Authentication is one of the most important—and misunderstood—parts of backend development. In 2025, developers building Python APIs with frameworks like FastAPI, Django, or Flask still face a critical decision:

Should I use JWT or Session authentication?

Let's break down how both work, what they're good at, and how to choose the right one for your project.

What Is Session Authentication?

Session authentication is the classic, cookie-based method used in web apps.

How it works:

1. User logs in
2. Server creates a session and stores it in memory or a database
3. Server sends a session ID back as a cookie
4. Client sends that cookie on every request
5. Server uses the session ID to validate and authorize the user

What Is JWT (JSON Web Token) Authentication?

JWT is a stateless authentication method that works perfectly for APIs, SPAs, and mobile apps.

How it works:

1. User logs in
2. Server signs a token (usually with user ID + expiry)
3. Token is sent to the client (usually via Authorization header)
4. Client sends that token on every request