How to quickly onboard a programmer to a project - Chatgpt Codex vs. Gemini CLI

How to quickly onboard a programmer to a project

many years ago I would worked with https://doxygen.nl/ to document the code but today the easy way will be using Chatgpt Codex or Gemini CLI:

Lets look the differences between the tools:

Area	Gemini CLI	ChatGPT Codex CLI
Vendor / model	Google, Gemini 2.5 Pro	OpenAI, GPT-5 / GPT-5-Codex
Install	npm i -g @google/gemini-cli or brew install gemini-cli	npm i -g @openai/codex or brew install codex
Auth	Google login or Gemini/Vertex API key	Sign in with ChatGPT (Plus/Pro/Team/Enterprise/Edu) or API key
License	Apache-2.0, open source	Apache-2.0, open source
OS	macOS, Linux, Windows	macOS, Linux; Windows via WSL (experimental)
Core behavior	Terminal AI agent with ReAct loop; built-in tools and MCP servers	Local coding agent that reads/edits files and runs commands; MCP support; approval modes
Web / grounding	Built-in Google Search grounding and web fetch	Internet access available in Codex Cloud; CLI supports MCP and can delegate
Context / quotas	Free tier includes Gemini 2.5 Pro with 1M-token context and ~60 RPM / 1,000 req/day during preview	Uses GPT-5 by default; switchable to GPT-5-Codex; plan-based access via ChatGPT
Multimodal	Images/video via Imagen/Veo integrations	Accepts image inputs; IDE and cloud variants can execute and draft PRs
Integrations	VS Code (Code Assist agent mode), GitHub Actions, Zed (ACP)	IDE extensions (VS Code, Cursor, Windsurf), Codex Cloud

Bottom line:

• Pick Gemini CLI if you want generous free usage, built-in Google Search grounding, and easy MCP-based extensions. blog.google

• Pick Codex CLI if you want tighter ChatGPT + IDE integration, fine-grained local approvals, and GPT-5-Codex for deep, agentic coding. developers.openai.com

Do it from PowerShell in that folder.

1. Install and sign in

npm install -g @openai/codex    # installs Codex CLI
codex login                      # OAuth with your ChatGPT account


Work in c:\code\
Set-Location c:\code
codex                        # starts an interactive session scoped to this directory