MythxEngine

CI npm license: MIT node

A complete tabletop RPG engine your AI can run: deterministic dice, five-tier outcomes, FitD-style stress, combat tracking, situation clocks, investigations, NPC relationships, world generation — 134 tools over the Model Context Protocol, with four complete starter worlds bundled in.

Claude (or any MCP client) becomes your Game Master. The engine keeps the rules honest: same seed + same actions = same outcome.

Install

Claude Code — plugin (skills + tools, recommended)

/plugin marketplace add protoLabsAI/mythx-mcp
/plugin install rpg@mythx-plugins

One install delivers the MCP server plus play skills (/rpg:play, /rpg:combat-mode, /rpg:world-building-mode, …).

Claude Code — tools only

claude mcp add rpg -- npx -y @mythxengine/mcp-server

Requires Node >= 22.13.

Claude Desktop

Download mythxengine-<version>.mcpb from the latest release and drag it into Settings → Extensions. No other setup — Claude Desktop ships its own Node runtime.

Five-minute walkthrough

Everything below is one conversation with Claude after installing. Four worlds are imported on first run — Port Miriam (harbor noir), Aethelgard (mythic frontier), Threshold Deep (weird depths), and the Treehouse Anthology.

1. Pick a world, make a character.

Start a session in Port Miriam. I want to play a dockside fixer who owes the wrong people money.

The GM calls create_session, load_world_summary, and create_character — your fixer gets HP, stress, gear, and a spot in the world's faction web.

2. Roll with real stakes. Every test is framed with position (how bad is failure?) and effect (how much does success buy?):

I try to slip past the harbor patrol to reach the smuggler's skiff.

roll_test → risky position / standard effect → 11 vs 13: partial
"Yes, but..." — you reach the skiff, and a lantern swings your way.
GM move suggested: put_someone_in_spot

Five outcomes — critical success, success, partial, failure, critical failure — and on a bad roll you can push_roll (spend 2 stress for +1d6) or resist_consequence to buy the severity down. Stress is a real currency; overflow means trauma.

3. Fight a combat round.

Two dockhands block the gangway. I draw my hook-knife.

start_combat → roll_initiative → attack — initiative order, armor, conditions, and damage all tracked; every attack returns the same five-tier outcome with suggested GM moves on a miss.

4. Watch a clock tick. The patrol you dodged? The GM started a harbor-alert clock. Fail forward enough times (tick_clock fires automatically on partials/failures when wired) and the dock lights come on — consequences arrive on schedule, not by fiat.

That's the loop: fiction in, mechanics out, consequences on clocks.

What's in the box

Piece	What it gives you
packages/mcp-server	The MCP server — 134 tools, published as @mythxengine/mcp-server
packages/engine	Pure, seeded game mechanics (dice, outcomes, combat)
packages/types / worlds / rulebook	Zod-validated schemas for every entity, plus rules lookup
packages/storage	SQLite persistence (node:sqlite, zero native deps)
packages/tools / prompts	Transport-agnostic tool implementations + runtime GM skills
world-builder/	The rpg Claude Code plugin (skills, agents, hooks)
agent-kit/	Portable GM system prompt + operating manual for any agent
docs/	The rulebook + how the engine works
4 bundled worlds	~13 MB of NPCs, factions, locations, situations, arcs — the demo

Rules & docs

• docs/RULEBOOK.md — the complete rules, all 12 chapters (outcomes, position & effect, tests, stress, combat, conditions, gear, time & clocks, GM moves). Generated from packages/rulebook, the same source the lookup_rule and generate_rulebook tools serve — so it never drifts from the engine.
• docs/engine.md — how the engine works: the five-tier outcome system, position & effect, stress, deterministic RNG, and how the packages fit together.
• agent-kit/ — a portable GM system prompt and operating manual for running a game with any agent.

Want the full experience?

This repo is the engine. MythxEngine Desktop is the game: a visual frame-based play surface, scene imagery, generated world books, and polish. Follow along and get notified when it ships at mythxengine.com.

Development

pnpm install
pnpm build && pnpm test && pnpm lint && pnpm typecheck

See CONTRIBUTING.md — this repo is a downstream mirror of a private monorepo; issues are the fastest way to get fixes in.

License

MIT © protoLabs Studio