Every error response has the same shape:

{
  "error": {
    "code": "validation_failed",
    "message": "Request failed validation",
    "details": [{ "path": "installId", "message": "Invalid UUID" }]
  }
}

code is stable: switch on it.
message is for people and may change.
details appears on validation_failed, with one entry per field.

Status	`code`	When
400	`validation_failed`	The body is invalid. See `details`.
400	`platform_mismatch`	`platform` doesn't match the key's platform.
401	`missing_signature`	`X-Key-Id`, `X-Timestamp` or `X-Signature` is missing.
401	`bad_timestamp`	`X-Timestamp` isn't whole seconds.
401	`stale_timestamp`	More than 5 minutes from server time.
401	`invalid_key`	Unknown or revoked key, or the app was deleted.
401	`bad_signature`	The signature doesn't match the body.
413	`fst_err_ctp_body_too_large`	The body is over 64 KB.
500	`internal`	Something went wrong on our side. Retry later.

How your app should react to each: Handling errors.