mcp-csharp

name	mcp-csharp
description	Create, debug, test, and publish MCP (Model Context Protocol) servers in C# using the official ModelContextProtocol SDK — tools/prompts/resources, stdio vs HTTP transport, MapMcp hosting. Use when building or changing an MCP server in the .NET backend (relevant to your agentic pipeline).
allowed-tools	Read, Glob, Grep, Edit, Write, Bash, mcp__serena__initial_instructions, mcp__serena__get_symbols_overview, mcp__serena__find_symbol, mcp__serena__find_referencing_symbols, mcp__serena__find_declaration, mcp__serena__find_implementations, mcp__serena__search_for_pattern, mcp__serena__get_diagnostics_for_file, mcp__serena__list_dir, mcp__serena__find_file, mcp__serena__create_text_file, mcp__serena__replace_symbol_body, mcp__serena__insert_after_symbol, mcp__serena__insert_before_symbol, mcp__serena__replace_content, mcp__serena__rename_symbol, mcp__serena__safe_delete_symbol

Serena MCP is mandatory for C# code. First call mcp__serena__initial_instructions to load the Serena tool manual, then use the Serena tools for ALL .cs reading / searching / navigation / creation / editing — prefer symbol navigation (get_symbols_overview / find_symbol / find_referencing_symbols) over whole-file reads. Native Edit/Write on .cs is hook-blocked (the TS/React frontend uses the native tools).

Building MCP servers in C# ({{ProductName}})

For exposing tools/capabilities over the Model Context Protocol from the .NET backend (your agentic pipeline on Microsoft Agent Framework is the natural consumer/producer). C# (.cs) edits via Serena.

⚠ Approval required. ModelContextProtocol (core) and ModelContextProtocol.AspNetCore (HTTP) plus the Microsoft.McpServer.ProjectTemplates template are NuGet dependencies — need Dan's explicit approval.

Create

1. Prereqs: dotnet --version (need .NET 10+); dotnet new install Microsoft.McpServer.ProjectTemplates.
2. Transport: stdio (local CLI/IDE plugins, default) vs HTTP (cloud/web, multiple clients, containers).
3. Scaffold: stdio → dotnet new mcpserver -n <Name>; HTTP → dotnet new web -n <Name> + dotnet add package ModelContextProtocol.AspNetCore.
4. Tools — [McpServerToolType] on the class, [McpServerTool] + [Description] on the method, a [Description] on EVERY parameter, async tools take a CancellationToken:

   [McpServerToolType]
   public static class ProjectTools
   {
       [McpServerTool, Description("Summarize a project.")]
       public static async Task<string> Summarize(
           [Description("The project id.")] Guid projectId,
           CancellationToken cancellationToken = default) => /* ... */;
   }
   

   For DI/non-static tools, use an explicit constructor assigning readonly fields — NOT a primary constructor (the upstream Microsoft sample uses class ApiTools(HttpClient http, ILogger log); we don't).
5. Prompts/Resources: [McpServerPromptType]/[McpServerPrompt] (return ChatMessage); [McpServerResourceType]/[McpServerResource(UriTemplate="config://app", MimeType="application/json")].
6. Program.cs:
   • stdio: Host.CreateApplicationBuilder(args); builder.Logging.AddConsole(o => o.LogToStandardErrorThreshold = LogLevel.Trace); — logging to stdout corrupts JSON-RPC (the #1 error); AddMcpServer().WithStdioServerTransport().WithToolsFromAssembly();
   • HTTP: WebApplication.CreateBuilder(args); AddMcpServer().WithHttpTransport().WithToolsFromAssembly(); app.MapMcp(); app.MapGet("/health", () => "ok");
7. Verify: dotnet build / dotnet run.

Debug / Test / Publish (lifecycle)

• Debug: run locally, validate via the MCP Inspector UI and Copilot Agent Mode; remember stdio logs go to stderr.
• Test: unit-test tools + integration-test via the MCP client SDK. (The upstream skill uses Moq + FluentAssertions — both are libraries needing approval, and must match whatever test stack Dan approves for apps/api/tests.)
• Publish: NuGet packaging for stdio servers; Docker containerization for HTTP servers → deploy to our (EU) host's container runtime (NOT Azure Container Apps / App Service — strip those). MCP Registry publish is optional.

Ours

• Explicit constructors, no primary ctors; .cs via Serena; async + CancellationToken throughout.
• Tools that touch tenant data must carry the tenancy invariant; failures should flow through our Result<T>/error model where the tool contract allows.
• Pair with the otel-instrumentation skill — transport-config supports OpenTelemetry; trace tool calls.
• No library/template added without Dan's approval.