A tool call fails because the requested order ID (ORD-99999) does not exist. The tool returns: {"is_error": true, "content": "order not found"}. The model retries the call twice with the same order ID, both times failing. What is missing from the error response?
Answer: A retryable: false field — without it, the model treats the error as retryable
Without a retryable: false field in the error payload, the model has no signal that retrying with the same inputs will produce the same failure. It defaults to retry behavior, which produces the loop described in the question. Adding retryable: false tells the model this is a semantic error (the resource doesn't exist) rather than a transient error (retry later). The is_error flag (A) is correct — not-found is an error, not a success with empty content. HTTP status codes (C) are not part of the Claude tool result format. Exception types (D) are not recognized fields in the tool error payload schema.