From 9eae352bb10634e017e63dbc777d2dae3a7a72a5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marc-Andr=C3=A9=20Bombeck?= <mbombeck@gmail.com>
Date: Thu, 15 Oct 2026 23:12:59 +0200
Subject: [PATCH] feat(claim): hand over a managed profile through single
 sign-on

POST /api/auth/claim/sso checks the handover link and the username and seals
the link id, the username and a per-start binding into an hl_claim cookie.
GET /api/auth/oidc/login?intent=claim carries the binding into the encrypted
OIDC state, and the callback binds the provider subject to the profile in the
claim transaction, refusing a subject or verified email another account
holds, a link that is no longer usable, and a live session.

The handover status and the claim preview name the redemption methods; a
single-sign-on-only server hands over through the identity provider, and only
the password claim stays refused there.
---
 docs/api/openapi.yaml                         | 258 ++++++++++--------
 src/__tests__/contract-stub-inventory.test.ts |   3 +-
 src/__tests__/session-surface-guard.test.ts   |  48 +++-
 .../api/auth/claim/__tests__/route.test.ts    | 186 ++++++++++++-
 src/app/api/auth/claim/preview/route.ts       |  13 +-
 src/app/api/auth/claim/route.ts               |   2 +-
 src/app/api/auth/claim/sso/route.ts           |  83 +++++-
 src/app/api/auth/oidc/callback/route.ts       |  91 +++++-
 .../auth/oidc/login/__tests__/route.test.ts   |  47 ++++
 src/app/api/auth/oidc/login/route.ts          |  55 +++-
 .../managed-profiles/[id]/handover/route.ts   |  25 +-
 src/lib/managed-profiles/claim-cookie.ts      | 111 ++++++++
 src/lib/managed-profiles/claim-gate.ts        |  51 +++-
 src/lib/managed-profiles/claim-sso.ts         | 178 ++++++++++++
 src/lib/managed-profiles/handover.ts          | 107 +++++++-
 src/lib/openapi/error-codes.ts                |   1 +
 src/lib/openapi/routes/account-sharing.ts     |   2 +-
 src/lib/openapi/routes/auth.ts                |  19 +-
 src/lib/validations/claim-sso.ts              |   1 +
 .../managed-profile-handover.test.ts          |   2 +-
 20 files changed, 1075 insertions(+), 208 deletions(-)
 create mode 100644 src/lib/managed-profiles/claim-cookie.ts
 create mode 100644 src/lib/managed-profiles/claim-sso.ts

diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index 43407e243..192fc35c0 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -1873,9 +1873,15 @@ paths:
       description: "Web only. Checks the link and the username, sets a sealed `hl_claim` cookie (10 minutes, `httpOnly`) and
         answers where to continue: `/api/auth/oidc/login?intent=claim`. The OIDC callback binds the identity provider's
         subject to the profile and signs the new owner in; a subject already bound to an account, or a verified email
-        another account holds, is refused there. Works on a single-sign-on-only instance. 404 `profile_claim.invalid`
-        (no usable link) or `profile_claim.sso_unavailable` (no identity provider configured); 409 `profile_claim.taken`
-        (username) or `auth.already_authenticated`; rate-limited per source address like the password claim."
+        another account holds, is refused there. Works on a single-sign-on-only instance. When the callback refuses, the
+        browser lands on `/auth/claim?error=<reason>` with reason `claim_expired` (the link was used, withdrawn,
+        replaced or expired in the meantime, or the cookie is missing, expired or belongs to another start),
+        `claim_sso_subject_bound`, `claim_email_taken`, `claim_username_taken` (taken during the round trip),
+        `claim_signed_in`, or one of the sign-in reasons (`oidc_denied`, `oidc_state`, `oidc_failed`,
+        `oidc_rate_limited`, `oidc_disabled`); on success it lands on `/onboarding/handover`, where the new owner
+        decides each guardian's access. The cookie is cleared either way. 404 `profile_claim.invalid` (no usable link)
+        or `profile_claim.sso_unavailable` (no identity provider configured); 409 `profile_claim.taken` (username) or
+        `auth.already_authenticated`; rate-limited per source address like the password claim."
       requestBody:
         required: true
         content:
@@ -1904,15 +1910,6 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-        "501": &a30
-          description: "Not implemented on this server yet: the contract is published, the effect is not. `meta.errorCode` =
-            `not_implemented`, `meta.operation` names the operation. Authentication, module gates and request validation
-            already answer as documented. A client reaches this only by ignoring the server feature flag that guards the
-            operation."
-          content:
-            application/json:
-              schema:
-                $ref: "#/components/schemas/ErrorEnvelope"
   /api/auth/claim/preview:
     post:
       security: []
@@ -1921,7 +1918,8 @@ paths:
       summary: Preview a managed-profile handover link
       description: "What a handover link would hand over: the profile's display name, the link's expiry, and each guardian's
         proposal. Anonymous; the token travels in the body. An unknown, expired, used or withdrawn token, and a link
-        whose creator lost access, all answer the same 404. Rate-limited per source address."
+        whose creator lost access, all answer the same 404. Rate-limited per source address. Since v1.43 answered on a
+        single-sign-on-only instance too, with `methods` naming the ways in."
       requestBody:
         required: true
         content:
@@ -1936,12 +1934,6 @@ paths:
               schema:
                 $ref: "#/components/schemas/ProfileClaimPreviewEnvelope"
         "401": *a1
-        "403":
-          description: "The instance allows single sign-on only (`meta.errorCode: profile_claim.oidc_only_unsupported`)."
-          content:
-            application/json:
-              schema:
-                $ref: "#/components/schemas/ErrorEnvelope"
         "404":
           description: "No usable link behind this token (`meta.errorCode: profile_claim.invalid`). Every failure class answers
             identically."
@@ -3738,6 +3730,19 @@ paths:
             type: string
           description: "Browser leg only: where to land after login. Sanitised to an in-app path; ignored entirely on the native
             leg."
+        - in: query
+          name: intent
+          schema:
+            description: "v1.43 — `claim` continues a managed-profile claim started at POST /api/auth/claim/sso; the link and
+              username ride the sealed `hl_claim` cookie that call set, never this URL. Without a valid cookie for a
+              still-usable link the start redirects to `/auth/claim?error=claim_expired`. Browser leg only: with
+              `client=native` it fails with `oidc_invalid_request`. `next` is ignored."
+            type: string
+            const: claim
+          description: "v1.43 — `claim` continues a managed-profile claim started at POST /api/auth/claim/sso; the link and
+            username ride the sealed `hl_claim` cookie that call set, never this URL. Without a valid cookie for a
+            still-usable link the start redirects to `/auth/claim?error=claim_expired`. Browser leg only: with
+            `client=native` it fails with `oidc_invalid_request`. `next` is ignored."
         - in: query
           name: client_proto
           schema:
@@ -3762,7 +3767,8 @@ paths:
           description: "Browser leg only: `location.host` of that page, read together with `client_proto`."
       responses:
         "302":
-          description: Always a redirect. To the provider on success. On failure to `/auth/login?error=<reason>` (browser) or
+          description: Always a redirect. To the provider on success. On failure to `/auth/login?error=<reason>` (browser;
+            `/auth/claim?error=<reason>` with `intent=claim`, where `claim_expired` is one more reason) or
             `healthlog://oidc-callback?error=<reason>` (native), where reason is one of `oidc_disabled`,
             `oidc_rate_limited`, `oidc_invalid_request`, `oidc_failed`, and on the browser leg `insecure_transport` (see
             `client_proto`). The provider then returns to the callback (not listed here, because the identity provider
@@ -15171,7 +15177,7 @@ paths:
             schema:
               $ref: "#/components/schemas/OcrTextExtractRequest"
       responses:
-        "202": &a39
+        "202": &a38
           description: Queued for the background worker. Every refusal the route can answer quickly (capability, provider, rate
             limit, budget, validation) has already been answered; anything that goes wrong from here is reported on the
             run.
@@ -15453,7 +15459,15 @@ paths:
         "409": *a7
         "422": *a4
         "429": *a12
-        "501": *a30
+        "501": &a45
+          description: "Not implemented on this server yet: the contract is published, the effect is not. `meta.errorCode` =
+            `not_implemented`, `meta.operation` names the operation. Authentication, module gates and request validation
+            already answer as documented. A client reaches this only by ignoring the server feature flag that guards the
+            operation."
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
   /api/encounters:
     get:
       tags:
@@ -15666,7 +15680,7 @@ paths:
                 $ref: "#/components/schemas/GetEncounterEnvelope"
         "401": *a1
         "403": *a11
-        "404": &a31
+        "404": &a30
           description: Visit not found (or owned by another user).
           content:
             application/json:
@@ -15700,7 +15714,7 @@ paths:
                 $ref: "#/components/schemas/UpdateEncounterEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a31
+        "404": *a30
         "422": *a4
         "429": *a5
     delete:
@@ -15725,7 +15739,7 @@ paths:
                 $ref: "#/components/schemas/DeleteEncounterEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a31
+        "404": *a30
         "422": *a4
         "429": *a5
   /api/encounters/{id}/restore:
@@ -15751,7 +15765,7 @@ paths:
                 $ref: "#/components/schemas/RestoreEncounterEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a31
+        "404": *a30
         "422": *a4
         "429": *a5
   /api/encounters/{id}/links:
@@ -15781,7 +15795,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/LinkEncounterEnvelope"
         "401": *a1
-        "404": *a31
+        "404": *a30
         "422": *a4
         "429": *a5
     delete:
@@ -15810,7 +15824,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/UnlinkEncounterEnvelope"
         "401": *a1
-        "404": *a31
+        "404": *a30
         "422": *a4
         "429": *a5
   /api/practitioners:
@@ -15889,7 +15903,7 @@ paths:
                 $ref: "#/components/schemas/GetPractitionerEnvelope"
         "401": *a1
         "403": *a11
-        "404": &a32
+        "404": &a31
           description: Practitioner not found (or owned by another user).
           content:
             application/json:
@@ -15923,7 +15937,7 @@ paths:
                 $ref: "#/components/schemas/UpdatePractitionerEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a32
+        "404": *a31
         "422": *a4
         "429": *a5
     delete:
@@ -15947,7 +15961,7 @@ paths:
                 $ref: "#/components/schemas/DeletePractitionerEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a32
+        "404": *a31
         "422": *a4
         "429": *a5
   /api/practitioners/{id}/restore:
@@ -15973,7 +15987,7 @@ paths:
                 $ref: "#/components/schemas/RestorePractitionerEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a32
+        "404": *a31
         "422": *a4
         "429": *a5
   /api/vaccinations/custom:
@@ -16017,7 +16031,7 @@ paths:
                 $ref: "#/components/schemas/CreateCustomVaccineEnvelope"
         "401": *a1
         "403": *a11
-        "409": &a33
+        "409": &a32
           description: A live definition on the record already carries this name, compared without case
             (`vaccination.custom.name-taken`).
           content:
@@ -16054,13 +16068,13 @@ paths:
                 $ref: "#/components/schemas/UpdateCustomVaccineEnvelope"
         "401": *a1
         "403": *a11
-        "404": &a34
+        "404": &a33
           description: No live definition with this id on the record (`vaccination.custom.not-found`).
           content:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "409": *a33
+        "409": *a32
         "422": *a4
         "429": *a5
     delete:
@@ -16085,7 +16099,7 @@ paths:
                 $ref: "#/components/schemas/DeleteCustomVaccineEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a34
+        "404": *a33
         "422": *a4
         "429": *a5
   /api/vaccinations:
@@ -16187,7 +16201,7 @@ paths:
                 $ref: "#/components/schemas/GetVaccinationEnvelope"
         "401": *a1
         "403": *a11
-        "404": &a35
+        "404": &a34
           description: Vaccination not found (or owned by another user).
           content:
             application/json:
@@ -16222,7 +16236,7 @@ paths:
                 $ref: "#/components/schemas/UpdateVaccinationEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a35
+        "404": *a34
         "422": *a4
         "429": *a5
     delete:
@@ -16247,7 +16261,7 @@ paths:
                 $ref: "#/components/schemas/DeleteVaccinationEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a35
+        "404": *a34
         "422": *a4
         "429": *a5
   /api/vaccinations/{id}/restore:
@@ -16272,7 +16286,7 @@ paths:
                 $ref: "#/components/schemas/RestoreVaccinationEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a35
+        "404": *a34
         "422": *a4
         "429": *a5
   /api/vaccinations/{id}/booster:
@@ -16338,7 +16352,7 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
-        "404": *a35
+        "404": *a34
         "422": *a4
         "429": *a5
   /api/vaccinations/suggest:
@@ -16396,7 +16410,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/LinkVaccinationEnvelope"
         "401": *a1
-        "404": *a35
+        "404": *a34
         "422": *a4
         "429": *a5
     delete:
@@ -16425,7 +16439,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/UnlinkVaccinationEnvelope"
         "401": *a1
-        "404": *a35
+        "404": *a34
         "422": *a4
         "429": *a5
   /api/illness/episodes:
@@ -16540,7 +16554,7 @@ paths:
                 $ref: "#/components/schemas/GetIllnessEpisodeEnvelope"
         "401": *a1
         "403": *a11
-        "404": &a36
+        "404": &a35
           description: Illness episode not found (or owned by another user).
           content:
             application/json:
@@ -16575,7 +16589,7 @@ paths:
                 $ref: "#/components/schemas/UpdateIllnessEpisodeEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a36
+        "404": *a35
         "422": *a4
         "429": *a5
     delete:
@@ -16599,7 +16613,7 @@ paths:
                 $ref: "#/components/schemas/DeleteIllnessEpisodeEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a36
+        "404": *a35
         "422": *a4
         "429": *a5
   /api/illness/episodes/{id}/restore:
@@ -16627,7 +16641,7 @@ paths:
                 $ref: "#/components/schemas/RestoreIllnessEpisodeEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a36
+        "404": *a35
         "409": *a7
         "422": *a4
         "429": *a5
@@ -16660,7 +16674,7 @@ paths:
                 $ref: "#/components/schemas/ResolveIllnessEpisodeEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a36
+        "404": *a35
         "422": *a4
         "429": *a5
   /api/illness/episodes/{id}/day-logs:
@@ -16711,7 +16725,7 @@ paths:
                 $ref: "#/components/schemas/GetIllnessDayLogEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a36
+        "404": *a35
         "422": *a4
         "429": *a5
     post:
@@ -16748,7 +16762,7 @@ paths:
                 $ref: "#/components/schemas/UpsertIllnessDayLogCreatedEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a36
+        "404": *a35
         "422": *a4
         "429": *a5
   /api/illness/episodes/{id}/correlation:
@@ -16775,7 +16789,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/IllnessCorrelationEnvelope"
         "401": *a1
-        "404": *a36
+        "404": *a35
         "422": *a4
         "429": *a5
   /api/illness/insights:
@@ -16991,7 +17005,7 @@ paths:
                 $ref: "#/components/schemas/GetAllergyEnvelope"
         "401": *a1
         "403": *a11
-        "404": &a37
+        "404": &a36
           description: Allergy not found (or owned by another user).
           content:
             application/json:
@@ -17025,7 +17039,7 @@ paths:
                 $ref: "#/components/schemas/UpdateAllergyEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a37
+        "404": *a36
         "422": *a4
         "429": *a5
     delete:
@@ -17049,7 +17063,7 @@ paths:
                 $ref: "#/components/schemas/DeleteAllergyEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a37
+        "404": *a36
         "422": *a4
         "429": *a5
   /api/family-history:
@@ -17123,7 +17137,7 @@ paths:
                 $ref: "#/components/schemas/GetFamilyHistoryEnvelope"
         "401": *a1
         "403": *a11
-        "404": &a38
+        "404": &a37
           description: Family history entry not found (or owned by another user).
           content:
             application/json:
@@ -17157,7 +17171,7 @@ paths:
                 $ref: "#/components/schemas/UpdateFamilyHistoryEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a38
+        "404": *a37
         "422": *a4
         "429": *a5
     delete:
@@ -17181,7 +17195,7 @@ paths:
                 $ref: "#/components/schemas/DeleteFamilyHistoryEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a38
+        "404": *a37
         "422": *a4
         "429": *a5
   /api/documents/inbound:
@@ -17846,7 +17860,7 @@ paths:
           required: true
           schema:
             type: string
-        - &a40
+        - &a39
           name: Prefer
           in: header
           required: false
@@ -17871,9 +17885,9 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/InboundExtractEnvelope"
-        "202": *a39
+        "202": *a38
         "401": *a1
-        "403": &a41
+        "403": &a40
           description: "AI refusal: the `documentAi` capability is unavailable for this record. The envelope carries
             `meta.errorCode`, `meta.capability` and `meta.reason` (one of `AiUnavailableReason`).
             `assistant.disabled.documentAi` — the operator turned off reading documents, or every AI feature (the master
@@ -17897,7 +17911,7 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-        "503": &a42
+        "503": &a41
           description: "`ai.unavailable` — the inputs behind the AI capability could not be read, and the answer failed closed.
             Nothing was sent. Retry later."
           content:
@@ -17927,7 +17941,7 @@ paths:
           required: true
           schema:
             type: string
-        - *a40
+        - *a39
       requestBody:
         required: false
         content:
@@ -17963,12 +17977,12 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/DocumentSuggestEnvelope"
-        "202": *a39
+        "202": *a38
         "401": *a1
-        "403": *a41
+        "403": *a40
         "422": *a4
         "429": *a5
-        "503": *a42
+        "503": *a41
   /api/documents/inbound/{id}/summary:
     post:
       tags:
@@ -18018,7 +18032,7 @@ paths:
             type: boolean
             default: false
           description: With `persist=true`, explicitly replace an existing stored summary.
-        - *a40
+        - *a39
       requestBody:
         required: false
         content:
@@ -18054,12 +18068,12 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/DocumentSummaryEnvelope"
-        "202": *a39
+        "202": *a38
         "401": *a1
-        "403": *a41
+        "403": *a40
         "422": *a4
         "429": *a5
-        "503": *a42
+        "503": *a41
   /api/documents/inbound/{id}/index:
     post:
       tags:
@@ -18085,7 +18099,7 @@ paths:
           required: true
           schema:
             type: string
-        - *a40
+        - *a39
       requestBody:
         required: false
         content:
@@ -18121,12 +18135,12 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/DocumentIndexEnvelope"
-        "202": *a39
+        "202": *a38
         "401": *a1
-        "403": *a41
+        "403": *a40
         "422": *a4
         "429": *a5
-        "503": *a42
+        "503": *a41
   /api/documents/inbound/{id}/chat:
     post:
       tags:
@@ -18166,10 +18180,10 @@ paths:
                 type: string
                 description: "SSE frames: `data: <json>\\n\\n`. See the operation description for the per-`type` frame shapes."
         "401": *a1
-        "403": *a41
+        "403": *a40
         "422": *a4
         "429": *a5
-        "503": *a42
+        "503": *a41
     get:
       tags:
         - Documents
@@ -18232,10 +18246,10 @@ paths:
               schema:
                 $ref: "#/components/schemas/DocumentReindexEnvelope"
         "401": *a1
-        "403": *a41
+        "403": *a40
         "422": *a4
         "429": *a5
-        "503": *a42
+        "503": *a41
   /api/documents/inbound/{id}/facts/{factId}:
     patch:
       tags:
@@ -18342,7 +18356,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/CreateCustomMetricResponse"
         "401": *a1
-        "409": &a44
+        "409": &a43
           description: A custom metric with this name already exists for the caller.
           content:
             application/json:
@@ -18371,7 +18385,7 @@ paths:
                 $ref: "#/components/schemas/GetCustomMetricResponse"
         "401": *a1
         "403": *a11
-        "404": &a43
+        "404": &a42
           description: Custom metric not found (or owned by another user).
           content:
             application/json:
@@ -18405,8 +18419,8 @@ paths:
               schema:
                 $ref: "#/components/schemas/UpdateCustomMetricResponse"
         "401": *a1
-        "404": *a43
-        "409": *a44
+        "404": *a42
+        "409": *a43
         "422": *a4
         "429": *a5
     delete:
@@ -18429,7 +18443,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/DeleteCustomMetricResponse"
         "401": *a1
-        "404": *a43
+        "404": *a42
         "422": *a4
         "429": *a5
   /api/custom-metrics/{id}/entries:
@@ -18473,7 +18487,7 @@ paths:
                 $ref: "#/components/schemas/ListCustomMetricEntriesEnvelope"
         "401": *a1
         "403": *a11
-        "404": *a43
+        "404": *a42
         "422": *a4
         "429": *a5
     post:
@@ -18502,7 +18516,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/CreateCustomMetricEntryResponse"
         "401": *a1
-        "404": *a43
+        "404": *a42
         "409": *a7
         "422": *a4
         "429": *a12
@@ -18538,7 +18552,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/UpdateCustomMetricEntryResponse"
         "401": *a1
-        "404": &a45
+        "404": &a44
           description: Custom metric entry not found (or owned by another user).
           content:
             application/json:
@@ -18571,7 +18585,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/DeleteCustomMetricEntryResponse"
         "401": *a1
-        "404": *a45
+        "404": *a44
         "422": *a4
         "429": *a5
   /api/custom-metrics/{id}/entries/restore:
@@ -18895,7 +18909,7 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/nutrients/batch:
     post:
       tags:
@@ -20883,7 +20897,9 @@ paths:
                 $ref: "#/components/schemas/ManagedProfileHandoverEnvelope"
         "401": *a1
         "403":
-          description: "The instance allows single sign-on only (`meta.errorCode: profile_claim.oidc_only_unsupported`)."
+          description: "Neither a password nor single sign-on could redeem a link on this instance (`meta.errorCode:
+            profile_claim.oidc_only_unsupported`). Since v1.43 a single-sign-on-only instance hands over through the
+            identity provider, so a working configuration never answers this."
           content:
             application/json:
               schema:
@@ -23992,7 +24008,7 @@ paths:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/medications/{id}/ingredients:
     put:
       tags:
@@ -24034,7 +24050,7 @@ paths:
         "409": *a7
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/medications/{id}/ingredients/extract:
     post:
       tags:
@@ -24064,7 +24080,7 @@ paths:
               required:
                 - file
       responses:
-        "202": *a39
+        "202": *a38
         "401": *a1
         "403": *a19
         "413":
@@ -24075,7 +24091,7 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
         "503":
           description: "`aiRuns.workerUnavailable`: the run could not be handed to the background worker. Nothing was read and
             nothing was charged; try again."
@@ -24100,7 +24116,7 @@ paths:
         "403": *a11
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/anamnesis/life-circumstances/ai-sections:
     patch:
       tags:
@@ -24130,7 +24146,7 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/anamnesis/life-circumstances/confirm:
     post:
       tags:
@@ -24154,7 +24170,7 @@ paths:
         "401": *a1
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/anamnesis/audit-c:
     post:
       tags:
@@ -24187,7 +24203,7 @@ paths:
         "409": *a7
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/anamnesis/audit-c/{id}:
     delete:
       tags:
@@ -24216,7 +24232,7 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/dawarich/connect:
     post:
       tags:
@@ -24242,7 +24258,7 @@ paths:
         "403": *a19
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/dawarich/status:
     get:
       tags:
@@ -24259,7 +24275,7 @@ paths:
         "403": *a19
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/dawarich/places:
     get:
       tags:
@@ -24290,7 +24306,7 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
         "502":
           description: "`dawarich.unreachable`."
           content:
@@ -24320,7 +24336,7 @@ paths:
         "403": *a19
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/dawarich/test:
     post:
       tags:
@@ -24339,7 +24355,7 @@ paths:
         "403": *a19
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/dawarich/sync:
     post:
       tags:
@@ -24374,7 +24390,7 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/dawarich/disconnect:
     post:
       tags:
@@ -24403,7 +24419,7 @@ paths:
         "403": *a19
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/environment/travel/suggestions:
     get:
       tags:
@@ -24420,7 +24436,7 @@ paths:
         "403": *a19
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/environment/travel/suggestions/{id}/dismiss:
     post:
       tags:
@@ -24450,7 +24466,7 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/medications/{id}/links:
     post:
       tags:
@@ -24487,7 +24503,7 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
     delete:
       tags:
         - Links
@@ -24523,7 +24539,7 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/labs/{id}/links:
     post:
       tags:
@@ -24560,7 +24576,7 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
     delete:
       tags:
         - Links
@@ -24596,7 +24612,7 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
-        "501": *a30
+        "501": *a45
   /api/bugreport:
     post:
       security: []
@@ -35592,17 +35608,17 @@ components:
                 personal_records.invalid_query; practitioner — practitioner.invalid; profile —
                 profile.update.emailInUse, profile.update.emailRateLimited, profile.update.emailSecondFactorRequired,
                 profile.update.invalidBody, profile.update.nothingSaved; profile_claim — profile_claim.invalid,
-                profile_claim.oidc_only_unsupported, profile_claim.taken; 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; search — search.invalid; 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, share-link.selection.unknown_leaf; sharing —
-                sharing.accept.expired, sharing.accept.not_found, sharing.accept.not_pending, sharing.accept.revoked,
-                sharing.access.denied, sharing.invite.duplicate, sharing.invite.invalid, sharing.invite.invalid_scope,
-                sharing.invite.manage_browser_only, sharing.invite.refused, sharing.invite.self, sharing.not_permitted,
-                sharing.renounce.already_ended, sharing.renounce.not_found, sharing.revoke.already_ended,
-                sharing.revoke.not_found, sharing.session.changed, sharing.switch.invalid,
+                profile_claim.oidc_only_unsupported, profile_claim.sso_unavailable, profile_claim.taken; 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; search —
+                search.invalid; 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, share-link.selection.unknown_leaf;
+                sharing — sharing.accept.expired, sharing.accept.not_found, sharing.accept.not_pending,
+                sharing.accept.revoked, sharing.access.denied, sharing.invite.duplicate, sharing.invite.invalid,
+                sharing.invite.invalid_scope, sharing.invite.manage_browser_only, sharing.invite.refused,
+                sharing.invite.self, sharing.not_permitted, sharing.renounce.already_ended, sharing.renounce.not_found,
+                sharing.revoke.already_ended, sharing.revoke.not_found, sharing.session.changed, sharing.switch.invalid,
                 sharing.switch.wrong_transport; source-priority — source-priority.body.invalid_json,
                 source-priority.body.invalid_shape; symptoms — symptoms.definition.limitReached,
                 symptoms.definition.notFound, symptoms.episode.notOpen, symptoms.event.notFound, symptoms.invalid;
diff --git a/src/__tests__/contract-stub-inventory.test.ts b/src/__tests__/contract-stub-inventory.test.ts
index 0d71bcd34..6c39536c3 100644
--- a/src/__tests__/contract-stub-inventory.test.ts
+++ b/src/__tests__/contract-stub-inventory.test.ts
@@ -37,7 +37,6 @@ const CONTRACT_STUBS: Readonly<Record<string, readonly string[]>> = {
     "anamnesis.life_circumstances.confirm",
   ],
   "anamnesis/life-circumstances/route.ts": ["anamnesis.life_circumstances.get"],
-  "auth/claim/sso/route.ts": ["profile_claim.sso.start"],
   "auth/me/ai-usage/route.ts": ["auth.me.ai_usage.put"],
   "biomarkers/[id]/merge/route.ts": ["labs.biomarker.merge"],
   "dawarich/connect/route.ts": ["dawarich.connect"],
@@ -63,7 +62,7 @@ const CONTRACT_STUBS: Readonly<Record<string, readonly string[]>> = {
 };
 
