Auto-served routes

The FastAPI constructor registers a small set of routes for documentation and schema serving. Each can be disabled or relocated.

Default path	Default method	Constructor parameter	Source	Purpose
/openapi.json	GET	openapi_url	fastapi/applications.py	Serves the OpenAPI document produced by app.openapi(). Set to None to hide.
/docs	GET	docs_url	fastapi/openapi/docs.py:get_swagger_ui_html	Swagger UI page that loads openapi_url. Set to None to hide.
/docs/oauth2-redirect	GET	swagger_ui_oauth2_redirect_url	fastapi/openapi/docs.py:get_swagger_ui_oauth2_redirect_html	Tiny page that bridges OAuth2 authorization-code flow back to Swagger UI. Set to None to disable; OAuth2 still works for password/client-credentials flows.
/redoc	GET	redoc_url	fastapi/openapi/docs.py:get_redoc_html	ReDoc page that loads openapi_url. Set to None to hide.

All four endpoints have include_in_schema=False, so they do not appear in the OpenAPI document themselves.

Customising the docs UIs

FastAPI.__init__ accepts knobs for each:

• swagger_ui_init_oauth: dict — passed to Swagger UI's initOAuth(...) call. Useful for prefilling clientId, appName, etc.
• swagger_ui_parameters: dict — extra UI configuration, merged onto swagger_ui_default_parameters from fastapi/openapi/docs.py.
• swagger_js_url, swagger_css_url, swagger_favicon_url — override the CDN URLs.
• redoc_js_url, redoc_favicon_url — same for ReDoc.

For air-gapped environments, host the JS/CSS yourself and point the URLs at your static server.

Implementation

The constructor calls setup() which adds the routes when the corresponding *_url parameters are non-None. Each route is a path-operation function that returns an HTMLResponse (for /docs and /redoc) or a JSONResponse (for /openapi.json) built from app.openapi().

The OpenAPI dict is cached on app.openapi_schema. Mutating it after first generation is supported and survives across requests until you set app.openapi_schema = None or restart the app.

Mounted-app caveat

When a FastAPI instance is mounted at a sub-path (for example app.mount("/api", sub_app)), the auto-served routes use paths relative to the mount. Swagger UI inside sub_app will load /api/openapi.json. The OpenAPI generator handles root_path correctly through Starlette's request scope.