STS2 Modding MCP

A Model Context Protocol server for Slay the Spire 2 modding. Connects to any MCP-compatible AI assistant (Claude Code, Claude Desktop, Cursor, Windsurf, etc.) and provides 151 tools for reverse-engineering the game, generating mod code, building/deploying, live-inspecting the running Godot engine, and autonomously playtesting mods.

• Reverse engineering — decompiles C# assemblies with Roslyn syntax trees, indexes 3,048+ entities and 144 hooks, extracts 15,000+ Godot assets

• Code generation — production-ready C# for 30+ entity types, Harmony patches, Godot UI, VFX, network messages, and complete mod projects

• Code intelligence — hook recommendations from natural language, patch suggestions, call graphs, API compatibility checks

• Build and deploy — builds mods, packages Godot PCK files, deploys to the game, validates assets and localization, watches for changes

• Live scene inspection — browses the running Godot scene tree, reads/writes node properties, toggles visibility, animates with Tweens

• Automated playtesting — starts seeded runs, plays cards, navigates every screen, runs at 20x speed, captures screenshots, sets breakpoints

• 29 built-in guides — hooks, Harmony, localization, multiplayer, Godot UI, IL transpilers, combat, save files, and more

Your mileage will vary depending on which LLM you use. This project is a fun experiment — please ping me if you have issues, want to suggest a feature, or find a bug.

Prerequisites

• Python 3.11+ — check with python --version

• .NET SDK 9.0 — for building mods, the Roslyn code analyzer, and decompilation

• .NET 8.0 Runtime — required by ilspycmd (see note below)

• ilspycmd — dotnet tool install -g ilspycmd (for C# decompilation)

• GDRE Tools — for Godot asset extraction (optional, setup wizard can download it)

• Slay the Spire 2 — the game itself

.NET version note: ilspycmd targets .NET 8.0. If you only have .NET 9.0+ installed, decompilation will fail with a runtime error. The fix is to also install the .NET 8.0 runtime alongside your .NET 9.0 SDK. Both can coexist without issues.

Related MCP server: DesyncedMCP

Quick Start

git clone https://github.com/elliotttate/sts2-modding-mcp.git
cd sts2-modding-mcp
python -m venv venv

# Activate the virtual environment:
source venv/bin/activate         # macOS / Linux
# source venv/Scripts/activate   # Windows (Git Bash)
# venv\Scripts\activate.bat      # Windows cmd
# venv\Scripts\Activate.ps1      # Windows PowerShell

pip install .
python -m sts2mcp.setup          # auto-finds game, installs tools, decompiles

The setup wizard automatically finds your Steam install, installs ilspycmd if needed, decompiles the game source, optionally downloads GDRE Tools for asset extraction, and builds the Roslyn code analyzer. In CI or non-interactive shells, use python -m sts2mcp.setup -y to auto-accept all prompts.

Connect to an AI Assistant

The MCP server connects to any AI tool that supports the Model Context Protocol. Point the config at the venv's Python so dependencies are always available.

Replace /path/to/sts2-modding-mcp with the actual path where you cloned the repo.

Claude Code (CLI):

# macOS / Linux:
claude mcp add sts2-modding /path/to/sts2-modding-mcp/venv/bin/python -- /path/to/sts2-modding-mcp/run.py

# Windows:
claude mcp add sts2-modding C:\path\to\sts2-modding-mcp\venv\Scripts\python.exe -- C:\path\to\sts2-modding-mcp\run.py

Claude Desktop — edit your config file (%APPDATA%\Claude\claude_desktop_config.json on Windows, ~/Library/Application Support/Claude/claude_desktop_config.json on macOS):

{
  "mcpServers": {
    "sts2-modding": {
      "command": "/path/to/sts2-modding-mcp/venv/bin/python",
      "args": ["/path/to/sts2-modding-mcp/run.py"]
    }
  }
}

On Windows, use full paths with backslashes: "command": "C:\\Users\\YourName\\sts2-modding-mcp\\venv\\Scripts\\python.exe".

Cursor / Windsurf / Other MCP Clients — most editors use the same JSON config format. Check your editor's MCP documentation for where to place it.

Verify It Works

• Claude Desktop: Click the hammer icon at the bottom of the chat input — you should see sts2-modding tools listed. Try: "What modding guides are available?"

• Claude Code: Run /mcp — you should see sts2-modding listed as connected. Try: "Use get_game_info to show me the server status."

If the server isn't connecting, run python run.py directly in the activated venv to check for startup errors.

Usage Examples

Once connected, ask your AI assistant things like:

• "Create a new mod called FlameForge with a card that deals 20 damage and applies 2 Vulnerable"

• "Which hook should I use to add extra card draw?"

• "Generate a relic that gives 3 Strength the first time you take damage each combat"

• "Build my mod and install it, then start a test run and play through a combat"

• "Show me the source code for the Bash card"

• "How do Harmony IL transpilers work?"

Updating After Game Patches

When STS2 updates:

1. C# source — run decompile_game (or manually re-run ilspycmd) to refresh the decompiled source. The Roslyn index automatically rebuilds on the next query.

2. Godot assets — run recover_game_project to re-extract scenes, textures, resources, and GDScript from the updated PCK.

Documentation

Detailed reference material is in the docs/ directory:

• Tool Reference — all 151 tools organized by category

• Complex Workflows — multi-step project editing, bridge automation, example sequences

• Advanced Generators — community-inspired generators and scaffold tools

• Modding Guides — all 29 built-in guide topics

• Project Structure — repository layout, generated mod structure, BaseLib integration

• Detailed Setup — manual decompilation, GDRE Tools, path configuration, scoped configs

License

MIT