> You will need at least a Gemini API key, and for best performance also an OpenAI API key.

Could you explain why both / why not also Claude (why not all three?)

Gemini is required for the context management sub-agent. You can use any of OpenAI, Anthropic, or Gemini for the main planning and coding agents, but GPT-5 performs the best in my experience. Claude 4 Sonnet works well too, but it's about twice as expensive.