uvicorn Error during WebSocket handshake: Unexpected

The Fix

Upgrade to version 0.14.0 or later.

Based on closed Kludex/uvicorn issue #797 · 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%.

Jump to Verify Open PR/Commit

@@ -266,6 +266,13 @@ def handle_upgrade(self, event):
             msg = "Unsupported upgrade request."
             self.logger.warning(msg)
+
+            from uvicorn.protocols.websockets.auto import AutoWebSocketsProtocol
+

repro.py

❯ uvicorn fapiws:app --reload
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [8960] using watchgod
INFO:     Started server process [8970]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     127.0.0.1:43636 - "GET / HTTP/1.1" 200 OK
INFO:     ('127.0.0.1', 43640) - "WebSocket /ws" [accepted]

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 0.14.0 or later.\nWhen NOT to use: This fix is not applicable if the application does not require WebSocket functionality.\n\n

Why This Fix Works in Production

Trigger: Error during WebSocket handshake: Unexpected response code: 400 (anonymous) @ (index):16
Mechanism: The new packaging does not install websocket or h11 by default, leading to handshake errors
Why the fix works: Improves user feedback when no WebSocket library is installed, providing a warning message. (first fixed release: 0.14.0).

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

Shows up under Python 3.8.2 in real deployments (not just unit tests).
The new packaging does not install websocket or h11 by default, leading to handshake errors
Surfaces as: Error during WebSocket handshake: Unexpected response code: 400 (anonymous) @ (index):16

Proof / Evidence

GitHub issue: #797
Fix PR: https://github.com/kludex/uvicorn/pull/926
First fixed release: 0.14.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.62

Discussion

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

Jump to Sources Open on GitHub

“ok the new packaging does not install websocket or h11 by default so with a simple pip install uvicorn no ws will be handled”

@euri10 · 2020-10-05 · source

“> In zsh on macos 10.15.5 i got the error > zsh: no matches found: uvicorn[standard] > > I had to escape the square brackets:…”

@euri10 · 2020-10-05 · source

“Definitely should provide a gentle message...”

@konoufo · 2020-11-15 · source

“In zsh on macos 10.15.5 i got the error zsh: no matches found: uvicorn[standard] I had to escape the square brackets: pip3 install uvicorn\[standard\]”

@KoehlerClem · 2020-10-05 · source

Failure Signature (Search String)

Error during WebSocket handshake: Unexpected response code: 400 (anonymous) @ (index):16

Error Message

Stack trace

error.txt

Error Message
-------------
Error during WebSocket handshake: Unexpected response code: 400 (anonymous) @ (index):16

Minimal Reproduction

repro.py

❯ uvicorn fapiws:app --reload
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [8960] using watchgod
INFO:     Started server process [8970]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     127.0.0.1:43636 - "GET / HTTP/1.1" 200 OK
INFO:     ('127.0.0.1', 43640) - "WebSocket /ws" [accepted]

Environment

Python: 3.8.2

What Broke

WebSocket connections fail with a 400 error during handshake, causing application functionality issues.

Why It Broke

The new packaging does not install websocket or h11 by default, leading to handshake errors

Fix Options (Details)

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

Upgrade to version 0.14.0 or later.

When NOT to use: This fix is not applicable if the application does not require WebSocket functionality.

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/926

First fixed release: 0.14.0

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

When NOT to Use This Fix

This fix is not applicable if the application does not require WebSocket functionality.

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

Version	Status
0.14.0	Fixed

Related Issues

No related fixes found.

Cluster: uvicorn:configuration-error All hubs All clusters

Related clusters: Data consistency Race condition SSL handshake

Sources

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

uvicorn Error during WebSocket handshake: Unexpected response code: 400 (anonymous) @ (index):16 (Fix)