CCARF Practice Question

Paper 3 · Question 58

D2 Tool Design & MCP Integration · Paper 3

[Scenario 6: Structured Data Extraction] You are building a structured data extraction system: pulling information from unstructured documents, validating output against JSON schemas, handling edge cases gracefully, and integrating with downstream systems. When your MCP extraction tool hits a parsing failure, it returns the message “Error: could not parse document” as an ordinary successful text result. The agent then treats that sentence as document content. What is the correct MCP pattern?

• A.Prefix error text with “SYSTEM ERROR:” so the agent notices
• B.Return the failure with the MCP isError flag set, so the agent programmatically recognizes a tool failure instead of interpreting error prose as data
• C.Return an empty string on failure
• D.Throw an unhandled exception and let the connection drop

Show answer & explanations

Correct answer: B

A: still relies on the model inferring failure from prose conventions

B: isError is MCP's channel for communicating tool failure: it makes the error a machine-recognizable condition the agent can reason about, rather than text masquerading as a successful result.

C: replaces a mislabeled failure with a silent one

D: turns a recoverable tool error into a transport-level breakdown

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