The Fix
Upgrade to version 0.17.0 or later.
Based on closed Kludex/starlette issue #1255 · PR/commit linked
Production note: This tends to surface only under concurrency. Reproduce with load tests and watch for lock contention/cancellation paths.
@@ -23,17 +23,25 @@ async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
async def call_next(request: Request) -> Response:
+ app_exc: typing.Optional[Exception] = None
send_stream, recv_stream = anyio.create_memory_object_stream()
repro.py
import httpx
import asyncio
from starlette.applications import Starlette
from starlette.routing import Route
from starlette.middleware.base import BaseHTTPMiddleware
from starlette.middleware import Middleware
async def error(request):
raise RuntimeError("oops")
class MyMiddleware(BaseHTTPMiddleware):
async def dispatch(self, request, call_next):
return await call_next(request)
app = Starlette(routes=[Route("/error", error)], middleware=[Middleware(MyMiddleware)])
async def main():
async with httpx.AsyncClient(app=app) as client:
_ = await client.get("http://testserver/error")
asyncio.run(main())
verify
Re-run: python example.py
fix.md
Option A — Upgrade to fixed release\nUpgrade to version 0.17.0 or later.\nWhen NOT to use: This fix should not be used if the application relies on the previous behavior of raising ExceptionGroup.\n\n
Why This Fix Works in Production
- Trigger: $ python example.py
- Mechanism: The test_custom_middleware fails due to an ExceptionGroup raised by anyio in error views under BaseHTTPMiddleware
- Why the fix works: Prevent anyio.ExceptionGroup in error views under a BaseHTTPMiddleware, resolving the issue with test_custom_middleware failing. (first fixed release: 0.17.0).
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 3.9 in real deployments (not just unit tests).
- The test_custom_middleware fails due to an ExceptionGroup raised by anyio in error views under BaseHTTPMiddleware
- Surfaces as: $ python example.py
Proof / Evidence
- GitHub issue: #1255
- Fix PR: https://github.com/kludex/starlette/pull/1262
- First fixed release: 0.17.0
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.29
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Indeed, have been seeing this persistently even since an upgrade to 0.17.1: There's no pattern I can discern after a few hours of trying to…”
“@bmwiedemann I am getting the same issue on an asyncio setup as soon as any custom middleware is combined with a view that raises an…”
“@florimondmanca this error is happening to me too, is there any solution you'd recommend if we don't want to downgrade Starlette?”
“@Kludex awesome, do you know when that will be?”
Failure Signature (Search String)
- $ python example.py
Error Message
Stack trace
error.txt
Error Message
-------------
$ python example.py
Traceback (most recent call last):
File "/Users/florimond/Desktop/test.py", line 26, in <module>
asyncio.run(main())
File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/asyncio/runners.py", line 44, in run
return loop.run_until_complete(main)
File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/asyncio/base_events.py", line 642, in run_until_complete
return future.result()
File "/Users/florimond/Desktop/test.py", line 23, in main
_ = await client.get("http://testserver/error")
File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/site-packages/httpx/_client.py", line 1722, in get
return await self.request(
File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/site-packages/httpx/_client.py", line 1481, in request
response = await self.send(
File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/site-packages/httpx/_client.py", line 1568, in send
response = await self._send_handling_auth(
File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/site-packages/httpx/_client.py", line 1604, in _send_handling_auth
response = await self._send_handling_redirects(
File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/site-packages/httpx/_client.py", line 1640, in _send_handling_redirects
response = await self._send_single_request(request, timeout)
File "/Users/florimond/.pyenv/
... (truncated) ...
Stack trace
error.txt
Error Message
-------------
File \".../lib/python3.7/site-packages/anyio/streams/memory.py\", line 81, in receive
return self.receive_nowait()
File \".../lib/python3.7/site-packages/anyio/streams/memory.py\", line 76, in receive_nowait
raise WouldBlock
anyio.WouldBlock
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File \".../lib/python3.7/site-packages/starlette/middleware/base.py\", line 41, in call_next
message = await recv_stream.receive()
File \".../lib/python3.7/site-packages/anyio/streams/memory.py\", line 101, in receive
raise EndOfStream
anyio.EndOfStream
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File \"...", line 324, in dispatch
response = await call_next(request)
File \".../lib/python3.7/site-packages/starlette/middleware/base.py\", line 45, in call_next
raise RuntimeError(\"No response returned.\")
Minimal Reproduction
repro.py
import httpx
import asyncio
from starlette.applications import Starlette
from starlette.routing import Route
from starlette.middleware.base import BaseHTTPMiddleware
from starlette.middleware import Middleware
async def error(request):
raise RuntimeError("oops")
class MyMiddleware(BaseHTTPMiddleware):
async def dispatch(self, request, call_next):
return await call_next(request)
app = Starlette(routes=[Route("/error", error)], middleware=[Middleware(MyMiddleware)])
async def main():
async with httpx.AsyncClient(app=app) as client:
_ = await client.get("http://testserver/error")
asyncio.run(main())
Environment
- Python: 3.9
What Broke
The test_custom_middleware fails, causing CI/CD pipeline failures and impacting deployment.
Why It Broke
The test_custom_middleware fails due to an ExceptionGroup raised by anyio in error views under BaseHTTPMiddleware
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
Upgrade to version 0.17.0 or later.
When NOT to use: This fix should not be used if the application relies on the previous behavior of raising ExceptionGroup.
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Fix reference: https://github.com/kludex/starlette/pull/1262
First fixed release: 0.17.0
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be used if the application relies on the previous behavior of raising ExceptionGroup.
Verify Fix
verify
Re-run: python example.py
Did This Fix Work in Your Case?
Quick signal helps us prioritize which fixes to verify and improve.
Prevention
- Add a TLS smoke test that performs a real handshake in CI (include CA bundle validation and hostname checks).
- Alert on handshake failures by error string and endpoint to catch cert/CA changes quickly.
- 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.
Version Compatibility Table
| Version | Status |
|---|---|
| 0.17.0 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.