Summer Engine: the AI game engine

Build real 2D and 3D games through conversation. No coding required. Export to Steam, desktop, mobile, and web. Built on the Godot team's work, customized and honed so AI agents and humans can collaborate on great games.

Summer is the MIT open-source agent layer that connects your AI coding agent to Summer Engine. It is the Summer CLI, the Summer MCP server, and the Summer agent skills, hooks, and plugin manifests, all in one package. First-class setup works in Claude Code, Cursor, Codex, Windsurf, Cline, Roo Code, Gemini CLI, GitHub Copilot CLI, GitHub Copilot in VS Code, and OpenCode. Factory Droid uses the plugin marketplace path.

• Source: github.com/SummerEngine/summer-engine-agent
• CLI setup: www.summerengine.com/cli
• MCP setup: www.summerengine.com/mcp
• Docs: docs.summerengine.com

It just works. Open your agent, say "let's make an FPS in Summer Engine," and it does.

Summer CLI, Summer MCP, and the Summer agent

Three names, one npm package (summer-engine):

• Summer CLI (npx -y summer-engine@latest): installs the engine, signs you in, scaffolds and runs projects, and writes your agent's config. See www.summerengine.com/cli.
• Summer MCP: the local MCP server that gives your agent 44 engine tools to build, run, and debug a real game. See www.summerengine.com/mcp.
• Summer agent layer: the game-dev skills, hooks, and plugin manifests that give your AI agent judgment, not just a chat box.

All MIT, all free to use. One paste sets up all three.

Summer Cloud (new in 2.6.0)

summer cloud syncs your whole game project across machines: scenes, scripts, settings, and the big binary assets that never fit in git. Files are stored by content hash, every push becomes a restorable version, and local checkpoints are taken before anything destructive touches your disk.

summer cloud init      # bind this project to your account
summer cloud push      # upload what changed (only missing bytes travel)
summer cloud pull      # make this machine match the cloud, verified by hash
summer cloud status    # what differs between here and the cloud
summer cloud restore   # roll the project back to any version

The same operations are available to agents as MCP tools (summer_cloud_push, summer_cloud_pull, and friends). See www.summerengine.com/cloud.

Get started: one prompt

Open your AI agent (Claude Code, Cursor, Codex, Copilot, Windsurf, etc.) and paste:

Install Summer Engine and let's make a game. Read github.com/SummerEngine/summer-engine-agent, check what I already have installed, install whatever is missing, log me in, and scaffold a starter project so we can start building.

That's it. The agent reads this README, detects what's already on your machine, and runs only what's missing via Bash. No /plugin slash commands. No manual installs. One paste.

The first run will:

1. Install the summer-engine npm package (~3 MB) if it's not on PATH.
2. Install Summer's game-dev skills into ~/.claude/skills/ (or your agent's user-skill directory) so the agent loads them next session.
3. Download the Summer Engine app (~1 GB, including the engine binary, Git, and other runtime tools so users without them aren't blocked) if it's not installed.
4. Open your browser for one-click sign-in.
5. Scaffold a starter project and open the engine.

At that point the agent has the skills, the MCP server, and your engine running. Ask it to build something.

AGENTS READING THIS: install playbook

You are an AI coding agent. The user told you to install Summer Engine. Run this playbook via your shell: Bash on macOS/Linux or PowerShell on Windows. npx -y is shell-agnostic and works identically in both. Detection first, then only install what's missing.

Step -1: Verify Node.js is on PATH

node --version

If this command does not print Node.js 18 or newer, stop. Tell the user: "Summer needs Node.js 18+. Install from https://nodejs.org and re-run the prompt." Do NOT try to install Node yourself.

Step 0: Pre-detection (no commits to disk, just check state)

npx -y summer-engine@latest doctor --json

The first invocation downloads the npm package transparently to npx's cache (~3 MB, ~5 sec). Subsequent calls are fast. The -y flag auto-confirms npx's "ok to install summer-engine?" prompt so the call doesn't hang in a non-interactive shell.

