battlecast-engine

A D&D 5e (2024 SRD) rules engine and combat state engine, with a built-in MCP server. Extracted from BattleCast, the browser-based tactical combat simulator that runs this engine in production - every rule here is battle-tested against thousands of simulated encounters.

Any LLM agent connected over MCP can build encounters, run tactically simulated combat, and make rules-validated state edits. Any TypeScript program can use the same engine directly as a library. Battles are deterministic when seeded: same seed, same result, byte for byte - which makes it equally suited to DM assistants, encounter balancing, regression testing, and LLM benchmarks.

What is inside

• Dice, attacks, advantage/disadvantage, criticals, saving throws
• Full damage pipeline: resistances, immunities, vulnerabilities, temp HP, Undead Fortitude, Death Ward, death saves, the dying state
• Conditions with durations and save-ends timers, ongoing effects
• Footprint-aware movement and collision on a grid (Medium through Gargantuan), terrain blocking, line of sight
• AoE geometry (cones, lines, spheres), spellcasting, concentration, buffs
• A complete combat AI (targeting, multiattack, smart movement, legendary actions) with four team tactics
• 317 SRD monsters and 12 hero classes at levels 1-20
• Seeded, reproducible battles: same seed, same result, byte for byte

Documentation

• docs/rules-coverage.md - exactly which 5e rules are implemented, verified against the source, including the honest gaps
• docs/api.md - the library API
• docs/mcp-tools.md - every MCP tool with parameters and an example session

MCP server

npx battlecast-engine mcp

Claude Code:

claude mcp add battlecast -- npx battlecast-engine mcp

Or in any MCP client config:

{
  "mcpServers": {
    "battlecast": { "command": "npx", "args": ["battlecast-engine", "mcp"] }
  }
}

Tools

A typical session: create_encounter (optionally seeded), add_creature a few times, start_battle, then either run_round for narrated round-by-round play or run_battle for an instant result. get_state returns a compact snapshot designed to be read by a language model. Round narration comes straight from the engine's battle log:

Round 1:
[R1] Fighter L5 moves 30 ft toward Goblin Warrior 2.
[R1] Fighter L5 hits Goblin Warrior 2 with Javelin (15 vs AC 15) for 10 piercing damage!
[R1] Goblin Warrior 2 has been slain!
[R1] Fighter L5 uses Action Surge for an extra attack action!

Library

import { Encounter } from 'battlecast-engine';

const enc = new Encounter({ gridSize: 20, seed: 42 });
enc.addCreature({ monster: 'Goblin Warrior', team: 'red', count: 4 });
enc.addCreature({ heroClass: 'Fighter', heroLevel: 5, team: 'blue' });
enc.start();

const round = enc.runRound();        // AI plays one full round
console.log(round.logs.map(l => l.details).join('\n'));

enc.damage('goblin-warrior-red-1', 8, 'fire');   // full rules pipeline
const result = enc.runToCompletion(50);
console.log(result.winner);

Lower-level engine primitives (applyDamage, resolveAttack, moveToward, executeRound, monster and hero data) are exported too - see docs/api.md and src/index.ts.

Built with battlecast-engine?

The code is MIT, so you owe nothing beyond keeping the license file. But if this engine saved you from reimplementing opportunity attacks, a link back means a lot and helps the project:

• Link to battlecast.gg or this repo in your README or credits
• A "powered by battlecast-engine" line in your app's about screen is the gold standard

If you build something with it, open an issue and say hi - cool projects get listed here.

Development

npm install
npm run build       # tsc
npm test            # vitest - engine suite ported from BattleCast + api tests

Read DESIGN.md for the architecture, the sync contract with BattleCast, and the roadmap.

License and SRD attribution

Code is MIT licensed (see LICENSE). Monster, spell, and class data derive from the Dungeons & Dragons 5.2 System Reference Document, released under CC-BY-4.0 by Wizards of the Coast.