Hearth

An AI-native 2D game engine — a real editor for humans, and the whole engine as commands your coding agent already knows how to use.

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Hearth is a real engine, not an AI game generator. Humans get a normal visual editor. Coding agents (Claude Code, Codex, OpenCode with local models, any MCP client) get the entire editor surface as structured, validated, permission-checked commands, and they arrive already knowing how to use it: a per-project skill teaches the working loop and game-feel craft, headless playtests and screenshots let an agent prove its work runs, and every command lands in a disk-backed journal you can read and revert. No AI runs inside the engine, nothing needs an API key or a cloud account, and your project is a folder of readable JSON, scripts, and assets on your own disk.

   Human ──▶ Editor UI ──┐
                         ├──▶  one shared command system  ──▶  project files
   Agent ──▶ CLI / MCP ──┘      (validate · execute · diff)     scenes · scripts · assets

The connection is plumbing; the point is the loop it enables. An agent snapshots the project, inspects what's there, builds a level, wires input, writes behavior scripts, imports and slices art, runs a deterministic headless playtest to check it actually works, and hands you a structural diff to review in the editor, all without guessing at file formats. You stay in charge: permission modes gate what agents may touch, every session can be snapshotted, diffed, and reverted, and every command any agent runs, whether from the CLI, MCP, or the editor's embedded terminal, is journaled (hearth log, or live in the editor's Agent panel).

Coding with your agent in minutes

The fastest path, no build step:

1. Get Hearth. Download the desktop app (macOS/Windows/Linux), or grab the two standalone agent files (Node 20+):

curl -LO https://github.com/echoo19/hearth/releases/latest/download/hearth-cli.mjs
curl -LO https://github.com/echoo19/hearth/releases/latest/download/hearth-mcp.mjs

2. Scaffold a game. hearth init (or node hearth-cli.mjs init) writes a small playable project (a scene with a camera, ground, and a player that already falls and lands) plus an AGENTS.md and a project skill any agent that visits will read:

node hearth-cli.mjs init "Star Catcher" --template platformer

3. Point your agent at it. In the desktop app, open the project, focus the Agent panel, and type claude, codex, opencode, hermes, or another agent command. The terminal starts at the project root with hearth already on PATH. MCP is optional; connect it by hand when you want typed tools:

claude mcp add hearth -- node $PWD/hearth-mcp.mjs --project $PWD/star_catcher

Then describe the game. The agent calls get_agent_instructions, follows the snapshot → inspect → edit → validate → playtest → diff loop from its skill, and hands you changes to review. Per-agent walkthroughs: Claude Code, Codex, OpenCode + Ollama (local models), Hermes, and any MCP client or CLI.

4. See it and ship it. Open the folder in the editor, press Play, then hearth export web --zip --allow build for an itch.io-ready build or hearth export desktop --allow build for a native app. See the Quickstart for the full ten-minute version.

Related MCP server: Unity-MCP-Vibe

What's in the engine

Runtime. A fixed-timestep, deterministic 2D runtime: sprites and spritesheets, tilemaps with auto-generated colliders and 47-blob autotiling, text, 2D lighting, polylines, deterministic particles, sprite animation plus animation state machines, gamepad input with analog virtual axes, camera effects (shake/flash/fade/zoom punch), a screen-space post-processing stack (bloom/CRT/vignette/chromatic-aberration/pixelate/color-grade, up to 8 stacked) plus per-sprite outline/hit-flash/dissolve effects, and screen-space game UI with layout containers, sliders, toggles, and focus navigation. Physics covers mass, restitution, friction, named collision layers, one-way platforms, and box/circle/convex-polygon colliders. Audio includes sound effects, procedural synthesis, and a streamed music channel that survives scene switches. The same runtime runs in the editor preview, headlessly in Node, and in exported games, including every post-processing/sprite effect. See docs/effects.md.

Scripting. Lua by default (sandboxed, seed-deterministic), JavaScript equally supported, both against the same ctx API: timers, tweens, seeded RNG, pub/sub events, math helpers, grid pathfinding, camera control and effects, per-sprite hit-flash, animation state machine control, UI focus, save data, and scene switching for building your own menus. hearth inspect api prints the whole surface, the editor's Code panel autocompletes off that same reference, and check-script syntax-checks a draft before you save it. See docs/scripting.md.

Editor. A dockable workspace: scene view with tilemap paint tool and direct-manipulation transform handles, hierarchy with "Save as prefab", a schema-driven inspector (typed pickers and enum dropdowns for every field, no raw JSON textareas), an Animator editor for state-machine assets, an asset browser with spritesheet slicing and whole-folder drag-drop import, live preview with Pause/Step, a Code panel, a read-only Live runtime inspector, console, Input and Game Settings panels, undo/redo with a history panel, a plain-language Checkpoint/Review/Changes loop for agent sessions, and an Agent panel: a real embedded project terminal where you run your own agent CLI, with hearth already on PATH, a live activity timeline, and Checkpoint/Review/Revert one click away (see docs/agent-panel.md). The editor live-follows changes from any external agent session, whether CLI, MCP, or another editor, reloading automatically instead of going stale, and its local project server enforces Origin/Host on every request. Runs in the browser during development or as a packaged desktop app (see docs/editor.md).

