Building MCP servers in .NET

This skill helps you write production-quality MCP servers and basic clients in C#/.NET against the official ModelContextProtocol NuGet packages, maintained by Microsoft and the MCP project. It targets the stable 2.x line and the current spec (2026-07-28).

When this skill earns its keep

The .NET MCP SDK had years of preview packages (0.x-preview) before reaching 1.0, and v2 flipped several defaults. Without help, the model tends to:

• Pin a stale preview version that won't compile against current samples.
• Apply 1.x-era defaults that v2 reversed (HTTP was stateful by default; in 2.x Stateless defaults to true).
• Recommend capabilities the 2026-07-28 spec deprecates (roots, sampling, MCP-channel logging — now [Obsolete], warning MCP9005).
• Miss recent spec features (multi-round-trip input_required, discovery-first negotiation, MCP Apps/Tasks extension packages, elicitation URL mode, structured content blocks).
• Get HTTP transport details wrong (stateful/stateless, proxy buffering, OAuth wiring).
• Forget the STDIO stdout/stderr trap.

If the task is one of those, load the matching reference and follow it. If it's truly trivial (e.g. "rename this tool method"), you don't need to read everything — the cardinal rules below are the minimum.

Mental model in 30 seconds

A .NET MCP server is an ordinary Microsoft.Extensions.Hosting (or WebApplication) app that wires an MCP server through DI: