uvicorn ValueError: 'localhost' does not appear to be an

The Fix

Upgrade to version 0.12.2 or later.

Based on closed Kludex/uvicorn issue #836 · PR/commit linked

Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.

Jump to Verify Open PR/Commit

@@ -7,6 +7,7 @@
 import ssl
 import sys
+from enum import Enum
 from typing import List, Tuple

repro.py

# server.py
import uvicorn
from starlette.applications import Starlette

app = Starlette()

if __name__ == "__main__":
    uvicorn.run(app, host="localhost")

verify

Re-run: python debug/server.py

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.12.2 or later.\nWhen NOT to use: Do not use this fix if the application requires a different host configuration.\n\n

Why This Fix Works in Production

Trigger: $ python debug/server.py
Mechanism: The server fails to start due to a ValueError when using 'localhost' as the host
Why the fix works: Fix reload with ipv6 host addresses to prevent ValueError when using 'localhost'. (first fixed release: 0.12.2).

Production impact:

If left unfixed, retries/timeouts can trigger duplicate external side-effects (double charges, duplicate emails, repeated writes).

Why This Breaks in Prod

Shows up under Python 3.9 in real deployments (not just unit tests).
The server fails to start due to a ValueError when using 'localhost' as the host
Surfaces as: $ python debug/server.py

Proof / Evidence

GitHub issue: #836
Fix PR: https://github.com/encode/uvicorn/pull/803
First fixed release: 0.12.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.40

Discussion

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

Jump to Sources Open on GitHub

“@euri10 Oops, just found out that there's a fix in master for this already — #827”

@florimondmanca · 2020-10-24 · source

Failure Signature (Search String)

$ python debug/server.py

Error Message

Stack trace

error.txt

Error Message
-------------
$ python debug/server.py                   
INFO:     Started server process [86782]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
Traceback (most recent call last):
  File "/Users/florimond/Developer/python-projects/httpx/debug/server.py", line 7, in <module>
    uvicorn.run(
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.9/site-packages/uvicorn/main.py", line 391, in run
    server.run()
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.9/site-packages/uvicorn/main.py", line 419, in run
    loop.run_until_complete(self.serve(sockets=sockets))
  File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/asyncio/base_events.py", line 642, in run_until_complete
    return future.result()
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.9/site-packages/uvicorn/main.py", line 436, in serve
    await self.startup(sockets=sockets)
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.9/site-packages/uvicorn/main.py", line 526, in startup
    if isinstance(ip_address(config.host), IPv6Address):
  File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/ipaddress.py", line 53, in ip_address
    raise ValueError('%r does not appear to be an IPv4 or IPv6 address' %
ValueError: 'localhost' does not appear to be an IPv4 or IPv6 address

Minimal Reproduction

repro.py

# server.py
import uvicorn
from starlette.applications import Starlette

app = Starlette()

if __name__ == "__main__":
    uvicorn.run(app, host="localhost")

Environment

Python: 3.9

What Broke

Server fails to start, raising an exception when attempting to bind to 'localhost'.

Why It Broke

The server fails to start due to a ValueError when using 'localhost' as the host

Fix Options (Details)

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

Upgrade to version 0.12.2 or later.

When NOT to use: Do not use this fix if the application requires a different host configuration.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Option D — Guard side-effects with OnceOnly Guardrail for side-effects

Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.

Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.
This is most useful when retries/timeouts can re-trigger the same external call.

Show example snippet (optional)

onceonly.py

from onceonly import OnceOnly
import os

once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)

# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..."  # replace
key = f"stripe:webhook:{event_id}"

res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
    return {"status": "already_processed"}

# Safe to execute the side-effect exactly once.
handle_event(event_id)

See OnceOnly SDK

When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Fix reference: https://github.com/encode/uvicorn/pull/803

First fixed release: 0.12.2

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

When NOT to Use This Fix

Do not use this fix if the application requires a different host configuration.
Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Verify Fix

verify

Re-run: python debug/server.py

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Add a stress test that runs high-concurrency workloads and fails on thread dumps / blocked locks.
Enable watchdog dumps in prod (faulthandler, thread dump endpoint) to capture deadlocks quickly.

Version Compatibility Table

Version	Status
0.12.2	Fixed

Related Issues

No related fixes found.

Cluster: uvicorn:duplicate-processing All hubs All clusters

Related clusters: Configuration error Data consistency Race condition

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

uvicorn ValueError: 'localhost' does not appear to be an IPv4 or IPv6 address (Fix)