uvicorn Uvicorn fails to resolve host names in version

The Fix

Upgrade to version 0.12.3 or later.

Based on closed Kludex/uvicorn issue #825 · 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

@@ -33,6 +33,31 @@ def install_signal_handlers(self):
 
 
+def test_run_hostname():
+    class App:
+        def __init__(self, scope):

repro.py

import uvicorn

async def app(scope, receive, send):
    assert scope['type'] == 'http'

    await send({
        'type': 'http.response.start',
        'status': 200,
        'headers': [
            [b'content-type', b'text/plain'],
        ],
    })
    await send({
        'type': 'http.response.body',
        'body': b'Hello, world!',
    })

uvicorn.run(app, host='localhost', port=8083)

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 0.12.3 or later.\nWhen NOT to use: Do not use this fix if you require support for IPv6 addresses without explicit handling.\n\n

Why This Fix Works in Production

Trigger: return _run_code(code, main_globals, None,
Mechanism: Uvicorn fails to resolve host names due to improper handling of IP addresses in the code
Why the fix works: Fixes an issue where Uvicorn fails to resolve host names when using uvicorn.run with explicit hostnames. (first fixed release: 0.12.3).

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 in real deployments (not just unit tests).
Uvicorn fails to resolve host names due to improper handling of IP addresses in the code
Surfaces as: .py thon/debugpy/launcher 61232 -- /Users/paulafernandez/Sandbox/test/test/start

Proof / Evidence

GitHub issue: #825
Fix PR: https://github.com/kludex/uvicorn/pull/827
First fixed release: 0.12.3
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.38

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)

return _run_code(code, main_globals, None,

Error Message

Stack trace

error.txt

Error Message
-------------
.py thon/debugpy/launcher 61232 -- /Users/paulafernandez/Sandbox/test/test/start 
INFO:     Started server process [43171]
INFO:     Waiting for application startup.
INFO:     ASGI 'lifespan' protocol appears unsupported.
INFO:     Application startup complete.
Traceback (most recent call last):
  File "/usr/local/Cellar/[email protected]/3.8.5/Frameworks/Python.framework/Versions/3.8/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/local/Cellar/[email protected]/3.8.5/Frameworks/Python.framework/Versions/3.8/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/Users/paulafernandez/.vscode/extensions/ms-python.python-2020.9.114305/pythonFiles/lib/python/debugpy/__main__.py", line 45, in <module>
    cli.main()
  File "/Users/paulafernandez/.vscode/extensions/ms-python.python-2020.9.114305/pythonFiles/lib/python/debugpy/../debugpy/server/cli.py", line 430, in main
    run()
  File "/Users/paulafernandez/.vscode/extensions/ms-python.python-2020.9.114305/pythonFiles/lib/python/debugpy/../debugpy/server/cli.py", line 267, in run_file
    runpy.run_path(options.target, run_name=compat.force_str("__main__"))
  File "/usr/local/Cellar/[email protected]/3.8.5/Frameworks/Python.framework/Versions/3.8/lib/python3.8/runpy.py", line 265, in run_path
    return _run_module_code(code, init_globals, run_name,
  Fil
... (truncated) ...

Minimal Reproduction

repro.py

import uvicorn

async def app(scope, receive, send):
    assert scope['type'] == 'http'

    await send({
        'type': 'http.response.start',
        'status': 200,
        'headers': [
            [b'content-type', b'text/plain'],
        ],
    })
    await send({
        'type': 'http.response.body',
        'body': b'Hello, world!',
    })

uvicorn.run(app, host='localhost', port=8083)

Environment

Python: 3.8

What Broke

Uvicorn throws an error when attempting to start with host names, causing application startup failures.

Why It Broke

Uvicorn fails to resolve host names due to improper handling of IP addresses in the code

Fix Options (Details)

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

Upgrade to version 0.12.3 or later.

When NOT to use: Do not use this fix if you require support for IPv6 addresses without explicit 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/827

First fixed release: 0.12.3

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

When NOT to Use This Fix

Do not use this fix if you require support for IPv6 addresses without explicit handling.

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
0.12.3	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 Uvicorn fails to resolve host names in version 0.12.2 (Fix)