FastAPI API server runs but does not respond on localhost (Uvicorn, Supabase)

[Lukas-Hi]

Problem:
I am running a FastAPI application with Uvicorn and Supabase as the database.
The server starts successfully (Uvicorn running on http://127.0.0.1:8000), but I cannot access it via Invoke-RestMethod or curl.
I keep getting "Connection could not be established" or "500 Internal Server Error".

What I have tried:
✅ Checked that no other processes are using port 8000 (netstat -ano | findstr :8000)
✅ Restarted Uvicorn with different options:

bash

uvicorn app:app --host 0.0.0.0 --port 8000 --log-level debug
uvicorn app:app --host 127.0.0.1 --port 8001 --log-level debug
✅ Disabled Windows Firewall temporarily to test access
✅ Verified that Supabase responds correctly via Python:

python

from config import supabase
response = supabase.table("code_snippets").select("*").execute()
print(response.data)
✅ Ran FastAPI with a minimal app.py to isolate the issue:

python

from fastapi import FastAPI
app = FastAPI()

@app.get("/")
async def root():
return {"message": "API is running!"}
✅ Tested with curl and Invoke-RestMethod, but still getting errors.

What I expected:

The FastAPI application should be accessible via 127.0.0.1:8000/
The /test endpoint should return { "status": "ok" }
Database queries should work correctly with Supabase
Error Message from Uvicorn:
(Paste your most recent Uvicorn error message here)

System Information:

OS: Windows 10
Python Version: 3.10.11
FastAPI Version: Latest
Uvicorn Version: Latest
Supabase SDK Version: Latest
Question:
Has anyone experienced a similar issue where FastAPI runs but does not respond?
Could there be a specific Windows networking issue that is blocking the API access?
Are there any known issues with Supabase that could interfere with FastAPI startup?

[silentworks]

This is a bit confusing as you are asking about a FastAPI/Windows issue on the Supabase forum.

[Lukas-Hi]

Hi Andrew,

Thank you for responding to my question.

I was running a FastAPI application using Uvicorn as the ASGI server and Supabase as the database.

✅ The server starts successfully (Uvicorn running on http://127.0.0.1:xxxx).
❌ But I could not access it via curl or Invoke-RestMethod.
❌ I get either:

"Connection could not be established"

"500 Internal Server Error"

Here you can find the whole history to this issue:

https://stackoverflow.com/beta/discussions/79444035/fastapi-api-server-runs-but-does-not-respond-on-localhost-uvicorn-supabase/79452267

As you are a real superpro - does that make any sense to you?

with Respect
Lukas-Hi