Read the JSON output. If top-level ok is true, everything is installed and you can skip straight to Step 5. Otherwise use the checks array to run only the remediation steps that need attention.

The relevant check ids are node-version, cli-version, cli-version-current, skills-version-stale, login, engine-install, local-api, project-memory, and mcp-server. For any check that is not ok, run the action in the table below, then re-run doctor.

| Check id | If ok | Action when attention is needed | |---|---|---| | node-version | continue | Step -1 already covered this | | cli-version | always ok via npx | no action | | cli-version-current | continue | run npx clear-npx-cache && npx -y summer-engine@latest doctor --json (forces a fresh resolve) | | skills-version-stale | skip Step 1 | run Step 1 with --force | | engine-install | skip Step 2 | run Step 2 | | login | skip Step 3 | run Step 3 | | local-api | skip Step 5b wait-loop | run wait-loop after summer run | | project-memory | continue | if a project is open, run summer:brainstorm-game before building | | mcp-server | skip Step 1 | run Step 1 |

On version drift: if either cli-version-current or skills-version-stale needs attention, refresh before proceeding. Tell the user once: "There's a newer Summer (X.Y.Z to A.B.C); updating before we start." Then run the recommended action from details.recommendedAction and re-run doctor. Don't ask the user to choose; they paste "install" and want the latest.

Step 1: Install user-level skills (idempotent, fast)

npx -y summer-engine@latest setup claude-code --yes --force

Replace claude-code with the user's actual agent: codex, cursor, windsurf, cline, roo-code, gemini, github-copilot, vscode-copilot, or opencode. Use github-copilot for Copilot CLI and vscode-copilot for GitHub Copilot Chat/Agent in VS Code. Factory Droid still uses the plugin marketplace path in the install section below. This writes the current Summer skill bundle to ~/.claude/skills/<name>/SKILL.md (or the agent's equivalent user-skill directory) AND writes the MCP server config so the agent can talk to the engine. The --force flag wipes any stale skill content first, so re-runs always end up with the latest copy.

First-install detection: if ~/.claude/skills/ didn't exist before this command, Claude Code wasn't watching it and won't auto-detect the new files this session. Tell the user once: "Skills installed - restart your agent so they load." On subsequent installs (directory already existed), skills auto-detect mid-session and no restart is needed. You can detect first-install vs upgrade by checking if ~/.claude/skills/ existed before Step 1; record the result before running setup.

Do NOT run /plugin install or any slash command. The CLI path is the canonical install. The marketplace path is an alternative for users who specifically prefer the official plugin TUI; we don't route agents there.

Step 2: Install the engine app (only if engine-install needs attention)

npx -y summer-engine@latest install

~1 GB. Downloads from Summer's signed releases. The bundle includes the engine binary plus Git and a handful of other runtime tools so users who don't already have them aren't blocked. The CLI prints the URL and size before touching disk. Tell the user "downloading the engine app, ~1 GB, this takes a couple minutes" so they don't bail thinking it stalled.

Linux note: the engine app currently supports macOS and Windows only. On Linux, summer install exits with a "Linux support coming soon" message. Surface the manual download URL to the user (https://summerengine.com/download) and stop. The rest of the playbook still works once they've installed manually.

Step 3: Sign in (only if login needs attention)

npx -y summer-engine@latest login

This opens the user's default browser. Tell the user: "Your browser is opening now. Click sign-in once and come back to this terminal." The CLI waits up to 120 seconds for the auth callback, writes the token to ~/.summer/auth-token, and returns. If the user takes longer than 120 seconds, re-run npx -y summer-engine@latest login and tell them to come back to the terminal quickly. Don't loop indefinitely. The same token is shared with the engine; they sign in once, both surfaces accept it.

Step 4: Re-run doctor to confirm

npx -y summer-engine@latest doctor --json

Should now return ok: true for cli-version, login, and engine-install. local-api may still need attention because the engine is not running yet. If a setup check still needs attention, surface the specific message to the user and do not loop or paper over it.

