cli-contracts

A guide for designing command-line tools as stable interfaces that people and scripts can rely on. It covers commands, options, output streams, errors, exit codes, and shell completion.

In plain words
What is it for?
Use it to define command names, arguments, options, defaults, validation, JSON output, help text, cancellation, and compatibility rules before writing the implementation.
Why use it?
It prevents command behavior, machine-readable output, and errors from changing in confusing ways. This makes the tool easier to use interactively and safer to call from scripts.

Skill for Claude CodeCodex

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add skills/fmind/dotfiles/cli-contracts
Any agent
npx skills add fmind/dotfiles --skill cli-contracts
Clone the repo
git clone --depth 1 https://github.com/fmind/dotfiles

Made for: Claude Code, Codex.

Per session 34 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 594 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00034 $0.00594
Opus 5 $0.00017 $0.00297
Sonnet 5 $0.00007 $0.00119
Haiku 4.5 $0.00003 $0.00059

Measured yesterday against content hash 4de8c8762459, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

cli-contracts scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured yesterday.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

skills/cli-contracts/SKILL.md · 36 lines

How it starts

The opening of the file, as written. The whole thing — 36 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Engineer CLI Contracts

Define a command-line interface as a stable human and machine contract before implementing it. Pair with go-stack or python-stack for language-specific code and systematic-debugging for unknown failures.

Contract

  1. Name the jobs: Give each command one purpose, a predictable noun/verb hierarchy, and aliases only when they cannot trigger a different class of action.
  2. Specify inputs: Define positional arguments, flags, environment variables, config precedence, defaults, validation, mutual exclusions, and behavior without a TTY.
  3. Separate streams: Send requested data to stdout and diagnostics or progress to stderr. Keep --json machine-readable, schema-stable, and free of decoration.
  4. Define outcomes: Map success, usage errors, authentication failures, remote failures, partial results, and cancellation to documented exit codes. Never convert an error into an empty success.
  5. Make help executable: Ensure root and subcommand help, examples, deprecations, and shell completions are generated from the same command definition.
  6. Protect sensitive context: Render safe command and argument context for actionable errors, but never include collected values, tokens, provider response bodies, or unredacted stderr.
  7. Handle process behavior: Respect signals, cancellation, timeouts, piping, broken pipes, and cleanup. Destructive commands need a confirmation mechanism or explicit non-interactive opt-in, while agent execution still requires separate target-scoped user authority; a --yes flag is never authority.
  8. Preserve compatibility deliberately: Treat command names, flags, JSON fields, exit codes, and stdout text consumed by scripts as public API. Prefer deprecation and migration guidance over silent changes.

Verification

  1. Write contract tests for help, parsing, validation, streams, exit codes, JSON schema, cancellation, and representative success and failure paths.
  2. Exercise the installed binary from a clean shell, including piped and non-TTY use; do not rely only on direct function tests.
  3. Run the focused tests, completion generation, documentation checks, and the repository's full gate.

Read the full file on GitHub · 36 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. yesterday First seen · 36 lines · 34 tokens per session scan A 4de8c8762459

Subscribe to this mod's changes

cli-contracts is a skill published in the GitHub repository fmind/dotfiles (4 stars, last pushed 2d ago), licensed MIT. It adds 34 tokens to every session and 594 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

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