mcp-debugger

mcp-debugger

A headless, agentic debugger over MCP — let your AI agents debug running programs in eight languages.

🎯 Overview

mcp-debugger is a Model Context Protocol (MCP) server that exposes step-through debugging as structured tool calls. It lets AI agents set breakpoints, inspect variables, evaluate expressions, and step through running programs across eight languages — driving real language debuggers through the Debug Adapter Protocol (DAP).

No IDE required. mcp-debugger runs anywhere Node.js runs: CI runners, Docker containers, Kubernetes pods, SSH boxes, and the sandboxes that cloud coding agents live in. It's the debugger for where IDEs can't go.

When to use mcp-debugger vs an IDE-bound debug server

Microsoft's DebugMCP exposes VS Code's debugger over MCP and is a good choice when your agent works inside a running VS Code. The two projects make different structural trade-offs:

If your agent runs in a terminal, a pipeline, or a cloud sandbox — or needs to attach to a process on another machine — you want mcp-debugger.

🆕 v0.24.0 — C/C++ debugging lands (CodeLLDB: prebuilt binaries, auto-compiled single sources, attach by PID), alongside statement/function-addressed breakpoints, logpoints, restart_debugging, breakpoint management tools, buffered get_output, break-on-uncaught-exceptions by default, a read-only IDE mirror (expose_session), default-on secret redaction, and a multi-language Docker image (Python, JS, Java, Rust, C/C++ native; Ruby attach). See the CHANGELOG for the full release history.

✨ Key Features

• 🌐 Multi-language support – Clean adapter pattern for any language
• 🐍 Python debugging via debugpy – Full DAP protocol support
• 💎 Ruby debugging via rdbg – Launch and attach workflows, including remote attach to containers and Kubernetes pods
• 🟨 JavaScript (Node.js) debugging via js-debug – VSCode's proven debugger
• 🦀 Rust debugging via CodeLLDB – Debug Rust & Cargo projects (Linux/macOS; Windows needs the GNU toolchain — see Rust on Windows)
• 🐹 Go debugging via Delve – Full DAP support for Go programs
• ☕ Java debugging via JDI bridge – Launch and attach modes with JDK 21+
• 🔷 .NET/C# debugging via netcoredbg – Debug .NET applications with full DAP support
• ⚙️ C/C++ debugging via CodeLLDB – Launch prebuilt binaries or lone source files (auto-compiled), attach by PID; core dumps and gdbserver/rr targets via config pass-through
• 🧪 Mock adapter for testing – Test without external dependencies
• 🛰️ Out-of-IDE & remote attach – Attach over host/port to a process on another machine or inside a container (Python via debugpy, Ruby via rdbg, Java via JDWP) with source-path mapping, or by PID for native code (C/C++) — direct-connect attach needs no local toolchain, and list_supported_languages reports per-mode availability with reasons
• 🎯 Breakpoints that survive edits – Address by content (statement: "total = sum(prices)"), by symbol (function: "main"), or assert line content with expectedContent; anchors re-resolve across restart_debugging and weak matches warn loudly
• 🪵 Logpoints – set_breakpoint with logMessage: "x={x}" streams interpolated values into get_output without pausing — prod-safe value watching on hot paths
• 🧰 Full breakpoint lifecycle – list_breakpoints / remove_breakpoint / clear_breakpoints work live mid-run; restart_debugging relaunches with the same config and re-applies everything in one call
• 📡 Buffered program output – get_output returns debuggee stdout/stderr with a cursor, and each session exposes its transcript as a subscribable MCP resource
• 💥 Crash-state debugging by default – Launch sessions pause on uncaught exceptions with stack and locals live (breakOnExceptions; exception class/message surfaced via lastStop)
• 🪞 Read-only IDE mirror – expose_session opens a loopback, token-gated DAP endpoint so a human's IDE can inspect the agent's live session without taking control
• ☸️ Kubernetes debugging – port-forward attach for interpreted runtimes, kubectl debug --target + attach-by-PID for native processes (recipe, turnkey manifests)
• 🔌 STDIO and Streamable HTTP transports – Works with any MCP client (legacy SSE transport is deprecated)
• 📦 Zero-runtime dependencies – Self-contained bundles via esbuild + tsup
• ⚡ npx ready – Run directly with npx @debugmcp/mcp-debugger - no installation needed
• 🐳 Docker and npm packages – Deploy anywhere
• 🤖 Built for AI agents – Structured JSON responses for easy parsing
• 🔒 Secret redaction on by default – Credential-shaped values (API keys, tokens, private keys) are masked as labeled placeholders in variable, evaluate, and output results before they reach the agent (details; opt out with DEBUG_MCP_NO_REDACT=1)
• 🛡️ Path validation – Prevents crashes from non-existent files
• 📝 AI-aware line context – Intelligent breakpoint placement with code context
• ✅ Comprehensive test suite – unit, integration, and end-to-end coverage across every adapter (CI status)

