Choosing the right hosting model is the most important architectural decision in a Blazor project.
The app runs on the server. UI events are sent to the server over a SignalR connection. The server executes logic and sends back UI diffs.
**Pros:** Fast initial load, full access to server resources, small download size.
**Cons:** High server latency, requires a persistent connection, high server memory usage per user.
The app, the .NET runtime, and all dependencies are downloaded to the browser.
**Pros:** Zero server load, works offline (PWA), predictable performance.
**Cons:** Large initial download (several MBs), slower startup time, browser resource constraints.
Introduced in .NET 8, the Auto model gives you the best of both. The app starts using Blazor Server for an instant load and then silently downloads the WASM bundle in the background. Subsequent interactions happen on the client. It's the 'Holy Grail' of web hosting.
Q: "When should I still choose Server-only?"
Architect Answer: "Choose Server-only for internal, high-security line-of-business apps where you don't want any code or sensitive business logic to ever leave your server. Choose WASM for public, consumer-facing apps where you need to scale to thousands of users without paying for massive server clusters."