uvicorn RuntimeError in uvicorn for some requests (Fix)

The Fix

Upgrade to version 0.11.4 or later.

Based on closed Kludex/uvicorn issue #111 · PR/commit linked

Production note: Watch p95/p99 latency and retry volume; timeouts can turn into retry storms and duplicate side-effects.

Jump to Verify Open PR/Commit

@@ -340,6 +340,17 @@ async def __call__(self, receive, send):
 
 
[email protected]("protocol_cls", [HttpToolsProtocol, H11Protocol])
+def test_early_disconnect(protocol_cls):
+    def app(scope):

repro.py

ERROR: Exception in callback UVTransport._call_connection_lost
handle: <Handle UVTransport._call_connection_lost>
Traceback (most recent call last):
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_state.py", line 249, in _fire_event_triggered_transitions
    new_state = EVENT_TRIGGERED_TRANSITIONS[role][state][event_type]
KeyError: <class 'h11._events.ConnectionClosed'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "uvloop/cbhandles.pyx", line 70, in uvloop.loop.Handle._run
  File "uvloop/handles/basetransport.pyx", line 202, in uvloop.loop.UVBaseTransport._call_connection_lost
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/uvicorn/protocols/http/h11.py", line 79, in connection_lost
    self.conn.send(event)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_connection.py", line 464, in send
    data_list = self.send_with_data_passthrough(event)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_connection.py", line 490, in send_with_data_passthrough
    self._process_event(self.our_role, event)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_connection.py", line 238, in _process_event
    self._cstate.process_event(role, type(event), server_switch_event)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_state.py", line 238, in process_event
    self._fire_event_triggered_transitions(role, event_type)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_state.py", line 253, in _fire_event_triggered_transitions
    .format(event_type, role, self.states[role]))
h11._util.LocalProtocolError: can't handle event type <class 'h11._events.ConnectionClosed'> for SERVER in state SEND_BODY

ERROR: Exception in ASGI application
Traceback (most recent call last):
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/uvicorn/protocols/http/h11.py", line 171, in run_asgi
    result = await asgi(self.receive, self.send)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/channels/http.py", line 190, in __call__
    await self.handle(body)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/asgiref/sync.py", line 110, in __call__
    return await asyncio.wait_for(future, timeout=None)
  File "/usr/lib/python3.6/asyncio/tasks.py", line 339, in wait_for
    return (yield from fut)
  File "/usr/lib/python3.6/concurrent/futures/thread.py", line 56, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/asgiref/sync.py", line 125, in thread_handler
    return self.func(*args, **kwargs)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/channels/http.py", line 229, in handle
    self.send(response_message)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/asgiref/sync.py", line 64, in __call__
    return call_result.result()
  File "/usr/lib/python3.6/concurrent/futures/_base.py", line 432, in result
    return self.__get_result()
  File "/usr/lib/python3.6/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/asgiref/sync.py", line 78, in main_wrap
    result = await self.awaitable(*args, **kwargs)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/uvicorn/protocols/http/h11.py", line 261, in send
    output = protocol.conn.send(event
... (truncated) ...

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 0.11.4 or later.\nWhen NOT to use: Do not apply this fix if the application requires strict connection handling.\n\n

Why This Fix Works in Production

Trigger: ERROR: Exception in ASGI application
Mechanism: RuntimeError occurs when attempting to send a response after the connection has been closed
Why the fix works: Addresses a RuntimeError in uvicorn by preventing logging errors on early disconnects. (first fixed release: 0.11.4).

Production impact:

If left unfixed, tail latency can spike under load and surface as timeouts/retries (amplifying incident impact).

