ghstack is a Python command-line tool for managing stacked pull requests on GitHub. It enables users to create, update, and merge stacks of PRs where each PR corresponds to a Git commit, handling the complexity of branch naming, PR state synchronization, and dependency tracking. The architecture is modular and asynchronous, using Click for the CLI, asyncio for I/O-bound operations, and abstract endpoints for GitHub API interactions with real and fake implementations for production and testing. Key features include a SQLite-based cache for API responses, shell command abstraction, commit trailer manipulation, and type-safe ID handling. The tool also includes a Chrome extension to modify GitHub's PR UI and a comprehensive test suite simulating GitHub/git interactions. The codebase demonstrates strong engineering practices: clear separation of concerns, dependency inversion via abstract endpoints, comprehensive testing with a custom harness, and attention to type safety. Reusable components include the caching layer, async shell runner, trailer parser, and type definitions. Maintenance is active with recent commits and ongoing issue triage. Technical drift is medium due to flexible dependency version ranges and a copied GraphQL schema requiring manual updates to track GitHub API evolution, though the core design remains robust and well-suited for its purpose.