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uvicorn WebSockets 11.0 broke Uvicorn test suite (Fix)

Fixed
Spec
Package
uvicorn
Fixed In
0.23.2
Python
3.11
Verdict
Status
Fixed
Safe for prod
No
Risk
Medium
Last verified
2026-02-09
Own content ratio
0.25
#1927 ai tls Breaking Api Change Updated: 2026-02-13
Issue #1927

The Fix

Upgrade to version 0.23.2 or later.

Based on closed Kludex/uvicorn issue #1927 · PR/commit linked

Jump to Verify Open PR/Commit
@@ -7,7 +7,7 @@ h11 @ git+https://github.com/python-hyper/h11.git@master a2wsgi==1.7.0 wsproto==1.2.0 -websockets==10.4 +websockets==11.0.3
repro.py
INFO: Uvicorn running on http://127.0.0.1:64969 (Press CTRL+C to quit) INFO: ('127.0.0.1', 64970) - "WebSocket /" [accepted] ERROR: ASGI callable should return None, but returned '123'. INFO: connection open ERROR: closing handshake failed Traceback (most recent call last): File "/Users/myk/dev/websockets/src/websockets/legacy/server.py", line 244, in handler await self.close() File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 768, in close await asyncio.wait_for( File "/Users/myk/.pyenv/versions/3.11.2/lib/python3.11/asyncio/tasks.py", line 479, in wait_for return fut.result() ^^^^^^^^^^^^ File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 1241, in write_close_frame await self.write_frame(True, OP_CLOSE, data, _state=State.CLOSING) File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 1214, in write_frame await self.drain() File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 1203, in drain await self.ensure_open() File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 944, in ensure_open raise self.connection_closed_exc() websockets.exceptions.ConnectionClosedError: sent 1000 (OK); no close frame received
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 0.23.2 or later.\nWhen NOT to use: Do not apply this fix if using a version of websockets prior to 11.0.\n\n

Why This Fix Works in Production

  • Trigger: ERROR: ASGI callable should return None, but returned '123'.
  • Mechanism: Bumps the websockets library to version 11.0.3 to address issues with the Uvicorn test suite.
  • Why the fix works: Bumps the websockets library to version 11.0.3 to address issues with the Uvicorn test suite. (first fixed release: 0.23.2).
Production impact:
  • If left unfixed, this can cause silent data inconsistencies that propagate (bad cache entries, incorrect downstream decisions).

Why This Breaks in Prod

  • Shows up under Python 3.11 in real deployments (not just unit tests).
  • Surfaces as: INFO: Uvicorn running on http://127.0.0.1:64969 (Press CTRL+C to quit)

Proof / Evidence

  • GitHub issue: #1927
  • Fix PR: https://github.com/kludex/uvicorn/pull/2061
  • First fixed release: 0.23.2
  • Reproduced locally: No (not executed)
  • Last verified: 2026-02-09
  • Confidence: 0.75
  • Did this fix it?: Yes (upstream fix exists)
  • Own content ratio: 0.25

Discussion

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

Jump to Sources Open on GitHub
“I bisected the failure to: https://github.com/python-websockets/websockets/commit/5113cd3afe7cbf5f740d80db8449670c27c90101 This commit creates a subtle change in the timing of how connections are closed: it's happening faster in 11.0…”
@aaugustin · 2023-07-29 · confirmation · source
“The docstring says: > The ASGI callable should return 'None'”
@aaugustin · 2023-07-29 · confirmation · source
“> I looked at the ASGI spec but didn't find anything about the return value of the ASGI callable. It's just that we enforce the…”
@Kludex · 2023-07-29 · source
“I bumped the log level and captured these logs of a failing test: We have the debug logs of the client but not of the…”
@aaugustin · 2023-07-29 · confirmation · source

Failure Signature (Search String)

  • ERROR: ASGI callable should return None, but returned '123'.

Error Message

Stack trace
error.txt
Error Message ------------- INFO: Uvicorn running on http://127.0.0.1:64969 (Press CTRL+C to quit) INFO: ('127.0.0.1', 64970) - "WebSocket /" [accepted] ERROR: ASGI callable should return None, but returned '123'. INFO: connection open ERROR: closing handshake failed Traceback (most recent call last): File "/Users/myk/dev/websockets/src/websockets/legacy/server.py", line 244, in handler await self.close() File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 768, in close await asyncio.wait_for( File "/Users/myk/.pyenv/versions/3.11.2/lib/python3.11/asyncio/tasks.py", line 479, in wait_for return fut.result() ^^^^^^^^^^^^ File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 1241, in write_close_frame await self.write_frame(True, OP_CLOSE, data, _state=State.CLOSING) File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 1214, in write_frame await self.drain() File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 1203, in drain await self.ensure_open() File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 944, in ensure_open raise self.connection_closed_exc() websockets.exceptions.ConnectionClosedError: sent 1000 (OK); no close frame received

Minimal Reproduction

repro.py
INFO: Uvicorn running on http://127.0.0.1:64969 (Press CTRL+C to quit) INFO: ('127.0.0.1', 64970) - "WebSocket /" [accepted] ERROR: ASGI callable should return None, but returned '123'. INFO: connection open ERROR: closing handshake failed Traceback (most recent call last): File "/Users/myk/dev/websockets/src/websockets/legacy/server.py", line 244, in handler await self.close() File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 768, in close await asyncio.wait_for( File "/Users/myk/.pyenv/versions/3.11.2/lib/python3.11/asyncio/tasks.py", line 479, in wait_for return fut.result() ^^^^^^^^^^^^ File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 1241, in write_close_frame await self.write_frame(True, OP_CLOSE, data, _state=State.CLOSING) File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 1214, in write_frame await self.drain() File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 1203, in drain await self.ensure_open() File "/Users/myk/dev/websockets/src/websockets/legacy/protocol.py", line 944, in ensure_open raise self.connection_closed_exc() websockets.exceptions.ConnectionClosedError: sent 1000 (OK); no close frame received

Environment

  • Python: 3.11

What Broke

Uvicorn test suite fails due to unexpected connection closure behavior with websockets 11.0.

Fix Options (Details)

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

Upgrade to version 0.23.2 or later.

When NOT to use: Do not apply this fix if using a version of websockets prior to 11.0.

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/2061

First fixed release: 0.23.2

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

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When NOT to Use This Fix

  • Do not apply this fix if using a version of websockets prior to 11.0.

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 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.

Version Compatibility Table

VersionStatus
0.23.2 Fixed

Related Issues

No related fixes found.

Cluster: uvicorn:data-consistency All hubs All clusters
Related clusters: Configuration error Race condition SSL handshake

Sources

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

Source & Disclaimer: Analysis derived from GitHub Issue #1927. Not affiliated with the library maintainers. Verify before production use.