The Fix
Upgrade to version 0.11.4 or later.
Based on closed Kludex/uvicorn issue #509 · PR/commit linked
Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.
@@ -5,10 +5,7 @@ def auto_loop_setup():
import uvloop
except ImportError as exc: # pragma: no cover
- if sys.platform == "win32":
- from uvicorn.loops.iocp import iocp_setup as loop_setup
- else:
repro.py
INFO: Started server process [19676]
ERROR: Accept failed on a socket
socket: <asyncio.TransportSocket fd=696, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('127.0.0.1', 8000)>
Traceback (most recent call last):
File "C:\Users\f\scoop\apps\python\current\lib\asyncio\proactor_events.py", line 817, in loop
f = self._proactor.accept(sock)
File "C:\Users\f\scoop\apps\python\current\lib\asyncio\windows_events.py", line 545, in accept
self._register_with_iocp(listener)
File "C:\Users\f\scoop\apps\python\current\lib\asyncio\windows_events.py", line 714, in _register_with_iocp
_overlapped.CreateIoCompletionPort(obj.fileno(), self._iocp, 0, 0)
OSError: [WinError 87] The parameter is incorrect
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 you require uvloop functionality on non-Windows platforms.\n\n
Why This Fix Works in Production
- Trigger: ERROR: Accept failed on a socket
- Mechanism: The default event loop on Windows was set to IOCP, causing compatibility issues
- Why the fix works: Switches the default event loop on Windows from IOCP to Asyncio, addressing the issue where --reload was not functioning properly. (first fixed release: 0.11.4).
Production impact:
- If left unfixed, retries/timeouts can trigger duplicate external side-effects (double charges, duplicate emails, repeated writes).
Why This Breaks in Prod
- Shows up under Python 3.8 in real deployments (not just unit tests).
- The default event loop on Windows was set to IOCP, causing compatibility issues
- Surfaces as: INFO: Started server process [19676]
Proof / Evidence
- GitHub issue: #509
- Fix PR: https://github.com/kludex/uvicorn/pull/513
- 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: 0.39
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Is this supposed to now work on Python 3.8 on Windows? Because applying this patch to 0.10.8 doesn't seem to work:”
Failure Signature (Search String)
- ERROR: Accept failed on a socket
Error Message
Stack trace
error.txt
Error Message
-------------
INFO: Started server process [19676]
ERROR: Accept failed on a socket
socket: <asyncio.TransportSocket fd=696, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('127.0.0.1', 8000)>
Traceback (most recent call last):
File "C:\Users\f\scoop\apps\python\current\lib\asyncio\proactor_events.py", line 817, in loop
f = self._proactor.accept(sock)
File "C:\Users\f\scoop\apps\python\current\lib\asyncio\windows_events.py", line 545, in accept
self._register_with_iocp(listener)
File "C:\Users\f\scoop\apps\python\current\lib\asyncio\windows_events.py", line 714, in _register_with_iocp
_overlapped.CreateIoCompletionPort(obj.fileno(), self._iocp, 0, 0)
OSError: [WinError 87] The parameter is incorrect
Minimal Reproduction
repro.py
INFO: Started server process [19676]
ERROR: Accept failed on a socket
socket: <asyncio.TransportSocket fd=696, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('127.0.0.1', 8000)>
Traceback (most recent call last):
File "C:\Users\f\scoop\apps\python\current\lib\asyncio\proactor_events.py", line 817, in loop
f = self._proactor.accept(sock)
File "C:\Users\f\scoop\apps\python\current\lib\asyncio\windows_events.py", line 545, in accept
self._register_with_iocp(listener)
File "C:\Users\f\scoop\apps\python\current\lib\asyncio\windows_events.py", line 714, in _register_with_iocp
_overlapped.CreateIoCompletionPort(obj.fileno(), self._iocp, 0, 0)
OSError: [WinError 87] The parameter is incorrect
Environment
- Python: 3.8
What Broke
The server fails to accept connections, resulting in service unavailability.
Why It Broke
The default event loop on Windows was set to IOCP, causing compatibility issues
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 you require uvloop functionality on non-Windows platforms.
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/513
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 you require uvloop functionality on non-Windows platforms.
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
- Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
- Pin production dependencies and upgrade only with a reproducible test that hits the failing path.
Version Compatibility Table
| Version | Status |
|---|---|
| 0.11.4 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.