[FAQ] Why does my FastAPI backend return "detail: Not Found" at localhost:8000?

Course

ai-dev-tools-zoomcamp

Question

Why does http://localhost:8000 return {"detail":"Not Found"} after I start my FastAPI backend?

Answer

Your FastAPI server is running correctly. The {"detail":"Not Found"} response means that the application has no endpoint defined for the root path (/).

Open the interactive API documentation instead:

http://localhost:8000/docs

Or call an endpoint that your application defines, for example:

http://localhost:8000/v1/household/state

If you want http://localhost:8000/ to show a response, add a root route to your FastAPI application:

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def health_check():
    return {"message": "API is running"}

FastAPI only responds to paths you define with route decorators such as @app.get("/"). When no matching route exists, it returns a 404 Not Found response with {"detail":"Not Found"}.

Checklist

• I have searched existing FAQs and this question is not already answered
• The answer provides accurate, helpful information
• I have included any relevant code examples or links

Review the FAQ question and answer in this issue, including the /docs and /v1/household/state examples. Verify that the FastAPI behavior and root-route example are accurate, then consider the FAQ complete when the explanation, links, and code example clearly answer why localhost:8000 returns 404.