httpx h11._util.RemoteProtocolError: can't handle event

The Fix

Upgrade to version 0.13.0 or later.

Based on closed encode/httpx issue #96 · 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

@@ -4,6 +4,7 @@
 from ..concurrency.base import BaseSocketStream, ConcurrencyBackend, TimeoutFlag
 from ..config import TimeoutConfig, TimeoutTypes
+from ..exceptions import ConnectionClosed, ProtocolError
 from ..models import AsyncRequest, AsyncResponse
 from ..utils import get_logger

repro.py

from fastapi import FastAPI
from starlette.requests import Request
from starlette.responses import JSONResponse
import http3

app = FastAPI()
http_client = http3.AsyncClient()

@app.middleware('http')
async def sso_middleware(request: Request, call_next):
    r = await http_client.post('http://127.0.0.1:8001')
    if r.status_code != 200:
        return JSONResponse({'ok': 0, 'data': {'status_code': r.status_code}})
    ret = r.json()
    await r.close()
    print(ret)
    response = await call_next(request)
    return response

@app.get('/')
def index(request: Request):
    return {"ok": 1, "data": "welcome to test app!"}

if __name__ == "__main__":
    import uvicorn
    uvicorn.run(app, host="127.0.0.1", port=8000)
    pass

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 0.13.0 or later.\nWhen NOT to use: Do not apply this fix if the application relies on the original behavior of the connection handling.\n\n

Why This Fix Works in Production

Trigger: timeout=timeout,
Mechanism: The error occurs when the server closes the connection unexpectedly during response handling
Why the fix works: Addresses the RemoteProtocolError by raising a new ConnectionClosed exception if the socket is closed, otherwise reusing the ProtocolError exception. (first fixed release: 0.13.0).

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 3.7 in real deployments (not just unit tests).
The error occurs when the server closes the connection unexpectedly during response handling
Surfaces as: File "/project/venv/lib/python3.7/site-packages/http3/client.py", line 365, in post

Proof / Evidence

GitHub issue: #96
Fix PR: https://github.com/encode/httpx/pull/524
First fixed release: 0.13.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.75
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.23

Discussion

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

Jump to Sources Open on GitHub

“There are two web servers”

@lch277 · 2019-07-16 · source

“I can reproduce it”

@yeraydiazdiaz · 2019-07-23 · source

“Relevant snippet from the h11 docs: > Connection.next_event Raises: RemoteProtocolError – The peer has misbehaved. You should close the connection (possibly after sending some kind…”

@yeraydiazdiaz · 2019-07-23 · source

“I think this is caused by the client code in http3.”

@didip · 2019-06-18 · source

Failure Signature (Search String)

timeout=timeout,

Error Message

Stack trace

error.txt

Error Message
-------------
File "/project/venv/lib/python3.7/site-packages/http3/client.py", line 365, in post
    timeout=timeout,
  File "/project/venv/lib/python3.7/site-packages/http3/client.py", line 497, in request
    timeout=timeout,
  File "/project/venv/lib/python3.7/site-packages/http3/client.py", line 112, in send
    allow_redirects=allow_redirects,
  File "/project/venv/lib/python3.7/site-packages/http3/client.py", line 145, in send_handling_redirects
    request, verify=verify, cert=cert, timeout=timeout
  File "/project/venv/lib/python3.7/site-packages/http3/dispatch/connection_pool.py", line 121, in send
    raise exc
  File "/project/venv/lib/python3.7/site-packages/http3/dispatch/connection_pool.py", line 116, in send
    request, verify=verify, cert=cert, timeout=timeout
  File "/project/venv/lib/python3.7/site-packages/http3/dispatch/connection.py", line 59, in send
    response = await self.h11_connection.send(request, timeout=timeout)
  File "/project/venv/lib/python3.7/site-packages/http3/dispatch/http11.py", line 65, in send
    event = await self._receive_event(timeout)
  File "/project/venv/lib/python3.7/site-packages/http3/dispatch/http11.py", line 109, in _receive_event
    event = self.h11_state.next_event()
  File "/project/venv/lib/python3.7/site-packages/h11/_connection.py", line 439, in next_event
    exc._reraise_as_remote_protocol_error()
  File "/project/venv/lib/pyth
... (truncated) ...

