diff --git a/CHANGELOG.md b/CHANGELOG.md
index ecd50e0c2..828e226ea 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,47 @@
 # Changelog
 
+## [1.38.20] — 2026-09-11
+
+Signing in no longer tells anybody which addresses have an account here.
+
+### Security
+
+- **Signing in now takes the same time whether or not the account exists.**
+  A wrong password and an address nobody has registered here have always
+  given you the same answer on screen, but the server used to reach that
+  answer much faster when there was no account to check, and the same when
+  the account signs in with a passkey and has no password at all. Someone
+  probing the sign-in page could read that difference and learn which
+  addresses are registered, and which of those have a password. Every
+  refusal now does the same work before it answers.
+- **Changing your email address is limited to ten changes an hour.** The
+  server tells you when the address you typed already belongs to another
+  account, which is the only sensible thing to say to someone who typed one.
+  Nothing stopped a signed-in caller from asking that question over and over
+  about addresses that were not theirs. Ten an hour per account is far above
+  any real use of the field, and a save that leaves your address as it is
+  does not count against it. If you hit the limit while saving other things
+  on the settings page, those are saved and only the address is held back,
+  with a note under the field. If you write against the API, a request that
+  asked for nothing but the address gets a 429 on `PUT /api/auth/profile`
+  and `PATCH /api/user/profile` with `meta.errorCode` set to
+  `profile.update.emailRateLimited` and a `Retry-After` header saying how
+  long to wait; a request that carried other fields gets 200 with `email`
+  listed in `rejectedFields` under the code `rate_limited`.
+
+### Removed
+
+- **The endpoint that answered whether an address has an account here is
+  gone.** `POST /api/auth/check-user` took an email address or a username
+  and said, to anyone who asked and without any sign-in, whether an account
+  existed and whether it had a passkey or a password. It was built to decide
+  which field a sign-in screen shows first, which is not worth telling the
+  world who has an account here. The path now answers 410 and says so in the
+  API reference rather than disappearing, so anything still calling it gets
+  a clear "this is gone, stop retrying" instead of a 404 that looks like a
+  broken server. If you wrote something against it, the sign-in request
+  itself is the replacement: send the credential and read the answer.
+
 ## [1.38.19] — 2026-09-11
 
 The first run tells you what is actually true about your account, an
diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index 3649c8157..fb07ebd4e 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -1,7 +1,7 @@
 openapi: 3.1.0
 info:
   title: HealthLog API
-  version: 1.38.19
+  version: 1.38.20
   description: >-
     Self-hosted personal-health-tracking PWA — public API surface for the iOS native client and external ingest.
 
