An MCP tool calling an external catalog API sometimes gets "item not found" and sometimes gets a timeout. Today both come back to the agent as "Operation failed." Which change best helps the agent recover?

Answer: A — Return isError with an error category, an isRetryable flag, and a short description

A timeout is transient and worth retrying. 'Item not found' is not. Structured metadata (category, isRetryable, description) lets the agent retry one and fix its input or tell the user about the other (a). Blind retries on any error (b) waste calls on the not-found case and still end in a generic message. An empty list (c) hides the failure. 'Please try again' (d) wrongly suggests both are retryable.