🧠 Agent Skill

Tools tell an agent what it can do; a skill teaches it how to debug well. This repo ships an agent skill covering the session golden path, root-cause discipline (bisection over line-by-line stepping), attach/remote recipes, and per-language quirks:

# Claude Code (user-level)
cp -r skills/debugging ~/.claude/skills/mcp-debugger
# Cross-agent directories (Copilot CLI and friends)
cp -r skills/debugging ~/.agents/skills/mcp-debugger

The server also serves condensed guidance in-band: MCP instructions on connect, plus a debugging-workflow prompt any MCP client can request. See skills/debugging/README.md for details.

🎬 See It In Action

• Auto-debug failing CI tests – a composite GitHub Action that launches mcp-debugger + an agent on a test failure and posts the root-cause analysis
• Sick pod walkthrough – attach to a misbehaving Python service in Kubernetes via port-forward (tutorial)
• Native sick pod – same story for compiled code: ephemeral debug sidecar + attach-by-PID, no in-process agent required

🚀 Quick Start

Requirements: Node.js 22+ for the server. Each language you debug also needs its own toolchain installed (Python + debugpy, Ruby + the debug gem / rdbg, Node.js, Go + Delve, JDK 21+, .NET SDK, the Rust toolchain, or a C/C++ compiler — g++/clang++, only needed for source-file launch). Not sure what's installed? Run npx @debugmcp/mcp-debugger doctor for a per-adapter toolchain report.

CodeLLDB platform note (npx/npm installs): the CodeLLDB debug engine ships as per-platform optional dependencies (@debugmcp/codelldb-win32-x64, -darwin-x64, -darwin-arm64, -linux-x64, -linux-arm64) — npm installs exactly the one matching your platform, so Rust and C/C++ debugging work out of the box everywhere npm serves. If you install with --omit=optional, set CODELLDB_PATH to a CodeLLDB release binary instead, or use the Docker image.

For MCP Clients (Claude Desktop, etc.)

Add to your MCP settings configuration:

{
  "mcpServers": {
    "mcp-debugger": {
      "command": "node",
      "args": ["C:/path/to/mcp-debugger/dist/index.js", "stdio", "--log-level", "debug", "--log-file", "C:/path/to/logs/debug-mcp-server.log"],
      "disabled": false,
      "autoApprove": ["create_debug_session", "set_breakpoint", "get_variables"]
    }
  }
}

For Claude Code CLI

For Claude Code users, we provide an automated installation script:

Prerequisite: The Claude CLI must be installed and available on your PATH before running the installation script. See Claude Code documentation for installation instructions.

# Clone the repository
git clone https://github.com/debugmcp/mcp-debugger.git
cd mcp-debugger

# Run the installation script
./scripts/install-claude-mcp.sh

# Verify the connection (use 'claude mcp list' if claude is on your PATH)
claude mcp list

Important: The stdio argument is required to prevent console output from corrupting the JSON-RPC protocol. See CLAUDE.md for detailed setup and troubleshooting.

