The Fix
Upgrade to version 0.12.0 or later.
Based on closed Kludex/uvicorn issue #788 · 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%.
@@ -33,6 +33,26 @@ Install using `pip`:
```
+This will install uvicorn with minimal (pure Python) dependencies.
+
+```shell
repro.py
$ pip3 install uvicorn==0.11.8 && pip3 freeze
click==7.1.2
h11==0.9.0
httptools==0.1.1
uvicorn==0.11.8
uvloop==0.14.0
websockets==8.1
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 0.12.0 or later.\nWhen NOT to use: This fix is not applicable if the application does not require optional dependencies.\n\n
Why This Fix Works in Production
- Trigger: Dependencies changed in 0.12.0
- Mechanism: Introduces optional package installs for Uvicorn, addressing the issue of missing dependencies in version 0.12.0.
- Why the fix works: Introduces optional package installs for Uvicorn, addressing the issue of missing dependencies in version 0.12.0. (first fixed release: 0.12.0).
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 3.8.3 in real deployments (not just unit tests).
- Production symptom (often without a traceback): Dependencies changed in 0.12.0
Proof / Evidence
- GitHub issue: #788
- Fix PR: https://github.com/encode/uvicorn/pull/666
- First fixed release: 0.12.0
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.74
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“I found https://github.com/encode/uvicorn/pull/666 which seems to be the source, so it solves my issue. It would be great if these changes was included in the…”
“Hi @vikahl! Thanks for reporting this :) I think given the tight constraints on developer resources for Uvicorn for now, and if you’ve got any…”
“> I think given the tight constraints on developer resources for Uvicorn for now, and if you’ve got any chance, a PR making the changelogs…”
Failure Signature (Search String)
- Dependencies changed in 0.12.0
- Version 0.12.0 removes some dependencies that causes our services to crash.
Copy-friendly signature
signature.txt
Failure Signature
-----------------
Dependencies changed in 0.12.0
Version 0.12.0 removes some dependencies that causes our services to crash.
Error Message
Signature-only (no traceback captured)
error.txt
Error Message
-------------
Dependencies changed in 0.12.0
Version 0.12.0 removes some dependencies that causes our services to crash.
Minimal Reproduction
repro.py
$ pip3 install uvicorn==0.11.8 && pip3 freeze
click==7.1.2
h11==0.9.0
httptools==0.1.1
uvicorn==0.11.8
uvloop==0.14.0
websockets==8.1
Environment
- Python: 3.8.3
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
Upgrade to version 0.12.0 or later.
When NOT to use: This fix is not applicable if the application does not require optional dependencies.
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Option D — Guard side-effects with OnceOnly Guardrail for side-effects
Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.
- Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
- Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
- Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.
Show example snippet (optional)
onceonly.py
from onceonly import OnceOnly
import os
once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)
# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..." # replace
key = f"stripe:webhook:{event_id}"
res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
return {"status": "already_processed"}
# Safe to execute the side-effect exactly once.
handle_event(event_id)
When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.
Fix reference: https://github.com/encode/uvicorn/pull/666
First fixed release: 0.12.0
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 optional dependencies.
- Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.
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 |
|---|---|
| 0.12.0 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.