mcp-netcoredbg

MCP server for .NET debugging via netcoredbg.

Enables AI agents (Claude, etc.) to set breakpoints, step through code, and inspect variables in .NET applications.

Architecture

┌─────────┐     MCP      ┌─────────────────┐     DAP      ┌─────────────┐
│  Claude │ ──────────►  │ mcp-netcoredbg  │ ──────────►  │ netcoredbg  │
│         │  (tools)     │   (this repo)   │  (stdio)     │  (Samsung)  │
└─────────┘              └─────────────────┘              └──────┬──────┘
                                                                │
                                                                ▼
                                                         ┌─────────────┐
                                                         │  .NET App   │
                                                         └─────────────┘

Tools

Session Management (NEW in v2.0)

Tool	Description
list_sessions	List all active debug sessions
select_session	Set the default session for commands
terminate_session	Terminate a specific session

Launch & Attach

Tool	Description
launch	Start debugging a .NET application (DLL path)
launch_watch	Start debugging with hot reload via dotnet watch
stop_watch	Stop hot reload debugging mode
attach	Attach to a running .NET process
invoke	Invoke a specific method in an assembly (with optional debugging)
restart	Restart the debugged program (for launch mode)
terminate	Stop debugging session

Breakpoints

Tool	Description
set_breakpoint	Set breakpoint at file:line (supports conditions)
remove_breakpoint	Remove a breakpoint
list_breakpoints	List all active breakpoints

Execution Control

Tool	Description
continue	Continue execution
pause	Pause execution
step_over	Step over current line
step_into	Step into function call
step_out	Step out of current function

Inspection

Tool	Description
stack_trace	Get current call stack
scopes	Get variable scopes for a stack frame
variables	Get variables from a scope
evaluate	Evaluate expression in debug context
threads	List all threads
output	Get recent program output
status	Get debugger status

Note: All tools accept an optional sessionId parameter to target a specific session. If omitted, the default session is used.

Multi-Session Debugging (v2.0)

Version 2.0 introduces support for multiple simultaneous debug sessions. This allows you to debug multiple .NET applications concurrently - for example, an API and a background worker.

Auto-Generated Session IDs

When you launch a debug session without specifying a sessionId, one is automatically derived from the project/program name:

• TopServer.Service.Api → session ID: api
• TopServer.Service.Worker → session ID: worker
• MyApp.Web → session ID: web
• Custom: specify sessionId parameter explicitly

Example: Debugging API + Worker Simultaneously

# Start API debugging
launch_watch projectPath=/path/to/MyApp.Api launchProfile=https
→ Session 'api' created

# Start Worker in a second session
launch_watch projectPath=/path/to/MyApp.Worker launchProfile=default
→ Session 'worker' created

# List all sessions
list_sessions
→ api (default): watch - /path/to/MyApp.Api [running]
   worker: watch - /path/to/MyApp.Worker [running]

# Set breakpoint in API
set_breakpoint file=/path/to/ApiController.cs line=42 sessionId=api

# Set breakpoint in Worker
set_breakpoint file=/path/to/WorkerService.cs line=100 sessionId=worker

# Continue API execution
continue sessionId=api

# Check Worker status
status sessionId=worker

# Switch default session
select_session sessionId=worker

# Now commands without sessionId go to worker
continue  # continues worker

Session Management Commands

Command	Description
list_sessions	Show all active sessions with status
select_session	Change which session receives commands by default
terminate_session	Stop a specific session

Backward Compatibility

If you only use one session, the behavior is unchanged from v1.x - no sessionId parameter needed.

Method Invocation (invoke)

The invoke tool lets you run a specific method from a .NET assembly without launching the full application. This is useful for:

• Testing individual methods in isolation
• Running utility functions
• Debugging specific code paths without going through the whole app

Parameters

Parameter	Type	Required	Description
assembly	string	Yes	Path to the .NET DLL
type	string	Yes	Fully qualified type name (e.g., MyApp.Services.Calculator)
method	string	Yes	Method name to invoke
args	array	No	Method arguments as JSON array
ctorArgs	array	No	Constructor arguments (for instance methods)
debug	boolean	No	Launch under debugger for breakpoint support (default: false)
cwd	string	No	Working directory

Examples

Static method:

{
  "assembly": "/path/to/MyApp.dll",
  "type": "MyApp.StringUtils",
  "method": "FormatName",
  "args": ["John", "Doe"]
}

Instance method with constructor arguments:

{
  "assembly": "/path/to/MyApp.dll",
  "type": "MyApp.Calculator",
  "method": "Add",
  "args": [5],
  "ctorArgs": [10]
}

With debugging (breakpoints supported):

{
  "assembly": "/path/to/MyApp.dll",
  "type": "MyApp.Calculator",
  "method": "Add",
  "args": [5],
  "debug": true
}

Features

• Static methods: Just provide type, method, and args
• Instance methods: Automatically constructs the type (provide ctorArgs if needed)
• Auto ILogger injection: ILogger<T> parameters are automatically resolved
• Async support: Automatically awaits Task-returning methods
• Console capture: Captures Console.WriteLine output
• Log capture: Captures ILogger calls made during execution
• Rich errors: On failure, shows available constructors/methods to help you fix the call

Output Format

The tool returns a structured JSON result:

{
  "success": true,
  "method": "MyApp.StringUtils.FormatName",
  "args": ["John", "Doe"],
  "returnType": "string",
  "returnValue": "Doe, John",
  "durationMs": 2.5,
  "logs": [
    {"level": "Information", "message": "Processing..."}
  ],
  "stdout": ""
}

On error, it provides helpful diagnostics:

{
  "success": false,
  "error": "Method not found",
  "errorDetails": {
    "type": "MyApp.StringUtils",
    "reason": "Method 'DoSomething' not found",
    "methods": [
      {"name": "FormatName", "params": ["string firstName", "string lastName"], "returnType": "string", "isStatic": true}
    ]
  }
}

Hot Reload Debugging (launch_watch)

The launch_watch tool enables debugging with hot reload support via dotnet watch. When you make code changes, the app automatically restarts and the debugger reconnects - preserving your breakpoints.

