Errors

Every refusal has one shape, on every surface:

{
  "error": {
    "code": "conflict",
    "message": "Someone saved this file first. Read it again, redo your change on the new version and save.",
    "retryable": true,
    "currentVersion": 4
  }
}

Where it appears. In a tool result with isError: true, as the text of the result; in the data of a JSON-RPC error, so error.data.error holds it and the JSON-RPC error.code stays numeric; and as the body of an upload or download link answer, with the HTTP status that goes with the code.
code. Lower snake_case, such as invalid_arguments, unknown_tool, organization_inactive, rate_limited, credits_exhausted or internal. Branch on code; the set is fixed.
message. One plain sentence in the language of the request, ending with the next step. It never starts with the code.
retryable. Always true or false. When true, repeat the call, after retryAfterSeconds when it is present.
Details. Next to the three fields, a refusal may carry currentVersion (the version to redo your change on), retryAfterSeconds, failingRecords (the records that failed, in a batch) and issues (each invalid field and what it expects).

A failure Siglata did not expect answers internal, retryable, with a generic message: it never carries database, provider or stack text.