CLI Development Guidelines

When to activate this skill

• You are designing, implementing, or reviewing a command-line tool.
• The user mentions (explicitly or implicitly): --help, flags, subcommands, exit codes, stdout/stderr, piping, JSON output, color, prompts, config files, env vars, “works in CI”, install/uninstall, telemetry.

What this skill produces

• A CLI contract (what users can rely on): commands, flags, IO behavior, exit codes, config/env, examples, and safety behavior.
• Draft help output and docs structure (example-first).
• A compliance audit (when runnable) using scripts/cli_audit.py.

Non-negotiable CLI citizenship

• Exit codes:

- 0 on success. - Non-zero on failure (and ideally meaningful, documented codes).

• Streams:

- stdout is for primary output and machine-readable output. - stderr is for errors, warnings, progress, and “what I’m doing” messaging.

• Discoverability:

- --help (and usually -h) shows help and exits. - --version prints version and exits.

• Interactivity:

- Prompts only when stdin is a TTY. - Provide --no-input to force non-interactive behavior.

• Scripting friendliness:

- No ANSI color / spinners when output isn’t a TTY. - Support NO_COLOR and --no-color. - Consider --json and --plain for stable output.

Workflow

Sketch the CLI contract first

• Start from the user’s jobs-to-be-done (what they’re trying to accomplish).
• Decide:

- Command shape: single command vs subcommands (noun verb is common). - Inputs: args vs flags vs stdin vs prompts vs config/env. - Outputs: human default, plus machine modes (--json, --plain, --quiet). - Safety: confirmations, --dry-run, --force, secret handling.

Use:

• [CLI reference](references/REFERENCE.md)
• [CLI spec template](templates/cli-command-spec-template.json)

Implement with safe defaults

• Use a CLI parsing library (don’t hand-roll).
• Make “boundary-crossing” actions explicit:

- Network calls - Writing files not explicitly provided - Mutating remote state

• Avoid footguns:

- Don’t accept secrets via flags or environment variables. - Don’t print stack traces by default. - Don’t assume TTY (detect it).

Validate and iterate

• Run an automated sanity check (when possible):

- python scripts/cli_audit.py -- <your-cli> [subcommand]

• Fix in this order:

- Broken stdout/stderr separation - Incorrect exit codes - Help that’s missing or undiscoverable - Unsafe defaults (destructive ops, secrets, hidden network writes) - Unscriptable output (no stable modes)

Use:

• [Checklist](references/CHECKLIST.md)
• scripts/cli_audit.py

Reference library

• Core reference: [references/REFERENCE.md](references/REFERENCE.md)
• Quick audit checklist: [references/CHECKLIST.md](references/CHECKLIST.md)
• Evaluation prompts: [references/EVALPROMPTS.md](references/EVALPROMPTS.md)

Templates and scripts

• CLI spec template: templates/cli-command-spec-template.json
• Help text template: templates/help-text-template.md
• Error message template: templates/error-message-template.md
• Audit a CLI: scripts/cli_audit.py