MarsAI is an API gateway designed to provide access to multiple frontier AI models through a single endpoint. It is compatible with both OpenAI and Anthropic client libraries, allowing developers to use their existing SDKs by simply changing the base URL to /v1. The service routes requests across private upstreams with automatic failover, and it handles billing through a prepaid credit system. New accounts receive 10 million free credits upon signup, which are valid for 7 days and require no credit card. Free access models incur no charges, while premium models deduct credits based on a burn multiplier per model. The platform includes two primary interfaces: a desktop application for Windows, macOS, and Linux, and a command line tool called MarsCLI. Both share the same credit balance and model catalog. The desktop app functions as a coding agent capable of editing files, running commands, and opening pull requests, and it self-updates. The CLI uses device code OAuth authentication and provides access to the full model catalog, subagents, and git workflows. The service also supports integration with third party tools such as OpenCode, Cline, and Codex through API key or device code authentication. MarsAI offers flexible credit plans. Premium credit packs are one time purchases with a defined validity period, and they can be stacked with free credits. Free credits are consumed first, then premium credits, at each model's specified burn rate. Premium credits expire with the pack, but renewals do not remove existing credits. The service includes daily check ins that grant bonus premium credits, referral grants, and a weekly leaderboard. Rate limits vary by plan, with starter plans offering 10 requests per minute and higher tiers offering 30 requests per minute. All published models support streaming, tool calling, and vision capabilities.