Using Docker

docker run -v $(pwd):/workspace debugmcp/mcp-debugger:latest

The Docker image debugs Python, JavaScript, Java, Rust, and C/C++ natively (toolchains + a shared vendored CodeLLDB are included), plus the mock adapter. Ruby is attach-only in the image (the adapter ships without a Ruby runtime — attach to any rdbg --open process, local or remote). Only Go and .NET are disabled in the container — run those via npm/npx next to your local toolchain. Host-built Rust/C++ binaries debugged in the container get an auto-derived source map back to /workspace. list_supported_languages reports per-mode availability (modes.launch / modes.attach) with reasons. See Docker support.

Using npm

npm install -g @debugmcp/mcp-debugger
mcp-debugger --help

Or use without installation via npx:

npx @debugmcp/mcp-debugger --help

📚 How It Works

mcp-debugger exposes debugging operations as MCP tools that can be called with structured JSON parameters:

// Tool: create_debug_session
// Request:
{
  "language": "python",  // or "ruby", "javascript", "rust", "go", "java", "dotnet", "cpp", or "mock" for testing
  "name": "My Debug Session"
}
// Response:
{
  "success": true,
  "sessionId": "a4d1acc8-84a8-44fe-a13e-28628c5b33c7",
  "message": "Created python debug session: My Debug Session"
}

🛠️ Available Tools

All 28 tools below are implemented — see the tool reference for parameters and response shapes.

🏗️ Architecture: Dynamic Adapter Loading

Version 0.10.0 introduces a clean adapter pattern that separates language-agnostic core functionality from language-specific implementations:

┌─────────────┐     ┌────────────────┐     ┌──────────────┐     ┌─────────────────┐
│ MCP Client  │────▶│ DebugMcpServer │────▶│SessionManager│────▶│ AdapterRegistry │
└─────────────┘     └────────────────┘     └──────────────┘     └─────────────────┘
                            │                      │
                            ▼                      ▼
                    ┌──────────────┐      ┌─────────────────┐
                    │ ProxyManager │◀─────│ Language Adapter│
                    └──────────────┘      └─────────────────┘
                                                  │
              ┌───────────┬───────────┬───────────┼───────────┬───────────┬───────────┬───────────┬───────────┐
              │           │           │           │           │           │           │           │           │
        ┌─────▼────┐┌─────▼────┐┌─────▼────┐┌─────▼────┐┌─────▼────┐┌─────▼────┐┌─────▼────┐┌─────▼────┐┌─────▼────┐
        │Python    ││Ruby      ││JavaScript││Rust      ││Go        ││Java      ││.NET      ││C/C++     ││Mock      │
        │Adapter   ││Adapter   ││Adapter   ││Adapter   ││Adapter   ││Adapter   ││Adapter   ││Adapter   ││Adapter   │
        └──────────┘└──────────┘└──────────┘└──────────┘└──────────┘└──────────┘└──────────┘└──────────┘└──────────┘

Adding Language Support

Want to add debugging support for your favorite language? Check out the Adapter Development Guide!

💡 Example: Debugging Python Code

Here's a complete debugging session example:

# buggy_swap.py
def swap_variables(a, b):
    a = b  # Bug: loses original value of 'a'
    b = a  # Bug: 'b' gets the new value of 'a'
    return a, b

Step 1: Create a Debug Session

// Tool: create_debug_session
// Request:
{
  "language": "python",
  "name": "Swap Bug Investigation"
}
// Response:
{
  "success": true,
  "sessionId": "a4d1acc8-84a8-44fe-a13e-28628c5b33c7",
  "message": "Created python debug session: Swap Bug Investigation"
}