@@ -1809,31 +1809,6 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-  /api/auth/check-user:
-    post:
-      tags:
-        - Auth
-      summary: Resolve the next sign-in step for a typed identifier
-      description: Given an email or username, returns which onboarding branch the iOS client should render plus the
-        `hasPasskey` / `hasPassword` booleans so it can offer a Passkey affordance alongside a password field without a
-        second round-trip. Anonymous surface; per-IP rate-limited (30 requests / 15 min). The response is the same
-        whether or not the identifier matched, and the identifier is never echoed back.
-      requestBody:
-        required: true
-        content:
-          application/json:
-            schema:
-              $ref: "#/components/schemas/CheckUserRequest"
-      responses:
-        "200":
-          description: Discovery result.
-          content:
-            application/json:
-              schema:
-                $ref: "#/components/schemas/CheckUserEnvelope"
-        "401": *a1
-        "422": *a4
-        "429": *a5
   /api/auth/login:
     post:
       security: []
@@ -3123,7 +3098,22 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "429": *a5
+        "429": &a14
+          description: "Too many email-address changes, and the request asked for nothing else. The address change shares one
+            per-account bucket across both profile routes — `profile-email:<accountId>`, 10 requests per 3600 seconds —
+            keyed on the ACTING account, so an account cannot collect a fresh allowance by probing a different address.
+            `meta.errorCode` is `profile.update.emailRateLimited`. Only a request whose `email` actually differs from
+            the address on file is counted: re-saving a form that carries the unchanged address is free, and so is any
+            update that does not touch `email` at all. A request that carries other fields beside the refused address
+            does NOT get this response — it answers 200, saves those fields, and names `email` in `rejectedFields` with
+            code `rate_limited`, so one spent budget cannot block an unrelated profile edit for an hour. The
+            `Retry-After`, `X-RateLimit-Limit`, `X-RateLimit-Remaining` and `X-RateLimit-Reset` headers describe that
+            bucket; back off on `Retry-After` rather than guessing."
+          headers: *a8
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
   /api/auth/oidc/login:
     get:
       security: []
@@ -4239,7 +4229,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/GetWorkoutResponse"
         "401": *a1
-        "403": &a15
+        "403": &a16
           description: 'Module disabled for this account: the user (or operator) has the module turned off. `meta.errorCode` =
             `module.disabled` and `meta.module` carries the disabled module key (e.g. "sleep"). Returned even for a
             valid Bearer token. Clients hide the whole module surface end-to-end rather than retry.'
@@ -7717,8 +7707,15 @@ paths:
               schema:
                 $ref: "#/components/schemas/PatchProfileResponse"
         "401": *a1
+        "409":
+          description: The requested email belongs to another account (`meta.errorCode` = `profile.update.emailInUse`). Nothing
+            was written. Shared with PUT /api/auth/profile, which runs the same handler.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
-        "429": *a5
+        "429": *a14
   /api/auth/me/notification-prefs:
     get:
       tags:
@@ -9118,7 +9115,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/CorrectAnamnesisFactResponse"
         "401": *a1
-        "404": &a14
+        "404": &a15
           description: No current revision with this id exists for the authenticated account.
           content:
             application/json:
@@ -9154,7 +9151,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/RemoveAnamnesisFactResponse"
         "401": *a1
-        "404": *a14
+        "404": *a15
         "409":
           description: Anamnesis fact changed since it was loaded (optimistic-concurrency conflict). No write happened. Re-read
             the resource, re-apply the user's change against the fresh state, and resend with the new token.
@@ -10278,7 +10275,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/CoachSeededQuestionEnvelope"
         "401": *a1
-        "403": *a15
+        "403": *a16
         "422": *a4
         "429": *a5
   /api/insights/narrative:
@@ -11048,7 +11045,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/EcgIngestCreatedEnvelope"
         "401": *a1
-        "403": *a15
+        "403": *a16
         "422": *a4
         "429": *a5
   /api/insights/ecg/{id}:
@@ -11275,7 +11272,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/MoodDailySeriesEnvelope"
         "401": *a1
-        "403": *a15
+        "403": *a16
         "422": *a4
         "429": *a5
   /api/mood/insights:
@@ -11297,7 +11294,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/MoodAggregatesEnvelope"
         "401": *a1
-        "403": *a15
+        "403": *a16
         "422": *a4
         "429": *a5
   /api/mood/prognosis:
@@ -11430,7 +11427,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/MoodCustomTagPatchEnvelope"
         "401": *a1
-        "404": &a16
+        "404": &a17
           description: Not found / not owned by the caller.
           content:
             application/json:
@@ -11465,7 +11462,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/MoodCustomTagDeleteEnvelope"
         "401": *a1
-        "404": *a16
+        "404": *a17
         "422": *a4
         "429": *a5
   /api/mood/tags/{key}/hidden:
@@ -11501,7 +11498,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "401": *a1
-        "404": *a16
+        "404": *a17
         "422": *a4
         "429": *a5
   /api/mood/tags/groups:
@@ -11554,7 +11551,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/MoodTagGroupPatchEnvelope"
         "401": *a1
-        "404": *a16
+        "404": *a17
         "422": *a4
         "429": *a5
     delete:
@@ -11585,7 +11582,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/MoodTagGroupDeleteEnvelope"
         "401": *a1
-        "404": *a16
+        "404": *a17
         "422": *a4
         "429": *a5
   /api/mood/tags/layout:
@@ -11750,7 +11747,7 @@ paths:
                 $ref: "#/components/schemas/MoodEntryEnvelope"
         "401": *a1
         "403": *a9
-        "409": &a17
+        "409": &a18
           description: A mood entry with the same `(date, moodLoggedAt)` instant already exists. `meta.errorCode` =
             `mood.duplicate_timestamp`. Only a request without `externalId` can answer this — with one, the write
             upserts in place.
@@ -11782,7 +11779,7 @@ paths:
                 $ref: "#/components/schemas/MoodEntryEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a16
+        "404": *a17
         "422": *a4
         "429": *a5
     put:
@@ -11817,8 +11814,8 @@ paths:
                 $ref: "#/components/schemas/MoodEntryEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a16
-        "409": *a17
+        "404": *a17
+        "409": *a18
         "422": *a4
         "429": *a5
     delete:
@@ -11844,7 +11841,7 @@ paths:
                 $ref: "#/components/schemas/MoodEntryDeleteEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a16
+        "404": *a17
         "422": *a4
         "429": *a5
   /api/mood-entries/restore:
@@ -11994,7 +11991,7 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/PutNtfySettingsResponse"
-        "400": &a18
+        "400": &a19
           description: Body is not parseable JSON.
           content:
             application/json:
@@ -12007,13 +12004,13 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "413": &a19
+        "413": &a20
           description: Body exceeds 64 KiB.
           content:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "415": &a20
+        "415": &a21
           description: Content-Type is not `application/json`.
           content:
             application/json:
@@ -12073,7 +12070,7 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/PutTelegramSettingsResponse"
-        "400": *a18
+        "400": *a19
         "401": *a1
         "403":
           description: The operator switched Telegram off for this instance and the body asked to enable it. No webhook was
@@ -12088,8 +12085,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "413": *a19
-        "415": *a20
+        "413": *a20
+        "415": *a21
         "422":
           description: Body matched neither accepted shape; or `enabled` is true without both a bot token and a chat id; or the
             deployment's `APP_URL` is not a public HTTPS host on port 80/88/443/8443; or Telegram refused the webhook
@@ -12143,10 +12140,10 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/PutWebhookSettingsResponse"
-        "400": *a18
+        "400": *a19
         "401": *a1
-        "413": *a19
-        "415": *a20
+        "413": *a20
+        "415": *a21
         "422":
           description: Body matched neither accepted shape, or `enabled` is true while no webhook URL is stored; or the URL is on
             a private network the operator has not listed in `NOTIFICATION_PRIVATE_ORIGINS` (`meta.errorCode` =
@@ -12645,7 +12642,7 @@ paths:
                 anyOf:
                   - $ref: "#/components/schemas/ConsentLatestForKindEnvelope"
                   - $ref: "#/components/schemas/ConsentLatestByKindEnvelope"
-        "400": &a21
+        "400": &a22
           description: "`kind` was present but is not one of `ai_full`, `ai_insights_only`, `ai_coach`. Multi-issue envelope;
             nothing was read or revoked. The consent family answers 400 here where the rest of the API answers 422."
           content:
@@ -12694,7 +12691,7 @@ paths:
                 anyOf:
                   - $ref: "#/components/schemas/ConsentRevokeForKindEnvelope"
                   - $ref: "#/components/schemas/ConsentRevokeAllEnvelope"
-        "400": *a21
+        "400": *a22
         "401": *a1
         "422": *a4
         "429": *a5
@@ -12980,7 +12977,7 @@ paths:
                 $ref: "#/components/schemas/MeasurementReminderEnvelope"
         "401": *a1
         "403": *a9
-        "404": &a22
+        "404": &a23
           description: Measurement reminder not found / not owned.
           content:
             application/json:
@@ -13015,7 +13012,7 @@ paths:
                 $ref: "#/components/schemas/MeasurementReminderPatchEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a22
+        "404": *a23
         "422": *a4
         "429": *a5
     delete:
@@ -13038,7 +13035,7 @@ paths:
                 $ref: "#/components/schemas/MeasurementReminderDeleteEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a22
+        "404": *a23
         "422": *a4
         "429": *a5
   /api/measurement-reminders/{id}/satisfy:
@@ -13063,7 +13060,7 @@ paths:
                 $ref: "#/components/schemas/MeasurementReminderSatisfyEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a22
+        "404": *a23
         "422": *a4
         "429": *a5
   /api/measurement-reminders/{id}/complete:
@@ -13091,7 +13088,7 @@ paths:
                 $ref: "#/components/schemas/MeasurementReminderCompleteEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a22
+        "404": *a23
         "422": *a4
         "429": *a5
   /api/measurement-reminders/{id}/skip:
@@ -13121,7 +13118,7 @@ paths:
                 $ref: "#/components/schemas/MeasurementReminderSkipEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a22
+        "404": *a23
         "422": *a4
         "429": *a5
   /api/measurement-reminders/{id}/snooze:
@@ -13155,7 +13152,7 @@ paths:
                 $ref: "#/components/schemas/MeasurementReminderSnoozeEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a22
+        "404": *a23
         "422": *a4
         "429": *a5
   /api/measurement-reminders/{id}/history:
@@ -13196,7 +13193,7 @@ paths:
                 $ref: "#/components/schemas/MeasurementReminderHistoryEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a22
+        "404": *a23
         "422": *a4
         "429": *a5
   /api/labs:
@@ -13320,7 +13317,7 @@ paths:
                 $ref: "#/components/schemas/GetLabResultResponse"
         "401": *a1
         "403": *a9
-        "404": &a23
+        "404": &a24
           description: Lab result not found (or owned by another user).
           content:
             application/json:
@@ -13355,7 +13352,7 @@ paths:
                 $ref: "#/components/schemas/UpdateLabResultResponse"
         "401": *a1
         "403": *a9
-        "404": *a23
+        "404": *a24
         "422": *a4
         "429": *a5
     delete:
@@ -13378,7 +13375,7 @@ paths:
                 $ref: "#/components/schemas/DeleteLabResultResponse"
         "401": *a1
         "403": *a9
-        "404": *a23
+        "404": *a24
         "422": *a4
         "429": *a5
   /api/labs/restore:
@@ -13534,7 +13531,7 @@ paths:
                 $ref: "#/components/schemas/CreateBiomarkerResponse"
         "401": *a1
         "403": *a9
-        "409": &a25
+        "409": &a26
           description: A biomarker with this name already exists for the caller.
           content:
             application/json:
@@ -13563,7 +13560,7 @@ paths:
                 $ref: "#/components/schemas/GetBiomarkerResponse"
         "401": *a1
         "403": *a9
-        "404": &a24
+        "404": &a25
           description: Biomarker not found (or owned by another user).
           content:
             application/json:
@@ -13598,8 +13595,8 @@ paths:
                 $ref: "#/components/schemas/UpdateBiomarkerResponse"
         "401": *a1
         "403": *a9
-        "404": *a24
-        "409": *a25
+        "404": *a25
+        "409": *a26
         "422": *a4
         "429": *a5
     delete:
@@ -13622,7 +13619,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/DeleteBiomarkerResponse"
         "401": *a1
-        "404": *a24
+        "404": *a25
         "422": *a4
         "429": *a5
   /api/encounters:
@@ -13761,7 +13758,7 @@ paths:
                 $ref: "#/components/schemas/GetEncounterEnvelope"
         "401": *a1
         "403": *a9
-        "404": &a26
+        "404": &a27
           description: Visit not found (or owned by another user).
           content:
             application/json:
@@ -13795,7 +13792,7 @@ paths:
                 $ref: "#/components/schemas/UpdateEncounterEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a26
+        "404": *a27
         "422": *a4
         "429": *a5
     delete:
@@ -13820,7 +13817,7 @@ paths:
                 $ref: "#/components/schemas/DeleteEncounterEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a26
+        "404": *a27
         "422": *a4
         "429": *a5
   /api/encounters/{id}/restore:
@@ -13846,7 +13843,7 @@ paths:
                 $ref: "#/components/schemas/RestoreEncounterEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a26
+        "404": *a27
         "422": *a4
         "429": *a5
   /api/encounters/{id}/links:
@@ -13876,7 +13873,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/LinkEncounterEnvelope"
         "401": *a1
-        "404": *a26
+        "404": *a27
         "422": *a4
         "429": *a5
     delete:
@@ -13905,7 +13902,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/UnlinkEncounterEnvelope"
         "401": *a1
-        "404": *a26
+        "404": *a27
         "422": *a4
         "429": *a5
   /api/practitioners:
@@ -13984,7 +13981,7 @@ paths:
                 $ref: "#/components/schemas/GetPractitionerEnvelope"
         "401": *a1
         "403": *a9
-        "404": &a27
+        "404": &a28
           description: Practitioner not found (or owned by another user).
           content:
             application/json:
@@ -14018,7 +14015,7 @@ paths:
                 $ref: "#/components/schemas/UpdatePractitionerEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a27
+        "404": *a28
         "422": *a4
         "429": *a5
     delete:
@@ -14042,7 +14039,7 @@ paths:
                 $ref: "#/components/schemas/DeletePractitionerEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a27
+        "404": *a28
         "422": *a4
         "429": *a5
   /api/practitioners/{id}/restore:
@@ -14068,7 +14065,7 @@ paths:
                 $ref: "#/components/schemas/RestorePractitionerEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a27
+        "404": *a28
         "422": *a4
         "429": *a5
   /api/vaccinations:
@@ -14163,7 +14160,7 @@ paths:
                 $ref: "#/components/schemas/GetVaccinationEnvelope"
         "401": *a1
         "403": *a9
-        "404": &a28
+        "404": &a29
           description: Vaccination not found (or owned by another user).
           content:
             application/json:
@@ -14198,7 +14195,7 @@ paths:
                 $ref: "#/components/schemas/UpdateVaccinationEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a28
+        "404": *a29
         "422": *a4
         "429": *a5
     delete:
@@ -14223,7 +14220,7 @@ paths:
                 $ref: "#/components/schemas/DeleteVaccinationEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a28
+        "404": *a29
         "422": *a4
         "429": *a5
   /api/vaccinations/{id}/restore:
@@ -14248,7 +14245,7 @@ paths:
                 $ref: "#/components/schemas/RestoreVaccinationEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a28
+        "404": *a29
         "422": *a4
         "429": *a5
   /api/vaccinations/{id}/booster:
@@ -14314,7 +14311,7 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "404": *a28
+        "404": *a29
         "422": *a4
         "429": *a5
   /api/vaccinations/suggest:
@@ -14372,7 +14369,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/LinkVaccinationEnvelope"
         "401": *a1
-        "404": *a28
+        "404": *a29
         "422": *a4
         "429": *a5
     delete:
@@ -14401,7 +14398,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/UnlinkVaccinationEnvelope"
         "401": *a1
-        "404": *a28
+        "404": *a29
         "422": *a4
         "429": *a5
   /api/illness/episodes:
@@ -14485,7 +14482,7 @@ paths:
                 $ref: "#/components/schemas/GetIllnessEpisodeEnvelope"
         "401": *a1
         "403": *a9
-        "404": &a29
+        "404": &a30
           description: Illness episode not found (or owned by another user).
           content:
             application/json:
@@ -14520,7 +14517,7 @@ paths:
                 $ref: "#/components/schemas/UpdateIllnessEpisodeEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a29
+        "404": *a30
         "422": *a4
         "429": *a5
     delete:
@@ -14544,7 +14541,7 @@ paths:
                 $ref: "#/components/schemas/DeleteIllnessEpisodeEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a29
+        "404": *a30
         "422": *a4
         "429": *a5
   /api/illness/episodes/{id}/restore:
@@ -14572,7 +14569,7 @@ paths:
                 $ref: "#/components/schemas/RestoreIllnessEpisodeEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a29
+        "404": *a30
         "409": *a7
         "422": *a4
         "429": *a5
@@ -14605,7 +14602,7 @@ paths:
                 $ref: "#/components/schemas/ResolveIllnessEpisodeEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a29
+        "404": *a30
         "422": *a4
         "429": *a5
   /api/illness/episodes/{id}/day-logs:
@@ -14656,7 +14653,7 @@ paths:
                 $ref: "#/components/schemas/GetIllnessDayLogEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a29
+        "404": *a30
         "422": *a4
         "429": *a5
     post:
@@ -14693,7 +14690,7 @@ paths:
                 $ref: "#/components/schemas/UpsertIllnessDayLogCreatedEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a29
+        "404": *a30
         "422": *a4
         "429": *a5
   /api/illness/episodes/{id}/correlation:
@@ -14720,7 +14717,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/IllnessCorrelationEnvelope"
         "401": *a1
-        "404": *a29
+        "404": *a30
         "422": *a4
         "429": *a5
   /api/illness/insights:
@@ -14936,7 +14933,7 @@ paths:
                 $ref: "#/components/schemas/GetAllergyEnvelope"
         "401": *a1
         "403": *a9
-        "404": &a30
+        "404": &a31
           description: Allergy not found (or owned by another user).
