Arco is a macOS application that enables users to interact with and compare responses from a wide range of artificial intelligence language models in a single interface. Designed for macOS 13 Ventura or later on Apple Silicon, the app functions as a local chat client that connects to the OpenRouter platform. Users provide their own OpenRouter API key, which grants access to over 400 models, including options such as GPT-4o, Claude, Gemini, and Mistral. All conversations are stored locally on the user’s device and are not uploaded to any external servers, ensuring that data remains private and under the user’s control. The core workflow involves typing a single prompt and sending it simultaneously to multiple models of the user’s choice. The app displays the responses in a configurable grid layout, supporting one, two, three, or four columns. Users can arrange panes to focus on specific models or hide others. After receiving responses, Arco offers a summarization feature: it sends all replies to a judge model (selected by the user) and generates a structured comparison that highlights key differences, identifies the strongest answer, and provides a recommendation. The summary is streamed in real time, can be copied to the clipboard, and can be regenerated with a different judge model. The application is free to start, with no subscription fees. Users pay only the per-token rates set by OpenRouter, without any markup from Arco. The free version includes up to three saved conversations, while an unlimited conversation option is available through a one-time payment that also covers future updates. Key technical requirements include a macOS device running Ventura or later with Apple Silicon. Typical use cases include comparing code debugging suggestions, evaluating API design proposals, and gathering multiple model perspectives on technical or creative questions, all without switching between separate chat websites.