In-process callback failures break tool calls and notification sends

Initial Checks

• I confirm that I am using the newest release of the 2.x line.
• I confirm that I searched the issue tracker before opening this issue.

Release line

2.x (current stable)

Description

Client(server) uses DirectDispatcher. If a progress callback raises, that exception crosses back into server tool execution, so a tool that otherwise returns successfully fails with UnexpectedToolError. DirectDispatcher also lets an inbound notification handler exception escape back to the sender's notify() call.

The stream-backed JSONRPCDispatcher logs and contains both callback failures. I expected the in-process path to keep the same boundary: log the callback failure, but let the protocol operation continue.

I have a small patch and regression tests for both paths and would like to fix this if maintainers want an outside pull request.

I used GitHub Copilot CLI to inspect the dispatcher paths, write the patch, and draft this report. I verified the reproduction and tests.

Example Code

import anyio

from mcp import Client
from mcp.server.mcpserver import Context, MCPServer

server = MCPServer("callback-repro")


@server.tool()
async def work(context: Context) -> str:
    await context.report_progress(1, 1)
    return "done"


async def broken_progress(progress: float, total: float | None, message: str | None) -> None:
    raise RuntimeError("consumer failed")


async def main() -> None:
    async with Client(server, mode="2026-07-28") as client:
        result = await client.call_tool("work", progress_callback=broken_progress)
        print(result.content[0].text)


anyio.run(main)

Current main ends with:

UnexpectedToolError: Error executing tool work

The tool should print done. The callback failure should still be logged.

Verification

With only the regression tests applied to clean main:

uv run --frozen pytest tests/shared/test_dispatcher.py \
  -k 'progress_callback_exception_does_not_fail_request or notification_handler_exception_does_not_reach_sender' -q
# 2 failed, 2 passed. Both DirectDispatcher cases failed.

With the two callback guards applied:

uv run --frozen pytest tests/shared/test_dispatcher.py \
  -k 'progress_callback_exception_does_not_fail_request or notification_handler_exception_does_not_reach_sender' -q
# 4 passed

Python & MCP Python SDK

Python 3.12.13
mcp 2.1.1
mcp 2.1.2.dev4+d060b36e at d060b36e1d095ef6e93e07ba5d59bb69b2ad449a
macOS

Commencez par les chemins de DirectDispatcher et comparez leur gestion des échecs de callback avec celle de JSONRPCDispatcher basé sur un flux. Exécutez tests/shared/test_dispatcher.py avec les deux cas -k nommés afin de reproduire les échecs. Le travail est terminé lorsque les deux cas de DirectDispatcher réussissent, que les appels d’outils réussis et les envois de notifications continuent de fonctionner, et que les échecs de callback sont consignés.