mcp-csharp-debug

by dotnet

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SKILL.md About

---
name: mcp-csharp-debug
description: >
  Run and debug C# MCP servers locally. Covers IDE configuration, MCP Inspector testing,
  GitHub Copilot Agent Mode integration, logging setup, and troubleshooting.
  USE FOR: running MCP servers locally with dotnet run, configuring VS Code or Visual Studio
  for MCP debugging, testing tools with MCP Inspector, testing with GitHub Copilot Agent Mode,
  diagnosing tool registration issues, setting up mcp.json configuration, debugging MCP
  protocol messages, configuring logging for stdio and HTTP servers.
  DO NOT USE FOR: creating new MCP servers (use mcp-csharp-create), writing automated tests
  (use mcp-csharp-test), publishing or deploying to production (use mcp-csharp-publish).
license: MIT
---

# C# MCP Server Debugging

Run, debug, and interactively test C# MCP servers. Covers local execution, IDE debugging with breakpoints, MCP Inspector for protocol-level testing, and GitHub Copilot Agent Mode integration.

## When to Use

- Running an MCP server locally for the first time
- Configuring VS Code or Visual Studio to debug an MCP server
- Testing tools interactively with MCP Inspector
- Verifying tools appear in GitHub Copilot Agent Mode
- Diagnosing issues: tools not discovered, protocol errors, server crashes
- Setting up `mcp.json` or `.mcp.json` configuration

## Stop Signals

- **No project yet?** → Use `mcp-csharp-create` first
- **Need automated tests?** → Use `mcp-csharp-test`
- **Production deployment issue?** → Use `mcp-csharp-publish`

## Inputs

| Input | Required | Description |
|-------|----------|-------------|
| Project path | Yes | Path to the `.csproj` file or project directory |
| Transport type | Recommended | `stdio` or `http` — detect from `.csproj` if not specified |
| IDE | Recommended | VS Code or Visual Studio — detect from environment if not specified |

**Agent behavior:** Detect transport type by checking the `.csproj` for a `PackageReference` to `ModelContextProtocol.AspNetCore`. If present → HTTP, otherwise → stdio.

## Workflow

### Step 1: Run the server locally

**stdio transport:**
```bash
cd <ProjectDir>
dotnet run
```
The process starts and waits for JSON-RPC messages on stdin. No output on stdout means it's working correctly.

**HTTP transport:**
```bash
cd <ProjectDir>
dotnet run
# Server listens on http://localhost:3001 (or configured port)
```

### Step 2: Generate MCP configuration

Detect the IDE and transport, then create the appropriate config file.

**For VS Code** — create `.vscode/mcp.json`:

stdio:
```json
{
  "servers": {
    "<ProjectName>": {
      "type": "stdio",
      "command": "dotnet",
      "args": ["run", "--project", "<path/to/ProjectFile.csproj>"]
    }
  }
}
```

HTTP:
```json
{
  "servers": {
    "<ProjectName>": {
      "type": "http",
      "url": "http://localhost:3001"
    }
  }
}
```

**For Visual Studio** — create `.mcp.json` at solution root (same JSON structure).

**For detailed IDE-specific configuration** (launch.json, environment variables, secrets), see [references/ide-config.md](references/ide-config.md).

### Step 3: Test with MCP Inspector

The MCP Inspector provides a UI for testing tools, viewing schemas, and inspecting protocol messages.

**stdio server:**
```bash
npx @modelcontextprotocol/inspector dotnet run --project <path/to/ProjectFile.csproj>
```

**HTTP server:**
1. Start your server: `dotnet run`
2. Run Inspector: `npx @modelcontextprotocol/inspector`
3. Connect to `http://localhost:3001`

**For detailed Inspector capabilities, usage, and troubleshooting**, see [references/mcp-inspector.md](references/mcp-inspector.md).

### Step 4: Test with GitHub Copilot Agent Mode

1. Open GitHub Copilot Chat → switch to **Agent** mode
2. Click **Select Tools** (wrench icon) → verify your server and tools are listed
3. Test with a prompt that should trigger your tool
4. Approve tool execution when prompted

**If tools don't appear — troubleshoot tool discovery:**

1. **Rebuild first** — stale builds are the #1 cause:
   ```bash
   dotnet build
   ```
   Then restart the MCP server (click Stop → Start in VS Code, or restart `dotnet run`).

