How to Access API Documentation in FastAPI Easily

FastAPI automatically generates interactive API documentation accessible at /docs for Swagger UI and /redoc for ReDoc. You just run your FastAPI app and open these URLs in your browser to explore and test your API.

Syntax

FastAPI provides two default documentation interfaces accessible via specific URLs:

• /docs: Shows Swagger UI, an interactive API explorer.
• /redoc: Shows ReDoc, a clean and readable API doc.

You do not need to add extra code to enable these; they are included by default when you create a FastAPI() app instance.

python

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
async def read_root():
    return {"message": "Hello World"}

# Run the app and visit http://127.0.0.1:8000/docs or /redoc

Example

This example shows a simple FastAPI app with one endpoint. After running it, you can access the docs at /docs or /redoc to see the API documentation and test the endpoint interactively.

python

from fastapi import FastAPI

app = FastAPI()

@app.get("/items/{item_id}")
async def read_item(item_id: int, q: str | None = None):
    return {"item_id": item_id, "q": q}

# Run with: uvicorn filename:app --reload
# Access docs at: http://127.0.0.1:8000/docs or /redoc

Output

Open your browser and go to http://127.0.0.1:8000/docs to see Swagger UI with the GET /items/{item_id} endpoint documented and testable.

Common Pitfalls

1. Docs not showing: This can happen if you disable docs by setting docs_url=None or run behind a proxy without correct URL paths.

2. Custom docs URLs: If you change docs_url or redoc_url in FastAPI(), you must visit the new URLs.

3. Missing dependencies: FastAPI uses swagger-ui and redoc from CDN, so no extra install is needed, but blocking internet access can cause docs not to load.

python

from fastapi import FastAPI

# Disabling docs example
app = FastAPI(docs_url=None, redoc_url=None)

@app.get("/")
async def root():
    return {"msg": "No docs available"}

Quick Reference

Feature	Default URL	Description
Swagger UI docs	/docs	Interactive API explorer with testing
ReDoc docs	/redoc	Clean, readable API documentation
OpenAPI JSON schema	/openapi.json	Raw API schema in JSON format
Disable docs	Set docs_url=None	Turn off docs if needed
Custom docs URL	docs_url='/custom'	Change docs path

Key Takeaways

FastAPI auto-generates docs at /docs (Swagger UI) and /redoc (ReDoc) by default.

Run your app and open these URLs in a browser to explore and test your API.

You can customize or disable docs URLs using FastAPI constructor parameters.

Docs require internet access to load UI assets from CDN unless self-hosted.

If docs don’t appear, check if docs_url or redoc_url were changed or disabled.