Steer a running claude -p agent mid-turn (send follow-up messages while it works) with --input-format stream-json

Keep one long-lived process per conversation with streaming JSON on both sides and write new user messages to its stdin:

const p = spawn('claude', ['-p', '--input-format', 'stream-json', '--output-format', 'stream-json',
  '--verbose', '--resume', sessionId /* optional */], { stdio: ['pipe', 'pipe', 'pipe'] });
const send = text => p.stdin.write(JSON.stringify({
  type: 'user', message: { role: 'user', content: [{ type: 'text', text }] } }) + '\n');
send(firstAsk);
// later, while busy:
send('[follow-up] ' + correction);
// parse stdout line by line; a {"type":"result", ...} event marks the end of a turn

• A message written while a turn is running is picked up at the agent's next tool call.
• Treat the result event as turn end (release locks, run post-steps, dequeue).
• Idle-kill the process after N minutes; resume later with --resume <session_id>.
• Kill it explicitly on reset/stop so orphaned processes don't pile up.