uvicorn UNIX domain socket raises 503 when workers are used

The Fix

Upgrade to version 0.15.0 or later.

Based on closed Kludex/uvicorn issue #428 · 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

@@ -355,3 +355,47 @@ def test_ws_max_size() -> None:
     config.load()
     assert config.ws_max_size == 1000
+
+
[email protected](

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.15.0 or later.\nWhen NOT to use: Do not use this fix if you require TCP sockets instead of UNIX domain sockets.\n\n

Why This Fix Works in Production

Trigger: UNIX domain socket raises 503 when workers are used
Mechanism: Uvicorn ignores the --uds argument and creates a TCP socket when using multiple workers
Why the fix works: Adapts the bind_socket function to support using UNIX domain sockets with multiple processes, resolving the issue where combining --workers and --uds raised a 503 error. (first fixed release: 0.15.0).

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

Uvicorn ignores the --uds argument and creates a TCP socket when using multiple workers
Production symptom (often without a traceback): UNIX domain socket raises 503 when workers are used

Proof / Evidence

GitHub issue: #428
Fix PR: https://github.com/kludex/uvicorn/pull/1009
First fixed release: 0.15.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.79

Discussion

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

Jump to Sources Open on GitHub

“It seems that when combine the --workers and --uds arguments”

@Hanaasagi · 2019-09-20 · source

Failure Signature (Search String)

UNIX domain socket raises 503 when workers are used
Is it Ok when a combination of `--workers ` and `--uds` raises 503 error via nginx? For example:

Copy-friendly signature

signature.txt

Failure Signature
-----------------
UNIX domain socket raises 503 when workers are used
Is it Ok when a combination of `--workers ` and `--uds` raises 503 error via nginx? For example:

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
UNIX domain socket raises 503 when workers are used
Is it Ok when a combination of `--workers ` and `--uds` raises 503 error via nginx? For example:

What Broke

Using multiple workers with UNIX domain sockets results in 503 errors from nginx.

Why It Broke

Uvicorn ignores the --uds argument and creates a TCP socket when using multiple workers

Fix Options (Details)

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

Upgrade to version 0.15.0 or later.

When NOT to use: Do not use this fix if you require TCP sockets instead of UNIX domain sockets.

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/1009

First fixed release: 0.15.0

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

When NOT to Use This Fix

Do not use this fix if you require TCP sockets instead of UNIX domain sockets.

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
0.15.0	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 UNIX domain socket raises 503 when workers are used (Fix)