| name | dotnet-mcp-builder |
| description | Build Model Context Protocol (MCP) servers in C#/.NET against the current ModelContextProtocol 1.x NuGet packages. Especially helps with cases the model often gets wrong without guidance — stale preview versions (it tends to pick 0.3 or 0.4 preview), MCP Apps (interactive UI rendered in the host), elicitation URL mode, per-session HTTP wiring, OAuth and reverse-proxy deploy specifics, and debugging concrete MapMcp / STDIO / Streamable-HTTP errors. Also covers the routine work — STDIO and Streamable HTTP transports (SSE is deprecated), tools, prompts, resources, sampling, roots, completions, logging — and a basic .NET MCP client. Trigger when the user says or implies any .NET MCP server work: ModelContextProtocol, McpServerTool, MapMcp, WithStdioServerTransport, "MCP server in C#", "MCP tool in dotnet", "expose this as MCP", or names a primitive (prompt/resource/elicitation/MCP App) in a .NET context. Skip for MCP work in other languages. |
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 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()
.WithToolsFromAssembly()
.WithPrompts<MyPrompts>()
.WithResources<MyResources>();
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)
- 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.
- 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.
- 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.
- SSE-only is deprecated. Use Streamable HTTP. Only enable legacy SSE (
EnableLegacySse = true) for an old client you must support, and call it out.
- Always
[Description] tools and parameters. This is what the LLM sees when picking and shaping calls. Vague descriptions are the #1 reason tools don't get used.
- Show the registration line every time you add a primitive. A new
[McpServerPromptType] class without .WithPrompts<...>() (or .WithPromptsFromAssembly()) is invisible.
- Don't invent APIs. If you're unsure a method exists, say so and check the API reference — wrong method names cause silent failures.
Working style
- Make minimal, additive changes. Add a method to the existing tool class rather than restructuring the project.
- For non-trivial setups, run
dotnet build. Catches missing usings, attribute typos, and TFM mismatches before the user sees them.
- Confirm transport + .NET version + primitives before scaffolding if context doesn't already make them obvious. Default to .NET 10 for new projects.
When the user is stuck
Walk this checklist before guessing:
- STDIO: something is writing to stdout (logger sink,
Console.WriteLine, library banner).
- HTTP 404: path mismatch —
app.MapMcp() is root, app.MapMcp("/mcp") puts it under /mcp.
- Tool not appearing: missing
[McpServerToolType] on the class, or no .WithToolsFromAssembly() / .WithTools<T>() registered.
- Args not bound: parameter names must match the JSON-RPC
arguments keys; complex types bind via System.Text.Json.
- Sampling/elicitation/roots failing: transport is stateless HTTP, or the client doesn't advertise the capability.
Still stuck? Point the user at the EverythingServer sample — it exercises every feature.
