diff --git a/api/openapi.yaml b/api/openapi.yaml
index 3d49b67b7..f7dcc584e 100644
--- a/api/openapi.yaml
+++ b/api/openapi.yaml
@@ -7801,6 +7801,7 @@ paths:
       tags:
         - domains
     post:
+      description: "Registering a domain the account already owns is idempotent: it returns the existing row (still 201) and does not count against the domain cap. Registering a domain owned by a different account is a 409 domain_taken conflict."
       operationId: registerDomain
       requestBody:
         content:
diff --git a/docs/api.md b/docs/api.md
index c4aad7739..0434e08fc 100644
--- a/docs/api.md
+++ b/docs/api.md
@@ -579,7 +579,10 @@ of `deleted: true`. Domain deletion adds the durable, open-set
 Custom sending/receiving domains and their DNS verification.
 
 - `GET /v1/domains`, `POST /v1/domains` — list / register (returns required MX +
-  TXT records and the DKIM selector/key).
+  TXT records and the DKIM selector/key). Registering a domain the account
+  already owns is idempotent: it returns the existing row (still 201) and
+  does not count against the domain cap. A different account's claim is a
+  409 `domain_taken` conflict.
 - `GET /v1/domains/{domain}`, `DELETE /v1/domains/{domain}?confirm=DELETE` —
   fetch / delete (delete deprovisions the sending identity; irreversible).
   The deletion receipt's open `sending_teardown` value is the DNS-release
diff --git a/docs/design/api-v1-redesign.md b/docs/design/api-v1-redesign.md
index bba87c4e6..799a12869 100644
--- a/docs/design/api-v1-redesign.md
+++ b/docs/design/api-v1-redesign.md
@@ -1017,7 +1017,8 @@ review diligence — a #206-style omission can't merge.
 >   exposes an optional `idempotency_key` tool arg so hosted-MCP callers can retry
 >   safely (the MCP hop is a network boundary the SDK auto-mint can't span). The
 >   create tools don't qualify: `create_agent` (email) and `register_domain`
->   (domain) have natural unique keys (retry → 409, never a duplicate), and creates
+>   (domain) have natural unique keys (a retry reclaims the existing row with
+>   201, never a duplicate), and creates
 >   are not the frequent/high-harm path — the rule is *mutating + duplicate-harmful
 >   + no natural dedup*, not call frequency. (`rotate_webhook_secret`, the one
 >   rare-but-dangerous non-create, is already idempotent from the GA-blockers work.)
@@ -1258,8 +1259,9 @@ worth making while we're reshaping the contract anyway, roughly in priority:
    adding it to `create_agent`/`create_webhook`/`register_domain`; on review,
    idempotency stays scoped to the send-family. The create tools don't meet the
    bar (*mutating + duplicate-harmful + no natural dedup*): `create_agent` (email)
-   and `register_domain` (domain) have natural unique keys → retry is a 409, not a
-   duplicate. The send-family already carries it (and exposes the optional tool
+   and `register_domain` (domain) have natural unique keys: a retry reclaims
+   the existing row (201), not a duplicate. The send-family already carries it (and
+   exposes the optional tool
    arg so hosted-MCP callers retry safely). Matches how mature email APIs scope it.
 8. **Consistent vocabulary — resolved.** `send_email` mixed "email" with
    `reply_to_message`/`forward_message`. Standardize the noun on the API
diff --git a/internal/httpapi/domains.go b/internal/httpapi/domains.go
index c8bef9076..9d6c5592b 100644
--- a/internal/httpapi/domains.go
+++ b/internal/httpapi/domains.go
@@ -314,7 +314,8 @@ func (s *Server) registerDomains() {
 	registerOp(s.API, huma.Operation{
 		OperationID: "registerDomain", Method: http.MethodPost, Path: "/v1/domains",
 		Summary: "Register a domain", Tags: []string{"domains"},
-		Security: []map[string][]string{{"bearer": {}}}, DefaultStatus: http.StatusCreated,
+		Description: "Registering a domain the account already owns is idempotent: it returns the existing row (still 201) and does not count against the domain cap. Registering a domain owned by a different account is a 409 domain_taken conflict.",
+		Security:    []map[string][]string{{"bearer": {}}}, DefaultStatus: http.StatusCreated,
 		Responses: map[string]*huma.Response{
 			"402": s.limitExceededResponse(),
 			"409": s.jsonResponse(reflect.TypeOf(ErrorEnvelope{}), "ErrorEnvelope",
diff --git a/sdks/python/src/e2a/v1/generated/api/domains_api.py b/sdks/python/src/e2a/v1/generated/api/domains_api.py
index 6a3cffeb3..7c3117af4 100644
--- a/sdks/python/src/e2a/v1/generated/api/domains_api.py
+++ b/sdks/python/src/e2a/v1/generated/api/domains_api.py
@@ -898,6 +898,7 @@ async def register_domain(
     ) -> DomainView:
         """Register a domain
 
+        Registering a domain the account already owns is idempotent: it returns the existing row (still 201) and does not count against the domain cap. Registering a domain owned by a different account is a 409 domain_taken conflict.
 