Step 2: Set Breakpoints

// Tool: set_breakpoint
// Request:
{
  "sessionId": "a4d1acc8-84a8-44fe-a13e-28628c5b33c7",
  "file": "buggy_swap.py",
  "line": 2
}
// Response:
{
  "success": true,
  "breakpointId": "28e06119-619e-43c0-b029-339cec2615df",
  "file": "C:\\path\\to\\buggy_swap.py",
  "line": 2,
  "verified": false,
  "message": "Breakpoint set at C:\\path\\to\\buggy_swap.py:2"
}

Step 3: Start Debugging

// Tool: start_debugging
// Request:
{
  "sessionId": "a4d1acc8-84a8-44fe-a13e-28628c5b33c7",
  "scriptPath": "buggy_swap.py"
}
// Response:
{
  "success": true,
  "state": "paused",
  "message": "Debugging started for buggy_swap.py. Current state: paused",
  "data": {
    "message": "Debugging started for buggy_swap.py. Current state: paused",
    "reason": "breakpoint"
  }
}

Step 4: Inspect Variables

First, get the scopes:

// Tool: get_scopes
// Request:
{
  "sessionId": "a4d1acc8-84a8-44fe-a13e-28628c5b33c7",
  "frameId": 3
}
// Response:
{
  "success": true,
  "scopes": [
    {
      "name": "Locals",
      "variablesReference": 5,
      "expensive": false,
      "presentationHint": "locals",
      "source": {}
    },
    {
      "name": "Globals", 
      "variablesReference": 6,
      "expensive": false,
      "source": {}
    }
  ]
}

Then get the local variables:

// Tool: get_variables
// Request:
{
  "sessionId": "a4d1acc8-84a8-44fe-a13e-28628c5b33c7",
  "scope": 5
}
// Response:
{
  "success": true,
  "variables": [
    {"name": "a", "value": "10", "type": "int", "variablesReference": 0, "expandable": false},
    {"name": "b", "value": "20", "type": "int", "variablesReference": 0, "expandable": false}
  ],
  "count": 2,
  "variablesReference": 5
}

📖 Documentation

• 🗺️ Roadmap – Path to 1.0: stability criteria, near-term themes, non-goals
• 📘 Tool Reference – Complete API documentation
• 🚦 Getting Started Guide – First-time setup
• 🏗️ Architecture Overview – Multi-language design
• 🔧 Adapter Development – Add new languages
• 🔌 Dynamic Loading Architecture – Runtime discovery, lazy loading, caching
• 🧩 Adapter API Reference – Adapter, factory, loader, and registry contracts
• 🔄 Migration Guide – Upgrading to v0.15.0 (dynamic loading)
• 🐍 Python Debugging Guide – Python-specific features
• 💎 Ruby Debugging Guide – Ruby debugging with rdbg, including remote attach
• 🔬 Case Study: the initialize response that never came – mcp-debugger debugging itself to root-cause a Ruby launch stall
• 🔬 Case Study: attaching the debugger to itself – live-patching a running mcp-debugger through its own evaluate_expression to catch a race strace couldn't see
• 🔬 Case Study: the pause that always succeeded – reading js-debug's own CDP trace to catch its smart-stepper eating user pauses on idle servers
• 🔬 Case Study: the trace that couldn't name its socket – measure-fix-measure on the diagnostics themselves: one scripted attach run, before and after, as the acceptance test
• 🟨 JavaScript Debugging Guide – JavaScript/TypeScript features
• 🐹 Go Debugging Guide – Go debugging with Delve
• ☕ Java Debugging Guide – Java debugging with JDI bridge
• 🔷 .NET Debugging Guide – .NET/C# debugging with netcoredbg
• ⚙️ C/C++ Debugging Guide – CodeLLDB launch, auto-compile, attach-by-PID, core dumps, remote stubs
• 🦀 Rust Debugging Guide – CodeLLDB setup (Windows specifics)
• 🐳 Docker Support – Container languages, attach modes, host-binary source mapping
• ☸️ Kubernetes Debugging – Turnkey attach recipes: registry-free manifests, per-language presets, ephemeral debug sidecar
• 🚑 JIT Diagnostics Tutorial – Debug live services in Kubernetes, incl. the ephemeral-sidecar flow for compiled code
• 🤖 Agent Debugging Guide – Correct tool usage patterns for AI agents
• ⚠️ Known Issues – Current caveats and workarounds
• 🩺 Diagnostics – mcp-debugger doctor, per-language prerequisites, failure signatures, env-var reference
• 🔧 Troubleshooting – Common issues & solutions

🤝 Contributing

We welcome contributions! See CONTRIBUTING.md for guidelines.

