Agent CLI

Agent-Friendly CLI Builder

Convert Python CLI scripts from human-only output to agent-consumable NDJSON, or scaffold a complete cli_utils package ready for open-source distribution.

Two Modes

Mode A: Convert an existing script

When the user points at a script and says "make this agent-friendly" or "add --json":

1. Scan the script for output points
2. Generate cli_utils.py if the project doesn't have one
3. Replace all output with structured helpers
4. Verify no raw output leaks in JSON mode

Mode B: Scaffold a complete package

When the user says "create a cli_utils package" or wants an open-source library:

1. Scaffold a full Python package with pyproject.toml, tests, license, README
2. Include all helpers: json_log, json_error, die, log, add_json_flag, enable_json, is_json
3. Add pytest test suite with full coverage
4. Add MIT license (or ask user preference)

Core Architecture

The fundamental pattern: every script gets a --json flag. When active, all stdout becomes newline-delimited JSON (NDJSON). Each line is a self-contained JSON object with a standard envelope.

The NDJSON Event Envelope

Every JSON line has at minimum:

{"event": "ready", "ts": "2026-04-30T14:00:00+00:00", "pid": 1234, "port": 8765}

• event — what happened (snake_case string)
• ts — ISO 8601 UTC timestamp
• Additional fields are event-specific kwargs

Why This Design

• NDJSON over JSON arrays: processable line-by-line, one bad line doesn't break the stream, works with grep/jq, low memory for long-running processes
• --json opt-in over default: preserves human DX, doesn't break existing scripts or habits
• Global mode flag over per-call checks: set once at startup, every helper respects it automatically
• die() over repeated if/else: the pattern if is_json(): json_error(); sys.exit(1) else: print(); sys.exit() appears constantly — die() collapses it to one line

The cli_utils.py Reference Implementation

When generating cli_utils.py, produce exactly this (adapt only if the project has specific needs):

"""Shared helpers for JSON CLI output."""
import json
import os
import sys
from datetime import datetime, timezone

_json_mode = False

def enable_json():
    global _json_mode
    _json_mode = True

def is_json():
    return _json_mode

def json_log(event: str, **kwargs):
    """Emit one NDJSON line to stdout."""
    obj = {"event": event, "ts": datetime.now(timezone.utc).isoformat(), **kwargs}
    print(json.dumps(obj, default=str), flush=True)

def json_error(message: str, **kwargs):
    """Emit a structured error event."""
    json_log("error", message=message, **kwargs)

def die(message: str, code: int = 1, **kwargs):
    """Print error and exit — JSON or human depending on mode."""
    if _json_mode:
        json_error(message, **kwargs)
    else:
        print(message, file=sys.stderr)
    sys.exit(code)

def add_json_flag(parser):
    """Add --json flag to an argparse parser."""
    parser.add_argument("--json", action="store_true",
                        help="NDJSON output for agent consumption")

def log(message: str, **json_kwargs):
    """Print human message normally, or emit JSON event if --json is active."""
    if _json_mode:
        json_log(json_kwargs.pop("event", "info"), message=message, **json_kwargs)
    else:
        print(message)

def json_ready(**kwargs):
    """Emit the readiness signal — only in JSON mode. Call early in daemon startup."""
    if _json_mode:
        json_log("ready", pid=os.getpid(), **kwargs)

Converting a Script — Step by Step

Step 1: Scan for output points

Search the target script for all places that produce output or exit:

grep -n 'print(\|sys\.exit\|exit(\|input(\|os\.system.*say' TARGET.py

Categorize each hit:

• Informational print → replace with log(message, event="descriptive_name")
• Error + exit → replace with die(message)
• Status line with \r → replace with if is_json(): json_log("status", ...) else: print("\r...", end="", flush=True)
• Interactive input() → guard with if not is_json(): or add --no-interactive flag
• Side effects (say, osascript, notifications) → guard with if not is_json():
• Import-time errors (before argparse runs) → use sys.exit("message") (writes to stderr)

Step 2: Add the import and flag

At the top of the script, after existing imports:

from cli_utils import add_json_flag, enable_json, is_json, json_log, log, die

In the if __name__ == "__main__" block, add to argparse:

add_json_flag(parser)
args = parser.parse_args()
if args.json:
    enable_json()

Step 3: Replace each output point

Apply the categorization from Step 1. Key patterns:

Simple informational:

# Before
print(f"Connected to {device}")

# After
log(f"Connected to {device}", event="connected", device=device)

Error + exit:

# Before
print("Device not found")
sys.exit(1)

# After
die("Device not found")

Daemon readiness (first output after initialization):

# Before
print(f"Server running on port {port}")

# After — json_ready() only emits in JSON mode, so always call it + human fallback
json_ready(port=port)
log(f"Server running on port {port}", event="ready", port=port)

Status lines (\r overwrite):

# Before
print(f"\r  HR {hr} RMSSD {rmssd:.1f}", end="", flush=True)

# After
if is_json():
    json_log("status", hr=hr, rmssd=rmssd)
else:
    print(f"\r  HR {hr} RMSSD {rmssd:.1f}", end="", flush=True)

Human-only output (banners, usage examples):

if not is_json():
    print("Usage: send {\"type\": \"join\", \"name\": \"Alice\"}")

Step 4: Verify

1. Run python script.py --help — confirm --json flag appears
2. Run python script.py --json — confirm first line is valid JSON
3. Grep for remaining raw print( calls — ensure each is guarded or intentional