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Summer Engine in Codex

Codex is OpenAI’s AI coding assistant that supports MCP (Model Context Protocol). With Summer Engine’s integration, Codex can control your game engine: add nodes to scenes, set properties, import assets, run the game, and debug, all from within your coding workflow. This guide covers the configuration. Setup takes about a minute.

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Prerequisites

  • Codex: OpenAI’s AI coding assistant (CLI or IDE extension)
  • Node.js: For running the MCP server (Node 18+)
  • Summer Engine: Installed and running with your project open

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Configuration

Codex uses TOML configuration. The config file is:
  • User-level: ~/.codex/config.toml (applies to all projects)
  • Project-level: .codex/config.toml in your project root (trusted projects only)
You can configure via the CLI or by editing the config file directly. Add Summer Engine with a single command: 
codex mcp add summer-engine -- npx summer-engine mcp

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Option 2: Edit config.toml

Create or edit ~/.codex/config.toml and add: 
[mcp_servers.summer-engine]
command = "npx"
args = ["summer-engine", "mcp"]
In the Codex IDE extension, you can open the config via MCP settings > Open config.toml from the gear menu.

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Step 3: Restart Codex

Close and reopen Codex (or the IDE extension) so it loads the new MCP config. The Summer Engine MCP server starts when Codex starts; it connects to the engine on the first tool call.

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Step 4: Verify

In the Codex TUI, use /mcp to see your active MCP servers. You should see “summer-engine” with 23 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 Codex

Once configured, Codex 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”
  • “Import this model and add it to the World node: [URL]”
  • “Run the game and capture a screenshot”
  • “Set the project’s main scene to res://main.tscn”
Engine must be running. Codex 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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MCP server not appearing

  • Config path: Verify ~/.codex/config.toml exists (or .codex/config.toml for project scope)
  • TOML syntax: Ensure valid TOML (no trailing commas, proper section headers)
  • Restart: Fully quit and reopen Codex

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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 23 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 install, 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.