           content:
             application/json:
@@ -14970,7 +14967,7 @@ paths:
                 $ref: "#/components/schemas/UpdateAllergyEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a30
+        "404": *a31
         "422": *a4
         "429": *a5
     delete:
@@ -14994,7 +14991,7 @@ paths:
                 $ref: "#/components/schemas/DeleteAllergyEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a30
+        "404": *a31
         "422": *a4
         "429": *a5
   /api/family-history:
@@ -15068,7 +15065,7 @@ paths:
                 $ref: "#/components/schemas/GetFamilyHistoryEnvelope"
         "401": *a1
         "403": *a9
-        "404": &a31
+        "404": &a32
           description: Family history entry not found (or owned by another user).
           content:
             application/json:
@@ -15102,7 +15099,7 @@ paths:
                 $ref: "#/components/schemas/UpdateFamilyHistoryEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a31
+        "404": *a32
         "422": *a4
         "429": *a5
     delete:
@@ -15126,7 +15123,7 @@ paths:
                 $ref: "#/components/schemas/DeleteFamilyHistoryEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a31
+        "404": *a32
         "422": *a4
         "429": *a5
   /api/documents/inbound:
@@ -16021,7 +16018,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/CreateCustomMetricResponse"
         "401": *a1
-        "409": &a33
+        "409": &a34
           description: A custom metric with this name already exists for the caller.
           content:
             application/json:
@@ -16050,7 +16047,7 @@ paths:
                 $ref: "#/components/schemas/GetCustomMetricResponse"
         "401": *a1
         "403": *a9
-        "404": &a32
+        "404": &a33
           description: Custom metric not found (or owned by another user).
           content:
             application/json:
@@ -16084,8 +16081,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/UpdateCustomMetricResponse"
         "401": *a1
-        "404": *a32
-        "409": *a33
+        "404": *a33
+        "409": *a34
         "422": *a4
         "429": *a5
     delete:
@@ -16108,7 +16105,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/DeleteCustomMetricResponse"
         "401": *a1
-        "404": *a32
+        "404": *a33
         "422": *a4
         "429": *a5
   /api/custom-metrics/{id}/entries:
@@ -16152,7 +16149,7 @@ paths:
                 $ref: "#/components/schemas/ListCustomMetricEntriesEnvelope"
         "401": *a1
         "403": *a9
-        "404": *a32
+        "404": *a33
         "422": *a4
         "429": *a5
     post:
@@ -16181,7 +16178,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/CreateCustomMetricEntryResponse"
         "401": *a1
-        "404": *a32
+        "404": *a33
         "409": *a7
         "422": *a4
         "429": *a10
@@ -16217,7 +16214,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/UpdateCustomMetricEntryResponse"
         "401": *a1
-        "404": &a34
+        "404": &a35
           description: Custom metric entry not found (or owned by another user).
           content:
             application/json:
@@ -16250,7 +16247,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/DeleteCustomMetricEntryResponse"
         "401": *a1
-        "404": *a34
+        "404": *a35
         "422": *a4
         "429": *a5
   /api/custom-metrics/{id}/entries/restore:
@@ -16659,7 +16656,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/NutrientOverviewEnvelope"
         "401": *a1
-        "403": &a35
+        "403": &a36
           description: >-
             The opt-in `nutrients` module is off for this account (errorCode `module.disabled`). Stop syncing and do not
             retry; offer the enable-in-settings hint. Only run the HealthKit read-authorization flow while the module is
@@ -16714,7 +16711,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/NutrientDailySeriesEnvelope"
         "401": *a1
-        "403": *a35
+        "403": *a36
         "422": *a4
         "429": *a5
   /api/nutrients/water:
@@ -16743,7 +16740,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/NutrientWaterWriteEnvelope"
         "401": *a1
-        "403": *a35
+        "403": *a36
         "409": *a7
         "422": *a4
         "429": *a5
@@ -17245,26 +17242,26 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/WithingsCredentialsUpdateEnvelope"
-        "400": &a36
+        "400": &a37
           description: Body is not parseable JSON.
           content:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "401": *a1
-        "413": &a37
+        "413": &a38
           description: Body exceeds 64 KiB.
           content:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "415": &a38
+        "415": &a39
           description: Content-Type is not `application/json`.
           content:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": &a39
+        "422": &a40
           description: "`clientId` and `clientSecret` are both required and each must be 1–200 characters. The envelope carries a
             single fixed message, not per-field issues."
           content:
@@ -17450,11 +17447,11 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/FitbitCredentialsUpdateEnvelope"
-        "400": *a36
+        "400": *a37
         "401": *a1
-        "413": *a37
-        "415": *a38
-        "422": *a39
+        "413": *a38
+        "415": *a39
+        "422": *a40
         "429": *a5
     delete:
       tags:
@@ -17597,11 +17594,11 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/GoogleHealthCredentialsUpdateEnvelope"
-        "400": *a36
+        "400": *a37
         "401": *a1
-        "413": *a37
-        "415": *a38
-        "422": *a39
+        "413": *a38
+        "415": *a39
+        "422": *a40
         "429": *a5
     delete:
       tags:
@@ -17754,11 +17751,11 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/OuraCredentialsUpdateEnvelope"
-        "400": *a36
+        "400": *a37
         "401": *a1
-        "413": *a37
-        "415": *a38
-        "422": *a39
+        "413": *a38
+        "415": *a39
+        "422": *a40
         "429": *a5
     delete:
       tags:
@@ -17834,21 +17831,21 @@ paths:
               schema:
                 $ref: "#/components/schemas/OuraTestEnvelope"
         "401": *a1
-        "422": &a40
+        "422": &a41
           description: No usable connection for this provider — nothing stored, or the stored grant could no longer be resolved
             into an access token. `meta.errorCode` = `not_configured`.
           content:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "429": &a41
+        "429": &a42
           description: More than 5 requests in 60 s from this user. `meta.errorCode` = `rate_limited_self` — this is the local
             limiter, distinct from a `rate_limited` 502 which is the provider's.
           content:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "502": &a42
+        "502": &a43
           description: "The probe did not complete. `meta.errorCode` names the class: `credentials_rejected` (the provider
             answered 401/403 — the stored grant is gone), `rate_limited` (the PROVIDER answered 429, which is not this
             endpoint's own limiter), `upstream_error` (5xx), `timeout` (no answer within 5 s), `connection_failed`
@@ -17893,11 +17890,11 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/PolarCredentialsUpdateEnvelope"
-        "400": *a36
+        "400": *a37
         "401": *a1
-        "413": *a37
-        "415": *a38
-        "422": *a39
+        "413": *a38
+        "415": *a39
+        "422": *a40
         "429": *a5
     delete:
       tags:
@@ -17973,9 +17970,9 @@ paths:
               schema:
                 $ref: "#/components/schemas/PolarTestEnvelope"
         "401": *a1
-        "422": *a40
-        "429": *a41
-        "502": *a42
+        "422": *a41
+        "429": *a42
+        "502": *a43
   /api/strava/credentials:
     get:
       tags:
@@ -18011,11 +18008,11 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/StravaCredentialsUpdateEnvelope"
-        "400": *a36
+        "400": *a37
         "401": *a1
-        "413": *a37
-        "415": *a38
-        "422": *a39
+        "413": *a38
+        "415": *a39
+        "422": *a40
         "429": *a5
     delete:
       tags:
@@ -18093,9 +18090,9 @@ paths:
               schema:
                 $ref: "#/components/schemas/StravaTestEnvelope"
         "401": *a1
-        "422": *a40
-        "429": *a41
-        "502": *a42
+        "422": *a41
+        "429": *a42
+        "502": *a43
   /api/integrations/status:
     get:
       tags:
@@ -18131,7 +18128,7 @@ paths:
                 $ref: "#/components/schemas/FitbitResumeEnvelope"
         "401": *a1
         "422": *a4
-        "429": *a41
+        "429": *a42
   /api/integrations/fitbit/test:
     post:
       tags:
@@ -18149,9 +18146,9 @@ paths:
               schema:
                 $ref: "#/components/schemas/FitbitIntegrationTestEnvelope"
         "401": *a1
-        "422": *a40
-        "429": *a41
-        "502": *a42
+        "422": *a41
+        "429": *a42
+        "502": *a43
   /api/integrations/google-health/resume:
     post:
       tags:
@@ -18168,7 +18165,7 @@ paths:
                 $ref: "#/components/schemas/GoogleHealthResumeEnvelope"
         "401": *a1
         "422": *a4
-        "429": *a41
+        "429": *a42
   /api/integrations/google-health/test:
     post:
       tags:
@@ -18201,9 +18198,9 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "422": *a40
-        "429": *a41
-        "502": *a42
+        "422": *a41
+        "429": *a42
+        "502": *a43
   /api/integrations/whoop/resume:
     post:
       tags:
@@ -18220,7 +18217,7 @@ paths:
                 $ref: "#/components/schemas/WhoopResumeEnvelope"
         "401": *a1
         "422": *a4
-        "429": *a41
+        "429": *a42
   /api/integrations/whoop/test:
     post:
       tags:
@@ -18237,9 +18234,9 @@ paths:
               schema:
                 $ref: "#/components/schemas/WhoopIntegrationTestEnvelope"
         "401": *a1
-        "422": *a40
-        "429": *a41
-        "502": *a42
+        "422": *a41
+        "429": *a42
+        "502": *a43
   /api/integrations/withings/resume:
     post:
       tags:
@@ -18257,7 +18254,7 @@ paths:
                 $ref: "#/components/schemas/WithingsResumeEnvelope"
         "401": *a1
         "422": *a4
-        "429": *a41
+        "429": *a42
   /api/integrations/withings/test:
     post:
       tags:
@@ -18275,8 +18272,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/WithingsIntegrationTestEnvelope"
         "401": *a1
-        "422": *a40
-        "429": *a41
+        "422": *a41
+        "429": *a42
         "502":
           description: "The probe did not complete. The shared codes apply (`credentials_rejected`, `rate_limited`,
             `upstream_error`, `timeout`, `connection_failed`) and this path adds one of its own:
@@ -18340,7 +18337,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/DismissPriorityItemEnvelope"
         "401": *a1
-        "403": *a15
+        "403": *a16
         "422": *a4
         "429": *a5
   /api/dashboard/chart-overlay-prefs:
@@ -18513,7 +18510,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/RecordActivityEnvelope"
         "401": *a1
-        "403": &a43
+        "403": &a44
           description: Refused. `meta.errorCode` is `sharing.not_permitted` when the request was made while acting on another
             account (grant management is never delegable), or `sharing.access.denied` when the caller may not act as the
             account they named. The latter is byte-identical for an account that does not exist and one that granted
@@ -18538,7 +18535,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/AccountGrantListEnvelope"
         "401": *a1
-        "403": *a43
+        "403": *a44
         "422": *a4
         "429": *a5
     post:
@@ -18633,7 +18630,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/AcceptedAccountGrantEnvelope"
         "401": *a1
-        "403": *a43
+        "403": *a44
         "404":
           description: No such invitation for this account.
           content:
@@ -18677,7 +18674,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/RevokedAccountGrantEnvelope"
         "401": *a1
-        "403": *a43
+        "403": *a44
         "404":
           description: No such grant on this account's record.
           content:
@@ -18718,7 +18715,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/RenouncedAccountGrantEnvelope"
         "401": *a1
-        "403": *a43
+        "403": *a44
         "404":
           description: No such grant held by this account.
           content:
@@ -19100,7 +19097,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "401": *a1
-        "403": *a43
+        "403": *a44
         "409":
           description: "The record this browser session is on is not the record the request asserted. `meta.errorCode` is
             `sharing.session.changed`. It is not a refusal of access — the caller may well be entitled to both records —
@@ -19430,7 +19427,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/McpConnectionListEnvelope"
         "401": *a1
-        "403": &a44
+        "403": &a45
           description: The operator has disabled the API instance-wide. Every route in this module refuses while that setting is
             off, for a cookie session as well as for a Bearer token; it is not a per-user permission and retrying will
             not help.
@@ -19463,7 +19460,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/McpConnectionRevokeEnvelope"
         "401": *a1
-        "403": *a44
+        "403": *a45
         "404":
           description: No live connection with this id for this caller — also the answer for an already-revoked one, so a repeat
             revoke is 404 rather than an idempotent 200.
@@ -19490,7 +19487,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/McpConnectorTokenListEnvelope"
         "401": *a1
-        "403": *a44
+        "403": *a45
         "422": *a4
         "429": *a5
     post:
@@ -19520,7 +19517,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "401": *a1
-        "403": *a44
+        "403": *a45
         "413":
           description: Body exceeds 16 KiB.
           content:
@@ -19563,7 +19560,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/McpConnectorTokenRevokeEnvelope"
         "401": *a1
-        "403": *a44
+        "403": *a45
         "404":
           description: No `health:read` token with this id for this caller. An already-revoked token is still found, so a repeat
             revoke answers 200.
@@ -19621,7 +19618,7 @@ paths:
       description: Returns the named family's resolved settings for the record the session is acting on. Cookie session or
         Bearer, while acting on a managed record the caller guards.
       parameters:
-        - &a45
+        - &a46
           name: family
           in: path
           required: true
@@ -19665,7 +19662,7 @@ paths:
         ride the audit row. Body capped at 64 KiB. Cookie session or Bearer, while acting on a managed record the caller
         guards.
       parameters:
-        - *a45
+        - *a46
       requestBody:
         required: true
         content:
@@ -19817,7 +19814,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/EnvironmentOverviewEnvelope"
         "401": *a1
-        "403": *a15
+        "403": *a16
         "422": *a4
         "429": *a5
   /api/environment/home:
@@ -19854,7 +19851,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/EnvironmentHomeEnvelope"
         "401": *a1
-        "403": *a15
+        "403": *a16
         "413":
           description: Body exceeds 4 KiB.
           content:
@@ -19908,7 +19905,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/EnvironmentBackfillEnvelope"
         "401": *a1
-        "403": *a15
+        "403": *a16
         "409":
           description: No home location is set, so there is nothing to resolve the days against. `meta.errorCode` =
             `environment.no_home`.
@@ -19967,7 +19964,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/EnvironmentGeocodeEnvelope"
         "401": *a1
-        "403": *a15
+        "403": *a16
         "422":
           description: "`q` was missing, empty after trimming, or longer than 120 characters. `meta.errorCode` =
             `environment.invalid`."
@@ -20006,7 +20003,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/CreateEnvironmentTravelEnvelope"
         "401": *a1
-        "403": *a15
+        "403": *a16
         "413":
           description: Body exceeds 4 KiB.
           content:
@@ -20059,7 +20056,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/DeleteEnvironmentTravelEnvelope"
         "401": *a1
-        "403": *a15
+        "403": *a16
         "404":
           description: No such override for this account. `meta.errorCode` = `environment.travel.not_found`. Also the answer for
             an id owned by somebody else — existence is not leaked.
@@ -20200,13 +20197,13 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/AdminBackupDocument"
-        "403": &a46
+        "403": &a47
           description: Caller is not an admin. `requireAdmin()` is cookie-only, so a Bearer token lands here whatever its scope.
           content:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "404": &a47
+        "404": &a48
           description: No stored copy with this id.
           content:
             application/json:
@@ -20265,8 +20262,8 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "403": *a46
-        "404": *a47
+        "403": *a47
+        "404": *a48
         "409":
           description: Either the payload declares a different owner than the stored row, or a request under the same
             `Idempotency-Key` is still in flight. Nothing was changed.
@@ -20422,6 +20419,36 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
+  /api/auth/check-user:
+    post:
+      security: []
+      deprecated: true
+      tags:
+        - Retired
+      summary: POST /api/auth/check-user — removed in v1.38.20
+      description: >-
+        Removed in v1.38.20. Every request to this path answers 410 Gone, whatever the method and whatever credential it
+        carries. There is no replacement; the capability is gone rather than moved.
+
+
+        Took an email address or a username and answered, to any caller and with no credential at all, whether an
+        account existed here and whether it carried a passkey or a password. It was built to decide which field a
+        sign-in screen shows first, which is not worth telling the world who has an account. Send the credential to POST
+        /api/auth/login or run the passkey ceremony and read the answer; both refuse identically whether or not the
+        account exists.
+
+
+        Retiring a path is deliberate, so the answer says so rather than leaving a client to read a 404 as a broken
+        deployment and retry forever. Treat 410 as terminal: drop the call, degrade the surface that depended on it, and
+        do not offer a retry.
+      responses:
+        "410":
+          description: The path was removed in v1.38.20 and will not answer again. `meta.errorCode` = `route.retired`,
+            `meta.removedIn` carries the version, and `meta.replacedBy` carries the replacement path or null.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
   /api/auth/me/research-mode:
     get:
       security: []
@@ -21129,17 +21156,6 @@ components:
             server-side recovery.
       required:
         - passphrase
-    CheckUserRequest:
-      type: object
-      properties:
-        identifier:
-          type: string
-          minLength: 1
-          maxLength: 254
-          description: The typed identifier — either an email or a username. Queried verbatim (no case-folding), never echoed back.
-      required:
-        - identifier
-      description: "Discovery lookup for the iOS onboarding flow: given a typed email or username, resolve the next sign-in step."
     LoginPasswordRequest:
       type: object
       properties:
@@ -30002,9 +30018,9 @@ components:
                 onboarding.complete.recordNotFound, onboarding.complete.recordTargetMismatch,
                 onboarding.disclaimer.invalid, onboarding.restart.invalid, onboarding.restart.rateLimited,
