FastAPI Query fields with numeric types appear not to

The Fix

pip install fastapi==0.128.4

Based on closed fastapi/fastapi issue #11580 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -91,7 +91,7 @@ def __init__(
             discriminator=discriminator,
             multiple_of=multiple_of,
-            allow_nan=allow_inf_nan,
+            allow_inf_nan=allow_inf_nan,
             max_digits=max_digits,

repro.py

import aiohttp
import asyncio
import math
import uvicorn

from fastapi import FastAPI, Query, Request, Response
from http import HTTPStatus
from typing import Annotated

app = FastAPI()

@app.get('/')
async def get(
        x: Annotated[float | None, Query(gt=0,                description='x')] = 1,
        y: Annotated[float | None, Query(allow_inf_nan=False, description='y')] = 0) -> str:

    assert x > 0
    assert not math.isnan(y) and not math.isinf(y)

    return 'OK'


async def main():

    config = uvicorn.Config(app, host='127.0.0.1', port=8001)
    server = uvicorn.Server(config)
    task = asyncio.create_task(server.serve())
    
    await asyncio.sleep(.1)

    async with aiohttp.ClientSession() as session:

        async with session.get('http://127.0.0.1:8001/?x=-1') as response:

            assert response.status == HTTPStatus.UNPROCESSABLE_ENTITY
    
        async with session.get('http://127.0.0.1:8001/?y=inf') as response:

            assert response.status == HTTPStatus.UNPROCESSABLE_ENTITY
    
    await server.shutdown()

if __name__ == '__main__':
    asyncio.get_event_loop().run_until_complete(main())

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\npip install fastapi==0.128.4\nWhen NOT to use: This fix should not be applied if backward compatibility with previous versions is required.\n\n

Why This Fix Works in Production

Trigger: ERROR: Exception in ASGI application
Mechanism: The allow_inf_nan parameter was ignored during parameter enforcement in FastAPI
Why the fix works: Fixes the issue where the allow_inf_nan parameter was ignored during parameter enforcement in FastAPI. (first fixed release: 0.128.4).

Production impact:

If left unfixed, this can cause silent data inconsistencies that propagate (bad cache entries, incorrect downstream decisions).

Why This Breaks in Prod

Shows up under Python 3.10 in real deployments (not just unit tests).
The allow_inf_nan parameter was ignored during parameter enforcement in FastAPI
Surfaces as: INFO: Started server process [4301]

Proof / Evidence

GitHub issue: #11580
Fix PR: https://github.com/fastapi/fastapi/pull/11867
First fixed release: 0.128.4
Reproduced locally: No (not executed)
Last verified: 2026-02-08
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.28

Discussion

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

Jump to Sources Open on GitHub

“This should be solved by https://github.com/fastapi/fastapi/pull/11867, the fix will be available in FastAPI 0.112.2 released in the next few hours. :tada:”

@tiangolo · 2024-08-24 · confirmation · source

“I made a PR for that at #11867 There was a little typo I suppose when passing the parameters to the pydantic object”

@giunio-prc · 2024-07-19 · source

Failure Signature (Search String)

ERROR: Exception in ASGI application

Error Message

Stack trace

error.txt

Error Message
-------------
INFO:     Started server process [4301]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8001 (Press CTRL+C to quit)
INFO:     127.0.0.1:58094 - "GET /?x=-1 HTTP/1.1" 422 Unprocessable Entity
INFO:     127.0.0.1:58094 - "GET /?y=inf HTTP/1.1" 500 Internal Server Error
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/layer_svc/test/env/lib/python3.10/site-packages/uvicorn/protocols/http/httptools_impl.py", line 411, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
  File "/layer_svc/test/env/lib/python3.10/site-packages/uvicorn/middleware/proxy_headers.py", line 69, in __call__
    return await self.app(scope, receive, send)
  File "/layer_svc/test/env/lib/python3.10/site-packages/fastapi/applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  File "/layer_svc/test/env/lib/python3.10/site-packages/starlette/applications.py", line 123, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/layer_svc/test/env/lib/python3.10/site-packages/starlette/middleware/errors.py", line 186, in __call__
    raise exc
  File "/layer_svc/test/env/lib/python3.10/site-packages/starlette/middleware/errors.py", line 164, in __call__
    await self.app(scope, receive, _send)
  File "/layer_svc/test/env/lib/py
... (truncated) ...

Minimal Reproduction

repro.py

import aiohttp
import asyncio
import math
import uvicorn

from fastapi import FastAPI, Query, Request, Response
from http import HTTPStatus
from typing import Annotated

app = FastAPI()

@app.get('/')
async def get(
        x: Annotated[float | None, Query(gt=0,                description='x')] = 1,
        y: Annotated[float | None, Query(allow_inf_nan=False, description='y')] = 0) -> str:

    assert x > 0
    assert not math.isnan(y) and not math.isinf(y)

    return 'OK'


async def main():

    config = uvicorn.Config(app, host='127.0.0.1', port=8001)
    server = uvicorn.Server(config)
    task = asyncio.create_task(server.serve())
    
    await asyncio.sleep(.1)

    async with aiohttp.ClientSession() as session:

        async with session.get('http://127.0.0.1:8001/?x=-1') as response:

            assert response.status == HTTPStatus.UNPROCESSABLE_ENTITY
    
        async with session.get('http://127.0.0.1:8001/?y=inf') as response:

            assert response.status == HTTPStatus.UNPROCESSABLE_ENTITY
    
    await server.shutdown()

if __name__ == '__main__':
    asyncio.get_event_loop().run_until_complete(main())

Environment

Python: 3.10

What Broke

Numeric query fields accepted invalid inf and nan values, leading to unexpected behavior.

Why It Broke

The allow_inf_nan parameter was ignored during parameter enforcement in FastAPI

Fix Options (Details)

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

pip install fastapi==0.128.4

When NOT to use: This fix should not be applied if backward compatibility with previous versions is required.

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

Fix reference: https://github.com/fastapi/fastapi/pull/11867

First fixed release: 0.128.4

Last verified: 2026-02-08. Validate in your environment.

When NOT to Use This Fix

This fix should not be applied if backward compatibility with previous versions is required.

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
0.128.4	Fixed

Related Issues

No related fixes found.

Cluster: fastapi:data-consistency FastAPI hub FastAPI best practices All hubs All clusters

Related clusters: Configuration error Rate limit Race condition

Sources

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

FastAPI Query fields with numeric types appear not to enforce allow_inf_nan=False, whereas gt=0, (Fix)