Clap

This walks you through building production-grade Rust CLIs with Clap's derive macros. It covers the full stack: subcommands with global flags, config layering (CLI over env over file), proper exit codes, and testing your binary surface with assert_cmd. The progressive disclosure is solid, token estimate looks right at 4500 for the full context. Most valuable for the subcommand patterns and the config merge precedence rules, which are easy to get wrong. Skips the builder API entirely and goes straight to derive, which is the right call for 2024. If you're past "hello world" CLIs and need something maintainers won't hate, this gives you the playbook.