What is zerostack?

zerostack is a lightweight coding agent that runs in the terminal, written in the Rust programming language. It is designed to be small and fast, and the project describes itself as inspired by two other coding agent tools called pi and opencode. A coding agent is a program that can read, write, and run code on your behalf while you describe what you want in plain language. The tool supports several different AI providers, including OpenRouter, OpenAI, Anthropic, Gemini, and Ollama, plus custom providers you configure yourself. It comes with the standard set of tools that coding agents typically use, such as reading and writing files or running commands. A permission system controls what the agent is allowed to do without asking first, ranging from a restrictive mode where nearly everything needs approval, to a fully automatic mode where all actions are approved without prompting. Sessions can be saved, resumed, and browsed later, and the agent automatically shortens old conversation history so it stays within the model's context limit. zerostack includes a set of built in prompt modes that change how the agent behaves for different tasks, such as a coding mode, a planning only mode, a code review mode, a debugging mode, and a read only mode for just asking questions. Users can switch between these with a command, or write their own custom prompts as text files. The project also supports connecting external tools through something called MCP, integrates a search tool for fetching and searching the web, and can move between different Git worktrees during a session. According to the README, zerostack is notably smaller and lighter than comparable agents written in JavaScript, using roughly 8 to 12 megabytes of memory compared to around 300 megabytes for some alternatives. It is installed using Rust's package manager, Cargo, or by downloading a pre-built binary from GitHub Releases. Windows support exists but the author notes it has not been tested.