If you’ve been using Swashbuckle and Swagger UI with ASP.NET APIs, you may have noticed a big change starting in .NET 9: the default tooling you were used to is suddenly gone. When I first ran into this, it felt a bit unsettling. Microsoft introduced first-party OpenAPI support with Microsoft.AspNetCore.OpenApi, reducing dependencies and making documentation for Minimal APIs much simpler. Now it feels like working with my APIs is just… smoother.

In this 20-minute session, I’ll walk through what these changes mean in practice. I’ll start with a high-level overview of the new native OpenAPI support and compare it with the Swashbuckle-approach that we have relied on for years. I’ll highlight some of my favorite .NET 10 additions, including OpenAPI 3.1 support, multiple output formats, and document transformers that let you tailor OpenAPI output for API groups and version. Finally, I’ll interact with the APIs using the Scalar testing UI, which stays responsive with large payloads.

Through a series of live demos, you’ll see how I migrated away from Swashbuckle and can take advantage of the modern .NET OpenAPI ecosystem in your projects.