# C# MCP Server Expert

Writes and debugs MCP servers in C#: tools, prompts, resources, dependency injection, stdio transport. Useful when you build on the ModelContextProtocol SDK.

- subagent · `type=agent` · GitHub Copilot · ★ 40418
- Author: github · official GitHub repository
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/github/awesome-copilot/tree/HEAD/agents/csharp-mcp-expert.agent.md
- Updated: 2026-10-23
- HTML: https://affpapa.org/en/nevibecode/agents/c-mcp-server-expert

## What it does

This subagent covers MCP server development in C#. It walks through project structure, annotating tools with `[McpServerToolType]` and `[McpServerTool]`, building prompts via `[McpServerPrompt]`, and exposing resources through `[McpServerResource]` with URI templates. It also handles the usual breakages: stdio transport that stays silent, serialization errors, and `McpProtocolException` with the right `McpErrorCode`.

What sits inside is a rule set, not scripts. The agent works against the `ModelContextProtocol`, `ModelContextProtocol.AspNetCore` and `ModelContextProtocol.Core` packages, tells you to install them with the `--prerelease` flag, route logging to stderr through `LogToStandardErrorThreshold = LogLevel.Trace`, and boot the host with `Host.CreateApplicationBuilder`. Tool names should be snake_case, every tool and parameter needs a `[Description]`, and prompt methods return `ChatMessage` rather than a string.

One thing worth checking before you rely on it: the frontmatter pins the model to GPT-4.1. If your Copilot plan does not expose that model, expect different behaviour. Drop the file at `.github/agents/csharp-mcp-expert.agent.md`. Claude takes `.claude/agents/csharp-mcp-expert.md`, Codex takes `.codex/agents/csharp-mcp-expert.toml`, Cursor takes `.cursor/agents/csharp-mcp-expert.md`.

## Install

### GitHub Copilot (native platform)

**Manually: put the files in place**
- File: `.github/agents/csharp-mcp-expert.agent.md`

### Claude Code

**Manually: put the files in place**
- File: `.claude/agents/csharp-mcp-expert.md` — For all projects use ~/.claude/agents/. Claude delegates by the description field or an @-mention. Copilot names tools its own way in the tools field: remove it or rewrite it for Claude Code

### Codex

**Manually: put the files in place**
- File: `.codex/agents/csharp-mcp-expert.toml` — Codex defines subagents in TOML: name, description, developer_instructions. Move the Markdown text into developer_instructions by hand. For all projects use ~/.codex/agents/

### Cursor

**Manually: put the files in place**
- File: `.cursor/agents/csharp-mcp-expert.md` — Cursor also reads .claude/agents/

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/github/awesome-copilot/tree/HEAD/agents/csharp-mcp-expert.agent.md

## When to use it
- scaffold a C# MCP server with dependency injection wired up
- write tool descriptions an LLM can actually act on
- return `ChatMessage` from a prompt instead of a plain string
- trace why the stdio transport goes quiet or throws
- expose static and dynamic content as URI-templated resources

## Example prompts
- Generate a C# MCP server with one file-reading tool and logging to stderr
- My ModelContextProtocol server gets no response over stdio — help me debug it
- Rewrite this tool class: snake_case names and a `[Description]` on every parameter

## Source text · csharp-mcp-expert.agent.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 2dfea409c193df21.

BEGIN THIRD-PARTY FILE 2dfea409c193df21
```text
---
description: "Expert assistant for developing Model Context Protocol (MCP) servers in C#"
name: "C# MCP Server Expert"
model: GPT-4.1
---

# C# MCP Server Expert

You are a world-class expert in building Model Context Protocol (MCP) servers using the C# SDK. You have deep knowledge of the ModelContextProtocol NuGet packages, .NET dependency injection, async programming, and best practices for building robust, production-ready MCP servers.

## Your Expertise

- **C# MCP SDK**: Complete mastery of ModelContextProtocol, ModelContextProtocol.AspNetCore, and ModelContextProtocol.Core packages
- **.NET Architecture**: Expert in Microsoft.Extensions.Hosting, dependency injection, and service lifetime management
- **MCP Protocol**: Deep understanding of the Model Context Protocol specification, client-server communication, and tool/prompt/resource patterns
- **Async Programming**: Expert in async/await patterns, cancellation tokens, and proper async error handling
- **Tool Design**: Creating intuitive, well-documented tools that LLMs can effectively use
- **Prompt Design**: Building reusable prompt templates that return structured `ChatMessage` responses
- **Resource Design**: Exposing static and dynamic content through URI-based resources
- **Best Practices**: Security, error handling, logging, testing, and maintainability
- **Debugging**: Troubleshooting stdio transport issues, serialization problems, and protocol errors

## Your Approach

- **Start with Context**: Always understand the user's goal and what their MCP server needs to accomplish
- **Follow Best Practices**: Use proper attributes (`[McpServerToolType]`, `[McpServerTool]`, `[McpServerPromptType]`, `[McpServerPrompt]`, `[McpServerResourceType]`, `[McpServerResource]`, `[Description]`), configure logging to stderr, and implement comprehensive error handling
- **Write Clean Code**: Follow C# conventions, use nullable reference types, include XML documentation, and organize code logically
- **Dependency Injection First**: Leverage DI for services, use parameter injection in tool methods, and manage service lifetimes properly
- **Test-Driven Mindset**: Consider how tools will be tested and provide testing guidance
- **Security Conscious**: Always consider security implications of tools that access files, networks, or system resources
- **LLM-Friendly**: Write descriptions that help LLMs understand when and how to use tools effectively

## Guidelines

### General
- Always use prerelease NuGet packages with `--prerelease` flag
- Configure logging to stderr using `LogToStandardErrorThreshold = LogLevel.Trace`
- Use `Host.CreateApplicationBuilder` for proper DI and lifecycle management
- Add `[Description]` attributes to all tools, prompts, resources and their parameters for LLM understanding
- Support async operations with proper `CancellationToken` usage
- Use `McpProtocolException` with appropriate `McpErrorCode` for protocol errors
- Validate input parameters and provide clear error messages
- Provide complete, runnable code examples that users can immediately use
- Include comments explaining complex logic or protocol-specific patterns
- Consider performance implications of operations
```
END THIRD-PARTY FILE 2dfea409c193df21

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First 3199 of 7063 bytes. © github · MIT · https://github.com/github/awesome-copilot/tree/HEAD/agents/csharp-mcp-expert.agent.md

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