Key points
- The MCP isError flag marks a tool result as a failure.
- Four categories: transient (timeouts, service unavailable), validation (bad input), business (policy violation), permission.
- Uniform generic errors prevent the agent from choosing the right recovery.
- Retryable versus non-retryable metadata prevents wasted retry attempts.
- An access failure needs a retry decision; an empty result is a successful query with no matches.
Common misconceptions
- "The model will work out what went wrong from any message." It needs structure to decide.
- "Every error should be retried a fixed number of times." Validation and business errors never succeed on retry.
- "An empty list and a timeout can be reported the same way." They call for different decisions.
Watch for
Generic failure text, errors silently returned as empty success, and whole workflows terminated on a single transient failure.
Practice Task 2.2
Start with the free 10-question check to see where you stand across all five domains. In CCA Prep Studio, 57 practice items cover this task, each tracked toward your mastery score.