         :param register_domain_request: (required)
         :type register_domain_request: RegisterDomainRequest
@@ -966,6 +967,7 @@ async def register_domain_with_http_info(
     ) -> ApiResponse[DomainView]:
         """Register a domain
 
+        Registering a domain the account already owns is idempotent: it returns the existing row (still 201) and does not count against the domain cap. Registering a domain owned by a different account is a 409 domain_taken conflict.
 
         :param register_domain_request: (required)
         :type register_domain_request: RegisterDomainRequest
@@ -1034,6 +1036,7 @@ async def register_domain_without_preload_content(
     ) -> RESTResponseType:
         """Register a domain
 
+        Registering a domain the account already owns is idempotent: it returns the existing row (still 201) and does not count against the domain cap. Registering a domain owned by a different account is a 409 domain_taken conflict.
 
         :param register_domain_request: (required)
         :type register_domain_request: RegisterDomainRequest
diff --git a/sdks/typescript/src/v1/generated/apis/DomainsApi.ts b/sdks/typescript/src/v1/generated/apis/DomainsApi.ts
index 4fcac12eb..7372fbf17 100644
--- a/sdks/typescript/src/v1/generated/apis/DomainsApi.ts
+++ b/sdks/typescript/src/v1/generated/apis/DomainsApi.ts
@@ -158,6 +158,7 @@ export class DomainsApiRequestFactory extends BaseAPIRequestFactory {
     }
 
     /**
+     * Registering a domain the account already owns is idempotent: it returns the existing row (still 201) and does not count against the domain cap. Registering a domain owned by a different account is a 409 domain_taken conflict.
      * Register a domain
      * @param registerDomainRequest 
      */
diff --git a/sdks/typescript/src/v1/generated/types/ObjectParamAPI.ts b/sdks/typescript/src/v1/generated/types/ObjectParamAPI.ts
index 1372a3a37..0d189dc96 100644
--- a/sdks/typescript/src/v1/generated/types/ObjectParamAPI.ts
+++ b/sdks/typescript/src/v1/generated/types/ObjectParamAPI.ts
@@ -1628,6 +1628,7 @@ export class ObjectDomainsApi {
     }
 
     /**
+     * Registering a domain the account already owns is idempotent: it returns the existing row (still 201) and does not count against the domain cap. Registering a domain owned by a different account is a 409 domain_taken conflict.
      * Register a domain
      * @param param the request object
      */
@@ -1636,6 +1637,7 @@ export class ObjectDomainsApi {
     }
 
     /**
+     * Registering a domain the account already owns is idempotent: it returns the existing row (still 201) and does not count against the domain cap. Registering a domain owned by a different account is a 409 domain_taken conflict.
      * Register a domain
      * @param param the request object
      */
diff --git a/sdks/typescript/src/v1/generated/types/ObservableAPI.ts b/sdks/typescript/src/v1/generated/types/ObservableAPI.ts
index 79e8d8390..86aee8e6c 100644
--- a/sdks/typescript/src/v1/generated/types/ObservableAPI.ts
+++ b/sdks/typescript/src/v1/generated/types/ObservableAPI.ts
@@ -1610,6 +1610,7 @@ export class ObservableDomainsApi {
     }
 
     /**
+     * Registering a domain the account already owns is idempotent: it returns the existing row (still 201) and does not count against the domain cap. Registering a domain owned by a different account is a 409 domain_taken conflict.
      * Register a domain
      * @param registerDomainRequest
      */
@@ -1634,6 +1635,7 @@ export class ObservableDomainsApi {
     }
 
     /**
+     * Registering a domain the account already owns is idempotent: it returns the existing row (still 201) and does not count against the domain cap. Registering a domain owned by a different account is a 409 domain_taken conflict.
      * Register a domain
      * @param registerDomainRequest
      */
diff --git a/sdks/typescript/src/v1/generated/types/PromiseAPI.ts b/sdks/typescript/src/v1/generated/types/PromiseAPI.ts
index fdd0aa185..dac391c85 100644
--- a/sdks/typescript/src/v1/generated/types/PromiseAPI.ts
+++ b/sdks/typescript/src/v1/generated/types/PromiseAPI.ts
@@ -1165,6 +1165,7 @@ export class PromiseDomainsApi {
     }
 
     /**
+     * Registering a domain the account already owns is idempotent: it returns the existing row (still 201) and does not count against the domain cap. Registering a domain owned by a different account is a 409 domain_taken conflict.
      * Register a domain
      * @param registerDomainRequest
      */
@@ -1175,6 +1176,7 @@ export class PromiseDomainsApi {
     }
 
     /**
+     * Registering a domain the account already owns is idempotent: it returns the existing row (still 201) and does not count against the domain cap. Registering a domain owned by a different account is a 409 domain_taken conflict.
      * Register a domain
      * @param registerDomainRequest
      */