Agent tooling. The hearth CLI wraps every command with --json envelopes, including undo/redo/history and log (the disk-backed command journal). Property writes validate the full dot-path against the component's real schema, with a did-you-mean suggestion on a typo instead of silently corrupting the write. The engine's command registry has 76 commands; the MCP server exposes 78 typed tools with per-session permission modes: 75 wrapping a core command, plus screenshot, capture, and get_agent_instructions. Playtests script input (including gamepad axes and pointer drags) and assert on game state, events, particles, audio, camera effects, post effects, and UI focus, entirely headless and CI-friendly. hearth screenshot renders real frames through headless Chromium so agents can see their own work. See docs/cli.md and docs/mcp.md.

Prefabs. Reusable, live-linked entity templates: serialize a subtree into a prefab asset, place instances, edit an instance and the change records as a per-instance override automatically, push a change back and every other instance auto-syncs (merging each instance's own overrides rather than discarding them), or revert one field/component/instance. CLI, MCP, and editor all the way through; scripts spawn prefabs live with ctx.scene.spawnPrefab. See docs/prefabs.md.

Export. hearth export web produces a static, self-contained build (one folder, one HTML file, or an itch.io zip) that boots straight into your first scene. hearth export desktop wraps the same build in a hardened Electron shell and zips a native app per platform (macOS arm64/x64, Windows, Linux). No engine splash, no branding, nothing of Hearth's chrome ships in your game. See docs/export.md, docs/ship-web-hosting.md, docs/shipping-to-itch.md, and docs/ship-desktop.md.

Procedural placeholders. Agents can create deterministic SVG sprites and WAV sound effects through commands, so a game is playable and audible before any real assets exist. Real art, sound, music, and fonts come in through the same asset pipeline: import, probe, slice, animate.

Status

Hearth is at v1.0.0, the production release — no longer a developer preview. The whole loop works end to end: project model, editor, runtime preview, CLI, MCP, headless playtests, diff review, and web and desktop export. Scripts have modules: they can require() each other in both Lua and JavaScript; a library is just a script that returns a table or exports an object, and hot-reload recompiles whatever depended on the file you changed. The Agent panel has its own full-height dock on the right beside the Inspector. It opens a project shell directly; a compact first-project guide explains how to type an agent command and can be dismissed. The Activity timeline collapses under the terminal. The roadmap keeps an honest list of what's next.

Install

Desktop app. Grab it from the latest release: macOS (Hearth-mac-arm64.dmg / Hearth-mac-x64.dmg), Windows (Hearth-win-x64.exe), Linux (Hearth-linux-x86_64.AppImage / Hearth-linux-amd64.deb). On macOS, first launch is right-click → Open (macOS 14 and earlier) or System Settings → Privacy & Security → Open Anyway (macOS 15+); if macOS claims the app is damaged, run xattr -cr /Applications/Hearth.app (desktop builds are ad-hoc signed, not notarized yet, see docs/desktop-app.md).

Agent tools, no install. The CLI and MCP server ship as single files that only need Node 20+:

curl -LO https://github.com/echoo19/hearth/releases/latest/download/hearth-cli.mjs
curl -LO https://github.com/echoo19/hearth/releases/latest/download/hearth-mcp.mjs
node hearth-cli.mjs --help
claude mcp add hearth -- node $PWD/hearth-mcp.mjs --project <your game>

The same two files are bundled inside the desktop app, and the editor's agent-setup panel shows their exact paths ready to copy.

The agent loop by hand

The workflow agents are taught is pleasant to run yourself:

alias hearth="node $PWD/packages/cli/dist/main.js"
cd packages/examples/mini-platformer

hearth snapshot                       # checkpoint for review
hearth inspect scene "Level 1" --json # what's here?
hearth create entity "Level 1" Gem \
  --position 620,300 --tags coin \
  --components '{"SpriteRenderer":{"shape":"diamond","color":"#9b59b6","width":20,"height":20},"Collider":{"shape":"circle","radius":12,"isTrigger":true}}'
hearth attach script "Level 1" Gem scripts/coin-pickup.js
hearth validate --json                # schema + reference checks
hearth test                           # headless playtests
hearth diff                           # what changed, human-readable
hearth revert --confirm               # undo the whole session

Permission modes from --mode read-only up to --mode all control what an agent may do; see docs/mcp.md.

Documentation

Organized as a journey: get started, build, script, ship, connect an agent, reference.

Design principles

1. The engine works without AI. Agents are a first-class client, but nothing in the engine depends on them.

2. One command system. If it isn't a registered command, neither the UI nor agents can do it, so capabilities stay legible and auditable.

3. Humans stay in charge. Permission modes gate what agents may touch, snapshots and structural diffs make every agent session reviewable and revertible, and playtests make "it works" checkable.

4. Local-first, readable files. Projects are JSON you can diff, art you can open in anything, and plain Lua or JavaScript scripts.

Contributing & building from source

Requires Node 20+.

git clone https://github.com/echoo19/hearth.git && cd hearth
npm install
npm run build:packages     # core → runtime → playtest → shipping → templates → cli → mcp-server
npm test                   # full suite, headless
npm run typecheck          # vitest does not typecheck; run this too
npm run dev                # editor at http://localhost:5173

Open an example from the launcher (try Mini Platformer) and press Play (arrows/WASD to move, Space to jump). For the desktop app, npm run app launches it and npm run app:dist packages an installer; see docs/desktop-app.md. Eleven example projects live in packages/examples, every one generated through the command system itself and covered by playtests in CI. They double as reference projects for everything the docs describe. Ground rules and the AI contribution policy are in CONTRIBUTING.md.

License

MIT. Dependencies are all open-source-friendly: zod, commander, PixiJS, React, wasmoon, and the MCP SDK.