cyclePeriodResponse = z.object({
+  cycle: menstrualCycleDto.nullable(),
+  dayLog: cycleDayLogDto.nullable(),
+});
+
+const cycleBulkEntryResult = z.object({
+  index: z.number().int(),
+  status: z.enum(["inserted", "duplicate", "updated", "skipped"]),
+  id: z.string().optional(),
+  externalId: z.string().optional(),
+  reason: z.string().optional(),
+});
+
+const cycleBulkResponse = z.object({
+  processed: z.number().int(),
+  inserted: z.number().int(),
+  updated: z.number().int(),
+  duplicates: z.number().int(),
+  skipped: z.number().int(),
+  entries: z.array(cycleBulkEntryResult),
+});
+
+// A reusable 403 the cycle routes carry (the feature gate).
+const cycleDisabledResponse = {
+  "403": {
+    description:
+      "Cycle tracking is not enabled for this account (errorCode `cycle.disabled`).",
+    content: { "application/json": { schema: errorEnvelope } },
+  },
+} as const;
+
 // ── Path table ───────────────────────────────────────────────────────
 
 export const openApiPaths: NonNullable<ZodOpenApiObject["paths"]> = {
+  "/api/cycle/day-logs": {
+    post: {
+      tags: ["Cycle"],
+      summary: "Capture a single cycle day-log (v1.15.0)",
+      description:
+        "Upserts on `(userId, source, externalId)` when externalId present, else `(userId, date)`. `note` encrypts at rest. 201 on insert, 200 on update. Gated: `cycle.disabled` 403 when the feature is off.",
+      requestBody: {
+        required: true,
+        content: { "application/json": { schema: cycleDayLogInputSchema } },
+      },
+      responses: {
+        "200": {
+          description: "Existing day-log updated.",
+          content: {
+            "application/json": {
+              schema: dataEnvelope(cycleDayLogDto, "CycleDayLogEnvelope"),
+            },
+          },
+        },
+        "201": {
+          description: "New day-log created.",
+          content: {
+            "application/json": {
+              schema: dataEnvelope(cycleDayLogDto, "CycleDayLogCreatedEnvelope"),
+            },
+          },
+        },
+        ...cycleDisabledResponse,
+        ...stdResponses,
+      },
+    },
+  },
+  "/api/cycle/day-logs/{id}": {
+    patch: {
+      tags: ["Cycle"],
+      summary: "Edit a single cycle day-log (v1.15.0)",
+      description:
+        "Partial edit; an omitted field is left untouched. Owner-scoped (a cross-user id 404s). Gated.",
+      requestParams: { path: z.object({ id: z.string() }) },
+      requestBody: {
+        required: true,
+        content: { "application/json": { schema: cycleDayLogPatchSchema } },
+      },
+      responses: {
+        "200": {
+          description: "Day-log updated.",
+          content: {
+            "application/json": {
+              schema: dataEnvelope(cycleDayLogDto, "CycleDayLogPatchEnvelope"),
+            },
+          },
+        },
+        "404": {
+          description: "Day-log not found / not owned.",
+          content: { "application/json": { schema: errorEnvelope } },
+        },
+        ...cycleDisabledResponse,
+        ...stdResponses,
+      },
+    },
+    delete: {
+      tags: ["Cycle"],
+      summary: "Soft-delete a cycle day-log (v1.15.0)",
+      description:
+        "Sets `deletedAt` + bumps `syncVersion`; surfaces as a tombstone on the next `/api/sync/changes` page. 204. Idempotent.",
+      requestParams: { path: z.object({ id: z.string() }) },
+      responses: {
+        "204": { description: "Soft-deleted (no body)." },
+        "404": {
+          description: "Day-log not found / not owned.",
+          content: { "application/json": { schema: errorEnvelope } },
+        },
+        ...cycleDisabledResponse,
+        ...stdResponses,
+      },
+    },
+  },
+  "/api/cycle/day-logs/bulk": {
+    post: {
+      tags: ["Cycle"],
+      summary: "Bulk drain cycle day-logs (Outbox / HealthKit) (v1.15.0)",
+      description:
+        "Up to 500 entries; `withIdempotency`; rate-limited `cycle:day-logs:bulk:<userId>` 60/min. Per-entry status: inserted | duplicate | updated | skipped. Always 200.",
+      requestBody: {
