Python MCP server on mcp 1.x dies when a tool call is cancelled: "AssertionError: Request already responded to", then every later call fails with "Connection closed". Upgrade to mcp 2.x, make the awaited work cancellable, or guard RequestResponder

AssertionError: Request already responded to. The server's stderr shows `File ".../mcp/server/lowlevel/server.py", line 806, in _handle_request: await message.respond(response)` -> `File ".../mcp/shared/session.py", line 129, in respond: assert not self._completed, "Request already responded to"` -> `AssertionError: Request already responded to`, inside an exception group from the server's task group, and the stdio server process exits with code 1. The trigger is the client cancelling an in-flight tool call: a timeout in an mcp 2.x ClientSession (which sends `notifications/cancelled` when a call is abandoned), Claude Desktop giving up on a long tool call after about 4 minutes, or a user interrupting a call in Claude Code (reported). The cancelled call comes back as cancelled or timed out, as expected. Every call after that fails with `MCPError: Connection closed` (Claude Desktop: the server disconnects) until the server is restarted. One cancelled slow call takes the whole session down.

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Incident v2 last verified 2026-10-07

Environment fingerprint

tool	mcp-python-sdk
tool_version	mcp 1.x up to and including 1.30.0 (latest 1.x, 2026-09-07; unchanged there). Not reproducible on mcp 2.3.0. PrefectHQ fastmcp 3.x (on mcp 1.x) affected; fastmcp 4 (mcp 2.3) not
harness	Python MCP servers on the mcp 1.x low-level Server, official FastMCP async tools that await non-cancellable work (anyio.to_thread.run_sync), or PrefectHQ fastmcp 3.x sync tools; clients that send notifications/cancelled (mcp 2.x clients, Claude Desktop, Claude Code)

What didn't work

Restarting or reconnecting the server: it works again, but the next cancelled slow call crashes it the same way
Treating it as a transport or client bug: the client only sees `Connection closed`. The cause is a server-side assertion that shows up only in the server's stderr log
Relying on the July 2025 fix for #1152 (PR #1153, "skip duplicate response on CancelledError"): that fix covers the path where the handler sees CancelledError. This bug is the path where the handler never sees it and returns normally
Guarding only respond(): a contributor's 3-step test on mcp 1.29.0 found that a repeated cancellation, or a cancel that arrives after the response, still puts a second terminal response on the wire. Guard both respond() and cancel()

Root cause

In mcp 1.x, a `notifications/cancelled` for an in-flight request makes `RequestResponder.cancel()` cancel the handler's cancel scope, set `_completed = True` and immediately send a "Request cancelled" error. If the handler is in work that cannot observe cancellation, it keeps running and returns normally. Examples: `anyio.to_thread.run_sync(...)` with its default `abandon_on_cancel=False`, a shielded scope, or a PrefectHQ fastmcp 3.x sync tool, which runs in an anyio worker thread. The server then calls `respond()` with the late result and hits `assert not self._completed`. The AssertionError escapes the server's task group and stops the session, and on stdio the process exits. The bug is still open for 1.x: the fix PRs (#2418, #2949, #3088) were closed or left unmerged, and 1.30.0 has the same code. mcp 2.0 replaced RequestResponder with a JSONRPCDispatcher that drops late results and late errors for requests the peer cancelled, so 2.x servers survive. Two patterns were not affected in our tests on 1.30.0: an official FastMCP `def` (sync) tool, which runs inline and blocks the event loop instead, and a tool that awaits `asyncio.to_thread(...)`, whose await is cancellable.

Solution

Server authors, pick one: 1) Move the server to mcp >= 2.0, or to PrefectHQ fastmcp 4 (built on mcp 2.3). We reproduced the crash on 1.30.0 and saw the same cancel-then-call sequence succeed on 2.3.0. 2.0 is a breaking release (renamed exceptions, decorator API removed); follow the official migration guide at https://py.sdk.modelcontextprotocol.io/migration/. 2) Staying on 1.x: make the awaited work cancellable, so the handler gets CancelledError and the SDK takes the path that is already guarded. Use `await anyio.to_thread.run_sync(fn, abandon_on_cancel=True)` or `await asyncio.to_thread(fn)`. The thread keeps running in the background and its result is discarded. Both variants survived our repro on 1.30.0. 3) Staying on 1.x without touching every tool: make both terminal paths idempotent with a small patch, imported before the server starts: ```python # mcp 1.x only: v2 has no RequestResponder from mcp.shared.session import RequestResponder _respond, _cancel = RequestResponder.respond, RequestResponder.cancel async def respond(self, response): if self._completed: # already answered by cancel() return await _respond(self, response) async def cancel(self): if self._completed: # already answered return await _cancel(self) RequestResponder.respond, RequestResponder.cancel = respond, cancel ``` With this patch, our repro on 1.30.0 (low-level Server and FastMCP) kept the session alive and the next call succeeded. It touches a private attribute, so remove it when you move to 2.x. Operators who cannot change the server: restart or reconnect it after the crash, avoid cancelling its long-running tools (a client timeout cancels too), and upgrade to a server release built on mcp 2.x or fastmcp 4 once one exists.

Evidence

https://github.com/modelcontextprotocol/python-sdk/issues/2416 (open; reproduced on 1.27.0, 1.29.0 and 1.30.0 by independent reporters: a Claude Code on Windows user, the hive stdio server, which carries a guard in production, and the mcp-server-datahub reporter's self-contained repro plus a respond()-only vs respond()+cancel() test). Proposed fixes https://github.com/modelcontextprotocol/python-sdk/pull/2418, #2949 and #3088 were all closed without merging (checked 2026-10-17); mcp 1.30.0 is still the latest 1.x. Earlier related path: https://github.com/modelcontextprotocol/python-sdk/issues/1152 (fixed by PR #1153, 2025-07-25). Downstream: https://github.com/oceanum-io/oceanum-mcp/pull/24 (merged 2026-10-05: crash with fastmcp 3.4.8 / mcp 1.30.0, gone with fastmcp 4.0.11 / mcp 2.3.0; points to the JSONRPCDispatcher code that drops late results); https://github.com/samuelgursky/davinci-resolve-mcp/issues/272 (Claude Desktop cancels after 4 min, then the server crashes with this assertion). Verified locally on 2026-10-07 with an mcp 2.3.0 client (call, 50 ms timeout, call): mcp 1.30.0 low-level Server and FastMCP async tools using anyio.to_thread.run_sync crash, and the third call gets `MCPError: Connection closed`. The same flow passes on mcp 2.3.0, with abandon_on_cancel=True or asyncio.to_thread, and with the guard above. Not affected in the same test: a FastMCP sync def tool, and an mcp 1.x client (it does not send cancellations on timeout).

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