Stdout/stderr separation

When a CLI mixes data output and status messages on stdout, any pipeline that consumes the output breaks silently. Running mytool list-users | jq '.[].email' will fail or produce corrupt results if progress text like "Fetching 42 users..." lands in the same stream as the JSON. Docker, xargs, tee, and CI log parsers all assume stdout carries only data. Sending error messages to stdout also defeats shell && semantics: a caller checking exit code sees 0 but its stdin feed contains garbage. This is classified as a critical reliability failure under ISO 25010:2011 usability.operability because the entire composability contract of the Unix tool model breaks at this seam.

Route all data to stdout and all messages (progress, warnings, errors) to stderr. Test the separation by redirecting each stream independently:

// Node.js — data to stdout, everything else to stderr
async function listUsers(format: string) {
  console.error('Fetching users...')  // status → stderr
  const users = await fetchUsers()
  process.stdout.write(JSON.stringify(users, null, 2) + '\n')  // data → stdout
  console.error(`Found ${users.length} users`)  // status → stderr
}

# Python — click.echo(err=True) for all non-data output
@cli.command()
def list_users():
    click.echo('Fetching users...', err=True)
    users = fetch_users()
    for u in users:
        click.echo(f'{u.name} <{u.email}>')  # data → stdout
    click.echo(f'Found {len(users)} users', err=True)

Verify with: mytool list-users > data.txt — the terminal should show status lines while data.txt contains only clean output.