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Summer Engine in Gemini CLI

Legacy. Gemini CLI was retired for individual and Google One accounts on 2026-06-18 and replaced by Antigravity. summer setup gemini still works and still warns; it is for Workspace and enterprise Gemini CLI users. Everyone else: Antigravity.
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 20+: For running the MCP server
  • Summer Engine: Open with your project, for engine work only
  • Sign in once: npx -y summer-engine@latest login. One browser sign-in also connects the store, publishing and Grow tools

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Configuration

Run the Summer setup command. It writes the Gemini extension, installs the recommended skills, and runs diagnostics:
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
To preview the exact config without writing files, run:

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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 tools need the engine. Open your project in Summer Engine first, or run npx summer-engine run from the project directory. Asset, plan, store and publishing tools do not need it.

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Troubleshooting

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”Summer Engine is not running” (engine_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. Every Summer error names its code and one next step (next). not_signed_in: run npx -y summer-engine login. no_credits: add credits at https://www.summerengine.com/studio/credits. All codes: How Summer answers.

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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 Value formats 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

Operations reference

Every operation with its inputs and an example

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
Need help or have questions? Reach out to our founders at founders@summerengine.com or join our community on Discord for fast responses.