mcp: reject prompts/get requests missing a required argument - #1297
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guglielmo-san merged 1 commit intoSep 28, 2026
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The prompts spec asks a server to answer -32602 when a required
argument is missing (server/prompts, "Error Handling", in 2025-11-25
and 2026-07-28). Server.getPrompt only checked that the prompt exists,
so a prompt that declares {Name: "code", Required: true} still had its
handler called without "code", and every handler had to repeat the
check.
Check the declared arguments before calling the handler. A required
argument that is present but empty still reaches the handler, and a nil
entry in Prompt.Arguments is skipped as before. As modelcontextprotocol#1080 did for
completion/complete, MCPGODEBUG=disablepromptargsvalidation=1 restores
the previous behavior.
Fixes modelcontextprotocol#1296
guglielmo-san
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Sep 28, 2026
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@po-et thank you for the contribution! |
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Fixes #1296.
Server.getPromptnow returns-32602when an argument the prompt declares as required is missing from the request, before calling the handler. The spec asks for this (server/prompts, "Error Handling": missing required arguments,-32602, in 2025-11-25 and 2026-07-28), and it saves every handler from repeating the check. A required argument that is present but empty still reaches the handler, and a nil entry inPrompt.Argumentsis skipped as before. TheAddPromptdoc comment now says thatRequiredis enforced.As #1080 did for
completion/complete,MCPGODEBUG=disablepromptargsvalidation=1restores the old behavior. It is documented in the 1.9.0 section ofdocs/mcpgodebug.md;go generateupdated it.TestGetPromptRequiresDeclaredArgumentscovers these cases:Prompt.Arguments;The conformance server suites give the same results as on main. On 2025-11-25 every scenario passes. On 2026-07-28 only the tasks scenarios fail, which this SDK does not implement yet.