Command name + one-sentence purpose.
Primary user: humans, scripts, or both.
Input sources: args vs stdin; files vs URLs; secrets (never via flags).
Output contract: human text, --json, --plain, exit codes.
Interactivity: prompts allowed? need --no-input? confirmations for destructive ops?
Config model: flags/env/config-file; precedence; XDG vs repo-local.
Platform/runtime constraints: macOS/Linux/Windows; single binary vs runtime.

Deliverables (what to output)

When designing a CLI, produce a compact spec the user can implement:

Command tree + USAGE synopsis.
Args/flags table (types, defaults, required/optional, examples).
Subcommand semantics (what each does; idempotence; state changes).
Output rules: stdout vs stderr; TTY detection; --json/--plain; --quiet/--verbose.
Error + exit code map (top failure modes).
Safety rules: --dry-run, confirmations, --force, --no-input.
Config/env rules + precedence (flags > env > project config > user config > system).
5–10 example invocations (common flows; include piped/stdin examples).

Default Conventions (unless user says otherwise)

-h/--help always shows help and ignores other args.
--version prints version to stdout.
Primary data to stdout; diagnostics/errors to stderr.
Add --json for machine output; consider --plain for stable line-based text.
Prompts only when stdin is a TTY; --no-input disables prompts.
Destructive operations: interactive confirmation + non-interactive requires --force or explicit --confirm=....
Respect NO_COLOR, TERM=dumb; provide --no-color.
Handle Ctrl-C: exit fast; bounded cleanup; be crash-only when possible.

Fill these sections, drop anything irrelevant:

Name: mycmd
One-liner: ...
USAGE:
- mycmd [global flags] <subcommand> [args]
Subcommands:
- mycmd init ...
- mycmd run ...
Global flags:
- -h, --help
- --version
- -q, --quiet / -v, --verbose (define exactly)
- --json / --plain (if applicable)
I/O contract:
- stdout:
- stderr:
Exit codes:
- 0 success
- 1 generic failure
- 2 invalid usage (parse/validation)
- (add command-specific codes only when actually useful)
Env/config:
- env vars:
- config file path + precedence:
Examples:
- …

Prefer recommending a parsing library (language-specific) only when asked; otherwise keep this skill language-agnostic.
If the request is “design parameters”, do not drift into implementation.