mcp: typed tool handlers lose wrapped JSON-RPC errors

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  • #1528 by @ktsoator — closed without merging

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
85/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
go
Domain
backend-api-design

Research direction

Start with toolForErr in the MCP package and the JSON-RPC error handling used by ClientSession.CallTool; compare direct *jsonrpc.Error handling with wrapped errors. Run the provided in-memory client/server reproduction, then verify that errors.As preserves the JSON-RPC code, message, and data and that the result is not incorrectly marked as an execution error. Add or update focused tests for wrapped and unwrapped protocol errors.

Written by the indexing model from the issue text.

Description

Checklist
  • I searched the existing issues, and this has not been reported before.
  • This is a real problem that I ran into myself, using the latest release of the SDK.
  • I ran into it in a production system, not in a toy example, a benchmark, or an automated or AI-driven scan of the code.
  • I (a human) examined the relevant SDK code, and wrote or reviewed every line of this report. It is not unreviewed AI output.
  • This report is brief and concise, and sticks to the facts needed to understand the problem.
What happened?

A typed tool handler returning a wrapped *jsonrpc.Error is treated as a tool execution error. ClientSession.CallTool returns a nil Go error and a result with IsError: true, so the JSON-RPC error code and data are lost. Returning the same protocol error directly works.

toolForErr checks err.(*jsonrpc.Error) instead of searching the error chain. The expected result is a JSON-RPC error with the underlying code, message, and data preserved when wrapped with %w.

How to reproduce

Run this program with github.com/modelcontextprotocol/go-sdk v1.8.0:


package main

import (
	"context"
	"errors"
	"fmt"

	"github.com/modelcontextprotocol/go-sdk/jsonrpc"
	"github.com/modelcontextprotocol/go-sdk/mcp"
)

func main() {
	ctx := context.Background()
	s := mcp.NewServer(&mcp.Implementation{Name: "server", Version: "1"}, nil)
	mcp.AddTool(s, &mcp.Tool{Name: "demo"}, func(context.Context, *mcp.CallToolRequest, map[string]any) (*mcp.CallToolResult, any, error) {
		return nil, nil, fmt.Errorf("backend: %w", &jsonrpc.Error{
			Code:    jsonrpc.CodeInvalidParams,
			Message: "bad input",
		})
	})

	st, ct := mcp.NewInMemoryTransports()
	ss, err := s.Connect(ctx, st, nil)
	if err != nil {
		panic(err)
	}
	defer ss.Close()

	c := mcp.NewClient(&mcp.Implementation{Name: "client", Version: "1"}, nil)
	cs, err := c.Connect(ctx, ct, nil)
	if err != nil {
		panic(err)
	}
	defer cs.Close()

	result, err := cs.CallTool(ctx, &mcp.CallToolParams{Name: "demo"})
	var rpcErr *jsonrpc.Error
	fmt.Printf("protocolError=%v, err=%v\n", errors.As(err, &rpcErr), err)
	if result != nil {
		fmt.Printf("isError=%v\n", result.IsError)
	}
}

Observed:

protocolError=false, err=<nil>
isError=true

Expected: protocolError=true, with JSON-RPC code -32602.

Where did you hit this?

Found during local code inspection and reproduced with an in-memory client and server.

Go MCP SDK version

v1.8.0

Go version

go1.26.10 darwin/arm64, also reproduced on main at 0fa5e8407aa3fda65166e4207156c03d084bfd44.

Proposed fix

Use errors.As and return the underlying protocol error.

Dominant language
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