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How to Control Unity with ChatGPT (Codex CLI)

Step-by-step setup guide for ChatGPT Plus users

Why is setup required?

ChatGPT doesn't support local MCP connections, so a slightly more complex setup is needed. Unlike Claude Desktop or Cursor, one-touch setup isn't available, but following this guide will get you up and running.

Before You Start

1Synaptic AI Pro package imported into Unity
2ChatGPT Plus subscription or higher
3
Node.js 18 or later installed(nodejs.org)

Import UnityPackage

Import the purchased UnityPackage file into Unity.

Open your Unity project
Drag and drop the UnityPackage file into the Assets folder
Click 'Import' when the import dialog appears

Install Node.js

Skip if already installed

Node.js is required to run the Synaptic MCP server. It's a JavaScript runtime that anyone can use - just install and you're ready.

Check if installed

node -v

If a version number appears (e.g., v20.10.0), you're good. If you see "command not found", installation is required.

Installation steps

nodejs.org Download the LTS version from nodejs.org
Run the installer (default settings are fine, just click Next)
After installation, close and reopen your terminal before proceeding

Install Codex CLI

Install Codex to use ChatGPT from your terminal.

Install command:

npm install -g @openai/codex

Verify installation:

codex --version

If a version number appears, you're good

Important: How to run Codex

After installation, run Codex with just "codex" - NOT "openai codex".

✓codexCorrect (use this)

✗openai codexWrong (won't work)

Launch Codex

After installation, launch Codex from your terminal.

codex

First-time launch requires signing in with your ChatGPT account.

💡

Model Selection Tip

Models with 'codex' in the name are code-optimized but may have weaker conversation abilities. If things aren't working well, try changing the model.

/modelto view and change models

Recommended: Latest general-purpose models like gpt-5.2 may work more reliably.

Reasoning Level

Models like gpt-5.2 let you select reasoning level. It's a tradeoff between performance and speed.

Low (fast) → Medium (default) → High → Extra High (accurate)

If responses feel slow, try Low. It's sufficient for simple tasks.

How to Use (6 Steps)

Open Setup Window

From Unity menu: Tools → Synaptic Pro → Setup Window

Switch to HTTP Tab

Click the HTTP Server tab to switch

Copy Prompt

Click Copy Prompt Template button

Paste to CLI

Paste the prompt to Codex

Key Point

Since it just hits HTTP endpoints, CLI can run from any directory. The server is started from Unity, so no cd commands needed in CLI.

Now just talk to the AI. Try saying "create a red cube" to test it out.

"Create a red cube""Create a Cube for Player""Move MainCamera back a bit"

💡Pro Tip: Automate with Skills

Using Codex CLI's Skills feature, you don't need to paste the template every time.

How to set up:

Copy prompt from Unity Setup Window's HTTP tab
Tell Codex: "Create a skill with this content"
From now on, the template is applied automatically

Thanks: Synaptic AI user

Start HTTP Server

Click Start HTTP Server button and you're done!

You're all set!

Connected to Unity! Now you can control it with just conversation. Try saying "create a red cube" to test it out.

Troubleshooting

'node: command not found' error

Node.js is not installed. Please install from nodejs.org.

Codex stuck loading

Check if your ChatGPT Plus subscription is active. Also check OpenAI status page for outages.

'Connection refused' error

Make sure Unity Editor is running and Synaptic AI Pro is installed.

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