diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index 71a91a954..fe7ca0be4 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -3331,10 +3331,12 @@ paths:
     get:
       tags:
         - Sync
-      summary: Measurements delta feed
-      description: "Incremental catch-up after the first-pair backfill (never a replacement for it). Pages over an opaque
-        keyset cursor; each page carries `tombstones` (soft-deleted rows, keyed on `externalId`) and `upserts` (live
-        rows). Apply tombstones before upserts within a page. `cursorExpired: true` forces a clean re-init."
+      summary: Multi-domain delta feed
+      description: "Incremental catch-up after the first-pair backfill (never a replacement for it). Pages over one opaque
+        keyset cursor spanning five domains — measurements, mood, medication intakes, cycle day logs and cycles. Each
+        domain carries `tombstones` (soft-deleted rows) and `upserts` (live rows); apply tombstones before upserts
+        within a domain. Tombstone identity is `externalId` for measurements and the server `id` everywhere else.
+        `cursorExpired: true` forces a clean re-init and the page is empty in every domain."
       parameters:
         - in: query
           name: cursor
@@ -37327,10 +37329,45 @@ components:
                 - upserts
                 - tombstones
               additionalProperties: false
+            cycleDays:
+              type: object
+              properties:
+                upserts:
+                  type: array
+                  items:
+                    $ref: "#/components/schemas/CycleDayLogDTO"
+                tombstones:
+                  type: array
+                  items:
+                    $ref: "#/components/schemas/SyncCycleDayTombstone"
+              required:
+                - upserts
+                - tombstones
+              additionalProperties: false
+              description: Cycle day logs. Present on every page, empty arrays when the account has the cycle module off.
+            cycles:
+              type: object
+              properties:
+                upserts:
+                  type: array
+                  items:
+                    $ref: "#/components/schemas/MenstrualCycleDTO"
+                tombstones:
+                  type: array
+                  items:
+                    $ref: "#/components/schemas/SyncCycleTombstone"
+              required:
+                - upserts
+                - tombstones
+              additionalProperties: false
+              description: Menstrual cycles, observed and forward-predicted. Present on every page, empty arrays when the account has
+                the cycle module off.
           required:
             - measurements
             - mood
             - intakes
+            - cycleDays
+            - cycles
           additionalProperties: false
       required:
         - serverNow
@@ -37339,9 +37376,10 @@ components:
         - cursorExpired
         - changes
       additionalProperties: false
-      description: "Multi-domain delta page (v1.7.0): measurements + mood + intakes. One opaque multi-domain keyset cursor;
-        tombstones apply before upserts within each domain. Tombstone identity: measurements key on externalId, mood +
-        intakes on server id. The iOS consumer is measurements-only this cycle; mood + intakes are forward-prep."
+      description: "Multi-domain delta page: measurements, mood, intakes, cycle day logs and cycles. One opaque multi-domain
+        keyset cursor; tombstones apply before upserts within each domain. Tombstone identity: measurements key on
+        externalId, every other domain on server id. All five keys are present on every response, including the
+        `cursorExpired` short-circuit — a domain with nothing to report sends empty arrays rather than being omitted."
     SyncMeasurementUpsert:
       type: object
       properties:
@@ -37683,6 +37721,60 @@ components:
         - updatedAt
       additionalProperties: false
       description: A soft-deleted medication intake, keyed on server `id`. Apply before upserts within the domain page.
+    SyncCycleDayTombstone:
+      type: object
+      properties:
+        id:
+          type: string
+        externalId:
+          anyOf:
+            - type: string
+            - type: "null"
+          description: The client's own key for the day log, when it set one.
+        syncVersion:
+          type: integer
+          minimum: 0
+          maximum: 9007199254740991
+        deletedAt:
+          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)))$
+        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
+        - externalId
+        - syncVersion
+        - deletedAt
+        - updatedAt
+      additionalProperties: false
+      description: A soft-deleted cycle day log, keyed on server `id`. Apply before upserts within the domain page.
+    SyncCycleTombstone:
+      type: object
+      properties:
+        id:
+          type: string
+        syncVersion:
+          type: integer
+          minimum: 0
+          maximum: 9007199254740991
+        deletedAt:
+          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)))$
+        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
+        - syncVersion
+        - deletedAt
+        - updatedAt
+      additionalProperties: false
+      description: A soft-deleted menstrual cycle, keyed on server `id`. Apply before upserts within the domain page.
     GetMeasurementsSeriesBatchResponse:
       type: object
       properties:
diff --git a/src/lib/openapi/routes/cycle.ts b/src/lib/openapi/routes/cycle.ts
index 903a8cd93..e32173f30 100644
--- a/src/lib/openapi/routes/cycle.ts
+++ b/src/lib/openapi/routes/cycle.ts
@@ -112,7 +112,7 @@ const cycleSymptomDto = z.object({
   severity: z.number().int().min(1).max(4).nullable(),
 });
 
