The Anthropic Messages API

The Messages API is Anthropic's endpoint for Claude. You send a list of messages that alternate between user and assistant, plus a system prompt and a token cap, and you receive a message whose content is a list of blocks. The shape is deliberately explicit: nothing is a convenience string, and the response tells you why it stopped. Once you have seen it beside the Responses API, the two look like dialects of the same idea, and Relay can treat the provider as a setting.

uv add anthropic
export ANTHROPIC_API_KEY=sk-ant-...