+        required: true,
+        content: { "application/json": { schema: cycleBulkSchema } },
+      },
+      responses: {
+        "200": {
+          description: "Batch processed (per-entry results).",
+          content: {
+            "application/json": {
+              schema: dataEnvelope(cycleBulkResponse, "CycleBulkEnvelope"),
+            },
+          },
+        },
+        ...cycleDisabledResponse,
+        ...stdResponses,
+      },
+    },
+  },
+  "/api/cycle/period": {
+    post: {
+      tags: ["Cycle"],
+      summary: "Period-boundary shortcut (v1.15.0)",
+      description:
+        "One-tap started/ended period. `start` opens a new cycle (closing the prior); `end` stamps periodEndDate. Writes the boundary day-log. Gated.",
+      requestBody: {
+        required: true,
+        content: { "application/json": { schema: cyclePeriodSchema } },
+      },
+      responses: {
+        "200": {
+          description: "Cycle + boundary day-log.",
+          content: {
+            "application/json": {
+              schema: dataEnvelope(cyclePeriodResponse, "CyclePeriodEnvelope"),
+            },
+          },
+        },
+        ...cycleDisabledResponse,
+        ...stdResponses,
+      },
+    },
+  },
+  "/api/cycle/calendar": {
+    get: {
+      tags: ["Cycle"],
+      summary: "Predicted cycle calendar (v1.15.0)",
+      description:
+        "Runs the deterministic engine to build `{ profile, prediction, days }`. Fertile-window fields are server-suppressed unless goal is TRYING_TO_CONCEIVE. Default range: today − 90d … +180d. Gated.",
+      requestParams: {
+        query: z.object({
+          from: z.string().optional(),
+          to: z.string().optional(),
+        }),
+      },
+      responses: {
+        "200": {
+          description: "Calendar grid + forecast.",
+          content: {
+            "application/json": {
+              schema: dataEnvelope(cycleCalendarResponse, "CycleCalendarEnvelope"),
+            },
+          },
+        },
+        ...cycleDisabledResponse,
+        ...stdResponses,
+      },
+    },
+  },
+  "/api/cycle/cycles": {
+    get: {
+      tags: ["Cycle"],
+      summary: "Cycle history + stats (v1.15.0)",
+      description:
+        "Most-recent cycles (newest first) + `{ avgLengthDays, lengthVariabilityDays (MAD), avgPeriodLengthDays, regularity }`. `limit` default 24. Gated.",
+      requestParams: {
+        query: z.object({
+          limit: z.coerce.number().int().min(1).max(60).optional(),
+        }),
+      },
+      responses: {
+        "200": {
+          description: "Cycle history.",
+          content: {
+            "application/json": {
+              schema: dataEnvelope(cycleHistoryResponse, "CycleHistoryEnvelope"),
+            },
+          },
+        },
+        ...cycleDisabledResponse,
+        ...stdResponses,
+      },
+    },
+  },
+  "/api/cycle/cycles/{id}": {
+    delete: {
+      tags: ["Cycle"],
+      summary: "Soft-delete a menstrual cycle (v1.15.0)",
+      description:
+        "Sets `deletedAt` + bumps `syncVersion`; tombstones on the next sync page. 204. Idempotent.",
+      requestParams: { path: z.object({ id: z.string() }) },
+      responses: {
+        "204": { description: "Soft-deleted (no body)." },
+        "404": {
+          description: "Cycle not found / not owned.",
+          content: { "application/json": { schema: errorEnvelope } },
+        },
+        ...cycleDisabledResponse,
+        ...stdResponses,
+      },
+    },
+  },
+  "/api/cycle/profile": {
+    get: {
+      tags: ["Cycle"],
+      summary: "Read the full cycle profile (v1.15.0)",
+      description: "Returns the resolved CycleProfileDTO. Gated.",
+      responses: {
+        "200": {
+          description: "Cycle profile.",
+          content: {
+            "application/json": {
+              schema: dataEnvelope(cycleProfileDto, "CycleProfileEnvelope"),
+            },
+          },
+        },
+        ...cycleDisabledResponse,
+        ...stdResponses,
+      },
+    },
+  },
+  "/api/auth/me/cycle-prefs": {
