Perch is an open-source semantic linter that uses AI decision models to review every pull request for bugs, security issues, and custom rules defined by code owners. It parses repositories using tree-sitter to map each method's callers and callees, then runs rule checks and security scans on every changed method. Users can write plain-language rules such as 'never log passwords' or 'store money in integer cents' in a simple YAML configuration file. Perch integrates with tools like Claude Code, Codex, Cursor, and GitHub, and comments directly on flagged code within the pull request. Pricing starts at roughly 20 cents per pull request, with the first five dollars of usage free and no credit card required.