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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.

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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.

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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

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Configuration

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

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Option 2: Manual configuration

If you prefer to write the file yourself, create ~/.gemini/extensions/summer-engine/gemini-extension.json:
gemini-extension.json
{
  "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: 
npx -y summer-engine@latest setup gemini --print

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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.

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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.

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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”
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.

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Troubleshooting

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”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.

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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

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Tools return errors

See the 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).

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Next Steps

Tools Reference

All 44 MCP tools with parameters and examples

Building a Game

How an AI builds a full game using MCP

MCP Setup Overview

Architecture, prerequisites, and other IDEs

CLI Reference

summer setup, doctor, run, create, and more
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