Build a Model Context Protocol (MCP) Server in C#

Montemagno builds a minimal MCP server with the new C# SDK: a console app hosting AddMcpServer().WithStdioServerTransport().WithToolsFromAssembly(), with tools defined as static methods tagged [McpServerTool] inside a class marked [McpServerToolType]. The SDK scans the assembly for those attributes and registers the methods automatically, using each method’s Description to tell a connecting client what the tool does.

He registers a MonkeyService (querying a small HTTP API for monkey data) via dependency injection and exposes it as GetMonkeys/GetMonkey tools returning JSON. In the walkthrough, GitHub Copilot’s Agent mode calls those tools to render the results as a table, a Mermaid diagram, or the XAML for a .NET MAUI screen. The post closes with packaging: containerizing the server with the .NET SDK’s built-in support, publishing to a registry, and a note on SSE transport for streaming server-to-client communication in remote MCP servers.

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• An MCP server built with the SDK is a console app: AddMcpServer().WithStdioServerTransport().WithToolsFromAssembly() sets up the transport and scans the assembly for tools.
• Tools are static methods marked [McpServerTool] inside a class marked [McpServerToolType]. The Description attribute on each becomes what a connecting client, GitHub Copilot’s Agent mode in the walkthrough, reads to decide which tool to call.
• The example grows from a toy echo tool into a MonkeyService that fetches real data over HTTP, registered via dependency injection and exposed as GetMonkeys/GetMonkey tools that return JSON.
• Once wired up, Copilot’s Agent mode can call the tools directly, asking for a monkey list as a table, a Mermaid diagram of the data type, or the XAML for a .NET MAUI screen built from it.
• Publishing is built into the .NET SDK: setting EnableSdkContainerSupport and target runtime identifiers, then running dotnet publish /t:PublishContainer, produces per-architecture container images that can be pushed to a registry.
• SSE transport streams server-to-client data over HTTP for remote MCP servers, with the SDK samples showing an SSE Monkey server and a deployment via Azure Functions.