 /** The size of the register when it was written; see the header. */
-const REGISTER_SIZE = 23;
+const REGISTER_SIZE = 22;
 
 function routeFiles(): string[] {
   return walkSourceFiles(API_ROOT, { floor: 400, extensions: [".ts"] })
diff --git a/src/__tests__/session-surface-guard.test.ts b/src/__tests__/session-surface-guard.test.ts
index d27126a2b..6fdcd8534 100644
--- a/src/__tests__/session-surface-guard.test.ts
+++ b/src/__tests__/session-surface-guard.test.ts
@@ -100,9 +100,11 @@ describe("S1 — the direct session-resolution set is frozen", () => {
     "app/api/auth/claim/sso/route.ts":
       "refuses to start a single-sign-on claim over a live session",
     // Anonymous IdP callback. Probes for an already-valid session so a
-    // replayed callback redirects instead of minting a second identity.
+    // replayed callback redirects instead of minting a second identity, and
+    // (v1.43, #959) so a single-sign-on claim is refused over a live session,
+    // as the claim routes refuse it.
     "app/api/auth/oidc/callback/route.ts":
-      "duplicate-callback probe on an otherwise anonymous route",
+      "duplicate-callback probe and the claim's live-session refusal on an otherwise anonymous route",
     // Public liveness probe. The session read only decides whether the body
     // carries operational detail; the status code is computed without it.
     "app/api/health/route.ts": "admin-only detail on a public probe",
@@ -370,6 +372,48 @@ describe("S2 — the URL-token authentication set is frozen", () => {
   });
 });
 
+describe("S4 — the single-sign-on claim cookie set is frozen", () => {
+  /**
+   * v1.43 (#959). A third way to act without a session: the sealed `hl_claim`
+   * cookie carries a managed-profile claim from `POST /api/auth/claim/sso`
+   * through the OIDC round trip, and the callback turns it — together with
+   * the provider's subject — into somebody's account. It is neither a session
+   * nor a URL credential, so neither leg above sees it.
+   *
+   * The handover token itself still only travels in the claim routes' bodies
+   * and the claim page's query string (S2); the cookie names the link by id.
+   * What this freezes is who may mint the cookie and who may act on it.
+   */
+  const CLAIM_COOKIE_ALLOWLIST: Record<string, string> = {
+    "app/api/auth/claim/sso/route.ts":
+      "mints the cookie, only after checking the token and the username",
+    "app/api/auth/oidc/login/route.ts":
+      "requires the cookie for intent=claim and binds it into the OIDC state",
+    "app/api/auth/oidc/callback/route.ts":
+      "binds the provider's subject to the profile the cookie names",
+  };
+
+  const CLAIM_COOKIE_USE =
+    /\b(readClaimCookie|setClaimCookie|claimMatchesBinding|clearClaimCookie)\s*\(|\bCLAIM_COOKIE\b|["']hl_claim["']/;
+
+  it("no file under src/app touches the claim cookie unlisted", () => {
+    const users = appFilesMatching(CLAIM_COOKIE_USE);
+    expect(users.length).toBeGreaterThan(0);
+    expect(users).toEqual(Object.keys(CLAIM_COOKIE_ALLOWLIST).sort());
+  });
+
+  it("only the single-sign-on start mints it", () => {
+    const minters = appFilesMatching(/\bsetClaimCookie\s*\(/);
+    expect(minters).toEqual(["app/api/auth/claim/sso/route.ts"]);
+  });
+
+  it("every allowlist entry carries a reason", () => {
+    for (const [rel, reason] of Object.entries(CLAIM_COOKIE_ALLOWLIST)) {
+      expect(reason.trim().length, `${rel} has no reason`).toBeGreaterThan(0);
+    }
+  });
+});
+
 describe("S3 — the two families stay distinct", () => {
   /**
    * A file that both resolves a session AND authenticates by a URL token is
diff --git a/src/app/api/auth/claim/__tests__/route.test.ts b/src/app/api/auth/claim/__tests__/route.test.ts
index b5b9722b1..49d2fc9c4 100644
--- a/src/app/api/auth/claim/__tests__/route.test.ts
+++ b/src/app/api/auth/claim/__tests__/route.test.ts
@@ -5,7 +5,11 @@
  * (`tests/integration/managed-profile-handover.test.ts`); here the order of
  * the refusals is pinned, because each one is a security property:
  *
- *   - single sign-on only refuses before anything else;
+ *   - single sign-on only refuses the PASSWORD claim before anything else,
+ *     while the preview answers there and names `sso` as the way in (v1.43);
+ *   - the single-sign-on start needs a configured provider, refuses a live
+ *     session before the rate limit, checks the link before the username,
+ *     and seals the link's id (never the token) into `hl_claim`;
  *   - a live session is refused BEFORE the rate limit (a state, not an
  *     attempt), and before any token is read;
  *   - the rate limit runs before the body is parsed;
@@ -24,7 +28,18 @@ vi.mock("@/lib/auth/session", () => ({
   createSession: vi.fn(),
   getSession: vi.fn(),
 }));
-vi.mock("@/lib/auth/oidc", () => ({ isOidcOnly: vi.fn() }));
+vi.mock("@/lib/auth/oidc", () => ({
+  isOidcOnly: vi.fn(),
+  isOidcConfigured: vi.fn(),
+  getOidcConfig: vi.fn(),
+}));
+vi.mock("@/lib/crypto", () => ({
+  encrypt: vi.fn((s: string) => `sealed(${s})`),
+  decrypt: vi.fn(),
+}));
+vi.mock("@/lib/auth/secure-cookie", () => ({
+  shouldEmitSecureCookie: () => true,
+}));
 vi.mock("@/lib/rate-limit", () => ({
   checkRateLimit: vi.fn(),
   checkAuthSurfaceRateLimit: vi.fn(),
@@ -49,6 +64,7 @@ vi.mock("@/lib/managed-profiles/handover", async () => {
   return {
     HandoverError: actual.HandoverError,
     previewHandover: vi.fn(),
+    usableHandoverForToken: vi.fn(),
     claimManagedProfile: vi.fn(),
   };
 });
@@ -67,9 +83,11 @@ vi.mock("next/headers", () => ({
 
 import { POST as claim } from "../route";
 import { POST as preview } from "../preview/route";
+import { POST as claimSso } from "../sso/route";
 import { prisma } from "@/lib/db";
 import { createSession, getSession } from "@/lib/auth/session";
-import { isOidcOnly } from "@/lib/auth/oidc";
+import { getOidcConfig, isOidcConfigured, isOidcOnly } from "@/lib/auth/oidc";
+import { auditLog } from "@/lib/auth/audit";
 import { checkAuthSurfaceRateLimit } from "@/lib/rate-limit";
 import { checkPasswordStrength, hashPassword } from "@/lib/auth/password";
 import { checkPasswordBreachIfEnabled } from "@/lib/password-breach-check";
@@ -77,6 +95,7 @@ import {
   claimManagedProfile,
   HandoverError,
   previewHandover,
+  usableHandoverForToken,
 } from "@/lib/managed-profiles/handover";
 import { notifyGuardiansOfHandover } from "@/lib/managed-profiles/handover-notify";
 
@@ -105,6 +124,12 @@ const PREVIEW = {
 beforeEach(() => {
   vi.resetAllMocks();
   vi.mocked(isOidcOnly).mockReturnValue(false);
+  vi.mocked(isOidcConfigured).mockReturnValue(false);
+  vi.mocked(getOidcConfig).mockReturnValue(null);
+  vi.mocked(usableHandoverForToken).mockResolvedValue({
+    handoverId: "h1",
+    profileId: "p1",
+  });
   vi.mocked(getSession).mockResolvedValue(null as never);
   vi.mocked(checkAuthSurfaceRateLimit).mockResolvedValue({
     allowed: true,
@@ -131,19 +156,62 @@ async function errorCode(res: Response): Promise<unknown> {
   return body.meta?.errorCode;
 }
 