# Development setup
git clone https://github.com/debugmcp/mcp-debugger.git
cd mcp-debugger

# Install dependencies and vendor debug adapters
pnpm install
# Vendored debug engines (Microsoft's js-debug; CodeLLDB, shared by Rust and C/C++)
# are downloaded automatically and verified against committed SHA-256 digest pins

# Build the project
pnpm build

# Run tests
pnpm test

# Check adapter vendoring status
pnpm vendor:status

# Force re-vendor all adapters (if needed)
pnpm vendor:force

Debug Adapter Vendoring

The project automatically vendors debug adapters during pnpm install:

• JavaScript: Downloads Microsoft's js-debug from GitHub releases
• Rust & C/C++: Download a single shared copy of CodeLLDB for the current platform (packages/codelldb-common)
• Integrity: Every download is verified against the pinned SHA-256 digests in the packages' vendor-manifest.json; mismatches fail the build
• CI Environment: Set SKIP_ADAPTER_VENDOR=true to skip vendoring

To manually manage adapters:

# Check current vendoring status
pnpm vendor:status

# Re-vendor all adapters
pnpm vendor

# Clean and re-vendor (force)
pnpm vendor:force

# Clean vendor directories only
pnpm clean:vendor

Running Container Tests Locally

We use Act to run GitHub Actions workflows locally:

# Build the Docker image first
docker build -t mcp-debugger:local .

# Run tests with Act (use WSL2 on Windows)
act -j build-and-test --matrix os:ubuntu-latest

See tests/README.md for detailed testing instructions.

📊 Project Status

• ✅ Production Ready: v0.24.0 with eight language adapters, 28 tools, and polished multi-language distribution
• ✅ Clean architecture with a dynamic adapter pattern
• ✅ Python · Ruby · JavaScript/TypeScript · Go · Java · .NET/C#: Full step-through debugging
• 🦀 Rust: Full support on Linux/macOS/Windows (Windows requires the GNU toolchain; MSVC is not supported by CodeLLDB)
• ⚙️ C/C++: Full step-through debugging via CodeLLDB (launch + attach-by-PID; on Windows prefer MinGW/DWARF — MSVC PDB fidelity is partial)
• 🟢 Runtime: Node.js 22+
• 📈 Active Development: Regular updates and improvements — see the Roadmap for the path to 1.0

🏛️ Who Maintains This

mcp-debugger is stewarded by Sycamore LLC and led by John Franklin (@debugmcpdev). The project uses an agent-first development model with human accountability: AI agents write most of the code; a human maintainer makes every merge, release, and security decision. See MAINTAINERS.md, GOVERNANCE.md, and SUPPORT.md (including commercial support).

Supply-chain posture: pinned CI actions, OIDC trusted publishing, sigstore provenance on every npm package, SBOMs attached to releases, and an OpenSSF Scorecard score we actively maintain — details in SUPPLY-CHAIN-SECURITY.md. Report vulnerabilities via SECURITY.md.

📄 License

MIT License - see LICENSE for details.

👥 Contributors

• @Poyraxx — Ruby adapter (rdbg)
• @swinyx — Go adapter (Delve)
• @roofpig95008 — Java adapter (JDI bridge)

🙏 Acknowledgments

Built with:

• Model Context Protocol by Anthropic
• Debug Adapter Protocol by Microsoft
• debugpy for Python debugging
• debug for Ruby debugging

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Give your AI agents a real debugger — in any language.