Surplus is a marketplace for AI model inference built on Robinhood Chain. It provides an API for Claude models, including Opus, Sonnet and Fable, that is compatible with the standard Anthropic SDK, so existing applications can switch to it by changing the base URL and API key. Usage is paid in USDG. On Surplus, model usage is measured in CREDIT, an on-chain unit in which 1 CREDIT stands for $1 of model usage. CREDIT is issued to $SURPLUS stakers, who can either spend it on inference or sell it to other users through an on-chain order book. The web app is at surplusllm.com and the API is at api.surplusllm.com.