+const OIDC_CONFIG = {
+  issuerUrl: "https://idp.example.test",
+  clientId: "c",
+  clientSecret: "s",
+  scopes: "openid email",
+  buttonLabel: "Example ID",
+};
+
+function singleSignOnOnly(): void {
+  vi.mocked(isOidcOnly).mockReturnValue(true);
+  vi.mocked(isOidcConfigured).mockReturnValue(true);
+  vi.mocked(getOidcConfig).mockReturnValue(OIDC_CONFIG);
+}
+
+describe("on a single-sign-on-only instance", () => {
+  it("refuses the password claim before anything else", async () => {
+    singleSignOnOnly();
+    vi.mocked(getSession).mockResolvedValue({ user: { id: "u1" } } as never);
+    const res = await claim(post("/api/auth/claim", BODY));
+    expect(res.status).toBe(403);
+    expect(await errorCode(res)).toBe("profile_claim.oidc_only_unsupported");
+    expect(previewHandover).not.toHaveBeenCalled();
+  });
+
+  it("answers the preview and names single sign-on as the only way in", async () => {
+    singleSignOnOnly();
+    const res = await preview(
+      post("/api/auth/claim/preview", { token: TOKEN }),
+    );
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as { data: Record<string, unknown> };
+    expect(body.data.methods).toEqual(["sso"]);
+    expect(body.data.ssoProviderName).toBe("Example ID");
+  });
+});
+
 describe.each([
   ["claim", claim, "/api/auth/claim", BODY],
   ["preview", preview, "/api/auth/claim/preview", { token: TOKEN }],
+  ["sso", claimSso, "/api/auth/claim/sso", { token: TOKEN, username: "alex" }],
 ] as const)(
   "POST %s — refusals that read no token",
-  (_n, route, path, body) => {
-    it("refuses on a single-sign-on-only instance before anything else", async () => {
-      vi.mocked(isOidcOnly).mockReturnValue(true);
-      vi.mocked(getSession).mockResolvedValue({ user: { id: "u1" } } as never);
-      const res = await route(post(path, body));
-      expect(res.status).toBe(403);
-      expect(await errorCode(res)).toBe("profile_claim.oidc_only_unsupported");
-      expect(previewHandover).not.toHaveBeenCalled();
+  (name, route, path, body) => {
+    beforeEach(() => {
+      if (name === "sso") {
+        vi.mocked(isOidcConfigured).mockReturnValue(true);
+        vi.mocked(getOidcConfig).mockReturnValue(OIDC_CONFIG);
+      }
+      if (name === "sso") {
+        vi.mocked(usableHandoverForToken).mockImplementation(async () => {
+          // Routes the sso start through the same "link" spy the other two
+          // use, so one set of assertions covers all three.
+          const found = await previewHandover(TOKEN);
+          return found ? { handoverId: "h1", profileId: "p1" } : null;
+        });
+      }
     });
 
     it("refuses over a live session before spending the rate limit", async () => {
@@ -234,7 +302,7 @@ describe("POST /api/auth/claim", () => {
     expect(res.status).toBe(201);
     expect(claimManagedProfile).toHaveBeenCalledWith(
       expect.objectContaining({
-        rawToken: TOKEN,
+        link: { rawToken: TOKEN },
         username: "alex",
         email: "alex@example.test",
         passwordHash: "argon2-hash",
@@ -260,7 +328,101 @@ describe("POST /api/auth/claim/preview", () => {
       "displayName",
       "expiresAt",
       "guardians",
+      "methods",
+      "ssoProviderName",
     ]);
+    expect(body.data.methods).toEqual(["password"]);
+    expect(body.data.ssoProviderName).toBeNull();
     expect(body.data.expiresAt).toBe("2030-01-01T00:00:00.000Z");
   });
 });
+
+describe("POST /api/auth/claim/sso", () => {
+  const SSO_BODY = { token: TOKEN, username: "alex" };
+
+  beforeEach(() => {
+    vi.mocked(isOidcConfigured).mockReturnValue(true);
+    vi.mocked(getOidcConfig).mockReturnValue(OIDC_CONFIG);
+  });
+
+  it("answers 404 sso_unavailable without an identity provider, before anything else", async () => {
+    vi.mocked(isOidcConfigured).mockReturnValue(false);
+    vi.mocked(getSession).mockResolvedValue({ user: { id: "u1" } } as never);
+    const res = await claimSso(post("/api/auth/claim/sso", SSO_BODY));
+    expect(res.status).toBe(404);
+    expect(await errorCode(res)).toBe("profile_claim.sso_unavailable");
+    expect(checkAuthSurfaceRateLimit).not.toHaveBeenCalled();
+    expect(usableHandoverForToken).not.toHaveBeenCalled();
+  });
+
+  it("works on a single-sign-on-only instance", async () => {
+    singleSignOnOnly();
+    const res = await claimSso(post("/api/auth/claim/sso", SSO_BODY));
+    expect(res.status).toBe(200);
+  });
+
+  it("draws on the password claim's bucket", async () => {
+    await claimSso(post("/api/auth/claim/sso", SSO_BODY));
+    expect(checkAuthSurfaceRateLimit).toHaveBeenCalledWith(
+      expect.anything(),
+      "auth:claim",
+      5,
+      15 * 60 * 1000,
+    );
+  });
+
+  it("checks the link before the username probe", async () => {
+    vi.mocked(usableHandoverForToken).mockResolvedValue(null);
+    vi.mocked(prisma.user.findUnique).mockResolvedValue({ id: "x" } as never);
+    const res = await claimSso(post("/api/auth/claim/sso", SSO_BODY));
+    expect(res.status).toBe(404);
+    expect(await errorCode(res)).toBe("profile_claim.invalid");
+    expect(prisma.user.findUnique).not.toHaveBeenCalled();
+    expect(res.headers.get("set-cookie")).toBeNull();
+  });
+
+  it("answers 409 taken for a username in use, and sets no cookie", async () => {
+    vi.mocked(prisma.user.findUnique).mockResolvedValue({ id: "x" } as never);
+    const res = await claimSso(post("/api/auth/claim/sso", SSO_BODY));
+    expect(res.status).toBe(409);
+    expect(await errorCode(res)).toBe("profile_claim.taken");
+    expect(res.headers.get("set-cookie")).toBeNull();
+  });
+
+  it("refuses an unknown field and a managed- username (strict body)", async () => {
+    for (const body of [
+      { ...SSO_BODY, email: "x@example.test" },
+      { ...SSO_BODY, username: "managed-alex" },
+    ]) {
+      const res = await claimSso(post("/api/auth/claim/sso", body));
+      expect(res.status).toBe(422);
+    }
+    expect(usableHandoverForToken).not.toHaveBeenCalled();
+  });
+
+  it("seals the link's id and the username, never the token, into hl_claim", async () => {
+    const res = await claimSso(post("/api/auth/claim/sso", SSO_BODY));
+    expect(res.status).toBe(200);
+    expect((await res.json()).data).toEqual({
+      redirectTo: "/api/auth/oidc/login?intent=claim",
+    });
+    const cookie = res.headers.get("set-cookie") ?? "";
+    expect(cookie).toMatch(/^hl_claim=/);
+    expect(cookie).toMatch(/Path=\/api\/auth\/oidc/);
+    expect(cookie).toMatch(/HttpOnly/i);
+    expect(cookie).toMatch(/Secure/i);
+    expect(cookie).toMatch(/SameSite=lax/i);
+    expect(cookie).toMatch(/Max-Age=600/);
+    const sealed = decodeURIComponent(cookie.split(";")[0]!.slice(9));
+    const payload = JSON.parse(sealed.replace(/^sealed\(/, "").slice(0, -1));
+    expect(payload.h).toBe("h1");
+    expect(payload.u).toBe("alex");
+    expect(typeof payload.b).toBe("string");
+    expect(cookie).not.toContain(TOKEN);
+    expect(JSON.stringify(vi.mocked(auditLog).mock.calls)).not.toContain(TOKEN);
+    expect(auditLog).toHaveBeenCalledWith(
+      "managed_profile.claim_sso.started",
+      expect.objectContaining({ userId: "p1", details: { handoverId: "h1" } }),
+    );
+  });
+});
diff --git a/src/app/api/auth/claim/preview/route.ts b/src/app/api/auth/claim/preview/route.ts
index 865f7fdfb..b5cb0b924 100644
--- a/src/app/api/auth/claim/preview/route.ts
+++ b/src/app/api/auth/claim/preview/route.ts
@@ -4,6 +4,8 @@
  * access (v1.42, #959). No health data.
  *
  * Anonymous by design: the person opening the link has no credentials yet.
+ * Answered on a single-sign-on-only server too (v1.43): `methods` then names
+ * only `sso`, and the page offers the identity provider.
  * The one-time `hlp_` token travels in the BODY, never in this request's URL,
  * so it never reaches an access log or a wide event's `http.path`. Every
  * unusable link — malformed, unknown, expired, used, withdrawn, a profile no
@@ -15,7 +17,12 @@ import { apiHandler } from "@/lib/api-handler";
 import { apiSuccess, returnAllZodIssues, safeJson } from "@/lib/api-response";
 import { getSession } from "@/lib/auth/session";
 import { annotate } from "@/lib/logging/context";
-import { claimGate, claimInvalid } from "@/lib/managed-profiles/claim-gate";
+import {
+  claimGate,
+  claimInvalid,
+  handoverMethods,
+  ssoProviderName,
+} from "@/lib/managed-profiles/claim-gate";
 import { previewHandover } from "@/lib/managed-profiles/handover";
 import { claimPreviewSchema } from "@/lib/validations/managed-profile-handover";
 
@@ -46,5 +53,9 @@ export const POST = apiHandler(async (request: NextRequest) => {
     displayName: preview.displayName,
     expiresAt: preview.expiresAt.toISOString(),
     guardians: preview.guardians,
+    // v1.43 (#959) — which ways in this server offers, so the page shows the
+    // password form, the provider button, or both.
+    methods: handoverMethods(),
+    ssoProviderName: ssoProviderName(),
   });
 });
diff --git a/src/app/api/auth/claim/route.ts b/src/app/api/auth/claim/route.ts
index 82bba356d..9ca4e09c2 100644
--- a/src/app/api/auth/claim/route.ts
+++ b/src/app/api/auth/claim/route.ts
@@ -104,7 +104,7 @@ export const POST = apiHandler(async (request: NextRequest) => {
   let claimed;
   try {
     claimed = await claimManagedProfile({
-      rawToken: token,
+      link: { rawToken: token },
       username,
       email,
       passwordHash,
diff --git a/src/app/api/auth/claim/sso/route.ts b/src/app/api/auth/claim/sso/route.ts
index 01b591758..21fd4938b 100644
--- a/src/app/api/auth/claim/sso/route.ts
+++ b/src/app/api/auth/claim/sso/route.ts
@@ -1,29 +1,47 @@
 /**
  * `POST /api/auth/claim/sso` — start claiming a managed profile through
- * single sign-on (#959).
+ * single sign-on (v1.43, #959).
  *
- * Anonymous, like the password claim beside it. `{ token, username }`: the
- * link is checked and the username reserved for the round trip, a sealed
- * `hl_claim` cookie (10 minutes, `httpOnly`, `Secure` through
- * `shouldEmitSecureCookie()`) carries `{ handoverId, username, exp }`, and the
- * answer names where to go: `{ redirectTo: "/api/auth/oidc/login?intent=claim" }`.
- * The OIDC callback binds the identity provider's subject to the profile. The
- * token never rides a URL. Works on a single-sign-on-only server; 404
- * `profile_claim.sso_unavailable` when no identity provider is configured.
+ * Anonymous, like the password claim beside it, and ordered like it:
+ *
+ *   1. refusals that read no token (`claimGate`: no identity provider, a live
+ *      session, the per-address bucket the password claim also draws on);
+ *   2. the body, strictly parsed, and the link checked;
+ *   3. the username probe, so the person learns a name is taken here rather
+ *      than after the round trip to the provider.
+ *
+ * Nothing about the account changes here. The answer is a sealed `hl_claim`
+ * cookie (`claim-cookie.ts`) naming the link by its id and carrying the
+ * chosen username, and `{ redirectTo }` for the browser to navigate to. The
+ * OIDC callback binds the provider's subject to the profile, under the lock,
+ * with every check repeated. The token never rides a URL and never reaches the
+ * cookie.
  */
 import { NextRequest } from "next/server";
 
 import { apiHandler } from "@/lib/api-handler";
-import { returnAllZodIssues, safeJson } from "@/lib/api-response";
+import {
+  apiError,
+  apiSuccess,
+  returnAllZodIssues,
+  safeJson,
+} from "@/lib/api-response";
+import { auditLog } from "@/lib/auth/audit";
 import { getSession } from "@/lib/auth/session";
-import { notImplemented } from "@/lib/http/not-implemented";
-import { claimGate } from "@/lib/managed-profiles/claim-gate";
+import { prisma } from "@/lib/db";
+import { annotate } from "@/lib/logging/context";
+import {
+  CLAIM_SSO_REDIRECT,
+  setClaimCookie,
+} from "@/lib/managed-profiles/claim-cookie";
+import { claimGate, claimInvalid } from "@/lib/managed-profiles/claim-gate";
+import { usableHandoverForToken } from "@/lib/managed-profiles/handover";
 import { claimSsoSchema } from "@/lib/validations/claim-sso";
 
 export const POST = apiHandler(async (request: NextRequest) => {
   // Refuses a signed-in caller and rate-limits the address, as the password
   // claim does.
-  const gate = await claimGate(request, "claim", (await getSession()) !== null);
+  const gate = await claimGate(request, "sso", (await getSession()) !== null);
   if (gate.refusal) return gate.refusal;
 
   const { data: body, error: jsonError } = await safeJson(request, {
@@ -32,6 +50,43 @@ export const POST = apiHandler(async (request: NextRequest) => {
   if (jsonError) return jsonError;
   const parsed = claimSsoSchema.safeParse(body);
   if (!parsed.success) return returnAllZodIssues(parsed.error, 422);
+  const { token, username } = parsed.data;
 
