uvicorn Can't handle event type ConnectionClosed when

The Fix

Upgrade to version 0.27.1 or later.

Based on closed Kludex/uvicorn issue #1637 · PR/commit linked

Production note: This tends to surface only under concurrency. Reproduce with load tests and watch for lock contention/cancellation paths.

Jump to Verify Open PR/Commit

@@ -176,6 +176,20 @@ def set_protocol(self, protocol):
 
 
+class MockTimerHandle:
+    def __init__(self, loop_later_list, delay, callback, args):
+        self.loop_later_list = loop_later_list

repro.py

import asyncio

import uvicorn
from fastapi import FastAPI

app = FastAPI()


@app.get("/")
async def root():
    await asyncio.sleep(3)
    return {"msg": "Hello World"}


async def main():
    # Start uvicorn in a background task
    config = uvicorn.Config(app, port=8000, timeout_keep_alive=1)
    server = uvicorn.Server(config)
    uvicorn_task = asyncio.create_task(server.serve())

    # After it starts, try making two HTTP requests.
    await asyncio.sleep(1)
    print("Sending requests")
    reader, writer = await asyncio.open_connection("localhost", 8000)
    writer.write(b"GET / HTTP/1.1\r\nHost: localhost\r\nConnection: keep-alive\r\n\r\n")
    writer.write(b"GET / HTTP/1.1\r\nHost: localhost\r\nConnection: keep-alive\r\n\r\n")
    await writer.drain()
    while data := await reader.read(100):
        print(data.decode("utf-8"))
    print("Server closed the connection")
    server.should_exit = True
    await uvicorn_task


if __name__ == "__main__":
    asyncio.run(main())

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.27.1 or later.\nWhen NOT to use: This fix should not be applied if the server's performance is consistently adequate.\n\n

Why This Fix Works in Production

Trigger: Exception in callback H11Protocol.timeout_keep_alive_handler()
Mechanism: The keep-alive timer was firing prematurely when processing pipelined requests
Why the fix works: Fixes spurious LocalProtocolError errors when processing pipelined requests, which were caused by the keep-alive timer firing prematurely. (first fixed release: 0.27.1).

Production impact:

If left unfixed, failures can be intermittent under concurrency (hard to reproduce; shows up as sporadic 5xx/timeouts).

Why This Breaks in Prod

Shows up under Python 3.7 in real deployments (not just unit tests).
The keep-alive timer was firing prematurely when processing pipelined requests
Surfaces as: Exception in callback H11Protocol.timeout_keep_alive_handler()

Proof / Evidence

GitHub issue: #1637
Fix PR: https://github.com/encode/uvicorn/pull/2243
First fixed release: 0.27.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.20

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“## >>> here is more information <<< ### how to start the server ### versions”

@Yangruipis · 2022-09-11 · source

“I've reopened. I'll check this in some days. Thanks.”

@Kludex · 2024-02-06 · source

“- Closed by https://github.com/encode/uvicorn/pull/2243”

@Kludex · 2024-02-10 · source

“It seems unregister keepalive before RequestResponseCycle not fix the root cause”

@hackrole · 2024-06-24 · source

Failure Signature (Search String)

Exception in callback H11Protocol.timeout_keep_alive_handler()

Error Message

Stack trace

error.txt

Error Message
-------------
Exception in callback H11Protocol.timeout_keep_alive_handler()
handle: <TimerHandle when=3544453.523044797 H11Protocol.timeout_keep_alive_handler()>
Traceback (most recent call last):
  File "/home/r.yang/miniconda3/envs/py37/lib/python3.7/asyncio/events.py", line 88, in _run
    self._context.run(self._callback, *self._args)
  File "/home/r.yang/miniconda3/envs/py37/lib/python3.7/site-packages/uvicorn/protocols/http/h11_impl.py", line 360, in timeout_keep_alive_handler
    self.conn.send(event)
  File "/home/r.yang/miniconda3/envs/py37/lib/python3.7/site-packages/h11/_connection.py", line 510, in send
    data_list = self.send_with_data_passthrough(event)
  File "/home/r.yang/miniconda3/envs/py37/lib/python3.7/site-packages/h11/_connection.py", line 535, in send_with_data_passthrough
    self._process_event(self.our_role, event)
  File "/home/r.yang/miniconda3/envs/py37/lib/python3.7/site-packages/h11/_connection.py", line 272, in _process_event
    self._cstate.process_event(role, type(event), server_switch_event)
  File "/home/r.yang/miniconda3/envs/py37/lib/python3.7/site-packages/h11/_state.py", line 289, in process_event
    self._fire_event_triggered_transitions(role, _event_type)
  File "/home/r.yang/miniconda3/envs/py37/lib/python3.7/site-packages/h11/_state.py", line 311, in _fire_event_triggered_transitions
    ) from None
h11._util.LocalProtocolError: can't handle e
... (truncated) ...