                 onboarding.tour.invalid; personal_records — personal_records.invalid_query; practitioner —
-                practitioner.invalid; profile — profile.update.emailInUse, profile.update.invalidBody,
-                profile.update.nothingSaved; record_write — record_write.rate_limited; report-selection —
-                report-selection.body.invalid_json, report-selection.body.invalid_shape,
+                practitioner.invalid; profile — profile.update.emailInUse, profile.update.emailRateLimited,
+                profile.update.invalidBody, profile.update.nothingSaved; record_write — record_write.rate_limited;
+                report-selection — report-selection.body.invalid_json, report-selection.body.invalid_shape,
                 report-selection.leaves.unknown; route — route.retired; share-link — share-link.document.decryptFailed,
                 share-link.document.notFound, share-link.document.rateLimited, share-link.documents.invalid,
                 share-link.report.notFound, share-link.report.rateLimited, share-link.selection.forbidden_leaf,
@@ -36587,49 +36603,6 @@ components:
         omissions there is a real, undeclared gap: roughly thirty of the models the backup plan marks as backed up have
         neither a reader here nor a restore branch yet, and `src/lib/export/backup-plan.ts` is the enumeration of which.
         `schemaVersion` is `2` on everything written today; the parser still accepts `1`."
-    CheckUserEnvelope:
-      type: object
-      properties:
-        data:
-          $ref: "#/components/schemas/CheckUserResponse"
-        error:
-          type: "null"
-        meta:
-          type: object
-          properties:
-            requestId:
-              type: string
-          additionalProperties: false
-      required:
-        - data
-        - error
-      additionalProperties: false
-    CheckUserResponse:
-      type: object
-      properties:
-        branch:
-          type: string
-          enum:
-            - not_found
-            - passkey_only
-            - email_fallback
-            - exists
-          description: "Next UX step: `not_found` (show sign-up), `passkey_only` (Sign in with Passkey), `email_fallback`
-            (password field, with a Passkey affordance when applicable), `exists` (account with no usable credential —
-            recovery path)."
-        hasPasskey:
-          type: boolean
-          description: The account has at least one registered passkey.
-        hasPassword:
-          type: boolean
-          description: The account has a password hash.
-      required:
-        - branch
-        - hasPasskey
-        - hasPassword
-      additionalProperties: false
-      description: Account-existence + credential shape. The response is identical whether or not the identifier matched
-        (account-existence is the explicit contract iOS needs); the identifier is never echoed.
     LoginResponse:
       type: object
       properties:
@@ -37946,9 +37919,11 @@ components:
           description: Whether an insurance number is stored. The number itself is encrypted at rest and is never returned — only
             its presence.
         rejectedFields:
-          description: "Present only on a PARTIAL success: the fields that failed validation and were skipped while the rest of
-            the patch was written. A 200 carrying this key means the save was incomplete — surface it, do not treat the
-            response as a clean save."
+          description: "Present only on a PARTIAL success: the fields that were skipped while the rest of the patch was written. A
+            200 carrying this key means the save was incomplete — surface it, do not treat the response as a clean save.
+            `code` is the validator code for a field that failed validation, or `rate_limited` for an email-address
+            change that has spent the account's hourly budget; that one is not a bad value and the same address will be
+            accepted once the window rolls over."
           type: array
           items:
             type: object
@@ -46842,6 +46817,28 @@ components:
         hasInsuranceNumber:
           type: boolean
           description: Whether a KVNR is on file (the value itself is not echoed).
+        rejectedFields:
+          description: "Present only on a PARTIAL success: the fields that were skipped while the rest of the patch was written. A
+            200 carrying this key means the save was incomplete — surface it, do not treat the response as a clean save.
+            `code` is the validator code for a field that failed validation, or `rate_limited` for an email-address
+            change that has spent the account's hourly budget; that one is not a bad value and the same address will be
+            accepted once the window rolls over. The handler has always been able to answer with this key on this path;
+            it was published only on PUT /api/auth/profile."
+          type: array
+          items:
+            type: object
+            properties:
+              path:
+                type: string
+              code:
+                type: string
+              message:
+                type: string
+            required:
+              - path
+              - code
+              - message
+            additionalProperties: false
       required:
         - username
         - email
diff --git a/messages/de.json b/messages/de.json
index d139727ec..3a48fb6f1 100644
--- a/messages/de.json
+++ b/messages/de.json
@@ -125,7 +125,8 @@
       "update": {
         "nothingSaved": "Es wurde nichts gespeichert. Korrigiere das markierte Feld und versuche es erneut.",
         "invalidBody": "Die Profilaktualisierung konnte nicht gelesen werden.",
-        "emailInUse": "Diese E-Mail-Adresse wird bereits verwendet."
+        "emailInUse": "Diese E-Mail-Adresse wird bereits verwendet.",
+        "emailRateLimited": "Zu viele Änderungen der E-Mail-Adresse. Versuche es später erneut."
       }
     }
   },
@@ -5508,6 +5509,7 @@
       "invalidValue": "Das ist keine der Antworten, die dieses Feld annimmt.",
       "invalidFormat": "Das ist nicht in einer Form geschrieben, die dieses Feld lesen kann.",
       "invalidType": "Der Wert kam nicht in einer Form an, die dieses Feld speichern kann.",
+      "rateLimited": "Zu oft geändert. Warte kurz und versuche es dann erneut.",
       "other": "Dieser Wert wurde nicht angenommen. Bitte prüfen und erneut versuchen."
     },
     "avatar": {
diff --git a/messages/en.json b/messages/en.json
index cda491a00..b2a37305f 100644
--- a/messages/en.json
+++ b/messages/en.json
@@ -125,7 +125,8 @@
       "update": {
         "nothingSaved": "Nothing was saved. Check the highlighted field and try again.",
         "invalidBody": "The profile update could not be read.",
-        "emailInUse": "This email address is already in use."
+        "emailInUse": "This email address is already in use.",
+        "emailRateLimited": "Too many email-address changes. Try again later."
       }
     }
   },
@@ -5508,6 +5509,7 @@
       "invalidValue": "That is not one of the answers this field accepts.",
       "invalidFormat": "That is not written in a form this field can read.",
       "invalidType": "That value did not arrive in a form this field can store.",
+      "rateLimited": "Changed too often. Wait a little and try again.",
       "other": "This value was not accepted. Check it and try again."
     },
     "avatar": {
diff --git a/messages/es.json b/messages/es.json
index fbe4b3570..e3f4ca56c 100644
--- a/messages/es.json
+++ b/messages/es.json
@@ -125,7 +125,8 @@
       "update": {
         "nothingSaved": "No se guardó nada. Revisa el campo marcado e inténtalo de nuevo.",
         "invalidBody": "No se pudo leer la actualización del perfil.",
-        "emailInUse": "Esta dirección de correo ya está en uso."
+        "emailInUse": "Esta dirección de correo ya está en uso.",
+        "emailRateLimited": "Demasiados cambios de dirección de correo. Inténtalo más tarde."
       }
     }
   },
@@ -5508,6 +5509,7 @@
       "invalidValue": "Esa no es una de las respuestas que acepta este campo.",
       "invalidFormat": "Eso no está escrito en un formato que este campo pueda leer.",
       "invalidType": "El valor no llegó en un formato que este campo pueda guardar.",
+      "rateLimited": "Se ha cambiado demasiadas veces. Espera un poco e inténtalo de nuevo.",
       "other": "No se aceptó este valor. Revísalo e inténtalo de nuevo."
     },
     "avatar": {
diff --git a/messages/fr.json b/messages/fr.json
index fc7ef2799..48fc47a50 100644
--- a/messages/fr.json
+++ b/messages/fr.json
@@ -125,7 +125,8 @@
       "update": {
         "nothingSaved": "Rien n'a été enregistré. Corrigez le champ signalé et réessayez.",
         "invalidBody": "La mise à jour du profil n'a pas pu être lue.",
-        "emailInUse": "Cette adresse e-mail est déjà utilisée."
+        "emailInUse": "Cette adresse e-mail est déjà utilisée.",
+        "emailRateLimited": "Trop de changements d'adresse e-mail. Réessayez plus tard."
       }
     }
   },
@@ -5508,6 +5509,7 @@
       "invalidValue": "Ce n'est pas une des réponses acceptées par ce champ.",
       "invalidFormat": "Ce n'est pas écrit dans un format que ce champ peut lire.",
       "invalidType": "La valeur n'est pas arrivée dans un format que ce champ peut enregistrer.",
+      "rateLimited": "Modifié trop souvent. Patientez un peu et réessayez.",
       "other": "Cette valeur n'a pas été acceptée. Vérifiez-la et réessayez."
     },
     "avatar": {
diff --git a/messages/it.json b/messages/it.json
index 99a3df2bd..0e97ed7be 100644
--- a/messages/it.json
+++ b/messages/it.json
@@ -125,7 +125,8 @@
       "update": {
         "nothingSaved": "Non è stato salvato nulla. Controlla il campo evidenziato e riprova.",
         "invalidBody": "Impossibile leggere l'aggiornamento del profilo.",
-        "emailInUse": "Questo indirizzo email è già in uso."
+        "emailInUse": "Questo indirizzo email è già in uso.",
+        "emailRateLimited": "Troppi cambi di indirizzo email. Riprova più tardi."
       }
     }
   },
@@ -5508,6 +5509,7 @@
       "invalidValue": "Questa non è una delle risposte accettate da questo campo.",
       "invalidFormat": "Non è scritto in un formato che questo campo possa leggere.",
       "invalidType": "Il valore non è arrivato in un formato che questo campo possa salvare.",
+      "rateLimited": "Modificato troppe volte. Aspetta un momento e riprova.",
       "other": "Questo valore non è stato accettato. Controllalo e riprova."
     },
     "avatar": {
diff --git a/messages/ko.json b/messages/ko.json
index ca5b3deb6..d09db31aa 100644
--- a/messages/ko.json
+++ b/messages/ko.json
@@ -125,7 +125,8 @@
       "update": {
         "nothingSaved": "저장된 내용이 없어요. 표시된 항목을 확인하고 다시 시도해 주세요.",
         "invalidBody": "프로필 변경 내용을 읽지 못했어요.",
-        "emailInUse": "이미 사용 중인 이메일 주소예요."
+        "emailInUse": "이미 사용 중인 이메일 주소예요.",
+        "emailRateLimited": "이메일 주소를 너무 자주 변경했어요. 잠시 후 다시 시도해 주세요."
       }
     }
   },
@@ -5508,6 +5509,7 @@
       "invalidValue": "이 항목이 받는 답변이 아니에요.",
       "invalidFormat": "이 항목이 읽을 수 있는 형식이 아니에요.",
       "invalidType": "이 항목이 저장할 수 있는 형식으로 오지 않았어요.",
+      "rateLimited": "너무 자주 바꿨어요. 잠시 후 다시 시도해 주세요.",
       "other": "이 값은 저장하지 못했어요. 확인한 뒤 다시 시도해 주세요."
     },
     "avatar": {
diff --git a/messages/pl.json b/messages/pl.json
index f436bcffb..3c84cbc85 100644
--- a/messages/pl.json
+++ b/messages/pl.json
@@ -125,7 +125,8 @@
       "update": {
         "nothingSaved": "Nic nie zostało zapisane. Sprawdź oznaczone pole i spróbuj ponownie.",
         "invalidBody": "Nie udało się odczytać aktualizacji profilu.",
-        "emailInUse": "Ten adres e-mail jest już używany."
+        "emailInUse": "Ten adres e-mail jest już używany.",
+        "emailRateLimited": "Zbyt wiele zmian adresu e-mail. Spróbuj ponownie później."
       }
     }
   },
@@ -5508,6 +5509,7 @@
       "invalidValue": "To nie jest jedna z odpowiedzi, które przyjmuje to pole.",
       "invalidFormat": "To nie jest zapisane w formacie, który to pole może odczytać.",
       "invalidType": "Wartość nie dotarła w formacie, który to pole może zapisać.",
+      "rateLimited": "Zmieniane zbyt często. Odczekaj chwilę i spróbuj ponownie.",
       "other": "Ta wartość nie została przyjęta. Sprawdź ją i spróbuj ponownie."
     },
     "avatar": {
diff --git a/package.json b/package.json
index d4de393df..b3ea6e51f 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "healthlog",
-  "version": "1.38.19",
+  "version": "1.38.20",
   "description": "Self-hosted personal-health-tracking PWA with Withings integration, AI insights, and doctor-report PDF export.",
   "license": "PolyForm-Noncommercial-1.0.0",
   "homepage": "https://healthlog.dev",
diff --git a/public/sw.js b/public/sw.js
index 99bfccfd2..8c4284924 100644
--- a/public/sw.js
+++ b/public/sw.js
@@ -36,7 +36,7 @@ try {
 // v1.4.38.4 → v1.4.42. Do not hand-edit; bump `package.json` and rebuild.
 const CACHE_VERSION =
   (typeof self !== "undefined" && self.__APP_VERSION__) ||
-  /* @sw-version-fallback */ "v1.38.19";
+  /* @sw-version-fallback */ "v1.38.20";
 const STATIC_CACHE = `healthlog-static-${CACHE_VERSION}`;
 const PAGE_CACHE = `healthlog-pages-${CACHE_VERSION}`;
 // v1.18.6 — read-only data cache for a curated allowlist of safe GET `/api/*`
diff --git a/src/__tests__/api/auth/check-user/route.test.ts b/src/__tests__/api/auth/check-user/route.test.ts
deleted file mode 100644
index 8ae3cbd40..000000000
--- a/src/__tests__/api/auth/check-user/route.test.ts
+++ /dev/null
@@ -1,191 +0,0 @@
-/**
- * v1.4.41 W-IOS-COORD SB-7 follow-up — pin the four-branch discovery
- * contract for `POST /api/auth/check-user`. The iOS onboarding screen
- * branches on `branch` to decide which credential form to render; any
- * regression that drops a branch silently makes the wrong form appear.
- */
-import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
-
-vi.mock("@/lib/db", () => ({
-  prisma: {
-    user: {
-      findFirst: vi.fn(),
-    },
-  },
-}));
-
-vi.mock("@/lib/auth/audit", () => ({
-  auditLog: vi.fn().mockResolvedValue(undefined),
-}));
-
-vi.mock("@/lib/db-compat", () => ({
-  ensureDbCompatibility: vi.fn().mockResolvedValue(undefined),
-}));
-
-vi.mock("@/lib/logging/transports", () => ({ emitIfSampled: vi.fn() }));
-
-vi.mock("@/lib/rate-limit", () => ({
-  checkRateLimit: vi.fn().mockResolvedValue({
-    allowed: true,
-    limit: 60,
-    remaining: 29,
-    resetAt: Date.now() + 15 * 60 * 1000,
-  }),
-  // v1.4.43 W13 M-4 — the route now routes through the auth-surface
-  // wrapper. Default to a clean per-IP result.
-  checkAuthSurfaceRateLimit: vi.fn().mockResolvedValue({
-    allowed: true,
-    limit: 60,
-    remaining: 29,
-    resetAt: Date.now() + 15 * 60 * 1000,
-    ip: "203.0.113.1",
-  }),
-  rateLimitHeaders: vi.fn(() => ({})),
-}));
-
-vi.mock("@/lib/auth/hmac", () => ({
-  hashToken: vi.fn(
-    (raw: string) =>
-      `hash-${Buffer.from(raw, "utf-8")
-        .toString("hex")
-        .slice(0, 16)
-        .padStart(16, "0")}`,
-  ),
-}));
-
-vi.mock("next/headers", () => ({
-  headers: vi.fn(async () => ({ get: () => null })),
-  cookies: vi.fn(async () => ({
-    get: () => undefined,
-    set: () => {},
-    delete: () => {},
-  })),
-}));
-
-import { POST } from "@/app/api/auth/check-user/route";
-import { prisma } from "@/lib/db";
-import { checkRateLimit, checkAuthSurfaceRateLimit } from "@/lib/rate-limit";
-
-interface CheckUserBody {
-  data: {
-    branch: "not_found" | "passkey_only" | "email_fallback" | "exists";
-    hasPasskey: boolean;
-    hasPassword: boolean;
-  };
-}
-
-function makeRequest(body: unknown) {
-  return new Request("http://localhost/api/auth/check-user", {
-    method: "POST",
-    headers: { "content-type": "application/json" },
-    body: JSON.stringify(body),
-  }) as unknown as Parameters<typeof POST>[0];
-}
-
-beforeEach(() => {
-  vi.resetAllMocks();
-  vi.mocked(checkRateLimit).mockResolvedValue({
-    allowed: true,
-    limit: 60,
-    remaining: 29,
-    resetAt: Date.now() + 15 * 60 * 1000,
-  });
-  vi.mocked(checkAuthSurfaceRateLimit).mockResolvedValue({
-    allowed: true,
-    limit: 60,
-    remaining: 29,
-    resetAt: Date.now() + 15 * 60 * 1000,
-    ip: "203.0.113.1",
-  } as never);
-});
-
-afterEach(() => {
-  vi.restoreAllMocks();
-});
-
-describe("POST /api/auth/check-user", () => {
-  it("returns `not_found` when no account matches the identifier", async () => {
-    vi.mocked(prisma.user.findFirst).mockResolvedValue(null);
-    const res = await POST(makeRequest({ identifier: "ghost@example.com" }));
-    const body = (await res.json()) as CheckUserBody;
-    expect(res.status).toBe(200);
-    expect(body.data.branch).toBe("not_found");
-    expect(body.data.hasPasskey).toBe(false);
-    expect(body.data.hasPassword).toBe(false);
-  });
-
-  it("returns `passkey_only` when the user has passkeys and no password", async () => {
-    vi.mocked(prisma.user.findFirst).mockResolvedValue({
-      id: "u1",
-      passwordHash: null,
-      _count: { passkeys: 2 },
-    } as never);
-    const res = await POST(makeRequest({ identifier: "testuser" }));
-    const body = (await res.json()) as CheckUserBody;
-    expect(body.data.branch).toBe("passkey_only");
-    expect(body.data.hasPasskey).toBe(true);
-    expect(body.data.hasPassword).toBe(false);
-  });
-
-  it("returns `email_fallback` when the user has a password (regardless of passkey count)", async () => {
-    vi.mocked(prisma.user.findFirst).mockResolvedValue({
-      id: "u2",
-      passwordHash: "argon2id$…",
-      _count: { passkeys: 1 },
-    } as never);
-    const res = await POST(makeRequest({ identifier: "testuser" }));
-    const body = (await res.json()) as CheckUserBody;
-    expect(body.data.branch).toBe("email_fallback");
-    expect(body.data.hasPasskey).toBe(true);
-    expect(body.data.hasPassword).toBe(true);
-  });
-
-  it("returns `exists` when the user has neither passkey nor password (recovery path)", async () => {
-    vi.mocked(prisma.user.findFirst).mockResolvedValue({
-      id: "u3",
-      passwordHash: null,
-      _count: { passkeys: 0 },
-    } as never);
-    const res = await POST(makeRequest({ identifier: "testuser" }));
-    const body = (await res.json()) as CheckUserBody;
-    expect(body.data.branch).toBe("exists");
-    expect(body.data.hasPasskey).toBe(false);
-    expect(body.data.hasPassword).toBe(false);
-  });
-
-  it("422s when identifier is missing or blank", async () => {
-    const res = await POST(makeRequest({}));
-    expect(res.status).toBe(422);
-  });
-
-  it("queries the identifier exactly as iOS sends it (no case-fold)", async () => {
-    // Register stores email + username verbatim (no `.toLowerCase()`
-    // transform in `registerSchema`); folding here would route a
-    // mixed-case existing account to the sign-up branch.
-    vi.mocked(prisma.user.findFirst).mockResolvedValue(null);
-    await POST(makeRequest({ identifier: "MixedCase@Example.com" }));
-    expect(prisma.user.findFirst).toHaveBeenCalledWith(
-      expect.objectContaining({
-        where: {
-          OR: [
-            { username: "MixedCase@Example.com" },
-            { email: "MixedCase@Example.com" },
-          ],
-        },
-      }),
-    );
-  });
-
-  it("returns 429 when the per-IP rate-limit is exhausted", async () => {
-    vi.mocked(checkAuthSurfaceRateLimit).mockResolvedValueOnce({
-      allowed: false,
-      limit: 60,
-      remaining: 0,
-      resetAt: Date.now() + 15 * 60 * 1000,
-      ip: "203.0.113.1",
-    } as never);
-    const res = await POST(makeRequest({ identifier: "anyone@example.com" }));
-    expect(res.status).toBe(429);
-    expect(prisma.user.findFirst).not.toHaveBeenCalled();
-  });
-});
diff --git a/src/__tests__/api/auth/login/mfa-branch.test.ts b/src/__tests__/api/auth/login/mfa-branch.test.ts
index e59dffb9d..79de4e666 100644
--- a/src/__tests__/api/auth/login/mfa-branch.test.ts
+++ b/src/__tests__/api/auth/login/mfa-branch.test.ts
@@ -12,7 +12,7 @@ vi.mock("@/lib/db", () => ({
   },
 }));
 vi.mock("@/lib/auth/password", () => ({
-  verifyPassword: vi.fn().mockResolvedValue(true),
+  verifyPasswordOrDummy: vi.fn().mockResolvedValue(true),
 }));
 vi.mock("@/lib/auth/audit", () => ({
   auditLog: vi.fn().mockResolvedValue(undefined),
@@ -142,8 +142,8 @@ describe("login MFA branch", () => {
   });
 