-  return notImplemented("profile_claim.sso.start");
+  // The link first, so the username probe below is no oracle for anybody
+  // without a live link (the password claim's argument).
+  const link = await usableHandoverForToken(token);
+  if (!link) {
+    annotate({
+      action: { name: "profile_claim.sso.start" },
+      meta: { found: false },
+    });
+    return claimInvalid();
+  }
+
+  const existing = await prisma.user.findUnique({
+    where: { username },
+    select: { id: true },
+  });
+  if (existing) {
+    return apiError("Username already taken", 409, {
+      errorCode: "profile_claim.taken",
+    });
+  }
+
+  const response = apiSuccess({ redirectTo: CLAIM_SSO_REDIRECT });
+  setClaimCookie(response, { handoverId: link.handoverId, username });
+
+  // Filed under the record, with no actor: nobody is signed in yet. No token,
+  // no username — the username is not the person's until the claim lands.
+  await auditLog("managed_profile.claim_sso.started", {
+    userId: link.profileId,
+    actorUserId: null,
+    ipAddress: gate.ip,
+    details: { handoverId: link.handoverId },
+  });
+  annotate({
+    action: { name: "profile_claim.sso.start" },
+    meta: { found: true },
+  });
+  return response;
 });
diff --git a/src/app/api/auth/oidc/callback/route.ts b/src/app/api/auth/oidc/callback/route.ts
index 0c090f4a0..f5d155a55 100644
--- a/src/app/api/auth/oidc/callback/route.ts
+++ b/src/app/api/auth/oidc/callback/route.ts
@@ -38,6 +38,17 @@ import {
   mintNativeHandoff,
 } from "@/lib/auth/oidc-native-handoff";
 import { setPendingLinkCookie } from "@/lib/auth/oidc-pending-link";
+import {
+  CLAIM_COOKIE,
+  CLAIM_ERROR_PATH,
+  claimMatchesBinding,
+  clearClaimCookie,
+  readClaimCookie,
+} from "@/lib/managed-profiles/claim-cookie";
+import {
+  completeSsoClaim,
+  SSO_CLAIM_LANDING,
+} from "@/lib/managed-profiles/claim-sso";
 
 const LOGIN_ERROR_URL = "/auth/login";
 
@@ -102,6 +113,14 @@ interface StoredOidcState {
   native?: boolean;
   /** The app's S256 PKCE challenge, bound at login; verified at the exchange. */
   appCodeChallenge?: string;
+  /**
+   * v1.43 (#959) — set only when the login carried `intent=claim` and a valid
+   * `hl_claim` cookie. The callback then claims the managed profile the
+   * cookie names instead of signing in, and only when the cookie still
+   * carries this exact binding. Web only: never together with `native`.
+   */
+  intent?: "claim";
+  claimBinding?: string;
 }
 
 function isStoredOidcState(value: unknown): value is StoredOidcState {
@@ -122,6 +141,13 @@ function isStoredOidcState(value: unknown): value is StoredOidcState {
     if (v.native !== true) return false;
     if (typeof v.appCodeChallenge !== "string") return false;
   }
+  if (v.intent !== undefined) {
+    if (v.intent !== "claim") return false;
+    if (typeof v.claimBinding !== "string" || v.claimBinding.length === 0) {
+      return false;
+    }
+    if (v.native !== undefined) return false;
+  }
   return true;
 }
 
@@ -153,12 +179,24 @@ export const GET = apiHandler(async (req: NextRequest) => {
     }
   }
   const isNative = stored?.native === true;
+  const isClaim = stored?.intent === "claim";
 
   // Native-aware error redirect: the custom scheme for a decrypted native flow,
   // the web login page otherwise. Deletes the single-use state cookie either
   // way. An error redirect carries no code, ticket, or session, so routing it
   // to the scheme leaks nothing (spec §1).
   const failRedirect = (reason: string): NextResponse => {
+    if (isClaim) {
+      // A refused claim returns to the claim page, never the sign-in page,
+      // and takes the claim cookie with it: the person starts again from the
+      // link, which is still theirs to use if the refusal left it open.
+      const response = NextResponse.redirect(
+        oidcAppUrl(`${CLAIM_ERROR_PATH}?error=${reason}`),
+      );
+      deleteStateCookie(response);
+      clearClaimCookie(response);
+      return response;
+    }
     if (isNative) {
       const response = NextResponse.redirect(
         buildNativeCallbackUrl({ error: reason }),
@@ -198,7 +236,9 @@ export const GET = apiHandler(async (req: NextRequest) => {
   // Broken round-trip / missing-or-unreadable state blob / CSRF mismatch all
   // resolve to the web `oidc_state` fallback: the flow is fundamentally broken
   // and there is nothing safe to hand the app (spec §1 web-fallback).
-  if (!code || !state) return errorRedirect("oidc_state");
+  if (!code || !state) {
+    return isClaim ? failRedirect("oidc_state") : errorRedirect("oidc_state");
+  }
 
   if (!stored) {
     // Defence-in-depth idempotency for a duplicated callback. The single-use
@@ -240,7 +280,32 @@ export const GET = apiHandler(async (req: NextRequest) => {
     !timingSafeEqual(Buffer.from(state), Buffer.from(stored.state))
   ) {
     annotate({ meta: { reason: "csrf" } });
-    return errorRedirect("oidc_state");
+    return isClaim ? failRedirect("oidc_state") : errorRedirect("oidc_state");
+  }
+
+  // v1.43 (#959) — a claim is admitted only with its own sealed cookie, still
+  // bound to this sign-in, and never over a live session (the claim routes'
+  // rule: a Guardian finishing their own link would otherwise be signed out
+  // of their account and into the person's). Both before the code is
+  // redeemed, so a refused claim spends nothing at the provider.
+  const pendingClaim = isClaim
+    ? readClaimCookie(req.cookies.get(CLAIM_COOKIE)?.value)
+    : null;
+  if (isClaim) {
+    if (
+      !pendingClaim ||
+      !claimMatchesBinding(pendingClaim, stored.claimBinding as string)
+    ) {
+      annotate({ meta: { reason: "claim_cookie" } });
+      return failRedirect("claim_expired");
+    }
+    const liveSession = await validateSessionFromCookieValue(
+      req.cookies.get(SESSION_COOKIE_NAME)?.value,
+    );
+    if (liveSession) {
+      annotate({ meta: { reason: "claim_signed_in" } });
+      return failRedirect("claim_signed_in");
+    }
   }
 
   try {
@@ -274,6 +339,28 @@ export const GET = apiHandler(async (req: NextRequest) => {
       emailVerified = userinfo.emailVerified;
     }
 
+    // v1.43 (#959) — the claim. The provider has authenticated somebody; bind
+    // that subject to the profile the cookie names, or refuse. Never falls
+    // through to the sign-in below: a refused claim signs nobody in, and
+    // provisions and links nothing.
+    if (pendingClaim) {
+      const outcome = await completeSsoClaim({
+        handoverId: pendingClaim.handoverId,
+        username: pendingClaim.username,
+        issuer: metadata.issuer,
+        sub: identity.sub,
+        email,
+        emailVerified,
+        ipAddress: ip,
+        userAgent: req.headers.get("user-agent"),
+      });
+      if (!outcome.ok) return failRedirect(outcome.reason);
+      const response = NextResponse.redirect(oidcAppUrl(SSO_CLAIM_LANDING));
+      deleteStateCookie(response);
+      clearClaimCookie(response);
+      return response;
+    }
+
     // (1) Durable identity match — the (issuer, sub) pair stamped on a
     // previous login, link, or provision. Email is deliberately NOT
     // consulted here: after the stamp it is a display field, so an
diff --git a/src/app/api/auth/oidc/login/__tests__/route.test.ts b/src/app/api/auth/oidc/login/__tests__/route.test.ts
index 38c13a58f..1d9fc1584 100644
--- a/src/app/api/auth/oidc/login/__tests__/route.test.ts
+++ b/src/app/api/auth/oidc/login/__tests__/route.test.ts
@@ -214,3 +214,50 @@ describe("GET /api/auth/oidc/login", () => {
     });
   });
 });
+
+describe("GET /api/auth/oidc/login?intent=claim (v1.43, #959)", () => {
+  beforeEach(() => {
+    vi.mocked(getOidcConfig).mockReturnValue({
+      issuerUrl: "https://idp.example.com",
+      clientId: "client-1",
+      clientSecret: "secret-1",
+      scopes: "openid email profile",
+      buttonLabel: "SSO",
+    });
+  });
+
+  it("refuses a claim on the native leg with oidc_invalid_request", async () => {
+    const res = await GET(
+      makeRequest(
+        `/api/auth/oidc/login?intent=claim&client=native&code_challenge=${"a".repeat(43)}`,
+      ),
+    );
+    expect(res.headers.get("location")).toMatch(
+      /^healthlog:\/\/oidc-callback\?error=oidc_invalid_request$/,
+    );
+    expect(discoverOidcMetadata).not.toHaveBeenCalled();
+    expect(res.cookies.get("oidc_auth_state")).toBeUndefined();
+  });
+
+  it("sends a claim without its cookie back to the claim page, before the provider", async () => {
+    const res = await GET(makeRequest("/api/auth/oidc/login?intent=claim"));
+    expect(res.headers.get("location")).toBe(
+      "https://healthlog.example.com/auth/claim?error=claim_expired",
+    );
+    expect(discoverOidcMetadata).not.toHaveBeenCalled();
+    expect(res.cookies.get("oidc_auth_state")).toBeUndefined();
+  });
+
+  it("routes the claim's other failures to the claim page too", async () => {
+    vi.mocked(checkAuthSurfaceRateLimit).mockResolvedValue({
+      allowed: false,
+      remaining: 0,
+      reset: 0,
+      ip: "1.2.3.4",
+    } as never);
+    const res = await GET(makeRequest("/api/auth/oidc/login?intent=claim"));
+    expect(res.headers.get("location")).toBe(
+      "https://healthlog.example.com/auth/claim?error=oidc_rate_limited",
+    );
+  });
+});
diff --git a/src/app/api/auth/oidc/login/route.ts b/src/app/api/auth/oidc/login/route.ts
index 9e55d9c0a..dd0dc1cc8 100644
--- a/src/app/api/auth/oidc/login/route.ts
+++ b/src/app/api/auth/oidc/login/route.ts
@@ -21,6 +21,12 @@ import {
   OIDC_STATE_COOKIE_PATH,
   OIDC_STATE_TTL_MS,
 } from "@/lib/auth/oidc-cookie";
+import {
+  CLAIM_COOKIE,
+  CLAIM_ERROR_PATH,
+  readClaimCookie,
+} from "@/lib/managed-profiles/claim-cookie";
+import { isHandoverUsable } from "@/lib/managed-profiles/handover";
 
 /**
  * Redirects the browser to the configured OIDC provider's authorization
@@ -40,16 +46,34 @@ export const GET = apiHandler(async (req: NextRequest) => {
   // nothing — an error redirect carries no code, ticket, or session (spec §1).
   const isNative = req.nextUrl.searchParams.get("client") === "native";
   const appCodeChallenge = req.nextUrl.searchParams.get("code_challenge");
+  // v1.43 (#959) — `intent=claim`: the sign-in claims a managed profile. The
+  // link and the username ride the sealed `hl_claim` cookie set by
+  // `POST /api/auth/claim/sso`, never this URL; the intent only says to look
+  // for it. Web only.
+  const isClaim = req.nextUrl.searchParams.get("intent") === "claim";
 