2. **Check attributes and registration:**
   - Verify `[McpServerToolType]` on the class and `[McpServerTool]` on each public method
   - Methods can be `static` or instance (instance types need DI registration)
   - Verify `.WithTools<T>()` or `.WithToolsFromAssembly()` in Program.cs

3. **Check `mcp.json`** points to the correct project path

4. If still not appearing, reference the tool explicitly: `Using #tool_name, do X`

### Step 5: Set up breakpoint debugging

1. Set breakpoints in your tool methods
2. Launch with the debugger:
   - **VS Code:** F5 (requires `launch.json` — see [references/ide-config.md](references/ide-config.md))
   - **Visual Studio:** F5 or right-click project → Debug → Start
3. Trigger the tool (via Inspector, Copilot, or test client)
4. Execution pauses at breakpoints

**Critical:** Build in Debug configuration. Breakpoints won't hit in Release builds.

### Diagnosing Tool Errors

When a tool works standalone but fails through MCP, work through these checks:

1. **Check the MCP output channel** — In VS Code: View → Output → select your MCP server name. Shows protocol errors and server stderr. In Visual Studio: check the Output window for MCP-related messages.
2. **Attach a debugger** — Set a breakpoint in the failing tool method and step through execution (see Step 5). Check for exceptions being swallowed or unexpected parameter values.
3. **Test with MCP Inspector** — Call the tool directly through Inspector to isolate whether the issue is in the tool code or the client integration: `npx @modelcontextprotocol/inspector dotnet run --project <path>`
4. **Check stdout contamination (stdio only)** — Any `Console.WriteLine()` or logging to stdout corrupts the JSON-RPC protocol. Redirect all output to stderr (see Step 6).
5. **Check common culprits:**
   - **Serialization errors** — Return types must be JSON-serializable. Avoid circular references.
   - **DI registration** — Missing service registrations cause runtime exceptions. Check `Program.cs`.
   - **Parameter binding** — Ensure parameter names and types match the tool schema.
   - **Unhandled exceptions** — Wrap tool logic in try-catch and log to stderr or a file.
6. **Enable file logging** — For post-mortem analysis, log to a file:
   ```csharp
   builder.Logging.AddFile("mcp-debug.log"); // or use Serilog/NLog
   ```

### Step 6: Configure logging

**Critical for stdio transport:** Any output to stdout (including `Console.WriteLine`) **corrupts the MCP JSON-RPC protocol** and causes garbled responses or crashes. All logging and diagnostic output must go to stderr.

**stdio transport** — log to stderr only:
```csharp
builder.Logging.AddConsole(options =>
    options.LogToStandardErrorThreshold = LogLevel.Trace);
```

**HTTP transport** — For HTTP transport logging configuration, see [references/ide-config.md](references/ide-config.md).

**In tool methods** — inject `ILogger<T>` via constructor and use `logger.LogDebug()` / `logger.LogError()`. Logging through `ILogger` respects the stderr configuration above.

## Validation

- [ ] Server starts without errors via `dotnet run`
- [ ] MCP Inspector connects and lists all expected tools
- [ ] Tool calls via Inspector return expected results
- [ ] Breakpoints hit when debugging in IDE
- [ ] Tools appear in GitHub Copilot Agent Mode tool list
- [ ] stdio: no logging output on stdout (stderr only)

## Common Pitfalls

| Pitfall | Solution |
|---------|----------|
| Tools not appearing or stale after changes | **Rebuild first:** `dotnet build`, then restart the server. If still missing, verify `[McpServerToolType]` on class, `[McpServerTool]` on methods, and `WithTools<T>()` or `WithToolsFromAssembly()` in Program.cs |
| stdio server produces garbled output | `Console.WriteLine()` or logging is writing to stdout. All output **must** go to stderr. Set `LogToStandardErrorThreshold = LogLevel.Trace` on the console logger |
| HTTP server returns 404 at MCP endpoint | Missing `app.MapMcp()` in Program.cs |
| Breakpoints not hit | Building in Release mode. Rebuild in Debug: `dotnet build -c Debug`, then restart |
| Environment variables not passed to server | Add `"env"` section to `mcp.json`. For secrets in VS Code, use `"${input:var_id}"` syntax |
| MCP Inspector can't connect to HTTP server | Server not running, or wrong port. Check `dotnet run` output for the listening URL |

## Related Skills

- `mcp-csharp-create` — Create a new MCP server project
- `mcp-csharp-test` — Automated tests and evaluations
- `mcp-csharp-publish` — NuGet, Docker, Azure deployment

## Reference Files

- [references/mcp-inspector.md](references/mcp-inspector.md) — Detailed MCP Inspector usage: installation, connecting to servers, feature walkthrough, troubleshooting. **Load when:** user needs detailed Inspector guidance or is having connection issues.
- [references/ide-config.md](references/ide-config.md) — Complete VS Code and Visual Studio configuration: mcp.json templates, launch.json, environment variables, conditional breakpoints. **Load when:** setting up IDE debugging or configuring environment-specific settings.

## More Info

- [MCP Inspector](https://www.npmjs.com/package/@modelcontextprotocol/inspector/v/0.21.1) — Interactive debugging tool for MCP servers
- [VS Code MCP documentation](https://code.visualstudio.com/docs/copilot/chat/mcp-servers) — Configuring MCP servers in VS Code