Stack trace

error.txt

Error Message
-------------
Exception in callback H11Protocol.timeout_keep_alive_handler()
handle: <TimerHandle when=335245.867740771 H11Protocol.timeout_keep_alive_handler()>
Traceback (most recent call last):
  File "/usr/lib/python3.12/asyncio/events.py", line 84, in _run
    self._context.run(self._callback, *self._args)
  File "venv/lib/python3.12/site-packages/uvicorn/protocols/http/h11_impl.py", line 363, in timeout_keep_alive_handler
    self.conn.send(event)
  File "venv/lib/python3.12/site-packages/h11/_connection.py", line 512, in send
    data_list = self.send_with_data_passthrough(event)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.12/site-packages/h11/_connection.py", line 537, in send_with_data_passthrough
    self._process_event(self.our_role, event)
  File "venv/lib/python3.12/site-packages/h11/_connection.py", line 272, in _process_event
    self._cstate.process_event(role, type(event), server_switch_event)
  File "venv/lib/python3.12/site-packages/h11/_state.py", line 293, in process_event
    self._fire_event_triggered_transitions(role, _event_type)
  File "venv/lib/python3.12/site-packages/h11/_state.py", line 311, in _fire_event_triggered_transitions
    raise LocalProtocolError(
h11._util.LocalProtocolError: can't handle event type ConnectionClosed when role=SERVER and state=SEND_RESPONSE

Stack trace

error.txt

Error Message
-------------
INFO:     127.0.0.1:47416 - "GET / HTTP/1.1" 200 OK
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "venv/lib/python3.12/site-packages/uvicorn/protocols/http/h11_impl.py", line 408, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 84, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.12/site-packages/fastapi/applications.py", line 1106, in __call__
    await super().__call__(scope, receive, send)
  File "venv/lib/python3.12/site-packages/starlette/applications.py", line 122, in __call__
    await self.middleware_stack(scope, receive, send)
  File "venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 184, in __call__
    raise exc
  File "venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 162, in __call__
    await self.app(scope, receive, _send)
  File "venv/lib/python3.12/site-packages/starlette/middleware/exceptions.py", line 79, in __call__
    raise exc
  File "venv/lib/python3.12/site-packages/starlette/middleware/exceptions.py", line 68, in __call__
    await self.app(scope, receive, sender)
  File "venv/lib/python3.12/site-packages/fastapi/middleware/a
... (truncated) ...

Minimal Reproduction

repro.py

import asyncio

import uvicorn
from fastapi import FastAPI

app = FastAPI()


@app.get("/")
async def root():
    await asyncio.sleep(3)
    return {"msg": "Hello World"}


async def main():
    # Start uvicorn in a background task
    config = uvicorn.Config(app, port=8000, timeout_keep_alive=1)
    server = uvicorn.Server(config)
    uvicorn_task = asyncio.create_task(server.serve())

    # After it starts, try making two HTTP requests.
    await asyncio.sleep(1)
    print("Sending requests")
    reader, writer = await asyncio.open_connection("localhost", 8000)
    writer.write(b"GET / HTTP/1.1\r\nHost: localhost\r\nConnection: keep-alive\r\n\r\n")
    writer.write(b"GET / HTTP/1.1\r\nHost: localhost\r\nConnection: keep-alive\r\n\r\n")
    await writer.drain()
    while data := await reader.read(100):
        print(data.decode("utf-8"))
    print("Server closed the connection")
    server.should_exit = True
    await uvicorn_task


if __name__ == "__main__":
    asyncio.run(main())

Environment

Python: 3.7

What Broke

Clients experienced spurious LocalProtocolError errors during high request rates.

Why It Broke

The keep-alive timer was firing prematurely when processing pipelined requests

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

Upgrade to version 0.27.1 or later.

When NOT to use: This fix should not be applied if the server's performance is consistently adequate.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/encode/uvicorn/pull/2243

First fixed release: 0.27.1

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix should not be applied if the server's performance is consistently adequate.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Add a stress test that runs high-concurrency workloads and fails on thread dumps / blocked locks.
Enable watchdog dumps in prod (faulthandler, thread dump endpoint) to capture deadlocks quickly.
Make timeouts explicit and test them (unit + integration) to avoid silent behavior changes.
Instrument retries (attempt count + reason) and alert on spikes to catch dependency slowdowns.

Version Compatibility Table

Version	Status
0.27.1	Fixed

Related Issues

No related fixes found.

Cluster: uvicorn:race-condition All hubs All clusters

Related clusters: Configuration error Data consistency SSL handshake

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

uvicorn Can't handle event type ConnectionClosed when role=SERVER and state=SEND_RESPONSE (Fix)

The Fix

Why This Fix Works in Production

Why This Breaks in Prod

Proof / Evidence

Discussion

Failure Signature (Search String)

Error Message

Minimal Reproduction

Environment

What Broke

Why It Broke

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

Get updates

When NOT to Use This Fix

Verify Fix

Did This Fix Work in Your Case?

Prevention

Version Compatibility Table

Related Issues

Sources