+    get: {
+      tags: ["Cycle"],
+      summary: "Read cycle preferences (v1.15.0)",
+      description:
+        "Returns the resolved CycleProfileDTO. NOT gated — this is the surface that flips the gate.",
+      responses: {
+        "200": {
+          description: "Cycle preferences.",
+          content: {
+            "application/json": {
+              schema: dataEnvelope(cycleProfileDto, "CyclePrefsGetEnvelope"),
+            },
+          },
+        },
+        ...stdResponses,
+      },
+    },
+    patch: {
+      tags: ["Cycle"],
+      summary: "Update cycle preferences (v1.15.0)",
+      description:
+        "Deep-merges the supplied fields. `enabled` flips `cycleTrackingEnabled`. Returns the merged CycleProfileDTO. NOT gated.",
+      requestBody: {
+        required: true,
+        content: { "application/json": { schema: cyclePrefsSchema } },
+      },
+      responses: {
+        "200": {
+          description: "Merged cycle preferences.",
+          content: {
+            "application/json": {
+              schema: dataEnvelope(cycleProfileDto, "CyclePrefsPatchEnvelope"),
+            },
+          },
+        },
+        ...stdResponses,
+      },
+    },
+  },
   "/api/meta/capabilities": {
     get: {
       tags: ["Meta"],
diff --git a/src/lib/sync/__tests__/cursor.test.ts b/src/lib/sync/__tests__/cursor.test.ts
index e4020537e..d8faff9bc 100644
--- a/src/lib/sync/__tests__/cursor.test.ts
+++ b/src/lib/sync/__tests__/cursor.test.ts
@@ -42,21 +42,36 @@ describe("sync cursor codec", () => {
     expect(decodeCursor("")).toBeNull();
     // valid base64url but not the expected JSON shape
     expect(decodeCursor(Buffer.from("[]").toString("base64url"))).toBeNull();
-    // wrong version
+    // wrong version — v1.15.0 bumped CURSOR_VERSION to 2, so a v1 token
+    // (the pre-cycle format) now decodes to null → clean re-init.
     expect(
-      decodeCursor(Buffer.from('{"v":2,"d":{}}').toString("base64url")),
+      decodeCursor(Buffer.from('{"v":1,"d":{}}').toString("base64url")),
     ).toBeNull();
     // missing the `d` envelope
     expect(
-      decodeCursor(Buffer.from('{"v":1}').toString("base64url")),
+      decodeCursor(Buffer.from('{"v":2}').toString("base64url")),
     ).toBeNull();
   });
 
+  it("decodes a current-version empty envelope to a fresh-scan map", () => {
+    expect(
+      decodeCursor(Buffer.from('{"v":2,"d":{}}').toString("base64url")),
+    ).toEqual({});
+  });
+
+  it("round-trips the v1.15.0 cycle domains", () => {
+    const cursor: SyncCursor = {
+      cycleDays: { updatedAtMs: 1_717_001_000_000, id: "clx0cday" },
+      cycles: { updatedAtMs: 1_717_001_500_000, id: "clx0cyc" },
+    };
+    expect(decodeCursor(encodeCursor(cursor))).toEqual(cursor);
+  });
+
   it("drops a domain whose watermark is malformed but keeps valid ones", () => {
     // measurements has a string `u` (invalid); mood is well-formed.
     const token = Buffer.from(
       JSON.stringify({
-        v: 1,
+        v: 2,
         d: {
           measurements: { u: "x", i: "z" },
           mood: { u: 5, i: "clx0mood" },
diff --git a/src/lib/sync/cursor.ts b/src/lib/sync/cursor.ts
index bcf1925a6..b74d40152 100644
--- a/src/lib/sync/cursor.ts
+++ b/src/lib/sync/cursor.ts
@@ -20,12 +20,19 @@
  */
 
 /** The sync domains the feed serves. */
-export type SyncDomain = "measurements" | "mood" | "intakes";
+export type SyncDomain =
+  | "measurements"
+  | "mood"
+  | "intakes"
+  | "cycleDays"
+  | "cycles";
 
 export const SYNC_DOMAINS: readonly SyncDomain[] = [
   "measurements",
   "mood",
   "intakes",
+  "cycleDays",
+  "cycles",
 ];
 
 /** A single domain's keyset position. */
@@ -42,7 +49,11 @@ export interface DomainWatermark {
  */
 export type SyncCursor = Partial<Record<SyncDomain, DomainWatermark>>;
 
-const CURSOR_VERSION = 1;
+// v1.15.0 — bumped to 2 when the `cycleDays` + `cycles` domains were
+// added. An in-flight v1 token decodes to null (wrong version) → clean
+// re-init, which is acceptable because the cursor is opaque and the
+// client re-syncs from scratch.