Why This Breaks in Prod

Shows up under Python 3.6 in real deployments (not just unit tests).
RuntimeError occurs when attempting to send a response after the connection has been closed
Surfaces as: ERROR: Exception in ASGI application

Proof / Evidence

GitHub issue: #111
Fix PR: https://github.com/kludex/uvicorn/pull/114
First fixed release: 0.11.4
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.16

Discussion

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

Jump to Sources Open on GitHub

“Looks like it's a case when sending a response and the connection has closed prematurely. Might be resolved by if self.disconnected: return at the start…”

@lovelydinosaur · 2018-07-04 · confirmation · source

“could you do 2 things please to try nail that down, 1”

@euri10 · 2020-10-01 · repro detail · source

“FYI I am getting this error with uvicorn 0.11.5 and h11 0.9.0, on a Django channels project”

@oTree-org · 2020-06-09 · source

“I am not sure how to reproduce. In one project it is happening for every 5 requests or so and it is failing randomly. In…”

@ChillarAnand · 2018-07-04 · source

Failure Signature (Search String)

ERROR: Exception in ASGI application

Error Message

Stack trace

error.txt

Error Message
-------------
ERROR: Exception in ASGI application
Traceback (most recent call last):
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/uvicorn/protocols/http/httptools.py", line 196, in run_asgi
    result = await asgi(self.receive, self.send)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/channels/http.py", line 190, in __call__
    await self.handle(body)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/asgiref/sync.py", line 110, in __call__
    return await asyncio.wait_for(future, timeout=None)
  File "/usr/lib/python3.6/asyncio/tasks.py", line 339, in wait_for
    return (yield from fut)
  File "/usr/lib/python3.6/concurrent/futures/thread.py", line 56, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/asgiref/sync.py", line 125, in thread_handler
    return self.func(*args, **kwargs)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/channels/http.py", line 229, in handle
    self.send(response_message)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/asgiref/sync.py", line 64, in __call__
    return call_result.result()
  File "/usr/lib/python3.6/concurrent/futures/_base.py", line 432, in result
    return self.__get_result()
  File "/usr/lib/python3.6/concurrent/futures/_base.py", line 384, in __get_
... (truncated) ...

Stack trace

error.txt

Error Message
-------------
ERROR: Exception in callback UVTransport._call_connection_lost
handle: <Handle UVTransport._call_connection_lost>
Traceback (most recent call last):
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_state.py", line 249, in _fire_event_triggered_transitions
    new_state = EVENT_TRIGGERED_TRANSITIONS[role][state][event_type]
KeyError: <class 'h11._events.ConnectionClosed'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "uvloop/cbhandles.pyx", line 70, in uvloop.loop.Handle._run
  File "uvloop/handles/basetransport.pyx", line 202, in uvloop.loop.UVBaseTransport._call_connection_lost
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/uvicorn/protocols/http/h11.py", line 79, in connection_lost
    self.conn.send(event)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_connection.py", line 464, in send
    data_list = self.send_with_data_passthrough(event)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_connection.py", line 490, in send_with_data_passthrough
    self._process_event(self.our_role, event)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_connection.py", line 238, in _process_event
    self._cstate.process_event(role, type(event), server_switch_event)
  File "/home/chillar/.virtual
... (truncated) ...

Stack trace

error.txt

Error Message
-------------
ERROR:    Exception in callback H11Protocol.timeout_keep_alive_handler()
handle: <TimerHandle when=112443.109 H11Protocol.timeout_keep_alive_handler()>
Traceback (most recent call last):
  File "c:\otree\ve38\lib\site-packages\h11\_state.py", line 249, in _fire_event_triggered_transitions
    new_state = EVENT_TRIGGERED_TRANSITIONS[role][state][event_type]
KeyError: <class 'h11._events.ConnectionClosed'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\wicke\AppData\Local\Programs\Python\Python38\lib\asyncio\events.py", line 81, in _run
    self._context.run(self._callback, *self._args)
  File "c:\otree\ve38\lib\site-packages\uvicorn\protocols\http\h11_impl.py", line 339, in timeout_keep_alive_handler
    self.conn.send(event)
  File "c:\otree\ve38\lib\site-packages\h11\_connection.py", line 469, in send
    data_list = self.send_with_data_passthrough(event)
  File "c:\otree\ve38\lib\site-packages\h11\_connection.py", line 494, in send_with_data_passthrough
    self._process_event(self.our_role, event)
  File "c:\otree\ve38\lib\site-packages\h11\_connection.py", line 242, in _process_event
    self._cstate.process_event(role, type(event), server_switch_event)
  File "c:\otree\ve38\lib\site-packages\h11\_state.py", line 238, in process_event
    self._fire_event_triggered_transitions(role, event_type)
  Fil
... (truncated) ...