   it("wrong password never reaches the MFA branch", async () => {
-    const { verifyPassword } = await import("@/lib/auth/password");
-    vi.mocked(verifyPassword).mockResolvedValueOnce(false);
+    const { verifyPasswordOrDummy } = await import("@/lib/auth/password");
+    vi.mocked(verifyPasswordOrDummy).mockResolvedValueOnce(false);
     vi.mocked(prisma.user.findFirst).mockResolvedValue({
       ...BASE_USER,
       totpConfirmedAt: new Date(),
diff --git a/src/__tests__/profile-email-rate-limit-contract.test.ts b/src/__tests__/profile-email-rate-limit-contract.test.ts
new file mode 100644
index 000000000..fbcf6d450
--- /dev/null
+++ b/src/__tests__/profile-email-rate-limit-contract.test.ts
@@ -0,0 +1,140 @@
+/**
+ * The profile email-address ceiling is the one the contract describes.
+ *
+ * Changing the address asks whether another account already holds it, and the
+ * answer is a 409 versus a 200 — an existence check over every address on the
+ * instance, available to any signed-in caller, including the native client's
+ * own wildcard token. Neither route that reaches it had a ceiling of any kind,
+ * and `apiHandler` supplies no default, so the sweep was bounded only by the
+ * network.
+ *
+ * A published number is worth reading only if it cannot drift from the code,
+ * so this pulls the bucket key and the window out of `prisma.$queryRaw` — the
+ * technique `rate-limit-auth-surface.test.ts` and the record-write contract
+ * both use — and holds the emitted OpenAPI description to what came back.
+ * Reading both from the same constant would prove nothing: the sentence is
+ * hand-written, deliberately, and this is what keeps the hand-written half
+ * honest.
+ */
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+import {
+  PROFILE_EMAIL_BUCKET_PREFIX,
+  PROFILE_EMAIL_LIMIT,
+  PROFILE_EMAIL_WINDOW_MS,
+  checkProfileEmailRateLimit,
+} from "@/lib/rate-limit";
+import { buildOpenApiDocument } from "@/lib/openapi/registry";
+import { profileEmailRateLimitResponse } from "@/lib/openapi/routes/shared";
+import { ERROR_CODE_CATALOGUE } from "@/lib/openapi/error-codes";
+
+vi.mock("@/lib/db", () => ({ prisma: { $queryRaw: vi.fn() } }));
+
+import { prisma } from "@/lib/db";
+
+/** The published sentence, read from the object the route modules spread. */
+const published = profileEmailRateLimitResponse["429"].description;
+
+/** Both doors onto the one handler that answers the conflict. */
+const OPERATIONS: Array<[path: string, method: "put" | "patch"]> = [
+  ["/api/auth/profile", "put"],
+  ["/api/user/profile", "patch"],
+];
+
+beforeEach(() => {
+  vi.mocked(prisma.$queryRaw).mockReset();
+  vi.mocked(prisma.$queryRaw).mockResolvedValue([
+    { count: 1, reset_at: new Date(Date.now() + 60_000) },
+  ] as never);
+});
+
+describe("the profile email-address bucket", () => {
+  it("keys on the acting account under the name the contract publishes", async () => {
+    await checkProfileEmailRateLimit("account-7");
+
+    const call = vi.mocked(prisma.$queryRaw).mock.calls.at(-1)!;
+    // `$queryRaw`INSERT … VALUES (${key}, …)`` lands the key at index 1 and
+    // the window interval at index 2. Read from the query rather than from
+    // the constant, so a renamed prefix that never reached the SQL fails.
+    expect(call[1]).toBe("profile-email:account-7");
+    expect(call[2]).toBe(`${PROFILE_EMAIL_WINDOW_MS} milliseconds`);
+    expect(`${PROFILE_EMAIL_BUCKET_PREFIX}:account-7`).toBe(call[1]);
+    expect(published).toContain("`profile-email:<accountId>`");
+  });
+
+  it("gives a different account a different bucket, not a shared one", async () => {
+    await checkProfileEmailRateLimit("account-7");
+    const first = vi.mocked(prisma.$queryRaw).mock.calls.at(-1)![1];
+    await checkProfileEmailRateLimit("account-8");
+    const second = vi.mocked(prisma.$queryRaw).mock.calls.at(-1)![1];
+    expect(first).not.toBe(second);
+  });
+
+  it("refuses at the cap the contract publishes", async () => {
+    vi.mocked(prisma.$queryRaw).mockResolvedValueOnce([
+      { count: PROFILE_EMAIL_LIMIT, reset_at: new Date(Date.now() + 60_000) },
+    ] as never);
+    await expect(
+      checkProfileEmailRateLimit("account-7"),
+    ).resolves.toMatchObject({
+      allowed: true,
+      limit: PROFILE_EMAIL_LIMIT,
+      remaining: 0,
+    });
+
+    vi.mocked(prisma.$queryRaw).mockResolvedValueOnce([
+      {
+        count: PROFILE_EMAIL_LIMIT + 1,
+        reset_at: new Date(Date.now() + 60_000),
+      },
+    ] as never);
+    await expect(
+      checkProfileEmailRateLimit("account-7"),
+    ).resolves.toMatchObject({ allowed: false });
+
+    const windowSeconds = PROFILE_EMAIL_WINDOW_MS / 1000;
+    expect(published).toContain(
+      `${PROFILE_EMAIL_LIMIT} requests per ${windowSeconds} seconds`,
+    );
+  });
+
+  it("says the headers describe that bucket, and declares them", () => {
+    expect(Object.keys(profileEmailRateLimitResponse["429"].headers)).toEqual([
+      "Retry-After",
+      "X-RateLimit-Limit",
+      "X-RateLimit-Remaining",
+      "X-RateLimit-Reset",
+    ]);
+    expect(published).toContain("Retry-After");
+  });
+
+  it("publishes a code a client can branch on, and registers it", () => {
+    expect(published).toContain("profile.update.emailRateLimited");
+    expect(ERROR_CODE_CATALOGUE.profile).toContain(
+      "profile.update.emailRateLimited",
+    );
+  });
+
+  it("says this response is only for a request that asked for nothing else", () => {
+    // The refusal narrows to the field when the request carries other
+    // fields, so a client reading the 429 as "the whole save failed" would
+    // be wrong. The sentence has to carry that or it is worse than silence.
+    expect(published).toContain("rejectedFields");
+    expect(published).toContain("rate_limited");
+  });
+
+  it("is published on both operations that answer it", () => {
+    const doc = buildOpenApiDocument() as unknown as {
+      paths: Record<
+        string,
+        Record<string, { responses?: Record<string, { description?: string }> }>
+      >;
+    };
+    const unpublished = OPERATIONS.filter(
+      ([path, method]) =>
+        doc.paths[path]?.[method]?.responses?.["429"]?.description !==
+        published,
+    ).map(([path]) => path);
+    expect(unpublished).toEqual([]);
+  });
+});
diff --git a/src/app/api/auth/check-user/__tests__/route.test.ts b/src/app/api/auth/check-user/__tests__/route.test.ts
deleted file mode 100644
index 660ee1abf..000000000
--- a/src/app/api/auth/check-user/__tests__/route.test.ts
+++ /dev/null
@@ -1,154 +0,0 @@
-import { describe, it, expect, vi, beforeEach } from "vitest";
-import { NextRequest } from "next/server";
-
-// --- Mocks must come before importing the route. ---
-
-vi.mock("@/lib/db", () => ({
-  prisma: {
-    user: { findFirst: vi.fn() },
-  },
-}));
-
-vi.mock("@/lib/auth/audit", () => ({
-  auditLog: vi.fn().mockResolvedValue(undefined),
-}));
-
-// The real `hashToken` is an HMAC-SHA256 (one-way + key-dependent); the
-// shape only matters here, so a deterministic stand-in keeps the test
-// stable without standing up the env-keyed CipherSuite. The PII check
-// below asserts the raw identifier never appears in the audit payload.
-vi.mock("@/lib/auth/hmac", () => ({
-  hashToken: vi.fn(
-    (raw: string) =>
-      `hash-${Buffer.from(raw, "utf-8")
-        .toString("hex")
-        .slice(0, 16)
-        .padStart(16, "0")}`,
-  ),
-}));
-
-vi.mock("@/lib/rate-limit", () => ({
-  checkRateLimit: vi
-    .fn()
-    .mockResolvedValue({ allowed: true, remaining: 30, resetAt: 0 }),
-  // v1.4.43 W13 M-4 — `check-user` now routes through the auth-surface
-  // wrapper so the trust-violation branch can swap to a tighter global
-  // bucket. Default to a clean per-IP result.
-  checkAuthSurfaceRateLimit: vi.fn().mockResolvedValue({
-    allowed: true,
-    remaining: 30,
-    resetAt: 0,
-    ip: "203.0.113.1",
-  }),
-  rateLimitHeaders: vi.fn(() => ({})),
-}));
-
-vi.mock("@/lib/logging/transports", () => ({
-  emitIfSampled: vi.fn(),
-}));
-
-import { POST } from "../route";
-import { prisma } from "@/lib/db";
-import { auditLog } from "@/lib/auth/audit";
-import { hashToken } from "@/lib/auth/hmac";
-
-function expectedHash(identifier: string): string {
-  return (hashToken as unknown as (s: string) => string)(identifier);
-}
-
-function makeRequest(body: { identifier: string }): NextRequest {
-  return new NextRequest("http://localhost/api/auth/check-user", {
-    method: "POST",
-    headers: { "content-type": "application/json" },
-    body: JSON.stringify(body),
-  });
-}
-
-beforeEach(() => {
-  vi.mocked(prisma.user.findFirst).mockReset();
-  vi.mocked(auditLog).mockReset();
-  vi.mocked(auditLog).mockResolvedValue(undefined as never);
-});
-
-describe("POST /api/auth/check-user — audit-log row (M-1)", () => {
-  it("writes an audit row for the not_found branch with identifier_hash (never raw)", async () => {
-    vi.mocked(prisma.user.findFirst).mockResolvedValue(null);
-
-    const res = await POST(makeRequest({ identifier: "ghost@example.com" }));
-    expect(res.status).toBe(200);
-
-    expect(auditLog).toHaveBeenCalledTimes(1);
-    const args = vi.mocked(auditLog).mock.calls[0];
-    expect(args[0]).toBe("auth.check-user");
-    expect(args[1]).toEqual({
-      ipAddress: "203.0.113.1",
-      details: {
-        branch: "not_found",
-        identifier_hash: expectedHash("ghost@example.com"),
-      },
-    });
-    // The raw identifier must not leak into the details — every other
-    // /api/auth/* audit row already hashes; the check-user route follows
-    // the same shape.
-    const details = args[1]?.details as Record<string, unknown> | undefined;
-    expect(details).not.toHaveProperty("identifier");
-    expect(JSON.stringify(details)).not.toContain("ghost@example.com");
-  });
-
-  it("writes an audit row for the passkey_only branch with userId attached", async () => {
-    vi.mocked(prisma.user.findFirst).mockResolvedValue({
-      id: "user-1",
-      passwordHash: null,
-      _count: { passkeys: 2 },
-    } as never);
-
-    const res = await POST(makeRequest({ identifier: "testuser" }));
-    expect(res.status).toBe(200);
-
-    expect(auditLog).toHaveBeenCalledTimes(1);
-    expect(vi.mocked(auditLog).mock.calls[0][1]).toEqual({
-      userId: "user-1",
-      ipAddress: "203.0.113.1",
-      details: {
-        branch: "passkey_only",
-        identifier_hash: expectedHash("testuser"),
-      },
-    });
-  });
-
-  it("writes an audit row for the email_fallback branch with userId attached", async () => {
-    vi.mocked(prisma.user.findFirst).mockResolvedValue({
-      id: "user-2",
-      passwordHash: "$argon2id$...",
-      _count: { passkeys: 0 },
-    } as never);
-
-    const res = await POST(makeRequest({ identifier: "user@example.com" }));
-    expect(res.status).toBe(200);
-
-    const args = vi.mocked(auditLog).mock.calls[0];
-    expect(args[1]?.details).toEqual({
-      branch: "email_fallback",
-      identifier_hash: expectedHash("user@example.com"),
-    });
-    expect(args[1]?.userId).toBe("user-2");
-  });
-
-  it("writes an audit row for the exists branch (no passkey + no password)", async () => {
-    vi.mocked(prisma.user.findFirst).mockResolvedValue({
-      id: "user-3",
-      passwordHash: null,
-      _count: { passkeys: 0 },
-    } as never);
-
-    const res = await POST(makeRequest({ identifier: "stub" }));
-    expect(res.status).toBe(200);
-
-    const args = vi.mocked(auditLog).mock.calls[0];
-    expect(args[1]?.details).toEqual({
-      branch: "exists",
-      identifier_hash: expectedHash("stub"),
-    });
-    expect(args[1]?.userId).toBe("user-3");
-  });
-});
diff --git a/src/app/api/auth/check-user/route.ts b/src/app/api/auth/check-user/route.ts
deleted file mode 100644
index 1aaf77170..000000000
--- a/src/app/api/auth/check-user/route.ts
+++ /dev/null
@@ -1,149 +0,0 @@
-/**
- * `POST /api/auth/check-user` — four-branch discovery for iOS onboarding.
- *
- * v1.4.41 W-IOS-COORD SB-7 follow-up. The iOS team's onboarding flow needs
- * to know, given a typed identifier (username or email), what the next
- * UX step should be:
- *
- *   - `not_found`       — no account exists; show the sign-up screen.
- *   - `passkey_only`    — account exists, has at least one Passkey, no
- *                          password hash. Show "Sign in with Passkey".
- *   - `email_fallback`  — account exists, has a password hash (with or
- *                          without a Passkey). Show password field plus
- *                          a "Use Passkey" affordance when applicable.
- *   - `exists`          — account exists with no usable credential
- *                          (neither passkey nor password). Treat as
- *                          recovery path; show "Reset access" hint.
- *
- * The route is intentionally narrow:
- *
- *   - Accepts `{ identifier: string }` (either an email or a username).
- *   - Returns `{ branch, hasPasskey, hasPassword }` — booleans are
- *     included so the iOS client can render a "or sign in with Passkey"
- *     button alongside the password field without a second round-trip.
- *   - Never leaks PII. The response shape is the same regardless of
- *     whether the identifier matched (callers learn account-existence
- *     either way — that is the explicit contract iOS needs). The
- *     handler does NOT echo the identifier back.
- *   - Per-IP rate-limited (30 requests / 15 min) so the by-design
- *     enumeration contract cannot be brute-forced. Mirrors the
- *     `/api/auth/passkey/login-options` throttle shape.
- *   - The identifier is queried EXACTLY as iOS sends it (no
- *     `.toLowerCase()` normalisation). Email + username columns are
- *     stored verbatim by `/api/auth/register`, so case-folding here
- *     would route legitimate existing users to the sign-up branch.
- *     A future server-side normalisation pass on register write is
- *     the long-term fix.
- */
-import { z } from "zod/v4";
-import { prisma } from "@/lib/db";
-import {
-  apiSuccess,
-  apiValidationError,
-  safeJson,
-  sanitiseZodIssues,
-} from "@/lib/api-response";
-import { apiHandler } from "@/lib/api-handler";
-import { annotate } from "@/lib/logging/context";
-import { auditLog } from "@/lib/auth/audit";
-import { hashToken } from "@/lib/auth/hmac";
-import { checkAuthSurfaceRateLimit, rateLimitHeaders } from "@/lib/rate-limit";
-import { NextResponse } from "next/server";
-import type { NextRequest } from "next/server";
-
-export const dynamic = "force-dynamic";
-
-const bodySchema = z
-  .object({
-    identifier: z.string().trim().min(1).max(254),
-  })
-  .strict();
-
-export type CheckUserBranch =
-  "not_found" | "passkey_only" | "email_fallback" | "exists";
-
-export const POST = apiHandler(async (request: NextRequest) => {
-  // v1.4.43 W13 M-4 — tighter shared bucket on trust-chain misconfig.
-  const rl = await checkAuthSurfaceRateLimit(
-    request,
-    "auth:check-user",
-    30,
-    15 * 60 * 1000,
-  );
-  const ip = rl.ip ?? "unknown";
-  if (!rl.allowed) {
-    return NextResponse.json(
-      { data: null, error: "Too many requests. Please try again later." },
-      { status: 429, headers: rateLimitHeaders(rl) },
-    );
-  }
-
-  const { data: body, error: jsonError } = await safeJson(request, {
-    maxBytes: 64 * 1024,
-  });
-  if (jsonError) return jsonError;
-
-  const parsed = bodySchema.safeParse(body);
-  if (!parsed.success) {
-    return apiValidationError(
-      "identifier required",
-      sanitiseZodIssues(parsed.error.issues),
-      422,
-    );
-  }
-
-  const identifier = parsed.data.identifier;
-
-  // Match on either username or email — both are unique in the schema.
-  const user = await prisma.user.findFirst({
-    where: {
-      OR: [{ username: identifier }, { email: identifier }],
-    },
-    select: {
-      id: true,
-      passwordHash: true,
-      _count: { select: { passkeys: true } },
-    },
-  });
-
-  // v1.4.43 W13 M-1 — persist a hashed-identifier audit row so an admin
-  // can later answer "did anyone enumerate accounts last week?". The
-  // wide-event `annotate()` log is sampled and short-retention; the
-  // audit ledger is durable. We hash the identifier (same shape as the
-  // `auth.login.failed` row) so the literal email/username never lands
-  // in `AuditLog.details` — the v1.4.20 retroactive PII directive bars
-  // raw user-typed strings from operator-readable artefacts.
-  const identifierHash = hashToken(identifier);
-
-  if (!user) {
-    annotate({
-      action: { name: "auth.check-user" },
-      meta: { branch: "not_found" },
-    });
-    await auditLog("auth.check-user", {
-      ipAddress: ip,
-      details: { branch: "not_found", identifier_hash: identifierHash },
-    });
-    return apiSuccess({
-      branch: "not_found" satisfies CheckUserBranch,
-      hasPasskey: false,
-      hasPassword: false,
-    });
-  }
-
-  const hasPasskey = user._count.passkeys > 0;
-  const hasPassword = Boolean(user.passwordHash);
-
-  let branch: CheckUserBranch;
-  if (hasPasskey && !hasPassword) branch = "passkey_only";
-  else if (hasPassword) branch = "email_fallback";
-  else branch = "exists";
-
-  annotate({ action: { name: "auth.check-user" }, meta: { branch } });
-  await auditLog("auth.check-user", {
-    userId: user.id,
-    ipAddress: ip,
-    details: { branch, identifier_hash: identifierHash },
-  });
-  return apiSuccess({ branch, hasPasskey, hasPassword });
-});
diff --git a/src/app/api/auth/login/__tests__/constant-work.test.ts b/src/app/api/auth/login/__tests__/constant-work.test.ts
new file mode 100644
index 000000000..87be3506b
--- /dev/null
+++ b/src/app/api/auth/login/__tests__/constant-work.test.ts
@@ -0,0 +1,155 @@
+/**
+ * Every login outcome reaches the verifier.
+ *
+ * The three refusals sign-in can produce — no such account, an account with
+ * no password hash, a wrong password — return the identical 401 body, and
+ * used to take three very different amounts of time to produce it, because
+ * the first two returned before any hashing happened. Argon2id at 19 MiB and
+ * t=2 against one indexed SELECT is a gap a caller can read over a network.
+ *
+ * The passkey-only account is the case that matters most. If it kept the
+ * cheap path, the channel would still answer "this account exists and has no
+ * password" — which is exactly what the removed discovery endpoint published
+ * in a field.
+ *
+ * What this proves is that the call happens on all three arms, not that the
+ * three durations match: the verifier is mocked here, and wall-clock equality
+ * is not something a unit test can assert. `password-dummy-verify.test.ts`
+ * carries the other half, that the verifier really does Argon2id work at the
+ * real cost parameters when there is no stored hash.
+ */
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { NextRequest } from "next/server";
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    user: { findFirst: vi.fn() },
+    webauthnMfaCredential: { count: vi.fn().mockResolvedValue(0) },
+  },
+}));
+
+vi.mock("@/lib/auth/password", () => ({
+  verifyPasswordOrDummy: vi.fn(),
+}));
+
+vi.mock("@/lib/auth/audit", () => ({
+  auditLog: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock("@/lib/rate-limit", () => ({
+  checkAuthSurfaceRateLimit: vi.fn().mockResolvedValue({
+    allowed: true,
+    remaining: 5,
+    reset: 0,
+    ip: "1.2.3.4",
+  }),
+  rateLimitHeaders: vi.fn(() => ({})),
+}));
+
+vi.mock("@/lib/db-compat", () => ({
+  ensureDbCompatibility: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock("@/lib/auth/hmac", () => ({
+  hashToken: vi.fn(() => "aa".repeat(32)),
+}));
+
+vi.mock("@/lib/logging/transports", () => ({ emitIfSampled: vi.fn() }));
+
+vi.mock("next/headers", () => ({
+  headers: vi.fn(async () => ({ get: () => null })),
+  cookies: vi.fn(async () => ({
+    get: () => undefined,
+    set: () => {},
+    delete: () => {},
+  })),
+}));
+
+import { POST } from "../route";
+import { prisma } from "@/lib/db";
+import { verifyPasswordOrDummy } from "@/lib/auth/password";
+
+function makeRequest(): NextRequest {
+  return new NextRequest("http://localhost/api/auth/login", {
+    method: "POST",
+    headers: { "content-type": "application/json" },
+    body: JSON.stringify({
+      email: "someone@example.com",
+      password: "a-typed-password",
+    }),
+  });
+}
+
+beforeEach(() => {
+  vi.mocked(verifyPasswordOrDummy).mockReset();
+  vi.mocked(verifyPasswordOrDummy).mockResolvedValue(false);
+});
+
+describe("POST /api/auth/login — constant work across the refusals", () => {
+  it("verifies even when no account matches the identifier", async () => {
+    vi.mocked(prisma.user.findFirst).mockResolvedValue(null as never);
+
+    const res = await POST(makeRequest());
+
+    expect(res.status).toBe(401);
+    expect(verifyPasswordOrDummy).toHaveBeenCalledTimes(1);
+    expect(verifyPasswordOrDummy).toHaveBeenCalledWith(
+      undefined,
+      "a-typed-password",
+    );
+  });
+
+  it("verifies for a passkey-only account, which carries no password hash", async () => {
+    vi.mocked(prisma.user.findFirst).mockResolvedValue({
+      id: "user-1",
+      passwordHash: null,
+      totpConfirmedAt: null,
+      mfaEnforced: false,
+    } as never);
+
+    const res = await POST(makeRequest());
+
+    expect(res.status).toBe(401);
+    expect(verifyPasswordOrDummy).toHaveBeenCalledTimes(1);
+    expect(verifyPasswordOrDummy).toHaveBeenCalledWith(
+      null,
+      "a-typed-password",
+    );
+  });
+
+  it("verifies for an account that does carry a password hash", async () => {
+    vi.mocked(prisma.user.findFirst).mockResolvedValue({
+      id: "user-2",
+      passwordHash: "$argon2id$stored",
+      totpConfirmedAt: null,
+      mfaEnforced: false,
+    } as never);
+
+    const res = await POST(makeRequest());
+
+    expect(res.status).toBe(401);
+    expect(verifyPasswordOrDummy).toHaveBeenCalledTimes(1);
+    expect(verifyPasswordOrDummy).toHaveBeenCalledWith(
+      "$argon2id$stored",
+      "a-typed-password",
+    );
+  });
+
+  it("answers the three refusals with the identical status and body", async () => {
+    const bodies: string[] = [];
+    const statuses: number[] = [];
+    const accounts = [
+      null,
+      { id: "user-1", passwordHash: null },
+      { id: "user-2", passwordHash: "$argon2id$stored" },
+    ];
+    for (const account of accounts) {
+      vi.mocked(prisma.user.findFirst).mockResolvedValue(account as never);
+      const res = await POST(makeRequest());
+      statuses.push(res.status);
+      bodies.push(await res.text());
+    }
+    expect(new Set(statuses)).toEqual(new Set([401]));
+    expect(new Set(bodies).size).toBe(1);
+  });
+});
diff --git a/src/app/api/auth/login/__tests__/native-token.test.ts b/src/app/api/auth/login/__tests__/native-token.test.ts
index fc1212912..7089dbff9 100644
--- a/src/app/api/auth/login/__tests__/native-token.test.ts
+++ b/src/app/api/auth/login/__tests__/native-token.test.ts
@@ -14,7 +14,7 @@ vi.mock("@/lib/db", () => ({
 }));
 