Stack trace

error.txt

Error Message
-------------
ERROR: Exception in ASGI application
Traceback (most recent call last):
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\uvicorn\protocols\http\h11_impl.py", line 370, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\starlette\applications.py", line 133, in __call__
    await self.error_middleware(scope, receive, send)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\starlette\middleware\errors.py", line 122, in __call__
    raise exc from None
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\starlette\middleware\errors.py", line 100, in __call__
    await self.app(scope, receive, _send)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\starlette\middleware\base.py", line 25, in __call__
    response = await self.dispatch_func(request, self.call_next)
  File "subapp.py", line 56, in sso_middleware
    'code': auth_code
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\client.py", line 406, in post
    timeout=timeout,
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\client.py", line 548, in request
    timeout=timeout,
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\client.py", line 145, in send
    allow_redi
... (truncated) ...

Stack trace

error.txt

Error Message
-------------
INFO: ('127.0.0.1', 14501) - "GET / HTTP/1.1" 200
{'ok': 1, 'data': 'welcome to test app 11111111111!'}
INFO: ('127.0.0.1', 14501) - "GET / HTTP/1.1" 200
INFO: ('127.0.0.1', 14553) - "GET / HTTP/1.1" 500
ERROR: Exception in ASGI application
Traceback (most recent call last):
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\uvicorn\protocols\http\h11_impl.py", line 370, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\starlette\applications.py", line 133, in __call__
    await self.error_middleware(scope, receive, send)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\starlette\middleware\errors.py", line 122, in __call__
    raise exc from None
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\starlette\middleware\errors.py", line 100, in __call__
    await self.app(scope, receive, _send)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\starlette\middleware\base.py", line 25, in __call__
    response = await self.dispatch_func(request, self.call_next)
  File "testapp.py", line 13, in sso_middleware
    r = await http_client.post('http://127.0.0.1:8001')
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\client.py", line 406, in post
    timeout=timeout,
  File "C:\User
... (truncated) ...

Minimal Reproduction

repro.py

from fastapi import FastAPI
from starlette.requests import Request
from starlette.responses import JSONResponse
import http3

app = FastAPI()
http_client = http3.AsyncClient()

@app.middleware('http')
async def sso_middleware(request: Request, call_next):
    r = await http_client.post('http://127.0.0.1:8001')
    if r.status_code != 200:
        return JSONResponse({'ok': 0, 'data': {'status_code': r.status_code}})
    ret = r.json()
    await r.close()
    print(ret)
    response = await call_next(request)
    return response

@app.get('/')
def index(request: Request):
    return {"ok": 1, "data": "welcome to test app!"}

if __name__ == "__main__":
    import uvicorn
    uvicorn.run(app, host="127.0.0.1", port=8000)
    pass

Environment

Python: 3.7

What Broke

Intermittent errors during load testing led to failed HTTP requests and potential service disruptions.

Why It Broke

The error occurs when the server closes the connection unexpectedly during response handling

Fix Options (Details)

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

Upgrade to version 0.13.0 or later.

When NOT to use: Do not apply this fix if the application relies on the original behavior of the connection handling.

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

Fix reference: https://github.com/encode/httpx/pull/524

First fixed release: 0.13.0

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

When NOT to Use This Fix

Do not apply this fix if the application relies on the original behavior of the 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

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.
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
0.13.0	Fixed

Related Issues

No related fixes found.

Cluster: httpx:timeout All hubs All clusters

Related clusters: Configuration error Data consistency Other

Sources

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

httpx h11._util.RemoteProtocolError: can't handle event type ConnectionClosed when role=SERVER and (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