Build an agentic product in a prompt.

Code that runs at fixed points: after the model answers, before anything is written, after Xero confirms. It can pass, fix, send the model back with a reason, block, or ask you.

fn after_model(ctx, out) {
  let b = out.bill;
  if b.net + b.vat != b.total {
    return retry("net + VAT does not equal total; re-read the document");
  }
  if b.vat_rate == 20 && (b.net * 0.20 - b.vat).abs() > 0.01 {
    return retry("VAT is not 20% of net");
  }
  if !ctx.document.text.contains(b.invoice_number) {
    return retry("invoice number not present on the document");
  }
  pass()
}

• after_model
• before_effect · idempotency
• after_effect · read-back verify
• on_run_end · claims vs actions

Larger jobs as declared plans: steps, fan-out, approval gates, a checkpoint after every step so a run survives a restart, and compensation when a late step fails after an early write.

1. collect · inbox-triage since last close , done 23 bills
2. capture · for each bill· gate: approval , done 21 posted · 2 parked
3. reconcile · unallocated payments , done 1 to review
4. statements · for each landlord· gate: approval , waiting waiting · 3 of 3
5. checklist · month-end-check → panel , queued queued

Checkpoint after every step. Compensation: a bill that fails in Xero after approval notifies the approver; nothing is voided silently.