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mcp: send handler errors that carry no code as Internal error - #1407
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guglielmo-san merged 1 commit intoOct 16, 2026
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An error returned by a request handler without a JSON-RPC error code reached the peer with code 0. handleReceive now gives such an error the code -32603 (Internal error), keeping its message and keeping it in the chain for errors.Is and errors.As. Errors that already carry a code, and jsonrpc2.ErrNotHandled, are unchanged. The path is shared by both sides, so client handlers (sampling, elicitation) get the same treatment. MCPGODEBUG=nointernalerrorcode=1 restores code 0. Fixes modelcontextprotocol#1394 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
guglielmo-san
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Oct 16, 2026
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A request handler that fails with an ordinary error (no JSON-RPC code) answered the peer with error code 0. As suggested in #1394,
handleReceiveinmcp/shared.gonow gives such an error the code -32603 (Internal error), right after the method handler returns. The message is unchanged, and the original error stays in the chain forerrors.Is/errors.As. Errors that already carry a code keep it, andjsonrpc2.ErrNotHandledis left alone so it still becomes "method not found".handleReceiveserves both sides, so a failing client handler (sampling, elicitation) now reaches the server as -32603 too.This is a behavioral change, so
MCPGODEBUG=nointernalerrorcode=1restores code 0. The option is documented indocs/mcpgodebug.mdunder 1.9.0.TestHandlerErrorWithoutCodecovers a server prompt handler and a client sampling handler, with plain, wrapped, coded and wrapped-coded errors, under both settings; without the change it fails with code 0.Fixes #1394
AI disclosure: written with Claude Code; I reviewed the change and ran
go test ./...,go vet ./...,staticcheckand themcptests under-race.🤖 Generated with Claude Code