uvicorn WSGI middleware error in non-latin1 path (Fix)

The Fix

Upgrade to version 0.13.4 or later.

Based on closed Kludex/uvicorn issue #961 · 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%.

Jump to Verify Open PR/Commit

@@ -3,7 +3,7 @@
 import pytest
 
-from uvicorn.middleware.wsgi import WSGIMiddleware
+from uvicorn.middleware.wsgi import WSGIMiddleware, build_environ

repro.py

import flask
import uvicorn

app = flask.Flask(__name__)

@app.route("/<keyword>")
def home(keyword):
    return keyword

if __name__ == "__main__":
    uvicorn.run(app, interface="wsgi")

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 0.13.4 or later.\nWhen NOT to use: Do not use this fix if your application does not handle non-latin1 paths.\n\n

Why This Fix Works in Production

Trigger: ERROR: Exception in ASGI application
Mechanism: The WSGI middleware incorrectly encodes PATH_INFO in utf8 instead of latin1
Why the fix works: Fixes the WSGI middleware PATH_INFO encoding issue for non-latin1 paths, addressing issue #961. (first fixed release: 0.13.4).

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

Shows up under Python 3.8.5 in real deployments (not just unit tests).
The WSGI middleware incorrectly encodes PATH_INFO in utf8 instead of latin1
Surfaces as: ERROR: Exception in ASGI application

Proof / Evidence

GitHub issue: #961
Fix PR: https://github.com/kludex/uvicorn/pull/962
First fixed release: 0.13.4
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.37

Discussion

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

Jump to Sources Open on GitHub

“### Checklist  - [x] The bug is reproducible against the latest release and/or master. - [x] There are no similar issues or pull requests to fix it yet. #”

Issue thread · issue description · source

Failure Signature (Search String)

ERROR: Exception in ASGI application

Error Message

Stack trace

error.txt

Error Message
-------------
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "C:\Users\cools\AppData\Local\Programs\Python\Python38\lib\site-packages\uvicorn\protocols\http\h11_impl.py", line 394, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "C:\Users\cools\AppData\Local\Programs\Python\Python38\lib\site-packages\uvicorn\middleware\proxy_headers.py", line 45, in __call__
    return await self.app(scope, receive, send)
  File "C:\Users\cools\AppData\Local\Programs\Python\Python38\lib\site-packages\uvicorn\middleware\wsgi.py", line 64, in __call__
    await instance(receive, send)
  File "C:\Users\cools\AppData\Local\Programs\Python\Python38\lib\site-packages\uvicorn\middleware\wsgi.py", line 95, in __call__
    await asyncio.wait_for(wsgi, None)
  File "C:\Users\cools\AppData\Local\Programs\Python\Python38\lib\asyncio\tasks.py", line 455, in wait_for
    return await fut
  File "C:\Users\cools\AppData\Local\Programs\Python\Python38\lib\concurrent\futures\thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "C:\Users\cools\AppData\Local\Programs\Python\Python38\lib\site-packages\uvicorn\middleware\wsgi.py", line 134, in wsgi
    for chunk in self.app(environ, start_response):
  File "C:\Users\cools\AppData\Local\Programs\Python\Python38\lib\site-packages\flask\app.py", line 2464, in __call__
    return self.wsgi
... (truncated) ...

Minimal Reproduction

repro.py

import flask
import uvicorn

app = flask.Flask(__name__)

@app.route("/<keyword>")
def home(keyword):
    return keyword

if __name__ == "__main__":
    uvicorn.run(app, interface="wsgi")

Environment

Python: 3.8.5

What Broke

Sending non-latin1 paths results in a 500 Internal Server Error.

Why It Broke

The WSGI middleware incorrectly encodes PATH_INFO in utf8 instead of latin1

Fix Options (Details)

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

Upgrade to version 0.13.4 or later.

When NOT to use: Do not use this fix if your application does not handle non-latin1 paths.

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/962

First fixed release: 0.13.4

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 handle non-latin1 paths.

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
0.13.4	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.