-const cycleDayLogDto = z
+export const cycleDayLogDto = z
   .object({
     id: z.string(),
     date: z.string(),
@@ -145,7 +145,7 @@ const cycleDayLogDto = z
       "The canonical day-log row iOS mirrors. `note` is decrypted on read. Soft-deleted rows ride `/api/sync/changes` as tombstones.",
   });
 
-const menstrualCycleDto = z
+export const menstrualCycleDto = z
   .object({
     id: z.string(),
     startDate: z.string(),
diff --git a/src/lib/openapi/routes/sync.ts b/src/lib/openapi/routes/sync.ts
index 2f767327b..33071b202 100644
--- a/src/lib/openapi/routes/sync.ts
+++ b/src/lib/openapi/routes/sync.ts
@@ -9,6 +9,7 @@ import { z } from "zod/v4";
 import type { ZodOpenApiObject } from "zod-openapi";
 import { measurementResource } from "./measurements";
 import { moodContextWire } from "./mood";
+import { cycleDayLogDto, menstrualCycleDto } from "./cycle";
 import { dataEnvelope, stdResponses } from "./shared";
 
 // ── Sync (v1.7.0 offline / server-optional) ─────────────────────────
@@ -185,6 +186,36 @@ const syncIntakeTombstone = z
       "A soft-deleted medication intake, keyed on server `id`. Apply before upserts within the domain page.",
   });
 
+const syncCycleDayTombstone = z
+  .object({
+    id: z.string(),
+    externalId: z
+      .string()
+      .nullable()
+      .describe("The client's own key for the day log, when it set one."),
+    syncVersion: z.number().int().nonnegative(),
+    deletedAt: z.iso.datetime({ offset: true }),
+    updatedAt: z.iso.datetime({ offset: true }),
+  })
+  .meta({
+    id: "SyncCycleDayTombstone",
+    description:
+      "A soft-deleted cycle day log, keyed on server `id`. Apply before upserts within the domain page.",
+  });
+
+const syncCycleTombstone = z
+  .object({
+    id: z.string(),
+    syncVersion: z.number().int().nonnegative(),
+    deletedAt: z.iso.datetime({ offset: true }),
+    updatedAt: z.iso.datetime({ offset: true }),
+  })
+  .meta({
+    id: "SyncCycleTombstone",
+    description:
+      "A soft-deleted menstrual cycle, keyed on server `id`. Apply before upserts within the domain page.",
+  });
+
 const syncChangesQuery = z
   .object({
     cursor: z
@@ -233,12 +264,28 @@ const syncChangesResponse = z
         upserts: z.array(syncIntakeUpsert),
         tombstones: z.array(syncIntakeTombstone),
       }),
+      cycleDays: z
+        .object({
+          upserts: z.array(cycleDayLogDto),
+          tombstones: z.array(syncCycleDayTombstone),
+        })
+        .describe(
+          "Cycle day logs. Present on every page, empty arrays when the account has the cycle module off.",
+        ),
+      cycles: z
+        .object({
+          upserts: z.array(menstrualCycleDto),
+          tombstones: z.array(syncCycleTombstone),
+        })
+        .describe(
+          "Menstrual cycles, observed and forward-predicted. Present on every page, empty arrays when the account has the cycle module off.",
+        ),
     }),
   })
   .meta({
     id: "SyncChangesResponse",
     description:
-      "Multi-domain delta page (v1.7.0): measurements + mood + intakes. One opaque multi-domain keyset cursor; tombstones apply before upserts within each domain. Tombstone identity: measurements key on externalId, mood + intakes on server id. The iOS consumer is measurements-only this cycle; mood + intakes are forward-prep.",
+      "Multi-domain delta page: measurements, mood, intakes, cycle day logs and cycles. One opaque multi-domain keyset cursor; tombstones apply before upserts within each domain. Tombstone identity: measurements key on externalId, every other domain on server id. All five keys are present on every response, including the `cursorExpired` short-circuit — a domain with nothing to report sends empty arrays rather than being omitted.",
   });
 
 export const syncPaths: NonNullable<ZodOpenApiObject["paths"]> = {
@@ -264,9 +311,9 @@ export const syncPaths: NonNullable<ZodOpenApiObject["paths"]> = {
   "/api/sync/changes": {
     get: {
       tags: ["Sync"],
-      summary: "Measurements delta feed",
+      summary: "Multi-domain delta feed",
       description:
-        "Incremental catch-up after the first-pair backfill (never a replacement for it). Pages over an opaque keyset cursor; each page carries `tombstones` (soft-deleted rows, keyed on `externalId`) and `upserts` (live rows). Apply tombstones before upserts within a page. `cursorExpired: true` forces a clean re-init.",
+        "Incremental catch-up after the first-pair backfill (never a replacement for it). Pages over one opaque keyset cursor spanning five domains — measurements, mood, medication intakes, cycle day logs and cycles. Each domain carries `tombstones` (soft-deleted rows) and `upserts` (live rows); apply tombstones before upserts within a domain. Tombstone identity is `externalId` for measurements and the server `id` everywhere else. `cursorExpired: true` forces a clean re-init and the page is empty in every domain.",
       requestParams: {
         query: syncChangesQuery,
       },