   // Native-aware error redirect: the scheme for a native start, the web login
-  // page otherwise. Machine-readable `error=<reason>` in both.
+  // page otherwise, and the claim page for a claim. Machine-readable
+  // `error=<reason>` in all three.
   const loginError = (reason: string): NextResponse => {
     if (isNative) {
       return NextResponse.redirect(buildNativeCallbackUrl({ error: reason }));
     }
+    if (isClaim) {
+      return NextResponse.redirect(
+        oidcAppUrl(`${CLAIM_ERROR_PATH}?error=${reason}`),
+      );
+    }
     return NextResponse.redirect(oidcAppUrl(`/auth/login?error=${reason}`));
   };
 
+  // The app never claims a profile: the claim runs in a browser, and the new
+  // owner signs in to the app afterwards through the ordinary native leg.
+  if (isClaim && isNative) {
+    annotate({ meta: { reason: "claim_on_native" } });
+    return loginError("oidc_invalid_request");
+  }
+
   const config = getOidcConfig();
   if (!config) {
     return loginError("oidc_disabled");
@@ -90,6 +114,19 @@ export const GET = apiHandler(async (req: NextRequest) => {
     return loginError("oidc_invalid_request");
   }
 
+  // A claim needs its sealed cookie and a link that is still usable. Checked
+  // here, after the rate limit, so a dead link is told before the round trip
+  // to the provider; the callback checks both again.
+  let claimBinding: string | null = null;
+  if (isClaim) {
+    const claim = readClaimCookie(req.cookies.get(CLAIM_COOKIE)?.value);
+    if (!claim || !(await isHandoverUsable(claim.handoverId))) {
+      annotate({ meta: { reason: "claim_expired" } });
+      return loginError("claim_expired");
+    }
+    claimBinding = claim.binding;
+  }
+
   let metadata;
   try {
     metadata = await discoverOidcMetadata(config);
@@ -104,10 +141,12 @@ export const GET = apiHandler(async (req: NextRequest) => {
   const codeChallenge = s256Challenge(codeVerifier);
 
   // `next` is a post-login WEB path, meaningless to the app — pin it to "/" on
-  // the native branch (one less input to sanitize).
-  const safeNext = isNative
-    ? "/"
-    : sanitizeOidcNextPath(req.nextUrl.searchParams.get("next"), req.url);
+  // the native branch (one less input to sanitize). A claim lands on a fixed
+  // page the callback names, so its `next` is pinned too.
+  const safeNext =
+    isNative || isClaim
+      ? "/"
+      : sanitizeOidcNextPath(req.nextUrl.searchParams.get("next"), req.url);
 
   const redirectUri = getOidcRedirectUri();
   const authorizationUrl = buildAuthorizationUrl({
@@ -133,6 +172,12 @@ export const GET = apiHandler(async (req: NextRequest) => {
       ...(isNative
         ? { native: true as const, appCodeChallenge: appCodeChallenge! }
         : {}),
+      // The claim's binding, inside the same tamper-authenticated blob: the
+      // callback admits the claim only when the cookie it carries back holds
+      // this exact value.
+      ...(claimBinding !== null
+        ? { intent: "claim" as const, claimBinding }
+        : {}),
     }),
   );
   response.cookies.set(OIDC_STATE_COOKIE, cookiePayload, {
diff --git a/src/app/api/managed-profiles/[id]/handover/route.ts b/src/app/api/managed-profiles/[id]/handover/route.ts
index 3c702f200..1177988b2 100644
--- a/src/app/api/managed-profiles/[id]/handover/route.ts
+++ b/src/app/api/managed-profiles/[id]/handover/route.ts
@@ -30,8 +30,8 @@ import {
   safeJson,
 } from "@/lib/api-response";
 import { buildHandoverUrl } from "@/lib/auth/handover-token";
-import { isOidcOnly } from "@/lib/auth/oidc";
 import { annotate } from "@/lib/logging/context";
+import { handoverMethods } from "@/lib/managed-profiles/claim-gate";
 import {
   createHandover,
   HandoverError,
@@ -49,16 +49,15 @@ const notFound = () =>
   });
 
 /**
- * v1.42 refuses the handover on a single-sign-on-only instance: the claim
- * sets a password, and such an instance admits none. The SSO arm of the claim
- * is a separate piece of work.
+ * A link nobody could redeem is not minted. Since v1.43 the person can claim
+ * through the identity provider, so a single-sign-on-only server hands over
+ * too; this answers only when neither a password nor single sign-on is
+ * possible, which a working configuration never produces.
  */
-const oidcOnlyRefusal = () =>
-  apiError(
-    "Handing over a profile is not available while this server allows single sign-on only",
-    403,
-    { errorCode: "profile_claim.oidc_only_unsupported" },
-  );
+const noMethodRefusal = () =>
+  apiError("Handing over a profile is not available on this server", 403, {
+    errorCode: "profile_claim.oidc_only_unsupported",
+  });
 
 /** Same ceiling as creating, editing and inviting: ten an hour per caller. */
 const CREATE_LIMIT = 10;
@@ -79,8 +78,10 @@ export const GET = apiHandler(
       action: { name: "managed_profile.handover.read" },
       meta: { profile_id: id, open: status.open !== null },
     });
+    const methods = handoverMethods();
     return apiSuccess({
-      available: !isOidcOnly(),
+      available: methods.length > 0,
+      methods,
       open: status.open
         ? {
             id: status.open.id,
@@ -107,7 +108,7 @@ export const POST = apiHandler(
     if (!rateLimit.allowed) {
       return apiError("Too many handover links, try again later", 429);
     }
-    if (isOidcOnly()) return oidcOnlyRefusal();
+    if (handoverMethods().length === 0) return noMethodRefusal();
     const { id } = await params;
 
     const { data: body, error: jsonError } = await safeJson(request, {
diff --git a/src/lib/managed-profiles/claim-cookie.ts b/src/lib/managed-profiles/claim-cookie.ts
new file mode 100644
index 000000000..f99e05b55
--- /dev/null
+++ b/src/lib/managed-profiles/claim-cookie.ts
@@ -0,0 +1,111 @@
+/**
+ * v1.43 (#959) — the sealed cookie that carries a single-sign-on claim through
+ * the round trip to the identity provider.
+ *
+ * `POST /api/auth/claim/sso` checks the handover token and the chosen username
+ * and seals `{ handoverId, username, binding, exp }` into `hl_claim`. The
+ * token itself never leaves that request: the cookie names the link by its
+ * id, and it can only have been minted by a request that held the token.
+ *
+ *   - AES-256-GCM with the at-rest key (`encrypt`), so it can be neither read
+ *     nor forged, and it carries its own expiry on top of `Max-Age`.
+ *   - `HttpOnly`, `SameSite=Lax` (the browser returns from the provider on a
+ *     cross-site top-level GET and must carry it), `Secure` through
+ *     `shouldEmitSecureCookie()`.
+ *   - Scoped to `/api/auth/oidc`, the only two routes that read it.
+ *   - `binding` is a fresh random value per start. The login copies it into
+ *     the encrypted OIDC state, and the callback admits the claim only when
+ *     the cookie and the state carry the same one, so a claim cookie from a
+ *     different start (another tab, an older attempt) cannot be paired with
+ *     this sign-in.
+ */
+import { randomBytes, timingSafeEqual } from "node:crypto";
+import type { NextResponse } from "next/server";
+
+import { decrypt, encrypt } from "@/lib/crypto";
+import { shouldEmitSecureCookie } from "@/lib/auth/secure-cookie";
+
+export const CLAIM_COOKIE = "hl_claim";
+export const CLAIM_COOKIE_PATH = "/api/auth/oidc";
+export const CLAIM_COOKIE_TTL_MS = 10 * 60 * 1000;
+
+/**
+ * Where `POST /api/auth/claim/sso` sends the browser next. A fixed path:
+ * nothing caller-supplied ever reaches a redirect on this flow.
+ */
+export const CLAIM_SSO_REDIRECT = "/api/auth/oidc/login?intent=claim";
+
+/** Where every refusal on the single-sign-on claim lands, with `?error=`. */
+export const CLAIM_ERROR_PATH = "/auth/claim";
+
+export interface PendingSsoClaim {
+  handoverId: string;
+  username: string;
+  binding: string;
+}
+
+/** Attach a freshly sealed claim to a response. Returns what was sealed. */
+export function setClaimCookie(
+  response: NextResponse,
+  claim: { handoverId: string; username: string },
+  now: number = Date.now(),
+): PendingSsoClaim {
+  const binding = randomBytes(32).toString("base64url");
+  const sealed = encrypt(
+    JSON.stringify({
+      h: claim.handoverId,
+      u: claim.username,
+      b: binding,
+      e: now + CLAIM_COOKIE_TTL_MS,
+    }),
+  );
+  response.cookies.set(CLAIM_COOKIE, sealed, {
+    httpOnly: true,
+    secure: shouldEmitSecureCookie(),
+    sameSite: "lax",
+    maxAge: Math.floor(CLAIM_COOKIE_TTL_MS / 1000),
+    path: CLAIM_COOKIE_PATH,
+  });
+  return { ...claim, binding };
+}
+
+/** The claim a cookie value carries, or null when absent, forged or expired. */
+export function readClaimCookie(
+  raw: string | undefined,
+  now: number = Date.now(),
+): PendingSsoClaim | null {
+  if (!raw) return null;
+  try {
+    const parsed = JSON.parse(decrypt(raw)) as Record<string, unknown>;
+    if (
+      typeof parsed.h !== "string" ||
+      parsed.h.length === 0 ||
+      typeof parsed.u !== "string" ||
+      parsed.u.length === 0 ||
+      typeof parsed.b !== "string" ||
+      parsed.b.length === 0 ||
+      typeof parsed.e !== "number" ||
+      parsed.e <= now
+    ) {
+      return null;
+    }
+    return { handoverId: parsed.h, username: parsed.u, binding: parsed.b };
+  } catch {
+    return null;
+  }
+}
+
+/** Whether a claim belongs to the sign-in whose state carries `binding`. */
+export function claimMatchesBinding(
+  claim: PendingSsoClaim,
+  binding: string,
+): boolean {
+  const a = Buffer.from(claim.binding);
+  const b = Buffer.from(binding);
+  return a.length === b.length && timingSafeEqual(a, b);
+}
+
+/** Expire the claim cookie on a response; the path must repeat the set's. */
+export function clearClaimCookie(response: NextResponse): void {
+  response.cookies.delete({ name: CLAIM_COOKIE, path: CLAIM_COOKIE_PATH });
+}
diff --git a/src/lib/managed-profiles/claim-gate.ts b/src/lib/managed-profiles/claim-gate.ts
index 3c1a4ec47..2173fd6e0 100644
--- a/src/lib/managed-profiles/claim-gate.ts
+++ b/src/lib/managed-profiles/claim-gate.ts
@@ -1,11 +1,14 @@
 /**
- * v1.42 (#959) — the refusals both anonymous claim routes give before they
- * look at a token, in one place so the preview and the claim cannot disagree.
+ * v1.42 (#959) — the refusals the anonymous claim routes give before they look
+ * at a token, in one place so the preview and the claims cannot disagree.
  *
  * Order matters and is the register route's order:
  *
- *   1. A single-sign-on-only instance admits no password, and the claim sets
- *      one, so the handover is refused outright in v1.42.
+ *   1. What this server cannot do at all. The password claim sets a password,
+ *      so a single-sign-on-only instance refuses it outright; the
+ *      single-sign-on claim (v1.43) needs an identity provider, and answers
+ *      404 `profile_claim.sso_unavailable` without one. The preview is
+ *      refused by neither: it is what tells the page which way is open.
  *   2. A signed-in caller is refused with 409. Claiming creates a session for
  *      the new owner, and a Guardian testing their own link would otherwise be
  *      signed out of their account and into the person's. Refused before the
@@ -13,12 +16,14 @@
  *      would lock out the person the link is for, who may share the address.
  *   3. The per-address rate limit, the anonymous auth-surface pattern: a
  *      misconfigured proxy trust chain collapses every caller into one tight
- *      bucket rather than one free-for-all.
+ *      bucket rather than one free-for-all. Both claims share one bucket, so
+ *      switching between them buys no extra attempts.
  */
 import { NextResponse } from "next/server";
 
 import { apiError } from "@/lib/api-response";
-import { isOidcOnly } from "@/lib/auth/oidc";
+import { getOidcConfig, isOidcConfigured, isOidcOnly } from "@/lib/auth/oidc";
+import type { HandoverMethod } from "@/lib/validations/claim-sso";
 import { annotate } from "@/lib/logging/context";
 import { checkAuthSurfaceRateLimit, rateLimitHeaders } from "@/lib/rate-limit";
 