Step 5: Scaffold a project and run

Important: summer create writes the project into the current working directory. Pick a stable parent directory first, such as the user's home or a Projects folder, and cd there before running create. Don't let it land in a temp dir or wherever the agent happened to start.

cd ~/Projects   # or wherever the user keeps code

If the user said "let's make a game" without specifying a template, default to 3d-basic. Pick a sensible project name from the user's request, for example "FPS" to my-fps-game. Avoid spaces in the name.

npx -y summer-engine@latest create 3d-basic my-fps-game
npx -y summer-engine@latest run my-fps-game

summer run launches the engine app with the project loaded and polls the engine's local API for up to 20 seconds before returning.

Step 5b: Wait for the engine to be reachable (only if summer run printed "API not responding yet")

If the engine takes longer than 20s to boot (cold start, slow disk, etc.), summer run returns successfully but the local API isn't up yet. MCP tools may not connect until the local API is ready. Wait for it:

# Poll doctor until local-api flips to ok (max ~30s):
for i in 1 2 3 4 5 6; do
  result=$(npx -y summer-engine@latest doctor --json)
  echo "$result" | grep -q '"id":"local-api","status":"ok"' && break
  sleep 5
done

(Equivalent PowerShell: for ($i=0; $i -lt 6; $i++) { $r = npx -y summer-engine@latest doctor --json; if ($r -match '"id":"local-api","status":"ok"') { break }; Start-Sleep 5 })

Once local-api is ok, MCP tools are safe to call.

Step 6: Brainstorm the game, then build

Now you have skills + MCP. Don't jump into summer create content from a vague prompt. Call summer_start_game_task with the user's goal, then invoke the recommended skill. For vague "make a game" requests, that should usually be summer:brainstorm-game. It scopes mechanics, art direction, and the V1 cut list, and writes the result to .summer/GameSoul.md. Every other Summer skill reads from that file.

Project memory lives in .summer/ as readable Markdown. Use .summer/memory/ for locked facts that should not drift across sessions, such as character voice IDs, world canon, and provider bindings. Users can inspect it with summer memory; agents should read relevant memory surfaced by summer_get_project_context before changing creative/audio/dialogue/level/character work.

Only skip brainstorm if the user explicitly said "skip brainstorm" or "just build the X already." After brainstorm, the appropriate build skills (fps-controller, design-mechanic, design-level, make-game, etc.) auto-trigger from natural language.

Anti-patterns (do NOT do these)

• Don't run npm install -g summer-engine. Use npx -y summer-engine@latest ... instead. -g requires sudo on most Mac/Linux setups, so the user will get permission errors and you'll get stuck. npx works without root and the MCP config we ship already points at npx summer-engine mcp.
• Don't try to install Node.js for the user. If node --version is unavailable, point them at https://nodejs.org and stop.
• Don't ask the user "do you want the engine too?" or "do you also want the CLI?" There is one product, three pieces, all installed by this playbook.
• Don't double-install. Always run npx -y summer-engine@latest doctor --json first. Skip steps that already returned status: ok.
• Don't tell the user to type /plugin marketplace add or /plugin install. The CLI path is the canonical install. Those slash commands are an alternative for the marketplace TUI, not the primary flow.
• Don't run summer install or summer login in the background. They print user-facing output (engine download progress, browser-opening notice). Run them in the foreground so the user sees what's happening.
• Don't skip the --json flag on summer doctor. Text output is fragile to parse and changes per version.
• Don't skip the -y flag on npx. Without it, npx prompts "ok to install summer-engine?" interactively and the call hangs in a non-interactive shell.
• Don't drop @latest from the npx commands. npx -y summer-engine (no version pin) serves whatever's in npx's local cache, which can be a stale older version. The @latest tag forces npm to re-resolve from the registry. If a user reports "unknown command 'doctor'" or sees a much smaller skill list than expected, they got cache-served. Tell them to run npx clear-npx-cache then retry with @latest.
• Don't call MCP tools (summer_get_scene_tree, etc.) until local-api is ok. The engine takes a few seconds to boot after summer run. Use Step 5b's wait-loop.
• Don't summer create without cd'ing to a stable parent directory first. It writes into CWD; agents that start in a temp dir will leave the project there.
• Don't loop summer login if it times out at 120s. Re-run it once and tell the user to be quicker; loop forever and the user is stuck.
• Don't jump straight into summer create from a vague prompt. Invoke summer:brainstorm-game first (Step 6). The build skills assume .summer/GameSoul.md exists.

