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Correct answer: A. A tool execution error carrying what was wrong, so the model can adjust the argument and try again.
Explanation
The principle — Report a failure to whoever can fix it, in a form they can act on.
Why the key is correct — Tool Execution Errors contain actionable feedback that language models can use to self-correct and retry with adjusted parameters, and input validation errors — a date in the wrong format is the specification's own example — sit in that category. The client-side guidance completes the loop: clients SHOULD provide tool execution errors to language models to enable self-correction. A message saying which format was expected is usually enough for the next call to succeed.
Why the others are wrong — Hiding the error removes the information that would fix it. A protocol error would classify a fixable argument as a structural fault. And having the client reformat the value substitutes a guess for a correction the model can make itself.
Remember this — If the caller can fix it, tell the caller.
Sources — MCP first-party documentation.
Sources
“Tool Execution Errors contain actionable feedback that language models can use to self-correct and retry with adjusted parameters:”
“Clients SHOULD provide tool execution errors to language models to enable self-correction.”
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