Does OAuth2PasswordBearer verify the token?

No! This is a critical distinction. It only EXTRACTS the string from the header. It does not check if the token is an expired JWT, or if it belongs to a real user. You must write a secondary dependency to actually verify the string.

Why does the Swagger UI suddenly have an 'Authorize' button?

Because FastAPI reads your dependencies. When it sees `OAuth2PasswordBearer` injected into a route, it automatically updates the OpenAPI JSON schema to indicate that the route requires OAuth2 authentication. Swagger UI reads this schema and renders the Login UI automatically.

What if an endpoint needs an optional token?

If a route can be accessed by both guests and logged-in users, you can configure the `OAuth2PasswordBearer(auto_error=False)`. It will return the token string if present, or `None` if missing, without throwing a 401 error.

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Security via Dependency Injection

Learn how to architect security in FastAPI. Master the OAuth2PasswordBearer class, construct multi-layered authentication dependencies, and apply router-level security constraints to keep your API locked down.

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