+const CURSOR_VERSION = 2;
 
 /** Encode a per-domain keyset map into an opaque token. */
 export function encodeCursor(cursor: SyncCursor): string {
diff --git a/src/lib/validations/cycle.ts b/src/lib/validations/cycle.ts
new file mode 100644
index 000000000..e4f490f1b
--- /dev/null
+++ b/src/lib/validations/cycle.ts
@@ -0,0 +1,185 @@
+/**
+ * Cycle-tracking request/response validation (v1.15.0).
+ *
+ * Source of truth for the `/api/cycle/*` wire contract (ios-contract §2).
+ * The Zod schemas here are reused by the OpenAPI registry so the spec
+ * stays single-source. `userId` is NEVER a body field — it is narrowed
+ * from the session/Bearer in every route.
+ *
+ * Dates are `YYYY-MM-DD` strings (the MoodEntry tz-anchored convention);
+ * instants are ISO-8601 with offset.
+ */
+import { z } from "zod/v4";
+
+/* ── enums (mirror the Prisma enums) ─────────────────────────────── */
+
+export const flowLevelEnum = z.enum([
+  "NONE",
+  "SPOTTING",
+  "LIGHT",
+  "MEDIUM",
+  "HEAVY",
+]);
+
+export const ovulationTestEnum = z.enum([
+  "NEGATIVE",
+  "POSITIVE_LH_SURGE",
+  "ESTROGEN_SURGE",
+  "INDETERMINATE",
+]);
+
+export const cervicalMucusEnum = z.enum([
+  "DRY",
+  "STICKY",
+  "CREAMY",
+  "WATERY",
+  "EGG_WHITE",
+]);
+
+export const homeTestResultEnum = z.enum([
+  "NEGATIVE",
+  "POSITIVE",
+  "INDETERMINATE",
+]);
+
+export const contraceptiveKindEnum = z.enum([
+  "NONE",
+  "UNSPECIFIED",
+  "IMPLANT",
+  "INJECTION",
+  "IUD",
+  "INTRAVAGINAL_RING",
+  "ORAL",
+  "PATCH",
+  "EMERGENCY",
+]);
+
+export const cycleTrackingGoalEnum = z.enum([
+  "GENERAL_HEALTH",
+  "AVOID_PREGNANCY",
+  "TRYING_TO_CONCEIVE",
+  "PERIMENOPAUSE",
+  "OFF",
+]);
+
+/**
+ * Day-log source. MANUAL (web / single POST) or APPLE_HEALTH (the
+ * HealthKit batch ingest). Mirrors `MeasurementSource`; the wire spelling
+ * is APPLE_HEALTH, not "HEALTHKIT".
+ */
+export const cycleSourceEnum = z.enum(["MANUAL", "APPLE_HEALTH"]);
+
+const dateString = z
+  .string()
+  .regex(/^\d{4}-\d{2}-\d{2}$/, "Expected a YYYY-MM-DD date");
+
+/* ── day-log capture (single + bulk shared input) ────────────────── */
+
+/**
+ * The canonical day-log input. Shared by the single POST, the PATCH, and
+ * the bulk drain. `note` is encrypted at rest (`notesEncrypted`); every
+ * other field is queryable plaintext (feeds rollups / correlation).
