MythxEngine — tabletop RPG engine over MCP
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
/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 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