Using a different agent? Replace claude-code with any supported agent in Step 1: codex, cursor, windsurf, cline, roo-code, gemini, github-copilot, vscode-copilot, or opencode. Skill targets vary per agent (Cursor uses .cursor/rules/, Windsurf uses .windsurfrules, Cline + Roo use .clinerules/, Copilot uses ~/.copilot/skills or .github/skills, OpenCode uses agent definitions, etc.). The CLI handles the difference. After install, Cline and Roo Code users should restart VS Code so the extension reloads its MCP config. Gemini users may need to run gemini extensions enable summer-engine after the first install. VS Code Copilot users should start the summer-engine MCP server from Agent mode if VS Code does not autostart it. Factory Droid still has its own plugin path below.

Power-user note: if the user specifically wants summer on their PATH for everyday terminal use outside the AI agent, a global npm install is still possible. The agent flow doesn't need it.

Get started: alternative manual install via plugin marketplace

If you specifically prefer the official Claude Code plugin marketplace UI, you can install via:

/plugin marketplace add SummerEngine/summer-engine-agent
/plugin install summer@summer-engine

This is functionally equivalent to npx -y summer-engine@latest setup claude-code --yes for the skill install. You'll still need npx -y summer-engine@latest install and npx -y summer-engine@latest login to get the engine running. The agent-driven path above is faster and idempotent; this exists for users who want everything in the plugin TUI.

What just happened (the three pieces)

You installed one product. It has three parts. You don't have to think about them because Summer wires them up, but here's what each does:

• Summer Engine: the game engine app. Where you see, play, and debug your game. Installed by the agent via npx -y summer-engine@latest install. Proprietary, free to use.
• Summer agent layer: the skills, hooks, and plugin manifests that tell Claude Code, Codex, Cursor, and other agents how to build games. Installed by summer setup or the marketplace path. MIT, open source.
• Summer CLI and MCP bridge: the terminal command and local tool server that install the engine, scaffold projects, run them, sign you in, and let agents talk to the running engine. Usually invoked through npx -y summer-engine@latest ..., so no global install is needed. MIT, open source.

The agent layer gives the agent judgment. The CLI and MCP bridge give it hands. The engine is where the game actually lives.

What gets downloaded

We tell you before we touch your disk.

| What | Size | When | Source | |---|---|---|---| | summer-engine npm package (CLI + plugin source) | ~3 MB | first npx -y summer-engine@latest ... call | npmjs.com/package/summer-engine | | Summer Engine app | ~1 GB (engine + bundled Git/runtime tools) | npx -y summer-engine@latest install | Summer's signed releases | | Auth token | ~1 KB | npx -y summer-engine@latest login | Browser to ~/.summer/auth-token | | Skill files | < 50 KB | bundled in the npm package | no extra network call | | Generated assets (3D / image / audio / video) | varies | only on explicit summer_generate_* calls | Summer Engine Studio | | URL imports | varies | only on explicit summer_import_from_url calls | the URL you provide |

Not downloaded:

• No background telemetry. Diagnostics stay local.
• No silent engine updates. You run summer update manually.
• No model weights. AI generation runs in Summer Engine Studio (cloud), never on your machine.

What's open and what's not

| Thing | License | Source | |---|---|---| | Summer (this repo: CLI, MCP server, skills, hooks, plugin manifests) | MIT | SummerEngine/summer-engine-agent | | Summer Engine app (the desktop editor and runtime) | free to download, closed source for now | summerengine.com/download | | Hosted Summer AI and cloud services (asset generation, storage, multiplayer, orchestration) | paid Summer services | summerengine.com/pricing |

The desktop app is free to download. The AI-agent layer, meaning the CLI, MCP server, skills, and plugin manifests, is MIT open source. Hosted Summer AI, asset generation, storage, multiplayer, and cloud features are paid Summer services.