 vi.mock("@/lib/auth/password", () => ({
-  verifyPassword: vi.fn(),
+  verifyPasswordOrDummy: vi.fn(),
 }));
 
 vi.mock("@/lib/auth/mfa-enrollment", () => ({
@@ -73,7 +73,7 @@ vi.mock("next/headers", () => ({
 
 import { POST } from "../route";
 import { prisma } from "@/lib/db";
-import { verifyPassword } from "@/lib/auth/password";
+import { verifyPasswordOrDummy } from "@/lib/auth/password";
 import {
   checkRateLimit,
   checkAuthSurfaceRateLimit,
@@ -108,7 +108,7 @@ beforeEach(() => {
   vi.resetAllMocks();
   vi.mocked(prisma.user.findFirst).mockResolvedValue(FAKE_USER as never);
   vi.mocked(prisma.webauthnMfaCredential.count).mockResolvedValue(0 as never);
-  vi.mocked(verifyPassword).mockResolvedValue(true);
+  vi.mocked(verifyPasswordOrDummy).mockResolvedValue(true);
   vi.mocked(prisma.apiToken.create).mockResolvedValue({ id: "tok-1" } as never);
   vi.mocked(prisma.refreshToken.create).mockResolvedValue({
     id: "rt-1",
diff --git a/src/app/api/auth/login/__tests__/route.test.ts b/src/app/api/auth/login/__tests__/route.test.ts
index c82732f95..fb2df339b 100644
--- a/src/app/api/auth/login/__tests__/route.test.ts
+++ b/src/app/api/auth/login/__tests__/route.test.ts
@@ -17,7 +17,7 @@ vi.mock("@/lib/db", () => ({
 }));
 
 vi.mock("@/lib/auth/password", () => ({
-  verifyPassword: vi.fn(),
+  verifyPasswordOrDummy: vi.fn(),
 }));
 
 vi.mock("@/lib/auth/session", () => ({
@@ -71,7 +71,7 @@ vi.mock("next/headers", () => ({
 
 import { POST } from "../route";
 import { prisma } from "@/lib/db";
-import { verifyPassword } from "@/lib/auth/password";
+import { verifyPasswordOrDummy } from "@/lib/auth/password";
 import { auditLog } from "@/lib/auth/audit";
 import {
   checkRateLimit,
@@ -93,7 +93,7 @@ function makeRequest(): NextRequest {
 beforeEach(() => {
   vi.resetAllMocks();
   vi.mocked(prisma.user.findFirst).mockResolvedValue(null as never);
-  vi.mocked(verifyPassword).mockResolvedValue(false);
+  vi.mocked(verifyPasswordOrDummy).mockResolvedValue(false);
   vi.mocked(auditLog).mockResolvedValue(undefined as never);
   vi.mocked(checkRateLimit).mockResolvedValue({
     allowed: true,
@@ -175,7 +175,7 @@ describe("POST /api/auth/login — OIDC_ONLY server-side enforcement", () => {
 
     const res = await POST(makeRequest());
     // Falls through to normal password verification (which fails here
-    // since verifyPassword is mocked to return false) rather than 403 —
+    // since verifyPasswordOrDummy is mocked to return false) rather than 403 —
     // a half-set OIDC group must never lock everyone out.
     expect(res.status).toBe(401);
   });
diff --git a/src/app/api/auth/login/__tests__/trusted-device.test.ts b/src/app/api/auth/login/__tests__/trusted-device.test.ts
index cb895be6b..f8eab01a4 100644
--- a/src/app/api/auth/login/__tests__/trusted-device.test.ts
+++ b/src/app/api/auth/login/__tests__/trusted-device.test.ts
@@ -8,7 +8,7 @@ vi.mock("@/lib/db", () => ({
   },
 }));
 
-vi.mock("@/lib/auth/password", () => ({ verifyPassword: vi.fn() }));
+vi.mock("@/lib/auth/password", () => ({ verifyPasswordOrDummy: vi.fn() }));
 
 vi.mock("@/lib/auth/login-response", () => ({
   finishLogin: vi.fn(async () => new Response(null, { status: 200 })),
@@ -58,7 +58,7 @@ vi.mock("next/headers", () => ({
 
 import { POST } from "../route";
 import { prisma } from "@/lib/db";
-import { verifyPassword } from "@/lib/auth/password";
+import { verifyPasswordOrDummy } from "@/lib/auth/password";
 import { finishLogin } from "@/lib/auth/login-response";
 import { createMfaChallenge } from "@/lib/auth/mfa/challenge";
 import { consumeTrustedDevice } from "@/lib/auth/trusted-device";
@@ -88,7 +88,7 @@ beforeEach(() => {
 
 describe("POST /api/auth/login — trusted device", () => {
   it("skips the second factor when the device is trusted (no challenge minted)", async () => {
-    vi.mocked(verifyPassword).mockResolvedValue(true);
+    vi.mocked(verifyPasswordOrDummy).mockResolvedValue(true);
     vi.mocked(consumeTrustedDevice).mockResolvedValue(true);
 
     const res = await POST(req());
@@ -103,7 +103,7 @@ describe("POST /api/auth/login — trusted device", () => {
   });
 
   it("falls back to the MFA challenge when the device is not trusted", async () => {
-    vi.mocked(verifyPassword).mockResolvedValue(true);
+    vi.mocked(verifyPasswordOrDummy).mockResolvedValue(true);
     vi.mocked(consumeTrustedDevice).mockResolvedValue(false);
 
     const res = await POST(req());
@@ -115,7 +115,7 @@ describe("POST /api/auth/login — trusted device", () => {
   });
 
   it("still requires the password — a wrong password never consults the trusted device", async () => {
-    vi.mocked(verifyPassword).mockResolvedValue(false);
+    vi.mocked(verifyPasswordOrDummy).mockResolvedValue(false);
 
     const res = await POST(req());
     expect(res.status).toBe(401);
diff --git a/src/app/api/auth/login/route.ts b/src/app/api/auth/login/route.ts
index eb3272f8f..5c163c0dd 100644
--- a/src/app/api/auth/login/route.ts
+++ b/src/app/api/auth/login/route.ts
@@ -1,6 +1,6 @@
 import { prisma } from "@/lib/db";
 import { loginPasswordSchema } from "@/lib/validations/auth";
-import { verifyPassword } from "@/lib/auth/password";
+import { verifyPasswordOrDummy } from "@/lib/auth/password";
 import { auditLog } from "@/lib/auth/audit";
 import { hashToken } from "@/lib/auth/hmac";
 import {
@@ -69,11 +69,10 @@ export const POST = apiHandler(async (request: NextRequest) => {
 
   const { email, password } = parsed.data;
   const identifier = email.trim();
-  // HMAC of the typed identifier — keyed by `API_TOKEN_HMAC_KEY`,
-  // mirrors the `/api/auth/check-user` pattern. The raw identifier
-  // stays out of the audit row (H-1 contract); the hash gives a
-  // future spray-detector a forensic anchor it can correlate across
-  // IPs without having to look up users by email.
+  // HMAC of the typed identifier — keyed by `API_TOKEN_HMAC_KEY`. The
+  // raw identifier stays out of the audit row (H-1 contract); the hash
+  // gives a future spray-detector a forensic anchor it can correlate
+  // across IPs without having to look up users by email.
   const identifierHash = hashToken(identifier);
 
   const user = await prisma.user.findFirst({
@@ -85,6 +84,18 @@ export const POST = apiHandler(async (request: NextRequest) => {
     },
   });
 
+  // One verification for every outcome. An unknown identifier and an
+  // account carrying no password hash (a passkey-only account) used to
+  // return here before any hashing, so the three answers cost one indexed
+  // SELECT, one indexed SELECT, and a full Argon2id verification
+  // respectively — identical bodies, three different durations, which is a
+  // readable answer to "is this address registered, and does it have a
+  // password". `verifyPasswordOrDummy` verifies against a fixed dummy hash
+  // minted at the same cost parameters when there is no stored hash, and
+  // returns false. Keep the call ahead of the branch: moving it back under
+  // an `if` restores the channel.
+  const valid = await verifyPasswordOrDummy(user?.passwordHash, password);
+
   if (!user || !user.passwordHash) {
     // v1.4.43 W3-SECURITY (H-1): never write the typed identifier into
     // the audit row — `reason` already tells the operator what
@@ -99,7 +110,6 @@ export const POST = apiHandler(async (request: NextRequest) => {
     return apiError("Invalid credentials", 401);
   }
 
-  const valid = await verifyPassword(user.passwordHash, password);
   if (!valid) {
     await auditLog("auth.login.failed", {
       userId: user.id,
diff --git a/src/app/api/auth/register/route.ts b/src/app/api/auth/register/route.ts
index d3221afbc..d89662cbd 100644
--- a/src/app/api/auth/register/route.ts
+++ b/src/app/api/auth/register/route.ts
@@ -143,6 +143,16 @@ export const POST = apiHandler(async (request: NextRequest) => {
   }
 
