Agent-Friendly CLI Spec v0.1

When building or modifying CLI tools, follow these rules to make them safe and reliable for AI agents to use.

Overview

A comprehensive design specification for building AI-native CLI tools. It defines 98 rules across three certification levels (Agent-Friendly, Agent-Ready, Agent-Native) with prioritized requirements (P0/P1/P2). The spec covers structured JSON output, error handling, input contracts, safety guardrails, exit codes, self-description, and a feedback loop via a built-in issue system.

When to Use This Skill

• Use when building a new CLI tool that AI agents will invoke
• Use when retrofitting an existing CLI to be agent-friendly
• Use when designing command-line interfaces for automation pipelines
• Use when auditing a CLI tool's compliance with agent-safety standards

Core Philosophy

1. Agent-first -- default output is JSON; human-friendly is opt-in via --human
2. Agent is untrusted -- validate all input at the same level as a public API
3. Fail-Closed -- when validation logic itself errors, deny by default
4. Verifiable -- every rule is written so it can be automatically checked

Layer Model

This spec uses two orthogonal axes:

• Layer answers rollout scope: core, recommended, ecosystem
• Priority answers severity: P0, P1, P2

Use layers for migration and certification:

• core -- execution contract: JSON, errors, exit codes, stdout/stderr, safety
• recommended -- better machine UX: self-description, explicit modes, richer schemas
• ecosystem -- agent-native integration: agent/, skills, issue, inline context

Certification maps to layers:

• Agent-Friendly -- all core rules pass
• Agent-Ready -- all core + recommended rules pass
• Agent-Native -- all layers pass

How It Works

Step 1: Output Mode

Default is agent mode (JSON). Explicit flags to switch:

$ mycli list              # default = JSON output (agent mode)
$ mycli list --human      # human-friendly: colored, tables, formatted
$ mycli list --agent      # explicit agent mode (override config if needed)

• Default (no flag) -- JSON to stdout. Agent never needs to add a flag.
• --human -- human-friendly format (colors, tables, progress bars)
• --agent -- explicit JSON mode (useful when env/config overrides default)

Step 2: agent/ Directory Convention

Every CLI tool MUST have an agent/ directory at its project root. This is the tool's identity and behavior contract for AI agents.

agent/
  brief.md          # One paragraph: who am I, what can I do
  rules/            # Behavior constraints (auto-registered)
    trigger.md      # When should an agent use this tool
    workflow.md     # Step-by-step usage flow
    writeback.md    # How to write feedback back
  skills/           # Extended capabilities (auto-registered)
    getting-started.md

Step 3: Four Levels of Self-Description

1. --brief (business card, injected into agent config)
2. Every Command Response (always-on context: data + rules + skills + issue)
3. --help (full self-description: brief + commands + rules + skills + issue)
4. skills \<name\> (on-demand deep dive into a specific skill)

Certification Requirements

Each level includes all rules from the previous level. Priority tag [P0]=agent breaks without it, [P1]=agent works but poorly, [P2]=nice to have.

Level 1: Agent-Friendly (core -- 20 rules)

Goal: CLI is a stable, callable API. Agent can invoke, parse, and handle errors.

Output -- default is JSON, stable schema

• [P0] O1: Default output is JSON. No --json flag needed
• [P0] O2: JSON MUST pass jq . validation
• [P0] O3: JSON schema MUST NOT change within same version

Error -- structured, to stderr, never interactive

• [P0] E1: Errors -> {"error":true, "code":"...", "message":"...", "suggestion":"..."} to stderr
• [P0] E4: Error has machine-readable code (e.g. MISSING_REQUIRED)
• [P0] E5: Error has human-readable message
• [P0] E7: On error, NEVER enter interactive mode -- exit immediately
• [P0] E8: Error codes are API contracts -- MUST NOT rename across versions

Exit Code -- predictable failure signals

• [P0] X3: Parameter/usage errors MUST exit 2
• [P0] X9: Failures MUST exit non-zero -- never exit 0 then report error in stdout

Composability -- clean pipe semantics

• [P0] C1: stdout is for data ONLY
• [P0] C2: logs, progress, warnings go to stderr ONLY

Input -- fail fast on bad input

• [P1] I4: Missing required param -> structured error, never interactive prompt
• [P1] I5: Type mismatch -> exit 2 + structured error

Safety -- protect against agent mistakes

• [P1] S1: Destructive ops require --yes confirmation
• [P1] S4: Reject ../../ path traversal, control chars

Guardrails -- runtime input protection

• [P1] G1: Unknown flags rejected with exit 2
• [P1] G2: Detect API key / token pat