+ */
+export const cycleDayLogInputSchema = z.object({
+  date: dateString,
+  flow: flowLevelEnum.optional(),
+  intermenstrualBleeding: z.boolean().optional(),
+  basalBodyTempC: z.number().min(30).max(45).optional(),
+  ovulationTest: ovulationTestEnum.optional(),
+  cervicalMucus: cervicalMucusEnum.optional(),
+  sexualActivity: z.boolean().optional(),
+  protectedSex: z.boolean().nullable().optional(),
+  pregnancyTest: homeTestResultEnum.optional(),
+  progesteroneTest: homeTestResultEnum.optional(),
+  contraceptive: contraceptiveKindEnum.optional(),
+  symptoms: z
+    .array(
+      z.object({
+        key: z.string().min(1).max(80),
+        severity: z.number().int().min(1).max(4).optional(),
+      }),
+    )
+    .max(40)
+    .optional(),
+  note: z.string().max(500).optional(),
+  loggedAt: z.iso.datetime({ offset: true }),
+  source: cycleSourceEnum.optional().default("MANUAL"),
+  externalId: z.string().min(1).max(120).optional(),
+});
+
+export type CycleDayLogInput = z.infer<typeof cycleDayLogInputSchema>;
+
+/** PATCH body — every field optional; `date`/`source`/`externalId` are immutable on update. */
+export const cycleDayLogPatchSchema = z.object({
+  flow: flowLevelEnum.nullable().optional(),
+  intermenstrualBleeding: z.boolean().optional(),
+  basalBodyTempC: z.number().min(30).max(45).nullable().optional(),
+  ovulationTest: ovulationTestEnum.nullable().optional(),
+  cervicalMucus: cervicalMucusEnum.nullable().optional(),
+  sexualActivity: z.boolean().optional(),
+  protectedSex: z.boolean().nullable().optional(),
+  pregnancyTest: homeTestResultEnum.nullable().optional(),
+  progesteroneTest: homeTestResultEnum.nullable().optional(),
+  contraceptive: contraceptiveKindEnum.nullable().optional(),
+  symptoms: z
+    .array(
+      z.object({
+        key: z.string().min(1).max(80),
+        severity: z.number().int().min(1).max(4).optional(),
+      }),
+    )
+    .max(40)
+    .optional(),
+  note: z.string().max(500).nullable().optional(),
+  loggedAt: z.iso.datetime({ offset: true }).optional(),
+});
+
+export const MAX_CYCLE_BULK_ENTRIES = 500;
+
+export const cycleBulkSchema = z.object({
+  entries: z.array(cycleDayLogInputSchema).min(1).max(MAX_CYCLE_BULK_ENTRIES),
+});
+
+/* ── period-boundary shortcut ────────────────────────────────────── */
+
+export const cyclePeriodSchema = z.object({
+  action: z.enum(["start", "end"]),
+  date: dateString,
+  externalId: z.string().min(1).max(120).optional(),
+  loggedAt: z.iso.datetime({ offset: true }),
+});
+
+/* ── calendar + history queries ──────────────────────────────────── */
+
+export const cycleCalendarQuerySchema = z.object({
+  from: dateString.optional(),
+  to: dateString.optional(),
+});
+
+export const cycleHistoryQuerySchema = z.object({
+  limit: z.coerce.number().int().min(1).max(60).optional(),
+});
+
+/* ── cycle preferences (PATCH /api/auth/me/cycle-prefs) ──────────── */
+
+/**
+ * Partial deep-merge body for the cycle-prefs PATCH. Mirrors the
+ * notification-prefs precedent: an omitted field is left untouched.
+ */
+export const cyclePrefsSchema = z
+  .object({
+    enabled: z.boolean().optional(),
+    goal: cycleTrackingGoalEnum.optional(),
+    rawChartMode: z.boolean().optional(),
+    predictionEnabled: z.boolean().optional(),
+    discreetNotifications: z.boolean().optional(),
+    sensitiveCategoryEncryption: z.boolean().optional(),
+    typicalCycleLength: z.number().int().min(15).max(60).nullable().optional(),
+    typicalPeriodLength: z.number().int().min(1).max(15).nullable().optional(),
+    lutealPhaseLength: z.number().int().min(8).max(20).nullable().optional(),
+  })
+  .refine((v) => Object.keys(v).length > 0, {
+    message: "At least one preference field is required",
+  });
+
+export type CyclePrefsInput = z.infer<typeof cyclePrefsSchema>;
diff --git a/tests/integration/cycle-routes.test.ts b/tests/integration/cycle-routes.test.ts
new file mode 100644
index 000000000..d7855fe4d
--- /dev/null
+++ b/tests/integration/cycle-routes.test.ts
@@ -0,0 +1,362 @@
+/**
+ * Integration suite for the v1.15.0 cycle-tracking routes against a real
+ * Postgres in a testcontainer. Asserts the iOS-facing contract end-to-end:
+ *
+ *   - the feature gate (`cycle.disabled` 403) keys on gender + the
+ *     `cycleTrackingEnabled` toggle, even with a valid session
+ *   - single day-log capture upserts (201 insert → 200 update), encrypts
+ *     the note, and reads it back decrypted
+ *   - the bulk drain returns per-entry inserted / updated / duplicate
+ *   - the period shortcut opens a cycle + boundary day-log
+ *   - the calendar read runs the engine + goal-gates the fertile window
+ *   - the cycle-prefs PATCH flips the gate for a non-FEMALE account
+ *   - `/api/sync/changes` carries the cycleDays + cycles domains incl.
