.NET 11 explained: C# 15 unions, the MCP server template, faster tooling, and what to do about .NET 8 and 9

.NET 11 is the new release of Microsoft's .NET platform, which teams use to build web services with ASP.NET Core, cross-platform apps with .NET MAUI and Windows desktop apps, in C# and F#. Microsoft released it on 10 November 2026, at the start of .NET Conf 2026.

It is a Standard Term Support (STS) release, which means two years of support. Microsoft says STS and LTS releases get the same quality and servicing; the only difference is how long support lasts. .NET 10 remains the supported Long Term Support option.

The most urgent news is about older versions. .NET 8 and .NET 9 reached end of support on the same day, 10 November 2026, so they stop getting fixes, including security fixes. If you run either one, plan the move now: .NET 11 if you want the newest features, .NET 10 if a longer support window matters more to you.

The SDK now ships an MCP server template. MCP, the Model Context Protocol, is a common way for AI apps to discover and call tools. Running dotnet new mcpserver -o MyMcpServer gives you a project built on the official MCP C# SDK with a working sample tool, so you can expose your own functions to an AI app without installing a separate template first.

For apps that call models, Microsoft points to Microsoft.Extensions.AI, which gives you interfaces such as IChatClient and IEmbeddingGenerator that plug into dependency injection, caching and OpenTelemetry. Microsoft Agent Framework builds agents and workflows on top of that. Both are NuGet packages, so you can use them without moving your whole app to .NET 11.

Blazor gets an experimental package of AI components for streaming chat, showing tool calls, approval steps and shared state, which can talk to a remote agent over the AG-UI protocol. Microsoft says this package stays prerelease for all of .NET 11, so treat it as a preview.

Read next: MCP in production: running tool servers that agents can trust

Two kinds of speed. For developers, the SDK now uses a Native AOT CLI host by default, so commands such as help and version queries run without starting the managed CLI, and the MSBuild server stays warm between commands. For running apps, the JIT removes more bounds checks and branches, improves devirtualization and folds more work into constants, and Native AOT uses a shared helper for interface calls.

Microsoft's advice is to measure on your own workload, because throughput, startup time and memory depend on what your app does and how you publish it. Runtime Async, which moves async suspension and resumption into the runtime and gives debuggers a clearer call stack, is still a preview feature that you turn on per project.

The libraries add Zstandard compression in System.IO.Compression, one-line methods to run a process and capture its output, stream adapters that read existing memory without copying it, and JSON Lines output from async sequences.