AI Coding Tool Selection

This course uses Claude Code as the primary recommended tool, but most labs can also be completed with the alternative tools listed below.

Tool Comparison

Item	Claude Code	Gemini CLI	Codex CLI	OpenCode
Vendor	Anthropic	Google	OpenAI	Community (open-source)
Install	pnpm add -g @anthropic-ai/claude-code	pnpm add -g @google/gemini-cli	pnpm add -g @openai/codex	brew install opencode
Run	claude "prompt"	gemini (interactive)	codex "prompt"	opencode (TUI)
Headless	claude --print --no-color	pipe support	codex --approval-mode full-auto	opencode serve (API)
MCP Support	Full	Supported	Not supported	Limited
Project Instruction File	CLAUDE.md	GEMINI.md	AGENTS.md	—
Context Window	200K	1M	192K	Model-dependent
Cost	API key required	Free (1,000 req/day)	ChatGPT Plus or API key	Free (local models available)
Sandbox	None	None	Yes (safest)	None

Installation

Claude Code (Recommended)

pnpm add -g @anthropic-ai/claude-code
claude --version

Gemini CLI

pnpm add -g @google/gemini-cli
gemini --version

Codex CLI

pnpm add -g @openai/codex
codex --version

OpenCode

# macOS
brew install opencode

# or go install
go install github.com/opencode-ai/opencode@latest

API Key Setup

Claude Code

# Anthropic API key (https://console.anthropic.com)
export ANTHROPIC_API_KEY="sk-ant-..."

Gemini CLI

# Issued from Google AI Studio (https://aistudio.google.com)
# On first run, browser OAuth login, or:
export GEMINI_API_KEY="..."

Tip

Gemini CLI offers a free tier (1,000 req/day), so there is no API cost to worry about.

Codex CLI

# OpenAI API key (https://platform.openai.com)
export OPENAI_API_KEY="sk-..."

OpenCode

# Set based on your chosen model provider
# For OpenAI models:
export OPENAI_API_KEY="sk-..."
# For local models (Ollama, etc.) no key is needed

Headless Mode (for Ralph Loop)

When running a CLI non-interactively inside a Ralph Loop, the commands differ by tool.

Claude Code

claude --print --no-color --dangerously-skip-permissions "$(cat PROMPT.md)"

Gemini CLI

cat PROMPT.md | gemini

Codex CLI

codex --approval-mode full-auto "$(cat PROMPT.md)"

Tip

Codex CLI has a built-in sandbox, so it can run safely in automated mode without --dangerously-skip-permissions.

OpenCode

# Start in API server mode, then call via curl
opencode serve &
curl -X POST http://localhost:3000/api/chat \
  -d "{\"message\": \"$(cat PROMPT.md)\"}"

Project Instruction Files

Each tool reads a specific file in the project root to pass project-specific instructions to the agent.

Tool	File	Location
Claude Code	CLAUDE.md	Project root
Gemini CLI	GEMINI.md	Project root
Codex CLI	AGENTS.md	Project root
OpenCode	—	No dedicated file

Course Recommendations

• Lab instructions and code examples in this course are written for Claude Code.
• MCP-related labs (Lab 03) can only be fully completed with Claude Code or Gemini CLI.
• Codex CLI does not support MCP, so Lab 03 must be done with an alternative tool.
• If using a different tool, translate commands and flags to match that tool’s syntax.

Tool Abstraction via Environment Variables

When writing scripts that need to support multiple tools, you can use an environment variable pattern.

# Select tool via environment variable
export AI_CLI="${AI_CLI:-claude}"  # default: claude

# Example usage in harness.sh
case "$AI_CLI" in
  claude)
    claude --print --no-color --dangerously-skip-permissions "$(cat PROMPT.md)" ;;
  gemini)
    cat PROMPT.md | gemini ;;
  codex)
    codex --approval-mode full-auto "$(cat PROMPT.md)" ;;
  *)
    echo "Unsupported tool: $AI_CLI" && exit 1 ;;
esac