@@ -29,9 +34,28 @@ export function claimInvalid() {
   });
 }
 
+/**
+ * v1.43 (#959) — how a handover link can be redeemed on this server: with a
+ * password unless the server allows single sign-on only, and through the
+ * identity provider whenever one is configured. Never empty in practice
+ * (`OIDC_ONLY` only takes effect with a configured provider), but the Guardian
+ * side refuses on an empty list rather than assuming it.
+ */
+export function handoverMethods(): HandoverMethod[] {
+  const methods: HandoverMethod[] = [];
+  if (!isOidcOnly()) methods.push("password");
+  if (isOidcConfigured()) methods.push("sso");
+  return methods;
+}
+
+/** The provider's name for the "Continue with …" button, or null. */
+export function ssoProviderName(): string | null {
+  return getOidcConfig()?.buttonLabel ?? null;
+}
+
 export async function claimGate(
   request: Request,
-  surface: "preview" | "claim",
+  surface: "preview" | "claim" | "sso",
   /**
    * Whether the request carries a live session. Resolved by the ROUTE with
    * `getSession()` and handed in, rather than resolved here: a session read
@@ -41,15 +65,24 @@ export async function claimGate(
    */
   signedIn: boolean,
 ): Promise<{ refusal: Response } | { refusal: null; ip: string | null }> {
-  if (isOidcOnly()) {
+  if (surface === "claim" && isOidcOnly()) {
     return {
       refusal: apiError(
-        "Taking over a profile is not available while this server allows single sign-on only",
+        "This server allows single sign-on only; take the profile over through the identity provider",
         403,
         { errorCode: "profile_claim.oidc_only_unsupported" },
       ),
     };
   }
+  if (surface === "sso" && !isOidcConfigured()) {
+    return {
+      refusal: apiError(
+        "Single sign-on is not configured on this server",
+        404,
+        { errorCode: "profile_claim.sso_unavailable" },
+      ),
+    };
+  }
 
   if (signedIn) {
     annotate({
diff --git a/src/lib/managed-profiles/claim-sso.ts b/src/lib/managed-profiles/claim-sso.ts
new file mode 100644
index 000000000..6fbf8c843
--- /dev/null
+++ b/src/lib/managed-profiles/claim-sso.ts
@@ -0,0 +1,178 @@
+/**
+ * v1.43 (#959) — the callback's half of claiming a managed profile through
+ * single sign-on: the identity provider has authenticated somebody, the
+ * sealed claim cookie names the link and the chosen username, and this binds
+ * the one to the other.
+ *
+ * The refusals, in order, each one before anything about any account changes:
+ *
+ *   1. `claim_expired` — the link is no longer usable (used, withdrawn,
+ *      replaced, expired, its creator gone, the profile no longer managed).
+ *   2. `claim_sso_subject_bound` — the provider's `(issuer, sub)` already
+ *      signs in to an account here. One subject, one account: binding it a
+ *      second time would make the sign-in ambiguous, and re-pointing it would
+ *      hand the existing account's sign-in to the claimed profile.
+ *   3. `claim_email_taken` — the provider vouches (`email_verified: true`) for
+ *      an address another account holds. An unverified address is not used at
+ *      all: the account is created without one rather than with a claim
+ *      nobody checked.
+ *   4. `claim_username_taken` — the username chosen at the start was taken in
+ *      the ten minutes since.
+ *
+ * Then `claimManagedProfile` — the password claim's own transaction, under
+ * the managed-profile lock, with every link check repeated — sets the
+ * username, the address, no password and the subject; applies each Guardian's
+ * proposed access; resets disclaimer, onboarding and AI consent. A race lost
+ * at the unique indexes is re-read and answered as the refusal it turned out
+ * to be. The session is minted only after that commit.
+ */
+import { auditLog } from "@/lib/auth/audit";
+import { recordSignInDevice } from "@/lib/auth/login-alert";
+import { createSession } from "@/lib/auth/session";
+import { prisma } from "@/lib/db";
+import { annotate } from "@/lib/logging/context";
+import {
+  claimManagedProfile,
+  HandoverError,
+  isHandoverUsable,
+} from "@/lib/managed-profiles/handover";
+import { notifyGuardiansOfHandover } from "@/lib/managed-profiles/handover-notify";
+
+export type SsoClaimRefusal =
+  | "claim_expired"
+  | "claim_sso_subject_bound"
+  | "claim_email_taken"
+  | "claim_username_taken";
+
+export type SsoClaimOutcome =
+  { ok: true; profileId: string } | { ok: false; reason: SsoClaimRefusal };
+
+/** Where the new owner lands: the decision about each former Guardian. */
+export const SSO_CLAIM_LANDING = "/onboarding/handover";
+
+interface SsoClaimInput {
+  handoverId: string;
+  username: string;
+  issuer: string;
+  sub: string;
+  email: string | null;
+  emailVerified: boolean | undefined;
+  ipAddress: string;
+  userAgent: string | null;
+}
+
+/** Which of the three unique values is held by an account already. */
+async function collision(input: {
+  issuer: string;
+  sub: string;
+  email: string | null;
+  username: string;
+}): Promise<SsoClaimRefusal | null> {
+  const bound = await prisma.user.findFirst({
+    where: { oidcIssuer: input.issuer, oidcSub: input.sub },
+    select: { id: true },
+  });
+  if (bound) {
+    // Under the account that holds the subject, so its owner's history shows
+    // that somebody tried to bind their sign-in to a second record.
+    await auditLog("auth.oidc.claim_conflict", {
+      userId: bound.id,
+      actorUserId: null,
+    });
+    return "claim_sso_subject_bound";
+  }
+  if (input.email) {
+    const byEmail = await prisma.user.findFirst({
+      where: { email: { equals: input.email, mode: "insensitive" } },
+      select: { id: true },
+    });
+    if (byEmail) return "claim_email_taken";
+  }
+  const byName = await prisma.user.findUnique({
+    where: { username: input.username },
+    select: { id: true },
+  });
+  return byName ? "claim_username_taken" : null;
+}
+
+export async function completeSsoClaim(
+  input: SsoClaimInput,
+): Promise<SsoClaimOutcome> {
+  const refuse = (reason: SsoClaimRefusal): SsoClaimOutcome => {
+    annotate({
+      action: { name: "profile_claim.sso.refused" },
+      meta: { reason },
+    });
+    return { ok: false, reason };
+  };
+
+  if (!(await isHandoverUsable(input.handoverId))) {
+    return refuse("claim_expired");
+  }
+
+  const email =
+    input.email && input.emailVerified === true
+      ? input.email.toLowerCase()
+      : null;
+  const identity = {
+    issuer: input.issuer,
+    sub: input.sub,
+    email,
+    username: input.username,
+  };
+
+  const early = await collision(identity);
+  if (early) return refuse(early);
+
+  let claimed;
+  try {
+    claimed = await claimManagedProfile({
+      link: { handoverId: input.handoverId },
+      username: input.username,
+      email,
+      passwordHash: null,
+      oidc: { issuer: input.issuer, sub: input.sub },
+      ipAddress: input.ipAddress,
+    });
+  } catch (error) {
+    if (error instanceof HandoverError) {
+      if (error.code === "taken") {
+        // Lost a race at a unique index: say which one.
+        return refuse((await collision(identity)) ?? "claim_username_taken");
+      }
+      return refuse("claim_expired");
+    }
+    throw error;
+  }
+
+  // The new owner starts with onboarding owed, as after the password claim.
+  // The fifth argument stays null: single sign-on is a delegated factor and
+  // must never satisfy a step-up (the OIDC login's rule).
+  await createSession(
+    claimed.profileId,
+    true,
+    input.ipAddress,
+    input.userAgent,
+    null,
+  );
+  void recordSignInDevice({
+    userId: claimed.profileId,
+    ip: input.ipAddress,
+    userAgent: input.userAgent,
+    alertOnNew: false,
+  });
+  await auditLog("auth.claim", {
+    userId: claimed.profileId,
+    actorUserId: null,
+    ipAddress: input.ipAddress,
+    details: { method: "sso" },
+  });
+
+  notifyGuardiansOfHandover("claimed", claimed.displayName, claimed.guardians);
+
+  annotate({
+    action: { name: "profile_claim.complete" },
+    meta: { guardians: claimed.guardians.length, method: "sso" },
+  });
+  return { ok: true, profileId: claimed.profileId };
+}
diff --git a/src/lib/managed-profiles/handover.ts b/src/lib/managed-profiles/handover.ts
index 431bd30d8..b09ce6596 100644
--- a/src/lib/managed-profiles/handover.ts
+++ b/src/lib/managed-profiles/handover.ts
@@ -444,17 +444,19 @@ export interface HandoverPreview {
 }
 
 /**
- * What a link would hand over, or null for every kind of unusable link.
- *
- * Deliberately minimal: the profile's name, the expiry, and who keeps which
- * access. No health data, no date of birth, no usernames and no grant ids —
- * the person holding the link has not proved anything yet beyond holding it.
+ * The open link behind a stored row, or null when it can no longer be
+ * claimed: used, withdrawn, expired, a state this code cannot read, a profile
+ * no longer managed, or a creator who lost access. The one definition of
+ * "usable" the preview, the single-sign-on start and the callback share.
  */
-export async function previewHandover(
-  rawToken: string,
-  now: Date = new Date(),
-): Promise<HandoverPreview | null> {
-  const row = await findHandoverByToken(prisma, rawToken);
+async function usableHandover(
+  row: Awaited<ReturnType<typeof findHandoverByToken>>,
+  now: Date,
+): Promise<{
+  row: NonNullable<Awaited<ReturnType<typeof findHandoverByToken>>>;
+  state: HandoverState;
+  displayName: string | null;
+} | null> {
   if (!row || !isOpen(row, now)) return null;
   const state = parseHandoverState(row.proposalsJson);
   if (!state) return null;
@@ -467,6 +469,26 @@ export async function previewHandover(
   if (!(await isActiveGuardian(prisma, row.profileId, row.createdById, now))) {
     return null;
   }
+  return { row, state, displayName: profile.displayName };
+}
+
+/**
+ * What a link would hand over, or null for every kind of unusable link.
+ *
+ * Deliberately minimal: the profile's name, the expiry, and who keeps which
+ * access. No health data, no date of birth, no usernames and no grant ids —
+ * the person holding the link has not proved anything yet beyond holding it.
+ */
+export async function previewHandover(
+  rawToken: string,
+  now: Date = new Date(),
+): Promise<HandoverPreview | null> {
+  const usable = await usableHandover(
+    await findHandoverByToken(prisma, rawToken),
+    now,
+  );
+  if (!usable) return null;
+  const { row, state } = usable;
 
   const guardians = await prisma.accountGrant.findMany({
     where: { grantorId: row.profileId, ...activeGuardianWhere(now) },
@@ -477,7 +499,7 @@ export async function previewHandover(
     orderBy: { createdAt: "asc" },
   });
   return {
-    displayName: profile.displayName,
+    displayName: usable.displayName,
     expiresAt: row.expiresAt,
     guardians: guardians.map((g) => ({
       displayName: g.grantee.displayName?.trim() || null,
@@ -486,6 +508,39 @@ export async function previewHandover(
   };
 }
 