+ *     tombstones after a soft-delete
+ */
+import { NextRequest } from "next/server";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+import { cookieJar, headerJar } from "./mock-next-headers";
+import { getPrismaClient, truncateAllTables } from "./setup";
+
+process.env.API_TOKEN_HMAC_KEY ??=
+  "test-hmac-key-cycle-routes-integration-32-bytes-min-1234567890";
+// Crypto-at-rest needs a key for the note encrypt/decrypt round-trip.
+process.env.ENCRYPTION_KEY ??=
+  "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef";
+
+const FEMALE_USER_ID = "user-cycle-female";
+const MALE_USER_ID = "user-cycle-male";
+
+vi.mock("next/headers", async () => {
+  const { cookieJar, headerJar } = await import("./mock-next-headers");
+  return {
+    headers: vi.fn(async () => ({
+      get: (name: string) => headerJar.get(name.toLowerCase()) ?? null,
+    })),
+    cookies: vi.fn(async () => ({
+      get: (name: string) => {
+        const value = cookieJar.get(name);
+        return value ? { name, value } : undefined;
+      },
+      set: (name: string, value: string) => cookieJar.set(name, value),
+      delete: (name: string) => cookieJar.delete(name),
+    })),
+  };
+});
+
+vi.mock("@/lib/db-compat", () => ({
+  ensureDbCompatibility: vi.fn().mockResolvedValue(undefined),
+}));
+
+async function loginAs(userId: string): Promise<void> {
+  cookieJar.clear();
+  headerJar.clear();
+  const session = await getPrismaClient().session.create({
+    data: { userId, expiresAt: new Date(Date.now() + 60 * 60 * 1000) },
+  });
+  cookieJar.set("healthlog_session", session.id);
+}
+
+beforeEach(async () => {
+  await truncateAllTables(getPrismaClient());
+  cookieJar.clear();
+  headerJar.clear();
+  const prisma = getPrismaClient();
+  await prisma.user.create({
+    data: {
+      id: FEMALE_USER_ID,
+      username: "cycle-female",
+      email: "cycle-female@example.test",
+      gender: "FEMALE",
+    },
+  });
+  await prisma.user.create({
+    data: {
+      id: MALE_USER_ID,
+      username: "cycle-male",
+      email: "cycle-male@example.test",
+      gender: "MALE",
+    },
+  });
+});
+
+function jsonRequest(
+  path: string,
+  method: string,
+  body?: unknown,
+  opts: { idempotencyKey?: string } = {},
+): NextRequest {
+  const headers: Record<string, string> = {
+    "content-type": "application/json",
+  };
+  if (opts.idempotencyKey) headers["idempotency-key"] = opts.idempotencyKey;
+  return new NextRequest(`http://localhost${path}`, {
+    method,
+    headers,
+    body: body !== undefined ? JSON.stringify(body) : undefined,
+  });
+}
+
+describe("cycle routes — feature gate", () => {
+  it("403s a MALE account on every cycle route", async () => {
+    await loginAs(MALE_USER_ID);
+    const { POST } = await import("@/app/api/cycle/day-logs/route");
+    const res = await POST(
+      jsonRequest("/api/cycle/day-logs", "POST", {
+        date: "2026-03-01",
+        flow: "MEDIUM",
+        loggedAt: "2026-03-01T08:00:00.000Z",
+      }),
+    );
+    expect(res.status).toBe(403);
+    const json = await res.json();
+    expect(json.meta?.errorCode).toBe("cycle.disabled");
+  });
+
+  it("admits a FEMALE account by gender derivation", async () => {
+    await loginAs(FEMALE_USER_ID);
+    const { POST } = await import("@/app/api/cycle/day-logs/route");
+    const res = await POST(
+      jsonRequest("/api/cycle/day-logs", "POST", {
+        date: "2026-03-01",
+        flow: "MEDIUM",
+        loggedAt: "2026-03-01T08:00:00.000Z",
+      }),
+    );
+    expect(res.status).toBe(201);
+  });
+});
+
+describe("cycle day-logs — single capture", () => {
+  it("upserts (201 insert → 200 update) and round-trips the encrypted note", async () => {
+    await loginAs(FEMALE_USER_ID);
+    const { POST } = await import("@/app/api/cycle/day-logs/route");
+
+    const insert = await POST(
+      jsonRequest("/api/cycle/day-logs", "POST", {
+        date: "2026-03-02",
+        flow: "LIGHT",
+        note: "tender, mild cramps",
+        loggedAt: "2026-03-02T08:00:00.000Z",
+      }),
+    );
+    expect(insert.status).toBe(201);
+    const insertJson = await insert.json();
+    expect(insertJson.data.flow).toBe("LIGHT");
+    expect(insertJson.data.note).toBe("tender, mild cramps");
+
+    // The note column is ciphertext, not plaintext.
