The distinction between a resolved call and a successful operation is easy to miss. In addition to checking isError, a client can validate a strict success schema and treat absent required fields or an unexpected error envelope as an integration failure, not as an empty successful result. It would be useful to add malformed JSON and a valid JSON object with the wrong shape to the fixture, and assert that neither reaches the next tool. Keeping protocol errors, flagged tool failures, and schema failures as separate telemetry categories would also make version changes easier to diagnose.