How it works

Two pieces, plus the glue.

Skills. A bundled set of markdown playbooks. Each one is a discipline guide for the agent. They auto-fire on natural language:

• "Let's brainstorm a game" -> summer:brainstorm-game
• "It crashes when I press play" -> summer:debug
• "Add an FPS controller" -> summer:fps-controller
• "Make it look prettier" -> summer:art-direction
• "Set up multiplayer" -> summer:setup-multiplayer -> summer:host-authoritative-state

Skills don't list steps. They encode the order of operations: diagnose before editing, scope before building, ask before guessing. Agent Skills format, so any conformant tool picks them up.

MCP bridge. The summer-engine MCP server gives the agent 44 tools that talk to your local engine on localhost:6550:

| | | |---|---| | Scene | summer_add_node, summer_set_prop, summer_instantiate_scene, summer_replace_node, summer_get_scene_tree, summer_inspect_node, summer_batch | | Diagnostics | summer_get_script_errors, summer_get_diagnostics, summer_get_console, summer_get_debugger_errors, summer_get_debugger_warnings | | Runtime | summer_play, summer_stop, summer_is_running | | Project | summer_get_project_context, summer_open_main_scene, summer_project_setting, summer_input_map_bind | | Assets | summer_search_assets, summer_import_asset, summer_import_from_url, summer_generate_image, summer_generate_3d, summer_generate_audio, summer_generate_video |

File ops, git, shell, and grep already belong to your agent. We don't shadow them.

Lifecycle hooks. A session-start hook detects whether you're in a Summer Engine project and feeds the agent a one-line orientation. An opt-in pre-commit doctor runs summer doctor before git commit and blocks when the setup needs attention.

For the full architecture, see docs/OVERVIEW.md.

The basic workflow

1. using-summer loads on session start. Sets workflow priority and the red-flag list.
2. brainstorm-game scopes a new project. One question, one page, one direction.
3. scene-composition picks the right hierarchy before any node lands.
4. fps-controller / design-mechanic / design-level / setup-multiplayer / vfx produce the artifact.
5. gdscript-patterns guides every .gd write.
6. play runs the game and reports clean or broken.
7. debug runs the cheapest diagnostic, forms one specific hypothesis, asks before editing, re-verifies after the fix.
8. export-and-ship runs the pre-flight checklist before producing a release build.

The skill bundle replaces "agent flailing through tutorials" with measurable craft.

What's inside

Process: using-summer, brainstorm-game, debug, play

Project setup: new-project, browse-templates, make-game, scene-composition

Build: fps-controller, design-mechanic, design-level, design-npc

Multiplayer: setup-multiplayer, host-authoritative-state, peer-to-peer-multiplayer

Look & feel: art-direction, audio-direction, 3d-lighting, vfx, ui-basics

Code & assets: gdscript-patterns, asset-strategy

Performance & ship: tune-performance, export-and-ship

Skill authoring: skill-create, skill-improve, skill-test

C# is supported by the engine. A summer:csharp-patterns skill is on the roadmap; for now write C# from Godot 4.5 docs.

Full list of shipping skills: the skills: array in .claude-plugin/plugin.json.

Philosophy

• Skills auto-fire on natural language. No slash command required.
• Diagnose before guessing. Read the error before grepping the code.
• Process skills run before build skills.
• Fewer tools, sharper tools.
• The user owns decisions. The agent diagnoses and proposes.

Install: full instructions per harness

Claude Code

claude /plugin marketplace add SummerEngine/summer-engine-agent
claude /plugin install summer@summer-engine

When Summer lands on Anthropic's official marketplace, also:

claude /plugin install summer@claude-plugins-official

Codex CLI

/plugins

Search summer, install.

Codex App

Sidebar to Plugins to Coding, then click + next to Summer.

Cursor

/add-plugin summer

Or search summer in the Cursor plugin marketplace.

