The Fix
Upgrade to version 0.11.4 or later.
Based on closed Kludex/uvicorn issue #529 · PR/commit linked
Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.
@@ -48,7 +48,6 @@
"asyncio": "uvicorn.loops.asyncio:asyncio_setup",
"uvloop": "uvicorn.loops.uvloop:uvloop_setup",
- "iocp": "uvicorn.loops.iocp:iocp_setup",
}
INTERFACES = ["auto", "asgi3", "asgi2", "wsgi"]
repro.py
ERROR: Accept failed on a socket
socket: <asyncio.TransportSocket fd=764, 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: This fix is not suitable for applications that rely on IOCP for performance.\n\n
Why This Fix Works in Production
- Trigger: ERROR: Accept failed on a socket
- Mechanism: Removes IOCP support and sets the event loop to SelectorEventLoop for Windows with Python 3.8 and above, resolving the reload issue.
- Why the fix works: Removes IOCP support and sets the event loop to SelectorEventLoop for Windows with Python 3.8 and above, resolving the reload issue. (first fixed release: 0.11.4).
Production impact:
- If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.
Why This Breaks in Prod
- Triggered by an upgrade/regression window: 3.8 breaks; 0.11.4 is the first fixed release.
- Shows up under Python 3.8 in real deployments (not just unit tests).
- Surfaces as: ERROR: Accept failed on a socket
Proof / Evidence
- GitHub issue: #529
- Fix PR: https://github.com/kludex/uvicorn/pull/535
- First fixed release: 0.11.4
- Affected versions: 3.8
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.37
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“This does not repro on py37, reloading works fine there.”
“Right, so the problem here is to do with IOCP”
“~I can trigger the same on windows and 3.7.5~ passing --loop="asyncio" is fine on 3.7.5 passing --loop="asyncio" on 3.8.0 the bug still here”
“>Does this only occur when using multiple workers / reload, or does it always occur on Python 3.8 with Windows? I'm not sure I understand…”
Failure Signature (Search String)
- ERROR: Accept failed on a socket
Error Message
Stack trace
error.txt
Error Message
-------------
ERROR: Accept failed on a socket
socket: <asyncio.TransportSocket fd=764, 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
ERROR: Accept failed on a socket
socket: <asyncio.TransportSocket fd=764, 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
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
Upgrade to version 0.11.4 or later.
When NOT to use: This fix is not suitable for applications that rely on IOCP for performance.
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/535
First fixed release: 0.11.4
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix is not suitable for applications that rely on IOCP for performance.
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 |
|---|---|
| 3.8 | Broken |
| 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.