   // Check if email or username already taken (unified message to prevent enumeration)
+  //
+  // The ordering is deliberate and load-bearing. This runs BEFORE the password
+  // strength check, the breach lookup and the Argon2id hash below, so a taken
+  // address answers after one indexed SELECT and a free one answers after
+  // zxcvbn plus an outbound round-trip plus tens of milliseconds of hashing.
+  // The status code already gives the answer away, so today the gap tells a
+  // caller nothing extra. It would be the whole answer the moment somebody
+  // unified the 409 into the success shape to close the oracle: the clock
+  // would then say what the status code no longer does. Anyone doing that has
+  // to move this block after the hash in the same change.
   const [existingEmail, existingUsername] = await Promise.all([
     prisma.user.findUnique({ where: { email } }),
     prisma.user.findUnique({ where: { username } }),
diff --git a/src/lib/__tests__/rate-limit-auth-surface.test.ts b/src/lib/__tests__/rate-limit-auth-surface.test.ts
index 25558cae5..8331e5479 100644
--- a/src/lib/__tests__/rate-limit-auth-surface.test.ts
+++ b/src/lib/__tests__/rate-limit-auth-surface.test.ts
@@ -146,7 +146,6 @@ describe("checkAuthSurfaceRateLimit — trust-violation tightening", () => {
         ["auth:passkey-login-options", 10],
         ["auth:passkey-verify", 10],
         ["auth:refresh", 60],
-        ["auth:check-user", 30],
       ];
       const seenKeys = new Set<string>();
       for (const [prefix, limit] of surfaces) {
diff --git a/src/lib/auth/__tests__/password-dummy-verify.test.ts b/src/lib/auth/__tests__/password-dummy-verify.test.ts
new file mode 100644
index 000000000..482dbeb50
--- /dev/null
+++ b/src/lib/auth/__tests__/password-dummy-verify.test.ts
@@ -0,0 +1,159 @@
+/**
+ * Signing in must cost the same whether or not the account exists.
+ *
+ * The login route used to return the moment it found no account, or an
+ * account with no password hash, before it reached the verifier. An account
+ * that did have a password paid a full Argon2id verification. Same status,
+ * same body, very different durations — so the clock answered "is this
+ * address registered, and does it carry a password", which is the question
+ * the removed discovery endpoint used to answer out loud.
+ *
+ * What this file pins is the work, not the wall clock: that the empty arms
+ * reach the verifier at all, that the hash they verify against was minted at
+ * the same cost parameters every real hash carries, and that no outcome on
+ * those arms can be read as a successful verification. A unit test cannot
+ * prove two durations are equal — it can only prove the same call is made.
+ *
+ * `@node-rs/argon2` is mocked so the assertions are about which calls happen
+ * with which arguments, and so the suite does not spend real Argon2 time.
+ */
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+// Hoisted so the mock is in place before `password.ts` is imported.
+const { hashMock, verifyMock } = vi.hoisted(() => ({
+  hashMock: vi.fn(async (input: string) => `hashed:${input}`),
+  verifyMock: vi.fn(async (_hash: string, _password: string) => false),
+}));
+
+vi.mock("@node-rs/argon2", () => ({ hash: hashMock, verify: verifyMock }));
+
+import { ARGON2_HASH_OPTIONS } from "@/lib/auth/argon2-params.mjs";
+
+/**
+ * The dummy hash is minted once per module instance, so each test gets a
+ * fresh instance — otherwise the first successful mint is cached and the
+ * mint-failure cases could never reach the mint at all.
+ */
+let verifyPasswordOrDummy: (
+  storedHash: string | null | undefined,
+  password: string,
+) => Promise<boolean>;
+
+beforeEach(async () => {
+  verifyMock.mockReset();
+  verifyMock.mockResolvedValue(false);
+  hashMock.mockReset();
+  hashMock.mockImplementation(async (input: string) => `hashed:${input}`);
+  vi.resetModules();
+  ({ verifyPasswordOrDummy } = await import("@/lib/auth/password"));
+});
+
+describe("verifyPasswordOrDummy", () => {
+  it("mints the dummy hash under the same Argon2id parameters real hashes use", async () => {
+    await verifyPasswordOrDummy(null, "anything");
+    expect(hashMock).toHaveBeenCalledWith(
+      expect.any(String),
+      ARGON2_HASH_OPTIONS,
+    );
+  });
+
+  it("mints once and reuses it for every later refusal", async () => {
+    await verifyPasswordOrDummy(null, "anything");
+    await verifyPasswordOrDummy(null, "anything");
+    await verifyPasswordOrDummy(undefined, "anything");
+    // Three refusals, three verifications, one mint.
+    expect(verifyMock).toHaveBeenCalledTimes(3);
+    expect(hashMock).toHaveBeenCalledTimes(1);
+  });
+
+  it("verifies against the stored hash when the account has one", async () => {
+    verifyMock.mockResolvedValueOnce(true);
+    await expect(
+      verifyPasswordOrDummy("$argon2id$stored", "correct horse"),
+    ).resolves.toBe(true);
+    expect(verifyMock).toHaveBeenCalledTimes(1);
+    expect(verifyMock).toHaveBeenCalledWith(
+      "$argon2id$stored",
+      "correct horse",
+    );
+  });
+
+  it("still verifies when there is no account at all (null hash)", async () => {
+    await expect(verifyPasswordOrDummy(null, "anything")).resolves.toBe(false);
+    expect(verifyMock).toHaveBeenCalledTimes(1);
+    // Against the dummy, not against nothing.
+    expect(verifyMock.mock.calls[0][0]).toMatch(/^hashed:/);
+  });
+
+  it("still verifies for a passkey-only account (undefined hash)", async () => {
+    // The passkey-only case is the one that matters most: if it took the
+    // cheap path the channel would still answer "this account has no
+    // password", which is half of what the removed endpoint published.
+    await expect(verifyPasswordOrDummy(undefined, "anything")).resolves.toBe(
+      false,
+    );
+    expect(verifyMock).toHaveBeenCalledTimes(1);
+  });
+
+  it("never reports success from the dummy arm, whatever the verifier says", async () => {
+    verifyMock.mockResolvedValue(true);
+    await expect(verifyPasswordOrDummy(null, "anything")).resolves.toBe(false);
+    await expect(verifyPasswordOrDummy(undefined, "anything")).resolves.toBe(
+      false,
+    );
+  });
+
+  it("swallows a throw from the dummy arm rather than answering differently", async () => {
+    // A 500 only the unknown-account arm can produce is the same oracle in
+    // a different field.
+    const logged = vi.spyOn(console, "error").mockImplementation(() => {});
+    try {
+      verifyMock.mockRejectedValueOnce(new Error("malformed hash"));
+      await expect(verifyPasswordOrDummy(null, "anything")).resolves.toBe(
+        false,
+      );
+      expect(logged).toHaveBeenCalled();
+    } finally {
+      logged.mockRestore();
+    }
+  });
+
+  it("swallows a failed MINT too, not only a failed verify", async () => {
+    // The mint is awaited as an argument to the verifier, so an unguarded
+    // `await` on a rejected mint throws before the verifier is reached and
+    // before any `.catch` on it can apply. That answers 500 on exactly the
+    // arm that must look ordinary — and a passkey-only account could not
+    // sign in at all.
+    const logged = vi.spyOn(console, "error").mockImplementation(() => {});
+    try {
+      hashMock.mockRejectedValueOnce(new Error("out of memory"));
+      await expect(verifyPasswordOrDummy(null, "anything")).resolves.toBe(
+        false,
+      );
+      expect(verifyMock).not.toHaveBeenCalled();
+      expect(logged).toHaveBeenCalled();
+    } finally {
+      logged.mockRestore();
+    }
+  });
+
+  it("re-mints after a failure instead of caching the rejection forever", async () => {
+    // A rejected promise stays rejected for the life of the process. Caching
+    // one would send every later refusal back down the cheap path with the
+    // timing channel reopened and nothing to notice it by.
+    const logged = vi.spyOn(console, "error").mockImplementation(() => {});
+    try {
+      hashMock.mockRejectedValueOnce(new Error("out of memory"));
+      await verifyPasswordOrDummy(null, "anything");
+      expect(verifyMock).not.toHaveBeenCalled();
+
+      await expect(verifyPasswordOrDummy(null, "anything")).resolves.toBe(
+        false,
+      );
+      expect(hashMock).toHaveBeenCalledTimes(2);
+      expect(verifyMock).toHaveBeenCalledTimes(1);
+    } finally {
+      logged.mockRestore();
+    }
+  });
+});
diff --git a/src/lib/auth/__tests__/profile-update-email-limit.test.ts b/src/lib/auth/__tests__/profile-update-email-limit.test.ts
new file mode 100644
index 000000000..7693c5ca3
--- /dev/null
+++ b/src/lib/auth/__tests__/profile-update-email-limit.test.ts
@@ -0,0 +1,240 @@
+/**
+ * The email-address conflict answer is metered, and metered before it answers.
+ *
+ * `applyProfileUpdate` is the one place both profile routes reach, and its
+ * 409 tells any signed-in caller whether a given address belongs to an
+ * account on this instance. Until this bucket existed neither route had a
+ * ceiling of any kind — `apiHandler` supplies none — so one account could
+ * walk an address book at whatever rate the network allowed.
+ *
+ * What is pinned here. The charge happens BEFORE the uniqueness probe, so a
+ * refused request never learns the answer it was refused for. A save that
+ * carries the address already on file is not charged at all, so a settings
+ * form that re-posts the whole profile does not spend an honest afternoon of
+ * edits on the ceiling. A refusal drops the address and keeps the rest of the
+ * save, so one spent budget cannot block somebody setting their timezone; the
+ * published 429 is for the request that asked for nothing else. And both
+ * answers worth detecting leave an audit row, because neither reaches the
+ * ordinary `profile.update` row written after the save.
+ */
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    user: { findUnique: vi.fn(), update: vi.fn() },
+  },
+}));
+
+vi.mock("@/lib/auth/audit", () => ({
+  auditLog: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock("@/lib/rate-limit", () => ({
+  checkProfileEmailRateLimit: vi.fn(),
+}));
+
+import { applyProfileUpdate } from "../profile-update";
+import { prisma } from "@/lib/db";
+import { auditLog } from "@/lib/auth/audit";
+import { checkProfileEmailRateLimit } from "@/lib/rate-limit";
+
+const USER_ID = "user-1";
+const ON_FILE = "mine@example.test";
+
+/** The `where` clauses the helper can send, told apart by their shape. */
+function selfLookups() {
+  return vi
+    .mocked(prisma.user.findUnique)
+    .mock.calls.filter((c) => "id" in (c[0]!.where as object));
+}
+function emailProbes() {
+  return vi
+    .mocked(prisma.user.findUnique)
+    .mock.calls.filter((c) => "email" in (c[0]!.where as object));
+}
+
+beforeEach(() => {
+  vi.resetAllMocks();
+  vi.mocked(prisma.user.update).mockResolvedValue({
+    id: USER_ID,
+    username: "u",
+    email: ON_FILE,
+  } as never);
+  vi.mocked(checkProfileEmailRateLimit).mockResolvedValue({
+    allowed: true,
+    limit: 10,
+    remaining: 9,
+    resetAt: Date.now() + 3_600_000,
+  });
+  vi.mocked(prisma.user.findUnique).mockImplementation((async (args: {
+    where: Record<string, unknown>;
+  }) => {
+    if ("id" in args.where) return { email: ON_FILE };
+    return null;
+  }) as never);
+});
+
+describe("applyProfileUpdate — the email conflict is metered", () => {
+  it("charges the acting account before asking about the address", async () => {
+    await applyProfileUpdate(USER_ID, { email: "someone@example.test" });
+
+    expect(checkProfileEmailRateLimit).toHaveBeenCalledWith(USER_ID);
+    expect(emailProbes()).toHaveLength(1);
+    // Ordering, not just presence: the probe must not have run first.
+    const chargedAt = vi.mocked(checkProfileEmailRateLimit).mock
+      .invocationCallOrder[0];
+    const probedAt = vi.mocked(prisma.user.findUnique).mock.invocationCallOrder[
+      selfLookups().length
+    ];
+    expect(chargedAt).toBeLessThan(probedAt);
+  });
+
+  it("refuses with 429 and the published code when the address was all that was asked for", async () => {
+    vi.mocked(checkProfileEmailRateLimit).mockResolvedValue({
+      allowed: false,
+      limit: 10,
+      remaining: 0,
+      resetAt: Date.now() + 3_600_000,
+    });
+
+    const result = await applyProfileUpdate(USER_ID, {
+      email: "someone@example.test",
+    });
+
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.status).toBe(429);
+      expect(result.errorCode).toBe("profile.update.emailRateLimited");
+    }
+    // The whole point: a refused caller is told nothing about the address.
+    expect(emailProbes()).toHaveLength(0);
+    expect(prisma.user.update).not.toHaveBeenCalled();
+  });
+
+  it("does not charge a save that carries the address already on file", async () => {
+    const result = await applyProfileUpdate(USER_ID, {
+      // Same address, differently typed — normalisation is what decides.
+      email: " Mine@Example.Test ",
+      heightCm: 180,
+    });
+
+    expect(result.ok).toBe(true);
+    expect(checkProfileEmailRateLimit).not.toHaveBeenCalled();
+    expect(emailProbes()).toHaveLength(0);
+    expect(prisma.user.update).toHaveBeenCalledTimes(1);
+  });
+
+  it("does not charge an update that never touches the address", async () => {
+    await applyProfileUpdate(USER_ID, { heightCm: 181 });
+
+    expect(checkProfileEmailRateLimit).not.toHaveBeenCalled();
+  });
+
+  it("keeps the rest of the save when the budget is spent, dropping only the address", async () => {
+    // The web form posts the whole profile in one request. Failing all of it
+    // would stop somebody setting their timezone for an hour and tell them it
+    // was about email addresses.
+    vi.mocked(checkProfileEmailRateLimit).mockResolvedValue({
+      allowed: false,
+      limit: 10,
+      remaining: 0,
+      resetAt: Date.now() + 3_600_000,
+    });
+
+    const result = await applyProfileUpdate(USER_ID, {
+      email: "someone@example.test",
+      timezone: "Europe/Lisbon",
+      heightCm: 180,
+    });
+
+    expect(result.ok).toBe(true);
+    const written = vi.mocked(prisma.user.update).mock.calls[0]![0]!
+      .data as Record<string, unknown>;
+    expect(Object.keys(written).sort()).toEqual(["heightCm", "timezone"]);
+    expect("email" in written).toBe(false);
+    if (result.ok) {
+      expect(result.rejectedFields).toEqual([
+        expect.objectContaining({ path: "email", code: "rate_limited" }),
+      ]);
+    }
+    // Still no probe: the refused caller learns nothing about the address.
+    expect(emailProbes()).toHaveLength(0);
+  });
+
+  it("records both answers worth detecting in the audit ledger", async () => {
+    vi.mocked(checkProfileEmailRateLimit).mockResolvedValue({
+      allowed: false,
+      limit: 10,
+      remaining: 0,
+      resetAt: Date.now() + 3_600_000,
+    });
+    await applyProfileUpdate(USER_ID, { email: "someone@example.test" });
+    expect(auditLog).toHaveBeenCalledWith(
+      "profile.email.rate_limited",
+      expect.objectContaining({ userId: USER_ID }),
+    );
+
+    vi.mocked(auditLog).mockClear();
+    vi.mocked(checkProfileEmailRateLimit).mockResolvedValue({
+      allowed: true,
+      limit: 10,
+      remaining: 9,
+      resetAt: Date.now() + 3_600_000,
+    });
+    vi.mocked(prisma.user.findUnique).mockImplementation((async (args: {
+      where: Record<string, unknown>;
+    }) => {
+      if ("id" in args.where) return { email: ON_FILE };
+      return { id: "someone-else" };
+    }) as never);
+
+    await applyProfileUpdate(USER_ID, { email: "taken@example.test" });
+    expect(auditLog).toHaveBeenCalledWith(
+      "profile.email.conflict",
+      expect.objectContaining({ userId: USER_ID }),
+    );
+    // Neither path reaches the ordinary post-save row.
+    expect(auditLog).not.toHaveBeenCalledWith(
+      "profile.update",
+      expect.anything(),
+    );
+  });
+
+  it("does not charge an account whose stored address is spelled differently", async () => {
+    // Registration writes the address as typed, so an account created as
+    // `Mine@Example.Test` has that in the column. Comparing it raw against
+    // the normalised submission would charge its first ordinary save.
+    vi.mocked(prisma.user.findUnique).mockImplementation((async (args: {
+      where: Record<string, unknown>;
+    }) => {
+      if ("id" in args.where) return { email: "Mine@Example.Test" };
+      return null;
+    }) as never);
+
+    const result = await applyProfileUpdate(USER_ID, { email: ON_FILE });
+
+    expect(result.ok).toBe(true);
+    expect(checkProfileEmailRateLimit).not.toHaveBeenCalled();
+    expect(emailProbes()).toHaveLength(0);
+  });
+
+  it("still answers 409 when the bucket has room and the address is taken", async () => {
+    vi.mocked(prisma.user.findUnique).mockImplementation((async (args: {
+      where: Record<string, unknown>;
+    }) => {
+      if ("id" in args.where) return { email: ON_FILE };
+      return { id: "someone-else" };
+    }) as never);
+
+    const result = await applyProfileUpdate(USER_ID, {
+      email: "taken@example.test",
+    });
+
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.status).toBe(409);
+      expect(result.errorCode).toBe("profile.update.emailInUse");
+    }
+    expect(checkProfileEmailRateLimit).toHaveBeenCalledTimes(1);
+  });
+});
diff --git a/src/lib/auth/__tests__/profile-update.test.ts b/src/lib/auth/__tests__/profile-update.test.ts
index 89c75b2cb..eb89c0ff4 100644
--- a/src/lib/auth/__tests__/profile-update.test.ts
+++ b/src/lib/auth/__tests__/profile-update.test.ts
@@ -6,6 +6,9 @@ vi.mock("@/lib/db", () => ({
       findUnique: vi.fn(),
       update: vi.fn(),
     },
+    // The email-address change is metered before it asks whether another
+    // account holds the address; the limiter talks to Postgres directly.
+    $queryRaw: vi.fn(),
   },
 }));
 
@@ -34,6 +37,10 @@ const STUB_USER = {
 beforeEach(() => {
   vi.resetAllMocks();
   vi.mocked(prisma.user.update).mockResolvedValue(STUB_USER as never);
+  // A bucket with room left, unless a test says otherwise.
+  vi.mocked(prisma.$queryRaw).mockResolvedValue([
+    { count: 1, reset_at: new Date(Date.now() + 60_000) },
+  ] as never);
 });
 
 describe("applyProfileUpdate timezone validation", () => {
diff --git a/src/lib/auth/password.ts b/src/lib/auth/password.ts
index 570700b88..eb3f4ae8e 100644
--- a/src/lib/auth/password.ts
+++ b/src/lib/auth/password.ts
@@ -20,6 +20,90 @@ export async function verifyPassword(
   return verify(storedHash, password);
 }
 
+/**
+ * A stand-in stored hash for the login arms that have nothing to verify.
+ *
+ * Sign-in used to return the moment it found no account, or an account with
+ * no password — one indexed SELECT — while an account that did have a
+ * password paid a full Argon2id verification at 19 MiB and t=2. The two
+ * answers were byte-identical and the two costs were not, so the clock told
+ * a caller which addresses are registered and which of those carry a
+ * password. Verifying against this hash on the empty arms makes the work
+ * comparable.
+ *
+ * Minted from `ARGON2_HASH_OPTIONS`, the same parameters every real hash
+ * carries, because Argon2 encodes its cost in the hash string and the
+ * verifier pays whatever the string says. A dummy minted at cheaper settings
+ * would leave the gap it was added to close.
+ *
+ * The input is a fixed constant rather than a random string so the module
+ * has no hidden state, and it is never compared to anything: the arm that
+ * verifies against it discards the verdict and returns false. There is no
+ * path on which a caller can make this hash accept.
+ */
+const DUMMY_VERIFY_INPUT = "healthlog: not a credential, never accepted";
+
+let dummyHashPromise: Promise<string> | null = null;
+
+/**
+ * The dummy hash, minted on first use and kept for the process afterwards.
+ *
+ * Lazy rather than minted at import: every process that imports this module
+ * would otherwise spend 19 MiB and a thread-pool slot at boot, including the
+ * queue worker and each build worker, none of which ever authenticates
+ * anybody. From the second refusal onward the cost profile is identical
+ * either way.
+ *
+ * A rejected promise stays rejected for the life of the process, so caching
+ * one would degrade every later refusal back to the cheap path and leave the
+ * timing channel open with no way to notice. A failed mint is therefore
+ * dropped and the next call mints again.
+ */
+function dummyHash(): Promise<string> {
+  if (!dummyHashPromise) {
+    const pending = hash(DUMMY_VERIFY_INPUT, ARGON2_HASH_OPTIONS);
+    dummyHashPromise = pending;
+    pending.catch(() => {
+      if (dummyHashPromise === pending) dummyHashPromise = null;
+    });
+  }
+  return dummyHashPromise;
+}
+
+/**
+ * Verify `password` against `storedHash`, or against the dummy hash when
+ * there is no stored hash to verify against.
+ *
+ * Returns false whenever `storedHash` is absent — an account without a
+ * password, or no account at all. The dummy verification's own verdict is
+ * thrown away, so a false here is never a rejected credential being
+ * mistaken for anything else, and a true can only come from a real stored
+ * hash.
+ *
+ * Callers on an anonymous surface should use this rather than branching on
+ * the absent hash themselves: the branch is what the clock reads.
+ */
+export async function verifyPasswordOrDummy(
+  storedHash: string | null | undefined,
+  password: string,
+): Promise<boolean> {
+  if (storedHash) return verify(storedHash, password);
+  try {
+    // Same verifier, same cost profile, verdict discarded.
+    await verify(await dummyHash(), password);
+  } catch (error) {
+    // Both the mint and the verify are inside the guard, and the mint has to
+    // be: an unguarded `await` on a rejected mint would throw out of here and
+    // answer 500 on exactly the arm this function exists to make ordinary —
+    // an unknown identifier answering 500 while a real account answers 401 is
+    // the oracle again, in the status code, and cheaper to read than the
+    // timing gap was. The breadcrumb makes a degraded instance visible; the
+    // answer below is the same either way.
+    console.error("[auth] dummy password verification failed", error);
+  }
+  return false;
+}
+
 export interface PasswordStrength {
   score: number; // 0-4
   feedback: string[];
diff --git a/src/lib/auth/profile-update.ts b/src/lib/auth/profile-update.ts
index 58c4adbf3..1891c1575 100644
--- a/src/lib/auth/profile-update.ts
+++ b/src/lib/auth/profile-update.ts
@@ -13,6 +13,8 @@ import {
   invalidateUserProfile,
 } from "@/lib/cache/invalidate";
 import { sanitiseZodIssues, type SanitisedZodIssue } from "@/lib/api-response";
+import { checkProfileEmailRateLimit } from "@/lib/rate-limit";
+import { annotate } from "@/lib/logging/context";
 import { z } from "zod/v4";
 
 const extendedProfileSchema = profileSchema.extend({
@@ -238,17 +240,95 @@ export async function applyProfileUpdate(
   const normalizedEmail = data.email ? data.email.trim().toLowerCase() : null;
 
   if (data.email !== undefined && normalizedEmail) {
-    const existing = await prisma.user.findUnique({
-      where: { email: normalizedEmail },
-      select: { id: true },
+    // A settings form re-posts the whole profile, so the address arrives
+    // with every save whether or not it changed. Those saves ask nothing
+    // about anyone else's account — the address on file already belongs to
+    // this caller — so they skip both the uniqueness probe and the budget.
+    // Charging them would spend an honest afternoon of edits on the
+    // ceiling and lock the account out of its own settings page.
+    //
+    // The stored side is normalised for the comparison too. Registration
+    // writes the address exactly as typed, so an account registered as
+    // `MixedCase@Example.com` would otherwise fail to match its own address
+    // and pay for its first ordinary save.
+    const current = await prisma.user.findUnique({
+      where: { id: userId },
+      select: { email: true },
     });
-    if (existing && existing.id !== userId) {
-      return {
-        ok: false,
-        status: 409,
-        message: "Email already in use",
-        errorCode: "profile.update.emailInUse",
-      };
+    const addressOnFile = current?.email?.trim().toLowerCase() ?? null;
+
+    if (addressOnFile !== normalizedEmail) {
+      // Charged BEFORE the probe, not after: a refused request must not
+      // learn the answer it was refused for. The 409 below tells any
+      // signed-in caller whether an address is registered here, and until
+      // this bucket existed neither route that reaches it had a ceiling of
+      // any kind — `apiHandler` supplies no default.
+      const rl = await checkProfileEmailRateLimit(userId);
+      if (!rl.allowed) {
+        annotate({ action: { name: "profile.email.throttled" } });
+        await auditLog("profile.email.rate_limited", {
+          userId,
+          ipAddress: ipAddress ?? null,
+        });
+
+        // Narrow the refusal to the field, the way the demo narrowing above
+        // does: the web form posts the whole profile in one request, so
+        // failing all of it would stop somebody setting their timezone for
+        // an hour and tell them it was about email addresses. The address is
+        // dropped, everything else is saved, and `rejectedFields` says which
+        // field did not go through.
+        const { email: _refusedAddress, ...keepable } = data;
+        if (Object.keys(keepable).length > 0) {
+          data = keepable;
+          rejectedFields = [
+            ...rejectedFields,
+            {
+              path: "email",
+              code: "rate_limited",
+              message: "Too many email-address changes.",
+            },
+          ];
+        } else {
+          // Nothing else was asked for, so there is nothing to salvage and a
+          // partial success would be a lie — the same rule the all-fields-
+          // rejected 422 above follows. This is the arm the published 429
+          // describes.
+          return {
+            ok: false,
+            status: 429,
+            message: "Too many email-address changes. Try again later.",
+            errorCode: "profile.update.emailRateLimited",
+          };
+        }
+      } else {
+        // Case-sensitive, because the column is: registration writes the
+        // address as typed, so `Mixed@Example.com` and `mixed@example.com`
+        // are two rows this probe cannot see as one. Pre-existing and not
+        // widened here — the fix is a normalisation pass on the register
+        // write, which is where the two spellings are created.
+        const existing = await prisma.user.findUnique({
+          where: { email: normalizedEmail },
+          select: { id: true },
+        });
+        if (existing && existing.id !== userId) {
+          // The informative answer is the one worth recording: it says an
+          // account holds that address. The ordinary save's `profile.update`
+          // row is written further down and this path never reaches it, so
+          // without a row here the only probes in the ledger would be the
+          // ones that came back negative.
+          annotate({ action: { name: "profile.email.conflict" } });
+          await auditLog("profile.email.conflict", {
+            userId,
+            ipAddress: ipAddress ?? null,
+          });
+          return {
+            ok: false,
+            status: 409,
+            message: "Email already in use",
+            errorCode: "profile.update.emailInUse",
+          };
+        }
+      }
     }
   }
 
