Agent Skills: CLI Development Guidelines

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
• CLI spec template

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
• scripts/cli_audit.py

Reference library

• Core reference: references/REFERENCE.md
• Quick audit checklist: references/CHECKLIST.md
• Evaluation prompts: references/EVAL_PROMPTS.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