uvicorn Gunicorn deployments with `--preload` not working

The Fix

Upgrade to version 0.11.4 or later.

Based on closed Kludex/uvicorn issue #85 · 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

@@ -1,5 +1,5 @@
@@ -1,5 +1,5 @@
 from uvicorn.config import Config
-from uvicorn.main import main, run, Server
+from uvicorn.main import Server, main, run

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.11.4 or later.\nWhen NOT to use: Do not use this fix if your application does not utilize Gunicorn with preload.\n\n

Why This Fix Works in Production

Trigger: **Error:** `RuntimeError: Event loop is closed`
Mechanism: Gunicorn preloading causes each fork to receive a different event loop instance
Why the fix works: Addresses the issue with Gunicorn deployments using `--preload` that caused a `RuntimeError: Event loop is closed`. (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

Gunicorn preloading causes each fork to receive a different event loop instance
Production symptom (often without a traceback): **Error:** `RuntimeError: Event loop is closed`

Proof / Evidence

GitHub issue: #85
Fix PR: https://github.com/kludex/uvicorn/pull/312
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

“**Use case:** you want to launch an asgi app to a 128 core machine with 128 processes **Problem:** 128xprocess boot time takes a while, in dockered environments (like k8s) this becomes an issue, quick boots mean quick deploys **Preload:** i”

Issue thread · issue description · source

Failure Signature (Search String)

**Error:** `RuntimeError: Event loop is closed`

Copy-friendly signature

signature.txt

Failure Signature
-----------------
**Error:** `RuntimeError: Event loop is closed`

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
**Error:** `RuntimeError: Event loop is closed`

What Broke

Deployments fail with 'RuntimeError: Event loop is closed' during startup.

Why It Broke

Gunicorn preloading causes each fork to receive a different event loop instance

Fix Options (Details)

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

Upgrade to version 0.11.4 or later.

When NOT to use: Do not use this fix if your application does not utilize Gunicorn with preload.

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/312

First fixed release: 0.11.4

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

When NOT to Use This Fix

Do not use this fix if your application does not utilize Gunicorn with preload.

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.

uvicorn Gunicorn deployments with `--preload` not working properly. (Fix)