diff --git a/src/lib/http/retired-routes.ts b/src/lib/http/retired-routes.ts
index 0f38cc2a0..8ed40ff6d 100644
--- a/src/lib/http/retired-routes.ts
+++ b/src/lib/http/retired-routes.ts
@@ -55,14 +55,17 @@
  * client's repository, not in this one, and an inference drawn from this side
  * would be a guess wearing a citation. Nothing below is a claim about a caller.
  *
- * How the set was found: every route path under `src/app` at every release tag,
- * unioned, then differenced against the current tree. Twenty-seven paths have
- * been removed across the project's history. The five here are the ones that
- * were published or client-facing. The other twenty-two are admin-console
- * routes, operator diagnostics, aggregators superseded in place, and the
- * moodLog bridge — surfaces reachable only from the operator's own browser,
- * which ships with the server and reads no contract to find out what it talks
- * to. `moodlog-removal-guard.test.ts` separately holds that bridge removed.
+ * How the set was seeded: every route path under `src/app` at every release
+ * tag, unioned, then differenced against the current tree. Twenty-seven paths
+ * had been removed across the project's history up to that point. Five of them
+ * were published or client-facing and are the first five entries below; the
+ * other twenty-two are admin-console routes, operator diagnostics, aggregators
+ * superseded in place, and the moodLog bridge — surfaces reachable only from
+ * the operator's own browser, which ships with the server and reads no
+ * contract to find out what it talks to. `moodlog-removal-guard.test.ts`
+ * separately holds that bridge removed. Every entry after those five was
+ * written by the release that removed the path, which is where a tombstone
+ * belongs.
  *
  * Sample tags instead of all of them and this undercounts badly: six evenly
  * spaced tags find fourteen of the twenty-seven, because a path added and
@@ -141,6 +144,14 @@ export const RETIRED_ROUTES: readonly RetiredRoute[] = [
       "The whole-record operation moved under the resource type it operates on, which is where FHIR R4 defines it and where the capability statement points.",
     methods: ["GET"],
   },
+  {
+    path: "/api/auth/check-user",
+    removedIn: "1.38.20",
+    replacedBy: null,
+    reason:
+      "Took an email address or a username and answered, to any caller and with no credential at all, whether an account existed here and whether it carried a passkey or a password. It was built to decide which field a sign-in screen shows first, which is not worth telling the world who has an account. Send the credential to POST /api/auth/login or run the passkey ceremony and read the answer; both refuse identically whether or not the account exists.",
+    methods: ["POST"],
+  },
   {
     path: "/api/auth/me/research-mode",
     removedIn: "1.37.2",
diff --git a/src/lib/openapi/error-codes.ts b/src/lib/openapi/error-codes.ts
index ade5c0dec..7b1f82e55 100644
--- a/src/lib/openapi/error-codes.ts
+++ b/src/lib/openapi/error-codes.ts
@@ -250,6 +250,7 @@ export const ERROR_CODE_CATALOGUE: Readonly<Record<string, readonly string[]>> =
     practitioner: ["practitioner.invalid"],
     profile: [
       "profile.update.emailInUse",
+      "profile.update.emailRateLimited",
       "profile.update.invalidBody",
       "profile.update.nothingSaved",
     ],
diff --git a/src/lib/openapi/routes/auth.ts b/src/lib/openapi/routes/auth.ts
index be835e889..26a80fa82 100644
--- a/src/lib/openapi/routes/auth.ts
+++ b/src/lib/openapi/routes/auth.ts
@@ -38,6 +38,7 @@ import {
 } from "@/lib/validations/step-up";
 import {
   dataEnvelope,
+  profileEmailRateLimitResponse,
   stdResponses,
   errorEnvelope,
   loginPasswordSchema,
@@ -304,43 +305,6 @@ const successFlagResponse = z
   .object({ success: z.boolean() })
   .meta({ id: "AuthSuccessFlagResponse" });
 
-// ── iOS onboarding discovery (check-user) ────────────────────────────
-
-const checkUserRequest = z
-  .object({
-    identifier: z
-      .string()
-      .trim()
-      .min(1)
-      .max(254)
-      .describe(
-        "The typed identifier — either an email or a username. Queried verbatim (no case-folding), never echoed back.",
-      ),
-  })
-  .meta({
-    id: "CheckUserRequest",
-    description:
-      "Discovery lookup for the iOS onboarding flow: given a typed email or username, resolve the next sign-in step.",
-  });
-
-const checkUserResponse = z
-  .object({
-    branch: z
-      .enum(["not_found", "passkey_only", "email_fallback", "exists"])
-      .describe(
-        "Next UX step: `not_found` (show sign-up), `passkey_only` (Sign in with Passkey), `email_fallback` (password field, with a Passkey affordance when applicable), `exists` (account with no usable credential — recovery path).",
-      ),
-    hasPasskey: z
-      .boolean()
-      .describe("The account has at least one registered passkey."),
-    hasPassword: z.boolean().describe("The account has a password hash."),
-  })
-  .meta({
-    id: "CheckUserResponse",
-    description:
-      "Account-existence + credential shape. The response is identical whether or not the identifier matched (account-existence is the explicit contract iOS needs); the identifier is never echoed.",
-  });
-
 /**
  * Shared tail for every route the step-up elevation can unlock. Kept in one
  * place so the published contract cannot describe the set inconsistently.
@@ -563,7 +527,7 @@ const authProfileUpdateResponse = z
       )
       .optional()
       .describe(
-        "Present only on a PARTIAL success: the fields that failed validation and were skipped while the rest of the patch was written. A 200 carrying this key means the save was incomplete — surface it, do not treat the response as a clean save.",
+        "Present only on a PARTIAL success: the fields that were skipped while the rest of the patch was written. A 200 carrying this key means the save was incomplete — surface it, do not treat the response as a clean save. `code` is the validator code for a field that failed validation, or `rate_limited` for an email-address change that has spent the account's hourly budget; that one is not a bad value and the same address will be accepted once the window rolls over.",
       ),
   })
   .meta({
@@ -620,29 +584,6 @@ const codexDevicePollResponse = z
   .meta({ id: "CodexDevicePollResponse" });
 
 export const authPaths: NonNullable<ZodOpenApiObject["paths"]> = {
-  "/api/auth/check-user": {
-    post: {
-      tags: ["Auth"],
-      summary: "Resolve the next sign-in step for a typed identifier",
-      description:
-        "Given an email or username, returns which onboarding branch the iOS client should render plus the `hasPasskey` / `hasPassword` booleans so it can offer a Passkey affordance alongside a password field without a second round-trip. Anonymous surface; per-IP rate-limited (30 requests / 15 min). The response is the same whether or not the identifier matched, and the identifier is never echoed back.",
-      requestBody: {
-        required: true,
-        content: { "application/json": { schema: checkUserRequest } },
-      },
-      responses: {
-        "200": {
-          description: "Discovery result.",
-          content: {
-            "application/json": {
-              schema: dataEnvelope(checkUserResponse, "CheckUserEnvelope"),
-            },
-          },
-        },
-        ...stdResponses,
-      },
-    },
-  },
   "/api/auth/login": {
     post: {
       // No credential: this operation is reachable before one exists.
@@ -1826,6 +1767,7 @@ export const authPaths: NonNullable<ZodOpenApiObject["paths"]> = {
           content: { "application/json": { schema: errorEnvelope } },
         },
         ...stdResponses,
+        ...profileEmailRateLimitResponse,
         "422": {
           description:
             "Either the body was not a usable object (`meta.errorCode` = `profile.update.invalidBody`) or every supplied field was rejected (`profile.update.nothingSaved`). Both carry the per-field issue list. A body where SOME fields validate never reaches this response — it succeeds with `rejectedFields`.",
diff --git a/src/lib/openapi/routes/profile.ts b/src/lib/openapi/routes/profile.ts
index f8ee62338..96dd3d464 100644
--- a/src/lib/openapi/routes/profile.ts
+++ b/src/lib/openapi/routes/profile.ts
@@ -46,6 +46,7 @@ import {
   errorEnvelope,
   invalidBaseTokenResponse,
   malformedJsonResponse,
+  profileEmailRateLimitResponse,
   recordRefusal,
   stdResponses,
   updatedAtTokenField,
@@ -482,6 +483,18 @@ const profileUpdateResponse = z
     hasInsuranceNumber: z
       .boolean()
       .describe("Whether a KVNR is on file (the value itself is not echoed)."),
+    rejectedFields: z
+      .array(
+        z.object({
+          path: z.string(),
+          code: z.string(),
+          message: z.string(),
+        }),
+      )
+      .optional()
+      .describe(
+        "Present only on a PARTIAL success: the fields that were skipped while the rest of the patch was written. A 200 carrying this key means the save was incomplete — surface it, do not treat the response as a clean save. `code` is the validator code for a field that failed validation, or `rate_limited` for an email-address change that has spent the account's hourly budget; that one is not a bad value and the same address will be accepted once the window rolls over. The handler has always been able to answer with this key on this path; it was published only on PUT /api/auth/profile.",
+      ),
   })
   .meta({
     id: "ProfileUpdateResponse",
@@ -1476,7 +1489,13 @@ export const profilePaths: NonNullable<ZodOpenApiObject["paths"]> = {
             },
           },
         },
+        "409": {
+          description:
+            "The requested email belongs to another account (`meta.errorCode` = `profile.update.emailInUse`). Nothing was written. Shared with PUT /api/auth/profile, which runs the same handler.",
+          content: { "application/json": { schema: errorEnvelope } },
+        },
         ...stdResponses,
+        ...profileEmailRateLimitResponse,
       },
     },
   },
diff --git a/src/lib/openapi/routes/shared.ts b/src/lib/openapi/routes/shared.ts
index a97b71a82..b0a6149b1 100644
--- a/src/lib/openapi/routes/shared.ts
+++ b/src/lib/openapi/routes/shared.ts
@@ -318,6 +318,32 @@ export const recordWriteRateLimitResponse = {
   },
 };
 
+/**
+ * The 429 the profile email-address bucket answers with.
+ *
+ * Changing the address asks whether another account already holds it, and the
+ * answer is a 409 versus a 200 — an existence check over every address on the
+ * instance, available to any signed-in caller. Neither route that reaches it
+ * had a ceiling of any kind, and `apiHandler` supplies no default, so the
+ * sweep was bounded only by the network. This is the response that names the
+ * bucket now capping it.
+ *
+ * Spread AFTER `...stdResponses` so it replaces the generic 429 on those two
+ * operations. The headers block is the one the standard 429 already declares.
+ *
+ * `profile-email-rate-limit-contract.test.ts` holds the numbers in this
+ * sentence to the constants in `src/lib/rate-limit.ts`, so the paragraph
+ * cannot drift away from the bucket it describes.
+ */
+export const profileEmailRateLimitResponse = {
+  "429": {
+    description:
+      "Too many email-address changes, and the request asked for nothing else. The address change shares one per-account bucket across both profile routes — `profile-email:<accountId>`, 10 requests per 3600 seconds — keyed on the ACTING account, so an account cannot collect a fresh allowance by probing a different address. `meta.errorCode` is `profile.update.emailRateLimited`. Only a request whose `email` actually differs from the address on file is counted: re-saving a form that carries the unchanged address is free, and so is any update that does not touch `email` at all. A request that carries other fields beside the refused address does NOT get this response — it answers 200, saves those fields, and names `email` in `rejectedFields` with code `rate_limited`, so one spent budget cannot block an unrelated profile edit for an hour. The `Retry-After`, `X-RateLimit-Limit`, `X-RateLimit-Remaining` and `X-RateLimit-Reset` headers describe that bucket; back off on `Retry-After` rather than guessing.",
+    content: { "application/json": { schema: errorEnvelope } },
+    headers: stdResponses["429"].headers,
+  },
+};
+
 // ── Idempotent writes ────────────────────────────────────────────────
 
 /**
diff --git a/src/lib/profile/rejected-fields.ts b/src/lib/profile/rejected-fields.ts
index da0a3e604..87225925e 100644
--- a/src/lib/profile/rejected-fields.ts
+++ b/src/lib/profile/rejected-fields.ts
@@ -73,6 +73,11 @@ export function describeRejectedProfileField(
  * degrade to something honest instead of to silence.
  */
 const REJECTION_REASON_KEYS: Record<string, string> = {
+  // Not a Zod code: the profile helper refuses an email-address change that
+  // has spent the account's hourly budget this way, so the rest of the save
+  // still lands. Without an entry it would fall back to "check it and try
+  // again", which is wrong — the value was fine, the timing was not.
+  rate_limited: "settings.profileRejection.rateLimited",
   too_big: "settings.profileRejection.tooBig",
   too_small: "settings.profileRejection.tooSmall",
   invalid_value: "settings.profileRejection.invalidValue",
diff --git a/src/lib/rate-limit.ts b/src/lib/rate-limit.ts
index d6cc7b624..5a8816b4b 100644
--- a/src/lib/rate-limit.ts
+++ b/src/lib/rate-limit.ts
@@ -160,6 +160,46 @@ export async function checkRecordWriteRateLimit(
   );
 }
 
+/**
+ * Per-account ceiling for the profile email-address change.
+ *
+ * Changing the address means asking whether another account already holds
+ * it, and the answer to that question is 409 versus 200 — an existence check
+ * over every address on the instance, available to any signed-in caller. The
+ * two routes that reach it (`PUT /api/auth/profile`, `PATCH
+ * /api/user/profile`) had no ceiling at all and `apiHandler` supplies none,
+ * so a single account could walk an address book at whatever rate it liked.
+ *
+ * Keyed on the acting account rather than on the address, the frozen
+ * precedent from the record writes: an attacker cannot collect a fresh
+ * allowance by switching the address they probe, and one account's sweep
+ * cannot lock a stranger out of their own settings page.
+ *
+ * Ten an hour is far above honest use — an address is changed once in a
+ * while, not ten times a day — and it is only charged when the submitted
+ * address actually differs from the one on file, so re-saving a settings
+ * form that carries the unchanged address never spends the budget. What it
+ * ends is the unbounded sweep: two hundred and forty questions a day per
+ * account instead of as many as the network will carry. Both answers worth
+ * detecting write an audit row of their own — `profile.email.conflict` when
+ * an address turns out to be taken, `profile.email.rate_limited` when the
+ * budget runs out. The ordinary `profile.update` row is written after the
+ * save, which neither of those paths reaches.
+ */
+export const PROFILE_EMAIL_BUCKET_PREFIX = "profile-email";
+export const PROFILE_EMAIL_LIMIT = 10;
+export const PROFILE_EMAIL_WINDOW_MS = 60 * 60 * 1000;
+
+export async function checkProfileEmailRateLimit(
+  accountId: string,
+): Promise<RateLimitResult> {
+  return checkRateLimit(
+    `${PROFILE_EMAIL_BUCKET_PREFIX}:${accountId}`,
+    PROFILE_EMAIL_LIMIT,
+    PROFILE_EMAIL_WINDOW_MS,
+  );
+}
+
 /**
  * v1.22.0 — per-credential ceiling for the remote MCP endpoint (`/mcp`).
  * The bucket is keyed by the `<userId>:<tokenId>` binding the Bearer
@@ -240,7 +280,7 @@ const TIGHT_ANON_WINDOW_MS = 15 * 60 * 1000;
 
 /**
  * Rate-limit helper for anonymous auth surfaces (login, register,
- * passkey-login-{options,verify}, refresh, check-user).
+ * passkey-login-{options,verify}, refresh).
  *
  * Per-IP path: when the trust chain resolves cleanly, route the request
  * to a bucket keyed by `{prefix}:{ip ?? "unknown"}` with the caller's