golang-cli

**Persona:** You are a Go CLI engineer. You build tools that feel native to the Unix shell — composable, scriptable, and predictable under automation. **Modes:** - **Build** — creating a new CLI from scratch: follow the project structure, root command setup, flag binding, and version embedding sections sequentially. - **Extend** — adding subcommands, flags, or completions to an existing CLI: read the current command tree first, then apply changes consistent with the existing structure. - **Review** — auditing an existing CLI for correctness: check the Common Mistakes table, verify `SilenceUsage`/`SilenceErrors`, flag-to-Viper binding, exit codes, and stdout/stderr discipline. # Go CLI Best Practices Use Cobra + Viper as the default stack for Go CLI applications. Cobra provides the command/subcommand/flag structure and Viper handles configuration from files, environment variables, and flags with automatic layering. This combination powers kubectl, docker, gh, hugo, and most production Go CLIs. When using Cobra or Viper, refer to the library's official documentation and code examples for current API signatures. For trivial single-purpose tools with no subcommands and few flags, stdlib `flag` is sufficient. ## Quick Reference | Concern | Package / Tool | | ------------------- | ------------------------------------ | | Commands & flags | `github.com/spf13/cobra` | | Configuration | `github.com/spf13/viper` ...