Parameters

Parameter	Type	Required	Description
projectPath	string	Yes	Path to the .NET project directory (containing .csproj)
launchProfile	string	No	Name of a launch profile from Properties/launchSettings.json
args	array	No	Additional arguments to pass to dotnet watch

Examples

Basic usage:

{
  "projectPath": "/path/to/MyApp"
}

With launch profile (recommended for ASP.NET apps):

{
  "projectPath": "/path/to/MyApp.Api",
  "launchProfile": "https"
}

The launch profile is important for ASP.NET applications as it sets:

• ASPNETCORE_ENVIRONMENT (e.g., Development)
• applicationUrl (e.g., https://localhost:7179)
• Other environment variables needed for proper operation

How It Works

1. Starts dotnet watch run with the specified project
2. Waits for the app to build and start
3. Automatically attaches the debugger to the running process
4. When you edit code and save, dotnet watch rebuilds and restarts
5. The debugger detects the restart and reconnects automatically
6. Breakpoints are preserved across restarts

Stopping Hot Reload Mode

Use stop_watch to cleanly terminate both the debugger and the dotnet watch process.

Status Information

Use status to see hot reload specific information:

• Watch process PID
• Child app PID
• Whether the debugger is currently reconnecting

Prerequisites

• netcoredbg installed and in PATH
• Node.js 18+
• .NET SDK 8.0+ (for building the harness and target applications)

Installation

# Install netcoredbg (example for Linux x64)
curl -sLO https://github.com/Samsung/netcoredbg/releases/download/3.1.3-1062/netcoredbg-linux-amd64.tar.gz
tar xzf netcoredbg-linux-amd64.tar.gz
sudo mv netcoredbg /opt/netcoredbg
sudo ln -sf /opt/netcoredbg/netcoredbg /usr/local/bin/netcoredbg

# Build this MCP server
npm install
npm run build

# The method invocation harness is auto-built on first use

Usage with Claude Code

Quick install:

# Clone and build
git clone https://github.com/AerialByte/mcp-netcoredbg.git
cd mcp-netcoredbg && npm install && npm run build

# Add to Claude Code
claude mcp add netcoredbg -- node $(pwd)/dist/index.js

Or manually add to your Claude Code MCP settings:

{
  "mcpServers": {
    "netcoredbg": {
      "command": "node",
      "args": ["/path/to/mcp-netcoredbg/dist/index.js"]
    }
  }
}

Security

This tool launches and controls a debugger. By design, it can:

• Execute arbitrary .NET applications
• Evaluate expressions within the debugged process
• Inspect memory and variables

Only use this with code you trust. Do not debug untrusted applications.

Example Session

Full Application Debugging

1. Build your .NET app with debug symbols: dotnet build --configuration Debug
2. Launch debugger: launch with the DLL path
3. Set breakpoints: set_breakpoint at file:line
4. Continue/step through code
5. Inspect variables with scopes and variables
6. Evaluate expressions with evaluate
7. Terminate when done

Method Invocation (Quick Testing)

1. Build the target assembly: dotnet build
2. Use invoke with the type and method name
3. If it fails, check the error for available constructors/methods
4. For debugging: set breakpoints first, then use invoke with debug: true

Agent Guidelines

When using this MCP server as an AI agent:

Choosing Between launch and invoke

• Use invoke when you want to test a specific method in isolation
• Use launch when you need to run the full application or debug complex scenarios

Using invoke Effectively

1. Start simple: Try without ctorArgs first - the harness will use parameterless constructors or auto-inject ILogger<T>

2. Handle errors iteratively: If invocation fails, the error response includes available methods/constructors. Use this to correct your call.

3. For debugging specific methods:

   1. Set breakpoints in the source files first
   2. Call invoke with debug: true
   3. Use continue/step_over/step_into to navigate
   4. Use output to see the final result
   

4. Arguments are JSON: Pass args as a JSON array. The harness handles type conversion:

   • Strings: "hello"
   • Numbers: 42, 3.14
   • Booleans: true, false
   • Null: null
   • Objects: {"name": "Alice", "age": 30}

Common Patterns

Testing a utility method:

invoke assembly=/path/to.dll type=MyApp.Utils method=Parse args=["input"]

Testing with constructor injection:

invoke assembly=/path/to.dll type=MyApp.Service method=Process ctorArgs=[100] args=["data"]

Debugging a failing method:

1. set_breakpoint file=/path/to/Service.cs line=42
2. invoke assembly=/path/to.dll type=MyApp.Service method=Process args=["bad-input"] debug=true
3. (breakpoint hits)
4. variables variablesReference=1
5. continue
6. output

License

MIT