uvicorn Finesse "Connection: Upgrade" case. (Fix)

The Fix

Upgrade to version 0.11.4 or later.

Based on closed Kludex/uvicorn issue #123 · 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%.

Open PR/Commit

@@ -1,8 +1,11 @@
 click
 h11
+
+# Optional
 httptools

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.11.4 or later.\nWhen NOT to use: This fix should not be used if the application requires strict upgrade handling.\n\n

Why This Fix Works in Production

Trigger: * Return graceful response when "Upgrade" is an unsupported option.
Mechanism: The websocket implementation did not handle unsupported upgrade requests properly
Why the fix works: Refactor the websocket implementation to handle unsupported upgrade requests gracefully. (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

The websocket implementation did not handle unsupported upgrade requests properly
Production symptom (often without a traceback): * Return graceful response when "Upgrade" is an unsupported option.

Proof / Evidence

GitHub issue: #123
Fix PR: https://github.com/kludex/uvicorn/pull/174
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: 0.84

Discussion

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

Jump to Sources Open on GitHub

“* Return graceful response when "Upgrade" is an unsupported option. * Check for comma separated cases as in "Connection: Upgrade, Keep-Alive" Related to #121. I suspect we probably just hang on h11 for this case right now. Probably return a”

Issue thread · issue description · source

Failure Signature (Search String)

* Return graceful response when "Upgrade" is an unsupported option.
I suspect we probably just hang on h11 for this case right now.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
* Return graceful response when "Upgrade" is an unsupported option.
I suspect we probably just hang on h11 for this case right now.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
* Return graceful response when "Upgrade" is an unsupported option.
I suspect we probably just hang on h11 for this case right now.

What Broke

Requests with unsupported upgrade headers resulted in hanging connections.

Why It Broke

The websocket implementation did not handle unsupported upgrade requests properly

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 should not be used if the application requires strict upgrade handling.

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/174

First fixed release: 0.11.4

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

When NOT to Use This Fix

This fix should not be used if the application requires strict upgrade handling.

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.

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.