But my understanding is Claude Code is closed source and only support Clade API endpoint. How do they make it work?
You set the environment variable ANTHROPIC_BASE_URL to an OpenAI-compatible endpoint and ANTHROPIC_AUTH_TOKEN to the API token for the service.
I used Kimi-K2 on Moonshot [1] with Claude Code with no issues.
There's also Claude Code Router and similar apps for routing CC to a bunch of different models [2].