+/**
+ * v1.43 (#959) — the usable link behind a raw token, or null. The
+ * single-sign-on start seals its id, never the token, into the claim cookie.
+ */
+export async function usableHandoverForToken(
+  rawToken: string,
+  now: Date = new Date(),
+): Promise<{ handoverId: string; profileId: string } | null> {
+  const usable = await usableHandover(
+    await findHandoverByToken(prisma, rawToken),
+    now,
+  );
+  return usable
+    ? { handoverId: usable.row.id, profileId: usable.row.profileId }
+    : null;
+}
+
+/**
+ * v1.43 (#959) — whether the link with this id can still be claimed. The
+ * single-sign-on legs re-ask on every step, so a link withdrawn or replaced
+ * while the person is at the identity provider is refused before any account
+ * changes. The claim re-checks under the lock; this decides nothing alone.
+ */
+export async function isHandoverUsable(
+  handoverId: string,
+  now: Date = new Date(),
+): Promise<boolean> {
+  const row = await prisma.managedProfileHandover.findUnique({
+    where: { id: handoverId },
+  });
+  return (await usableHandover(row, now)) !== null;
+}
+
 export interface ClaimedProfile {
   profileId: string;
   username: string;
@@ -502,15 +557,33 @@ export interface ClaimedProfile {
  * strength: none of that work belongs inside the lock.
  */
 export async function claimManagedProfile(input: {
-  rawToken: string;
+  /**
+   * The link: the raw token from the password form, or (v1.43) the id the
+   * single-sign-on start sealed into the claim cookie after checking the
+   * token itself.
+   */
+  link: { rawToken: string } | { handoverId: string };
   username: string;
-  email: string;
-  passwordHash: string;
+  /** Null on a single-sign-on claim whose provider vouched for no address. */
+  email: string | null;
+  /** Null on a single-sign-on claim: the account signs in at the provider. */
+  passwordHash: string | null;
+  /**
+   * v1.43 (#959) — the identity provider's subject, bound to the profile in
+   * the same transaction. The `(issuer, sub)` unique index refuses a subject
+   * that another account holds, as `taken`.
+   */
+  oidc?: { issuer: string; sub: string };
   ipAddress?: string | null;
   now?: Date;
 }): Promise<ClaimedProfile> {
   const now = input.now ?? new Date();
-  const found = await findHandoverByToken(prisma, input.rawToken);
+  const found =
+    "rawToken" in input.link
+      ? await findHandoverByToken(prisma, input.link.rawToken)
+      : await prisma.managedProfileHandover.findUnique({
+          where: { id: input.link.handoverId },
+        });
   if (!found) throw new HandoverError("invalid");
 
   try {
@@ -551,6 +624,9 @@ export async function claimManagedProfile(input: {
             username: input.username,
             email: input.email,
             passwordHash: input.passwordHash,
+            ...(input.oidc
+              ? { oidcIssuer: input.oidc.issuer, oidcSub: input.oidc.sub }
+              : {}),
             disclaimerAcknowledgedAt: null,
             disclaimerAcknowledgedVersion: null,
             onboardingCompletedAt: null,
@@ -636,6 +712,7 @@ export async function claimManagedProfile(input: {
             })),
             withdrawnInvitations,
             consentsWithdrawn: consent.revoked.length,
+            method: input.oidc ? "sso" : "password",
           },
           client: tx,
         });
diff --git a/src/lib/openapi/error-codes.ts b/src/lib/openapi/error-codes.ts
index b2acc7860..da98bc184 100644
--- a/src/lib/openapi/error-codes.ts
+++ b/src/lib/openapi/error-codes.ts
@@ -350,6 +350,7 @@ export const ERROR_CODE_CATALOGUE: Readonly<Record<string, readonly string[]>> =
     profile_claim: [
       "profile_claim.invalid",
       "profile_claim.oidc_only_unsupported",
+      "profile_claim.sso_unavailable",
       "profile_claim.taken",
     ],
     record_write: ["record_write.rate_limited"],
diff --git a/src/lib/openapi/routes/account-sharing.ts b/src/lib/openapi/routes/account-sharing.ts
index 8f0d1ff35..eeea2cb37 100644
--- a/src/lib/openapi/routes/account-sharing.ts
+++ b/src/lib/openapi/routes/account-sharing.ts
@@ -736,7 +736,7 @@ export const accountSharingPaths: NonNullable<ZodOpenApiObject["paths"]> = {
         },
         "403": {
           description:
-            "The instance allows single sign-on only (`meta.errorCode: profile_claim.oidc_only_unsupported`).",
+            "Neither a password nor single sign-on could redeem a link on this instance (`meta.errorCode: profile_claim.oidc_only_unsupported`). Since v1.43 a single-sign-on-only instance hands over through the identity provider, so a working configuration never answers this.",
           content: { "application/json": { schema: errorEnvelope } },
         },
         "404": {
diff --git a/src/lib/openapi/routes/auth.ts b/src/lib/openapi/routes/auth.ts
index 4103ae971..2556bb59d 100644
--- a/src/lib/openapi/routes/auth.ts
+++ b/src/lib/openapi/routes/auth.ts
@@ -43,7 +43,6 @@ import {
 } from "@/lib/validations/step-up";
 import {
   dataEnvelope,
-  notImplementedResponse,
   profileEmailRateLimitResponse,
   stdResponses,
   errorEnvelope,
@@ -768,7 +767,7 @@ export const authPaths: NonNullable<ZodOpenApiObject["paths"]> = {
       tags: ["Auth"],
       summary: "Claim a managed profile through single sign-on (v1.43)",
       description:
-        "Web only. Checks the link and the username, sets a sealed `hl_claim` cookie (10 minutes, `httpOnly`) and answers where to continue: `/api/auth/oidc/login?intent=claim`. The OIDC callback binds the identity provider's subject to the profile and signs the new owner in; a subject already bound to an account, or a verified email another account holds, is refused there. Works on a single-sign-on-only instance. 404 `profile_claim.invalid` (no usable link) or `profile_claim.sso_unavailable` (no identity provider configured); 409 `profile_claim.taken` (username) or `auth.already_authenticated`; rate-limited per source address like the password claim.",
+        "Web only. Checks the link and the username, sets a sealed `hl_claim` cookie (10 minutes, `httpOnly`) and answers where to continue: `/api/auth/oidc/login?intent=claim`. The OIDC callback binds the identity provider's subject to the profile and signs the new owner in; a subject already bound to an account, or a verified email another account holds, is refused there. Works on a single-sign-on-only instance. When the callback refuses, the browser lands on `/auth/claim?error=<reason>` with reason `claim_expired` (the link was used, withdrawn, replaced or expired in the meantime, or the cookie is missing, expired or belongs to another start), `claim_sso_subject_bound`, `claim_email_taken`, `claim_username_taken` (taken during the round trip), `claim_signed_in`, or one of the sign-in reasons (`oidc_denied`, `oidc_state`, `oidc_failed`, `oidc_rate_limited`, `oidc_disabled`); on success it lands on `/onboarding/handover`, where the new owner decides each guardian's access. The cookie is cleared either way. 404 `profile_claim.invalid` (no usable link) or `profile_claim.sso_unavailable` (no identity provider configured); 409 `profile_claim.taken` (username) or `auth.already_authenticated`; rate-limited per source address like the password claim.",
       requestBody: {
         required: true,
         content: { "application/json": { schema: claimSsoRequest } },
@@ -795,7 +794,6 @@ export const authPaths: NonNullable<ZodOpenApiObject["paths"]> = {
           content: { "application/json": { schema: errorEnvelope } },
         },
         ...stdResponses,
-        ...notImplementedResponse,
       },
     },
   },
@@ -807,7 +805,7 @@ export const authPaths: NonNullable<ZodOpenApiObject["paths"]> = {
       tags: ["Auth"],
       summary: "Preview a managed-profile handover link",
       description:
-        "What a handover link would hand over: the profile's display name, the link's expiry, and each guardian's proposal. Anonymous; the token travels in the body. An unknown, expired, used or withdrawn token, and a link whose creator lost access, all answer the same 404. Rate-limited per source address.",
+        "What a handover link would hand over: the profile's display name, the link's expiry, and each guardian's proposal. Anonymous; the token travels in the body. An unknown, expired, used or withdrawn token, and a link whose creator lost access, all answer the same 404. Rate-limited per source address. Since v1.43 answered on a single-sign-on-only instance too, with `methods` naming the ways in.",
       requestBody: {
         required: true,
         content: { "application/json": { schema: claimPreviewRequest } },
@@ -821,11 +819,6 @@ export const authPaths: NonNullable<ZodOpenApiObject["paths"]> = {
             },
           },
         },
-        "403": {
-          description:
-            "The instance allows single sign-on only (`meta.errorCode: profile_claim.oidc_only_unsupported`).",
-          content: { "application/json": { schema: errorEnvelope } },
-        },
         "404": {
           description:
             "No usable link behind this token (`meta.errorCode: profile_claim.invalid`). Every failure class answers identically.",
@@ -2338,6 +2331,12 @@ export const authPaths: NonNullable<ZodOpenApiObject["paths"]> = {
             .describe(
               "Browser leg only: where to land after login. Sanitised to an in-app path; ignored entirely on the native leg.",
             ),
+          intent: z
+            .literal("claim")
+            .optional()
+            .describe(
+              "v1.43 — `claim` continues a managed-profile claim started at POST /api/auth/claim/sso; the link and username ride the sealed `hl_claim` cookie that call set, never this URL. Without a valid cookie for a still-usable link the start redirects to `/auth/claim?error=claim_expired`. Browser leg only: with `client=native` it fails with `oidc_invalid_request`. `next` is ignored.",
+            ),
           client_proto: z
             .enum(["http", "https"])
             .optional()
@@ -2356,7 +2355,7 @@ export const authPaths: NonNullable<ZodOpenApiObject["paths"]> = {
       responses: {
         "302": {
           description:
-            "Always a redirect. To the provider on success. On failure to `/auth/login?error=<reason>` (browser) or `healthlog://oidc-callback?error=<reason>` (native), where reason is one of `oidc_disabled`, `oidc_rate_limited`, `oidc_invalid_request`, `oidc_failed`, and on the browser leg `insecure_transport` (see `client_proto`). The provider then returns to the callback (not listed here, because the identity provider drives it rather than a client), which ends the native leg at `healthlog://oidc-callback` with `code=<hlh_…>`, with `mfa_ticket=<ticket>&methods=<list>`, or with `error=<reason>`, where reason is one of `oidc_disabled`, `oidc_rate_limited`, `oidc_denied`, `oidc_no_email`, `oidc_email_unverified`, `oidc_identity_conflict`, `oidc_link_required`, `oidc_registration_disabled`, `oidc_failed`. `oidc_link_required` means the provider's e-mail matches an existing local account that is not linked yet; the owner links it by signing in once with that account's own password or passkey in a browser. The reasons are this exact snake_case vocabulary on both legs; a client maps a value it does not know to a generic sign-in failure.",
+            "Always a redirect. To the provider on success. On failure to `/auth/login?error=<reason>` (browser; `/auth/claim?error=<reason>` with `intent=claim`, where `claim_expired` is one more reason) or `healthlog://oidc-callback?error=<reason>` (native), where reason is one of `oidc_disabled`, `oidc_rate_limited`, `oidc_invalid_request`, `oidc_failed`, and on the browser leg `insecure_transport` (see `client_proto`). The provider then returns to the callback (not listed here, because the identity provider drives it rather than a client), which ends the native leg at `healthlog://oidc-callback` with `code=<hlh_…>`, with `mfa_ticket=<ticket>&methods=<list>`, or with `error=<reason>`, where reason is one of `oidc_disabled`, `oidc_rate_limited`, `oidc_denied`, `oidc_no_email`, `oidc_email_unverified`, `oidc_identity_conflict`, `oidc_link_required`, `oidc_registration_disabled`, `oidc_failed`. `oidc_link_required` means the provider's e-mail matches an existing local account that is not linked yet; the owner links it by signing in once with that account's own password or passkey in a browser. The reasons are this exact snake_case vocabulary on both legs; a client maps a value it does not know to a generic sign-in failure.",
         },
       },
     },
diff --git a/src/lib/validations/claim-sso.ts b/src/lib/validations/claim-sso.ts
index e53466a1b..5e14ac63b 100644
--- a/src/lib/validations/claim-sso.ts
+++ b/src/lib/validations/claim-sso.ts
@@ -15,3 +15,4 @@ export type ClaimSsoInput = z.infer<typeof claimSsoSchema>;
 
 /** How a handover link can be redeemed on this server. */
 export const HANDOVER_METHODS = ["password", "sso"] as const;
+export type HandoverMethod = (typeof HANDOVER_METHODS)[number];
diff --git a/tests/integration/managed-profile-handover.test.ts b/tests/integration/managed-profile-handover.test.ts
index b40171a30..f7ab2d0a1 100644
--- a/tests/integration/managed-profile-handover.test.ts
+++ b/tests/integration/managed-profile-handover.test.ts
@@ -811,7 +811,7 @@ describe("claiming the profile", () => {
     // cookie jar, and the race is between the two transactions, not cookies.
     const [claimed, minted] = await Promise.allSettled([
       claimManagedProfile({
-        rawToken: first,
+        link: { rawToken: first },
         username: "racer",
         email: "racer@example.test",
         passwordHash: "hash",