^{Originally posted by falkoschindler December 3, 2025}

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Example Code

from functools import wraps
import uvicorn
from fastapi import FastAPI

app = FastAPI()

def my_decorator(func):
    """A decorator that wraps a sync function with an async handler."""

    @wraps(func)
    async def wrapper():
        func()
        return 'OK'

    return wrapper

@app.get('/')
@my_decorator
def index():
    """A simple sync page function."""
    print('Hello!')

if __name__ == '__main__':
    uvicorn.run(app)

Description

• Visit "/".
• You'll see "Internal Server Error".
• A ValueError appears:

  ValueError: [TypeError("'coroutine' object is not iterable"), TypeError('vars() argument must have __dict__ attribute')]
  

Since FastAPI 0.123.5 (PR #14434), using functools.wraps on an async wrapper around a sync function causes FastAPI to incorrectly treat the handler as sync, resulting in: ValueError: [TypeError("'coroutine' object is not iterable"), ...]

The issue is that Dependant.is_coroutine_callable now uses inspect.unwrap() which follows __wrapped__ back to the original sync function, instead of checking if the actual registered handler is async.

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.123.5

Pydantic Version

2.12.5

Python Version

3.11.11

Additional Context

No response