uvicorn Phase out `debug` in favour of `reload` (Fix)

The Fix

Upgrade to version 0.19.0 or later.

Based on closed Kludex/uvicorn issue #480 · 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

@@ -20,7 +20,6 @@ files =
     uvicorn/protocols/websockets/auto.py,
     uvicorn/supervisors/__init__.py,
-    uvicorn/middleware/debug.py,
     uvicorn/middleware/wsgi.py,
     uvicorn/supervisors/watchfilesreload.py,

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.19.0 or later.\nWhen NOT to use: Do not use this fix if your application relies on the `--debug` flag for error handling.\n\n

Why This Fix Works in Production

Trigger: * `--reload`/`reload=True` (A server-level concern. Reload the server on code changes.)
Mechanism: The `--debug` flag was hidden and not useful for applications using frameworks like Starlette
Why the fix works: Removes the `--debug` flag in favor of using `--reload`. (first fixed release: 0.19.0).

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

The `--debug` flag was hidden and not useful for applications using frameworks like Starlette
Production symptom (often without a traceback): * `--reload`/`reload=True` (A server-level concern. Reload the server on code changes.)

Proof / Evidence

GitHub issue: #480
Fix PR: https://github.com/kludex/uvicorn/pull/1640
First fixed release: 0.19.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.73

Discussion

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

Jump to Sources Open on GitHub

“Really we ought to phase out the usage of --debug/debug=True, in favour of... * --reload/reload=True (A server-level concern. Reload the server on code changes.) * app = Starlette(debug=True) (An application level concern. Issue nice HTML t”

Issue thread · issue description · source

Failure Signature (Search String)

* `--reload`/`reload=True` (A server-level concern. Reload the server on code changes.)
* `app = Starlette(debug=True)` (An application level concern. Issue nice HTML tracebacks for the "500 Server Error" case.)

Copy-friendly signature

signature.txt

Failure Signature
-----------------
* `--reload`/`reload=True` (A server-level concern. Reload the server on code changes.)
* `app = Starlette(debug=True)` (An application level concern. Issue nice HTML tracebacks for the "500 Server Error" case.)

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
* `--reload`/`reload=True` (A server-level concern. Reload the server on code changes.)
* `app = Starlette(debug=True)` (An application level concern. Issue nice HTML tracebacks for the "500 Server Error" case.)

What Broke

The presence of the `--debug` flag did not provide useful error information for clients.

Why It Broke

The `--debug` flag was hidden and not useful for applications using frameworks like Starlette

Fix Options (Details)

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

Upgrade to version 0.19.0 or later.

When NOT to use: Do not use this fix if your application relies on the `--debug` flag for error 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/1640

First fixed release: 0.19.0

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

When NOT to Use This Fix

Do not use this fix if your application relies on the `--debug` flag for error 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
0.19.0	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.