> ## Documentation Index
> Fetch the complete documentation index at: https://docs.summerengine.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Build Games in Gemini CLI with Summer Engine MCP

> Connect Summer Engine's MCP server to Gemini CLI and build games with Google's AI driving your engine. One-command setup.

## Summer Engine in Gemini CLI

Gemini CLI is Google's open-source terminal agent. With Summer Engine's integration, Gemini can control your game engine: add nodes to scenes, set properties, search and import assets, generate images and 3D models, run the game, and debug, all from the terminal.

Summer ships as a Gemini extension, so setup installs both the MCP bridge and a context primer (`GEMINI.md`) that teaches Gemini the Summer skills. Setup takes about a minute.

## Prerequisites

* **Gemini CLI**: Google's terminal agent
* **Node.js**: For running the MCP server (Node 18+)
* **Summer Engine**: Installed and running with your project open

## Configuration

### Option 1: One-command setup (recommended)

Run the Summer setup command. It writes the Gemini extension, installs the recommended skills, and runs diagnostics:

```bash theme={null}
npx -y summer-engine@latest setup gemini --yes
```

This creates the extension at `~/.gemini/extensions/summer-engine/`. Gemini extensions are user-scoped, so the extension applies across your projects.

### Option 2: Manual configuration

If you prefer to write the file yourself, create `~/.gemini/extensions/summer-engine/gemini-extension.json`:

```json gemini-extension.json theme={null}
{
  "name": "summer",
  "description": "Superpowers for AI game dev. MCP bridge to the local Summer Engine plus a context primer that teaches Gemini the Summer skills.",
  "contextFileName": "GEMINI.md",
  "mcpServers": {
    "summer-engine": {
      "command": "npx",
      "args": ["-y", "summer-engine@latest", "mcp"]
    }
  }
}
```

To preview the exact config without writing files, run:

```bash theme={null}
npx -y summer-engine@latest setup gemini --print
```

### Restart Gemini CLI

Close and restart Gemini CLI so it loads the new extension. The Summer Engine MCP server starts with Gemini; it connects to the engine on the first tool call.

### Verify

In Gemini CLI, list your MCP servers and confirm `summer-engine` appears with its tools. Then try a prompt:

* "Use Summer Engine to add a MeshInstance3D with a BoxMesh to the scene"

If the node appears in Summer Engine, the setup is working.

## Using Summer Engine with Gemini CLI

Once configured, Gemini can use Summer Engine's tools when you ask for game-related changes. Example prompts:

* "Add a Camera3D and DirectionalLight3D to the scene root"
* "Get the scene tree from Summer Engine"
* "Generate a low-poly treasure chest and import it under the World node"
* "Run the game and read the diagnostics"
* "Set the project's main scene to res\://main.tscn"

<Tip>
  **Engine must be running.** Gemini talks to Summer Engine over localhost. Open your project in the engine first, or run `npx summer-engine run` from the project directory.
</Tip>

## Troubleshooting

### "Summer Engine is not running"

Start Summer Engine and open your project. The MCP server connects to the engine's local API on port 6550. The engine writes a token to `~/.summer/api-token` when it starts.

### Extension not appearing

* **Config path:** Verify `~/.gemini/extensions/summer-engine/gemini-extension.json` exists
* **JSON validity:** Ensure no trailing commas and valid JSON
* **Restart:** Fully quit and reopen Gemini CLI
* **Diagnostics:** Run `npx -y summer-engine@latest doctor` to check the MCP server boot, login, and engine connection

### Tools return errors

See the [Tools Reference](/mcp/tools-reference) for parameter formats. Common issues: wrong node path format (`./World/Player`), invalid property values (use engine string format like `"Vector3(0, 10, 0)"`), or the game running when scene edits are attempted (stop it first with `summer_stop`).

## Next Steps

<CardGroup cols={2}>
  <Card title="Tools Reference" icon="wrench" href="/mcp/tools-reference">
    All 52 MCP tools with parameters and examples
  </Card>

  <Card title="Building a Game" icon="gamepad-2" href="/mcp/building-a-game">
    How an AI builds a full game using MCP
  </Card>

  <Card title="MCP Setup Overview" icon="plug" href="/mcp/setup">
    Architecture, prerequisites, and other IDEs
  </Card>

  <Card title="CLI Reference" icon="terminal" href="/mcp/cli-reference">
    summer setup, doctor, run, create, and more
  </Card>
</CardGroup>

***

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