Facing issue while loading FastAPI swagger URL

Question

I am trying to run FastAPI application using uvicorn and I am using Python for this. After running the command "uvicorn app.main:app --host 0.0.0.0 --port 8000 --log-level debug", I am able to get the "Hello" message while accessing the URL but the moment I try to access the swagger document it shows the below error:

Fetch error:

Failed to fetch http://0.0.0.0:8000/openapi.json

Fetch error:

Possible cross-origin (CORS) issue? The URL origin (http://0.0.0.0:8000) does not match the page (http://127.0.0.1:8000). Check the server returns the correct 'Access-Control-Allow-*' headers.

Could anyone please help as I am new to this and kind of struggling.

I am expecting the swagger document to load successfully.

What's the reason for using 0.0.0.0 at all and not just using 127.0.0.1? If you plan to use different (local) ips for accessing the service, you'll have to configure CORS as shown in the FastAPI docs for those alternative ips: fastapi.tiangolo.com/tutorial/cors — MatsLindh
– MatsLindh, Commented Aug 20, 2024 at 8:01
You need to add more details on your app. Do you see the same error if you run simple app like this fastapi.tiangolo.com/#example ? — Yurii Motov
– Yurii Motov, Commented Aug 20, 2024 at 8:21

Philipp1297 · Accepted Answer · 2025-08-04 06:12:19Z

0

You can try the following things:

  uvicorn app.main:app --host 0.0.0.0 --port 8000 --log-level debug

You can access the service now from any IP-Address. Maybe also check your firewall settings. You can try to accept incoming calls from the port 8000 (outcoming and incoming rules)

If you need to access the Swagger UI from a different origin or domain, you may need to configure CORS in your FastAPI application. You can do this by using the fastapi.middleware.cors.CORSMiddleware:

edited Aug 4 at 6:12

answered Aug 20, 2024 at 9:34

Philipp1297

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1 Comment

Abriti Ghosh Over a year ago

Thanks for your suggestion. I tried changing the URL and used the above configuration mentioned and it worked.

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