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mcp: a handler error without a code reaches the peer as JSON-RPC error code 0

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4/5
見積もり時間
3〜5日
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issue の種類
バグ
明瞭さ
明確に書かれている
活発さ
活発
技術スタック
go
領域
backend-api-design

調査の方向性

Start with internal/jsonrpc2/messages.go and toWireError, then read #1367 and #1393 for the related behavior. Compare the proposed default in toWireError with handling errors at MCP call sites, including the duplicate initialize case. Done means uncoded handler errors reach peers as -32603, duplicate initialize as -32600, existing coded errors retain their codes, and the agreed MCPGODEBUG option and 1.9.0 documentation are addressed.

索引モデルが issue の本文から書いたものです。

説明

What happens

When a handler returns an ordinary Go error, the peer receives it with "code": 0. Measured on main at b5d319b:

side where the plain error comes from response
server prompts/get handler {"code":0,"message":"prompt boom"}
server resources/read handler (resource) {"code":0,"message":"resource boom"}
server resources/read handler (template) {"code":0,"message":"template boom"}
server tools/call, raw ToolHandler from Server.AddTool {"code":0,"message":"raw boom"}
server CompletionHandler {"code":0,"message":"complete boom"}
server SubscribeHandler / UnsubscribeHandler {"code":0,"message":"subscribe boom"}
server receiving middleware {"code":0,"message":"middleware boom"}
server handler added with AddReceivingCustomMethod {"code":0,"message":"custom boom"}
server second initialize on a legacy session (SDK's own error) {"code":0,"message":"duplicate \"initialize\" received"}
client CreateMessageHandler {"code":0,"message":"sampling boom"}
client ElicitationHandler {"code":0,"message":"elicit boom"}
client receiving middleware {"code":0,"message":"client middleware boom"}

The server rows give the same answer over IOTransport on 2025-11-25, over IOTransport on 2026-07-28, and over stateless Streamable HTTP on 2026-07-28 (subscribe/unsubscribe and the second initialize on 2025-11-25 only). The client rows were run on 2025-11-25 over IOTransport. context.DeadlineExceeded returned from a handler is answered the same way.

Not affected:

  • A handler that returns a *jsonrpc.Error, or an error wrapping one, keeps its code.
  • ResourceNotFoundError is answered with -32602.
  • A typed ToolHandlerFor: its error becomes an isError result.

Why

Same cause as #1367. toWireError in internal/jsonrpc2/messages.go copies a code only from an error that is or wraps a WireError, so a plain error keeps the zero value. #1393 fixed the one site that issue was about.

What the specification says

  • Prompts, Error Handling (2025-11-25 and 2026-07-28): "Internal errors: -32603 (Internal error)".
  • Resources, 2026-07-28: "Servers SHOULD return -32603 for internal errors." 2025-11-25 lists "Internal errors: -32603".
  • Tools, Error Handling: protocol errors are "standard JSON-RPC errors for issues like … Server errors".

For comparison, the TypeScript SDK (1.32.1, shared/protocol.js) answers with -32603 when a handler throws an error that has no integer code.

Proposal

Answer with -32603 (jsonrpc.CodeInternalError) when an error that carries no code is about to be sent, keeping the message as it is. For the duplicate initialize, -32600, matching #1393.

Two ways to do it, and I would like your preference before writing the patch:

  1. In toWireError: default to -32603 whenever no code is set. One change, covers every row above and future sites, but changes every such answer at once.
  2. At the call sites in mcp only. Leaves the jsonrpc2 default alone, but it is a dozen sites today and each new handler type needs the same care.

Given how many sites there are, I lean towards 1. Either way gated behind an MCPGODEBUG option and documented under 1.9.0, as in #1393. I can send the PR for whichever you choose.

The only test I found that expects 0 is internal/jsonrpc2/wire_test.go, which builds the error with code 0 explicitly, so it would not change.

AI disclosure: investigated and drafted with Claude Code; I reproduced and reviewed it.

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