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The envelope

Error response

{
  "error": {
    "code": "DOMAIN_RULE",
    "message": "This class is full.",
    "details": {
      "capacity": 12
    },
    "requestId": "req_2f3a9c81d4b7"
  }
}

code is stable and machine-readable. New codes may be added; existing codes will not change meaning.
message is human-readable and may change wording at any time. Never branch on it.
details is optional structured context for some codes.
requestId identifies this exact request in our logs. It is also returned on every response, success included, in the x-request-id header.

Canonical codes

Code	Status	Meaning
`VALIDATION_ERROR`	400	The request shape failed validation. The message says which field.
`IDEMPOTENCY_KEY_REQUIRED`	400	The operation requires an Idempotency-Key header and none was sent.
`UNAUTHENTICATED`	401	Missing or invalid Bearer token.
`FORBIDDEN`	403	The token is valid but lacks the required scope or store access.
`NOT_FOUND`	404	The resource does not exist or is not visible to this caller.
`CONFLICT`	409	The write conflicts with the current state of the resource.
`DOMAIN_RULE`	422	The request is well-formed but a business rule rejected it.
`IDEMPOTENCY_KEY_REUSED`	422	The same Idempotency-Key was reused with a different request body.
`RATE_LIMITED`	429	Too many requests. Honour the Retry-After header.
`INTERNAL`	500	Something failed on our side. Safe to retry idempotent requests.
`DOWNSTREAM_UNAVAILABLE`	503	A dependency (for example the payment provider) is unavailable.
`RATE_LIMITER_UNAVAILABLE`	503	The rate limiter is unavailable and the operation fails closed. Retry after the Retry-After delay.

Domain codes

Some operations return their own specific codes for business-rule rejections, always with a 4xx status and always in the same envelope. Treat any unrecognised code the way you treat DOMAIN_RULE: the request was understood and refused, so do not retry it unchanged.

Handling errors well

4xx: fix the request. Retrying the same request gives the same answer, except 429, which asks you to wait.
5xx: retry with backoff. Writes are safe to retry when you send an Idempotency-Key, see the idempotency guide.
Honour Retry-After whenever it is present (on 429 and some 503 responses).