[Python] FastAPI route works locally with `/` but not on Vercel (and vice versa)

I have a FastAPI + Next.js project deployed on Vercel. Locally everything works, but on Vercel some routes break depending on whether I use a trailing slash in the route definition.

For example, in my usrs.py router:

from fastapi import APIRouter, Depends
from ..schemas.user import UserOut
from ..auth import require_admin
from ..repositories.users import list_users

user_router = APIRouter(prefix="/users", tags=["users"])

@user_router.get("/", response_model=list[UserOut])
def get_users(_: UserOut = Depends(require_admin)):
return list_users()


Locally (Uvicorn)

GET http://localhost:8001/api/users/ works. GET http://localhost:8001/api/users redirects to /users/ as expected.

On Vercel (serverless FastAPI):

GET https://myapp.vercel.app/api/users/ → 500 GET https://myapp.vercel.app/api/users 404 or .vercel.app/api/users/ net::ERR_TOO_MANY_REDIRECTS

So I tried removing the slash in the route definition:

@user_router.get("", response_model=list[UserOut])
def get_users(_: UserOut = Depends(require_admin)):
return list_users()


Now it works on Vercel, but fails locally (404).

What I’ve checked

DB and schema are fine (/api/db-check and /api/_schema_status return OK).

Other endpoints like /api/healthz and /api/users/login work fine.

My next.config.js proxies /api/ath* → FastAPI in dev, and Vercel vercel.json routes /api/(.*) → /api/index.py.

Question

1. Why is there this incompatibility between local and Vercel regarding trailing slashes?
2. How can I define my FastAPI routes so they work both locally and on Vercel without having to change code?

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