+    const row = await getPrismaClient().cycleDayLog.findFirstOrThrow({
+      where: { userId: FEMALE_USER_ID, date: "2026-03-02" },
+    });
+    expect(row.notesEncrypted).not.toBeNull();
+    expect(row.notesEncrypted).not.toContain("cramps");
+
+    const update = await POST(
+      jsonRequest("/api/cycle/day-logs", "POST", {
+        date: "2026-03-02",
+        flow: "HEAVY",
+        loggedAt: "2026-03-02T09:00:00.000Z",
+      }),
+    );
+    expect(update.status).toBe(200);
+    const updateJson = await update.json();
+    expect(updateJson.data.flow).toBe("HEAVY");
+    expect(updateJson.data.id).toBe(insertJson.data.id);
+  });
+});
+
+describe("cycle day-logs — bulk drain", () => {
+  it("returns per-entry inserted / updated / duplicate", async () => {
+    await loginAs(FEMALE_USER_ID);
+    const { POST } = await import("@/app/api/cycle/day-logs/bulk/route");
+
+    const first = await POST(
+      jsonRequest("/api/cycle/day-logs/bulk", "POST", {
+        entries: [
+          {
+            date: "2026-03-05",
+            flow: "MEDIUM",
+            source: "APPLE_HEALTH",
+            externalId: "hk-aaa",
+            loggedAt: "2026-03-05T08:00:00.000Z",
+          },
+        ],
+      }),
+    );
+    expect(first.status).toBe(200);
+    const firstJson = await first.json();
+    expect(firstJson.data.inserted).toBe(1);
+    expect(firstJson.data.entries[0].status).toBe("inserted");
+
+    // Re-post unchanged → duplicate; re-post changed → updated.
+    const second = await POST(
+      jsonRequest("/api/cycle/day-logs/bulk", "POST", {
+        entries: [
+          {
+            date: "2026-03-05",
+            flow: "MEDIUM",
+            source: "APPLE_HEALTH",
+            externalId: "hk-aaa",
+            loggedAt: "2026-03-05T08:00:00.000Z",
+          },
+          {
+            date: "2026-03-05",
+            flow: "HEAVY",
+            source: "APPLE_HEALTH",
+            externalId: "hk-aaa",
+            loggedAt: "2026-03-05T08:00:00.000Z",
+          },
+        ],
+      }),
+    );
+    const secondJson = await second.json();
+    expect(secondJson.data.entries[0].status).toBe("duplicate");
+    expect(secondJson.data.entries[1].status).toBe("updated");
+  });
+});
+
+describe("cycle period shortcut", () => {
+  it("opens a cycle + writes the boundary day-log", async () => {
+    await loginAs(FEMALE_USER_ID);
+    const { POST } = await import("@/app/api/cycle/period/route");
+    const res = await POST(
+      jsonRequest("/api/cycle/period", "POST", {
+        action: "start",
+        date: "2026-03-10",
+        loggedAt: "2026-03-10T08:00:00.000Z",
+      }),
+    );
+    expect(res.status).toBe(200);
+    const json = await res.json();
+    expect(json.data.cycle.startDate).toBe("2026-03-10");
+    expect(json.data.dayLog.flow).toBe("MEDIUM");
+  });
+});
+
+describe("cycle calendar", () => {
+  it("runs the engine and goal-gates the fertile window", async () => {
+    await loginAs(FEMALE_USER_ID);
+    const prisma = getPrismaClient();
+    // Three ~28d cycles → a real prediction.