Factory Droid

droid plugin marketplace add https://github.com/SummerEngine/summer-engine-agent
droid plugin install summer@summer-engine

Cline (VS Code)

npx -y summer-engine@latest setup cline --yes

Writes the MCP server config to VS Code's globalStorage for the Cline extension and drops project rules into .clinerules/. Restart VS Code so Cline reloads its MCP config.

Roo Code (VS Code)

npx -y summer-engine@latest setup roo-code --yes

Same shape as Cline (Roo Code is a Cline fork): writes globalStorage MCP config and .clinerules/ files. Restart VS Code afterward.

Gemini CLI

npx -y summer-engine@latest setup gemini --yes

Drops a Summer extension manifest at ~/.gemini/extensions/summer-engine/. If the extension isn't auto-enabled on next launch, run gemini extensions enable summer-engine. The marketplace path (gemini extensions install https://github.com/SummerEngine/summer-engine-agent) still works for users who prefer it. Both end up at the same place.

GitHub Copilot CLI

npx -y summer-engine@latest setup github-copilot --yes

Writes MCP config to ~/.copilot/mcp-config.json and skills to ~/.copilot/skills/. In an active Copilot CLI session, run /mcp reload and /skills reload; otherwise restart Copilot CLI.

GitHub Copilot in VS Code

npx -y summer-engine@latest setup vscode-copilot --yes

Writes MCP config to VS Code's user mcp.json and skills to ~/.copilot/skills/. Restart VS Code or run MCP: List Servers, then use Copilot Agent mode.

OpenCode

npx -y summer-engine@latest setup opencode --yes

Writes the MCP server entry into opencode.json (~/.config/opencode/opencode.json for user scope, ./opencode.json for project) using the array-shaped command: ["npx", "-y", "summer-engine@latest", "mcp"] format. Restart OpenCode. Full guide: .opencode/INSTALL.md.

Windsurf and others

npx -y summer-engine@latest setup <agent> --yes --force
npx -y summer-engine@latest doctor

CLI reference

| Command | What it does | |---|---| | summer install | Download Summer Engine. Prints URL and size first. | | summer login | Browser-based sign-in. | | summer logout | Clear auth tokens. | | summer status | Engine state, port, auth. | | summer doctor | Diagnose Node, login, engine, project memory, MCP. | | summer plan <goal> | Route a game-building goal into skills, MCP tools, gates, and verification. | | summer memory | Inspect project memory in .summer. | | summer memory show <file> | Print a project memory Markdown file. | | summer run [path] | Launch the engine. | | summer open <path> | Open a project in a running engine. | | summer create <template> [name] | Scaffold a project. | | summer list templates / projects | Browse. | | summer skills list | Show all skills. | | summer skills install <name> | Install one. | | summer skills install --recommended --agent <agent> | Install the recommended set. | | summer mcp | Start the MCP server. | | summer mcp setup <agent> | Write MCP config for an agent. | | summer setup <agent> [--yes] | One shot: MCP config + recommended skills + doctor. |

Agents: claude-code, codex, cursor, windsurf, cline, roo-code, gemini, opencode. Scopes: --scope user (default), --scope project.

Verify

In a fresh agent session:

Let's make an FPS in Summer Engine.

The agent should auto-load summer:fps-controller before writing code. If it doesn't, run summer doctor.

Templates

| Template | What you get | |---|---| | empty | Empty 3D project with a root node | | 3d-basic | 3D scene with camera, light, floor |

More coming.

Contributing

Skills evolve fast. Two ways to help:

1. Open an issue if a skill misfires. Quote the prompt and the response.
2. Submit a new SKILL.md under skills/<category>/<name>/. Register it in .claude-plugin/plugin.json and src/lib/skills-registry.ts. Run summer:skill-test against it before opening the PR.

docs/DEVELOPMENT.md is the contributor guide.

Per-harness docs

• Claude Code
• Codex
• Cursor
• OpenCode
• Skills overview
• Templates
• Architecture overview

License

MIT for everything in this repo. Summer Engine itself is proprietary. See What's open and what's not.

Links

• Website
• Download Summer Engine
• Documentation
• Community
• Issues