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Correct answer: D. It may forward them, but an unknown tool is a structural fault and rarely something the model can fix.
Explanation
The principle — Two error kinds get two different pieces of advice, and neither is a prohibition.
Why the key is correct — Protocol Errors indicate issues with the request structure itself that models are less likely to be able to fix, and an unknown tool is the first example given. The client-side guidance matches that: clients MAY provide protocol errors to language models, though these are less likely to result in successful recovery. So forwarding is allowed and often pointless, which is a useful thing to know before building a retry loop around it.
Why the others are wrong — The two kinds are not equally recoverable. Forwarding is permitted rather than forbidden. And an unknown tool is a structural fault, not business logic.
Remember this — You may show the model a protocol error; do not expect it to help. Build the retry loop around the errors the model can act on, and treat the rest as something a person will read.
Sources — MCP first-party documentation.
Sources
“Protocol Errors indicate issues with the request structure itself that models are less likely to be able to fix:”
“Clients MAY provide protocol errors to language models, though these are less likely to result in successful recovery.”
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