Minimal Reproduction

repro.py

ERROR: Exception in callback UVTransport._call_connection_lost
handle: <Handle UVTransport._call_connection_lost>
Traceback (most recent call last):
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_state.py", line 249, in _fire_event_triggered_transitions
    new_state = EVENT_TRIGGERED_TRANSITIONS[role][state][event_type]
KeyError: <class 'h11._events.ConnectionClosed'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "uvloop/cbhandles.pyx", line 70, in uvloop.loop.Handle._run
  File "uvloop/handles/basetransport.pyx", line 202, in uvloop.loop.UVBaseTransport._call_connection_lost
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/uvicorn/protocols/http/h11.py", line 79, in connection_lost
    self.conn.send(event)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_connection.py", line 464, in send
    data_list = self.send_with_data_passthrough(event)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_connection.py", line 490, in send_with_data_passthrough
    self._process_event(self.our_role, event)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_connection.py", line 238, in _process_event
    self._cstate.process_event(role, type(event), server_switch_event)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_state.py", line 238, in process_event
    self._fire_event_triggered_transitions(role, event_type)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/h11/_state.py", line 253, in _fire_event_triggered_transitions
    .format(event_type, role, self.states[role]))
h11._util.LocalProtocolError: can't handle event type <class 'h11._events.ConnectionClosed'> for SERVER in state SEND_BODY

ERROR: Exception in ASGI application
Traceback (most recent call last):
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/uvicorn/protocols/http/h11.py", line 171, in run_asgi
    result = await asgi(self.receive, self.send)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/channels/http.py", line 190, in __call__
    await self.handle(body)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/asgiref/sync.py", line 110, in __call__
    return await asyncio.wait_for(future, timeout=None)
  File "/usr/lib/python3.6/asyncio/tasks.py", line 339, in wait_for
    return (yield from fut)
  File "/usr/lib/python3.6/concurrent/futures/thread.py", line 56, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/asgiref/sync.py", line 125, in thread_handler
    return self.func(*args, **kwargs)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/channels/http.py", line 229, in handle
    self.send(response_message)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/asgiref/sync.py", line 64, in __call__
    return call_result.result()
  File "/usr/lib/python3.6/concurrent/futures/_base.py", line 432, in result
    return self.__get_result()
  File "/usr/lib/python3.6/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/asgiref/sync.py", line 78, in main_wrap
    result = await self.awaitable(*args, **kwargs)
  File "/home/chillar/.virtualenvs/library/lib/python3.6/site-packages/uvicorn/protocols/http/h11.py", line 261, in send
    output = protocol.conn.send(event
... (truncated) ...

Environment

Python: 3.6

What Broke

Some requests fail with a RuntimeError, causing intermittent application errors.

Why It Broke

RuntimeError occurs when attempting to send a response after the connection has been closed

Fix Options (Details)

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

Upgrade to version 0.11.4 or later.

When NOT to use: Do not apply this fix if the application requires strict connection handling.

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

Fix reference: https://github.com/kludex/uvicorn/pull/114

First fixed release: 0.11.4

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

When NOT to Use This Fix

Do not apply this fix if the application requires strict connection handling.

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

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
0.11.4	Fixed

Related Issues

No related fixes found.

Cluster: uvicorn:timeout All hubs All clusters

Related clusters: Configuration error Data consistency Race condition

Sources

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

uvicorn RuntimeError in uvicorn for some requests (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