+    for (const [start, end, len] of [
+      ["2026-01-01", "2026-01-28", 28],
+      ["2026-01-29", "2026-02-25", 28],
+    ] as const) {
+      await prisma.menstrualCycle.create({
+        data: {
+          userId: FEMALE_USER_ID,
+          startDate: start,
+          endDate: end,
+          lengthDays: len,
+          periodEndDate: null,
+        },
+      });
+    }
+    await prisma.menstrualCycle.create({
+      data: { userId: FEMALE_USER_ID, startDate: "2026-02-26" },
+    });
+
+    const { GET } = await import("@/app/api/cycle/calendar/route");
+    const res = await GET(
+      new NextRequest(
+        "http://localhost/api/cycle/calendar?from=2026-02-20&to=2026-04-15",
+      ),
+    );
+    expect(res.status).toBe(200);
+    const json = await res.json();
+    expect(json.data.prediction).not.toBeNull();
+    expect(json.data.days.length).toBeGreaterThan(0);
+    // GENERAL_HEALTH default → fertile window suppressed.
+    expect(json.data.prediction.fertileWindowStart).toBeNull();
+
+    // The cache row is persisted fire-and-forget (the route never blocks
+    // the read on it), so poll briefly for it to land.
+    let cache = null;
+    for (let i = 0; i < 20 && cache === null; i++) {
+      cache = await prisma.cyclePrediction.findUnique({
+        where: { userId: FEMALE_USER_ID },
+      });
+      if (cache === null) await new Promise((r) => setTimeout(r, 25));
+    }
+    expect(cache).not.toBeNull();
+  });
+});
+
+describe("cycle-prefs gate flip", () => {
+  it("enables a MALE account via the prefs PATCH, then admits cycle routes", async () => {
+    await loginAs(MALE_USER_ID);
+    const { PATCH } = await import("@/app/api/auth/me/cycle-prefs/route");
+    const patch = await PATCH(
+      jsonRequest("/api/auth/me/cycle-prefs", "PATCH", { enabled: true }),
+    );
+    expect(patch.status).toBe(200);
+    const patchJson = await patch.json();
+    expect(patchJson.data.cycleTrackingEnabled).toBe(true);
+
+    // Re-login (fresh session) and confirm the gate now opens.
+    await loginAs(MALE_USER_ID);
+    const { POST } = await import("@/app/api/cycle/day-logs/route");
+    const res = await POST(
+      jsonRequest("/api/cycle/day-logs", "POST", {
+        date: "2026-03-15",
+        flow: "LIGHT",
+        loggedAt: "2026-03-15T08:00:00.000Z",
+      }),
+    );
+    expect(res.status).toBe(201);
+  });
+});
+
+describe("sync/changes — cycle domains", () => {
+  it("carries cycleDays + cycles upserts and tombstones after a soft-delete", async () => {
+    await loginAs(FEMALE_USER_ID);
+    const prisma = getPrismaClient();
+
+    const cycle = await prisma.menstrualCycle.create({
+      data: { userId: FEMALE_USER_ID, startDate: "2026-03-20" },
+    });
+    const dayLog = await prisma.cycleDayLog.create({
+      data: {
+        userId: FEMALE_USER_ID,
+        date: "2026-03-20",
+        flow: "MEDIUM",
+        source: "APPLE_HEALTH",
+        externalId: "hk-sync-1",
+      },
+    });
+
+    const { GET } = await import("@/app/api/sync/changes/route");
+    const first = await GET(
+      new NextRequest("http://localhost/api/sync/changes"),
+    );
+    const firstJson = await first.json();
+    expect(firstJson.data.changes.cycleDays.upserts.length).toBe(1);
+    expect(firstJson.data.changes.cycles.upserts.length).toBe(1);
+    expect(firstJson.data.changes.cycleDays.upserts[0].externalId).toBe(
+      "hk-sync-1",
+    );
+
+    // Soft-delete both → next pull surfaces tombstones.
+    await prisma.cycleDayLog.update({
+      where: { id: dayLog.id },
+      data: { deletedAt: new Date(), syncVersion: { increment: 1 } },
+    });
+    await prisma.menstrualCycle.update({
+      where: { id: cycle.id },
+      data: { deletedAt: new Date(), syncVersion: { increment: 1 } },
+    });
+
+    const second = await GET(
+      new NextRequest("http://localhost/api/sync/changes"),
+    );
+    const secondJson = await second.json();
+    const dayTombs = secondJson.data.changes.cycleDays.tombstones;
+    const cycleTombs = secondJson.data.changes.cycles.tombstones;
+    expect(dayTombs.length).toBe(1);
+    expect(dayTombs[0].externalId).toBe("hk-sync-1");
+    expect(cycleTombs.length).toBe(1);
+    expect(cycleTombs[0].id).toBe(cycle.id);
+  });
+});