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bug: a request sent before initialize is answered with JSON-RPC error code 0 instead of -32600

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
82/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
go
Domain
api

Research direction

Start with the minimal program in the issue: run a server over IOTransport, send tools/list before initialize, and confirm the response carries code: 0. Find where the 'invalid during session initialization' error is constructed and check which code it sets. Done when that path returns -32600 and the message text is unchanged, with a test that asserts the code.

Written by the indexing model from the issue text.

Description

Summary

A request sent before initialize is answered with a JSON-RPC error whose code is 0. 0 is not a JSON-RPC 2.0 error code; the spec requires -32600 (Invalid Request) for a request that cannot be served.

Version

github.com/modelcontextprotocol/go-sdk v1.8.0 (also observed on main).

Reproduction

Run a server over a raw stdio transport (IOTransport) and, before sending initialize, send:

{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}

Observed response:

{"jsonrpc":"2.0","id":1,"error":{"code":0,"message":"method \"tools/list\" is invalid during session initialization"}}

Expected response (JSON-RPC 2.0):

{"jsonrpc":"2.0","id":1,"error":{"code":-32600,"message":"method \"tools/list\" is invalid during session initialization"}}

The message text is correct; only the code is wrong.

Minimal program

srv := mcp.NewServer(&mcp.Implementation{Name: "t", Version: "0"}, nil)
c2s, serverIn := io.Pipe()
serverOut, cFromS := io.Pipe()
go func() { _ = srv.Run(ctx, &mcp.IOTransport{Reader: c2s, Writer: cFromS}) }()
fmt.Fprintln(serverIn, `{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}`)
// read the first line of serverOut

Notes

  • ping is correctly allowed before initialize.
  • 0 is especially awkward for clients: it is indistinguishable from "no code set" and is not one of the standard codes (-32700, -32600, -32601, -32602, -32603).

Impact

Consumers must wrap the transport to rewrite code: 0 to -32600 before forwarding to clients. We do exactly that in ai-rulez (internal/mcp GuardLifecycle) and would like to delete the workaround once the SDK answers with -32600 itself.

Dominant language
Go
Stars
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Forks
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Avg merge
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Merged PRs (30d)
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