Sync tool exceeding its timeout returns successful result

A synchronous tool exceeding its configured timeout returns success instead of a timeout error. The timeout docs and #2872 promise timeout errors for sync tools too.

import asyncio
import time
from fastmcp import FastMCP

async def main():
    server = FastMCP()

    @server.tool(timeout=0.03)
    def slow() -> str:
        time.sleep(0.3)
        return "finished after deadline"

    result = await server.call_tool("slow")
    print(result.content)

asyncio.run(main())

Actual: successful TextContent containing finished after deadline. Expected: a timeout error; no thread preemption is assumed. The existing test_sync_timeout_exceeded is skipped.

Reproduced without network requests on main 9488a2619154b7728be80c725fc7473069a1e645, Python 3.12.12, Windows. Investigated with Codex assistance.

@HootRock

Hi @HootRock and maintainers, I’d like to tackle this. I verified the repro where sync worker thread execution ignores the configured timeout parameter and returns successful text content instead of a timeout error. I have a patch that wraps the thread runner future in a strict deadline check returning CallToolResult(isError=True) on expiry, along with un-skipping and passing test_sync_timeout_exceeded. Ready to open a PR.

@Jaswanth1902

Thanks for verifying the reproduction, @Jaswanth1902! A fix is already available in #5379, opened on October 1, including an enabled test_sync_timeout_exceeded and coverage for dependency lifetime.

The PR rejects successful results returned after the deadline while still waiting for a synchronous worker to finish, so its dependencies stay alive. It does not preempt the thread or guarantee a response at the deadline. Could you compare your patch with that PR and share any differences or failing cases there? That would help us converge on one repair for maintainers to review.

fix(tools): reject results returned after timeout deadlines (PrefectH…

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Sync tool exceeding its timeout returns successful result #5378

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Sync tool exceeding its timeout returns successful result #5378

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Jaswanth1902 commented on Oct 2, 2026

HootRock commented on Oct 3, 2026

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