Dotnet-Mcp-Builder

# Building MCP servers in .NET

This skill helps you write production-quality MCP servers and basic clients in C#/.NET against the **official** [`ModelContextProtocol`](https://www.nuget.org/profiles/ModelContextProtocol) NuGet packages, maintained by Microsoft and the MCP project. It targets the **stable 1.x** line and the current spec (2025-11-25).

When this skill earns its keep

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

- Pin a stale preview version that won't compile against current samples.

- Miss recent spec features (elicitation URL mode, MCP Apps, 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:

builder.Services
    .AddMcpServer()
    .WithStdioServerTransport()      // OR .WithHttpTransport(...)
    .WithToolsFromAssembly()         // discover [McpServerToolType] classes
    .WithPrompts<MyPrompts>()        // optional
    .WithResources<MyResources>();   // optional

Primitives are plain C# methods on classes marked with attributes (`[McpServerToolType]` + `[McpServerTool]`, `[McpServerPromptType]` + `[McpServerPrompt]`, `[McpServerResourceType]` + `[McpServerResource]`). Parameters bind from JSON-RPC; the SDK builds the JSON Schema from the signature plus `[Description]` attributes.

Server-to-client features (sampling, elicitation, roots, log/progress notifications) are methods on the injected `IMcpServer`.

Decision tree → which references to load

Always load `references/packages.md` if you're creating a new project or unsure of the current package version.

| Task | Load |

|---|---|

| New STDIO server | `references/transport-stdio.md` |

| New HTTP (Streamable) server | `references/transport-http.md` |

| Add/modify a tool | `references/tool-primitive.md` |

| Add/modify a prompt | `references/prompt-primitive.md` |

| Add/modify a resource | `references/resource-primitive.md` |

| Ask the user a question mid-tool | `references/elicitation.md` |

| Call the client's LLM from a tool | `references/sampling.md` |

| Read the user's project roots | `references/roots.md` |

| Return an interactive UI | `references/mcp-apps.md` |

| Argument completions, log/progress notifications, filters, server instructions | `references/server-features.md` |

| Write a .NET program that **consumes** an MCP server | `references/client.md` |

| MCP Inspector, in-memory tests, mocks, CI | `references/testing.md` |

For multi-primitive tasks, load several at once. For trivial edits in an existing file, you usually don't need any.

Cardinal rules (apply always; these prevent the highest-frequency breakages)

1. **Pin the current stable package, not a preview.** Use `ModelContextProtocol` / `ModelContextProtocol.AspNetCore` / `ModelContextProtocol.Core` at the latest **1.x**. If you find yourself writing `0.3-preview` or `0.4-preview`, stop and check NuGet — preview APIs have breaking differences.

2. **STDIO servers must not write to stdout.** Stdout is the JSON-RPC channel. Configure `LogToStandardErrorThreshold = LogLevel.Trace` before anything else and never `Console.WriteLine` from a tool.

3. **HTTP defaults to stateful.** For horizontally-scaled deployments without server-initiated traffic, set `options.Stateless = true`. Server-to-client features (sampling, elicitation, roots, unsolicited notifications) require stateful HTTP **or** STDIO — `Stateless = true` will break them at runtime.

4. **SSE-only is deprecated.** Use Streamable HTTP. Only enable legacy SSE (`EnableLegacyS