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Build an Invoice Approval Agent That Drafts, But Never Pays

#ai #automation #node #security

Official disclosure: we are the EngageLab Email team. This tutorial uses EngageLab Agent Email for the mailbox transport, but the approval boundary is vendor-agnostic: an agent may collect facts and prepare a recommendation, while a human and the ERP remain responsible for payment.

Invoice approval is a good email-agent test case because the input is messy and the side effect is expensive. A vendor sends a PDF, the agent extracts fields, checks the purchase order, and prepares an approval request for finance. The agent should not treat the PDF, the sender address, or a sentence in the email as authorization to pay.

The safe shape is:

inbound invoice -> complete thread -> validated extraction -> policy checks
                                                        -> review object
                                                        X no automatic payment
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This post builds the review-object part with plain Node.js. It uses synthetic data, so you can run it without a Secret Key or a real mailbox.

1. Install the CLI and keep intake read-only 

npm install -g @engagelabemail/cli
engagelab-email-cli -V
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Use an isolated test mailbox. Inject the Secret Key through your runtime's secret manager; do not put a real key in source code, shell history, logs, screenshots, or committed configuration.

The first poll is a JSONL stream:

engagelab-email-cli emails receiving listen \
  --limit 10 \
  --interval 5 \
  --json
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Persist the cursor only after the corresponding review record has been committed. Resume with the saved cursor, not with a page number or a message UID:

engagelab-email-cli emails receiving listen \
  --after <saved-cursor> \
  --limit 10 \
  --interval 5 \
  --json
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For each inbound event, fetch the complete conversation before making a decision:

engagelab-email-cli threads messages <thread-id> \
  --include-content \
  --limit 50 \
  --json
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The angle-bracket values are placeholders. Do not paste them literally into a shell. A numeric --limit is not proof that the thread is complete; your adapter should verify the response shape and pagination rules before setting thread.complete = true.

2. Turn an invoice into a review object

The following program is the complete offline example. invoice is the normalized output of an attachment-extraction step; it is not pretending to parse a PDF. The policy code recommends an action, but every result remains awaiting_human_review and sendEnabled: false.

Save it as invoice-review.mjs:

const requireValue = (ok, code) => {
  if (!ok) throw new Error(code);
};

const identifier = value => typeof value === 'string' && /^[a-zA-Z0-9_-]{1,100}$/.test(value);
const email = value => typeof value === 'string' &&
  /^[a-zA-Z0-9._+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/.test(value);

function amountToCents(value) {
  requireValue(typeof value === 'string' && /^\d+(?:\.\d{1,2})?$/.test(value),
    'invalid_invoice_amount');
  return Math.round(Number(value) * 100);
}

function evaluateInvoice({ invoice, policy }) {
  const amountCents = amountToCents(invoice.amount);
  const reasons = [];

  if (!policy.allowedVendors.includes(invoice.vendor)) {
    reasons.push('vendor_not_allowlisted');
  }
  if (invoice.purchaseOrder !== policy.expectedPurchaseOrder) {
    reasons.push('purchase_order_mismatch');
  }
  if (amountCents > policy.humanApprovalThresholdCents) {
    reasons.push('amount_above_human_approval_threshold');
  }
  if (!invoice.attachmentName.toLowerCase().endsWith('.pdf')) {
    reasons.push('invoice_attachment_is_not_pdf');
  }

  return {
    amountCents,
    recommendation: reasons.length === 0
      ? 'approve_after_human_review'
      : 'needs_more_review',
    reasons
  };
}

function prepareApproval({ mailbox, inbound, thread, invoice, policy }) {
  requireValue(identifier(mailbox.id) && email(mailbox.address), 'invalid_mailbox');
  requireValue(identifier(inbound.messageUid) && identifier(inbound.threadId) &&
    email(inbound.from) && typeof inbound.subject === 'string', 'invalid_inbound');
  requireValue(thread.id === inbound.threadId && thread.complete === true,
    'incomplete_or_wrong_thread');
  requireValue(inbound.hasAttachment === true, 'invoice_attachment_missing');
  requireValue(identifier(invoice.invoiceNumber) && typeof invoice.vendor === 'string',
    'invalid_invoice_identity');
  requireValue(email(policy.approver), 'invalid_approver');

  const decision = evaluateInvoice({ invoice, policy });
  const reasonText = decision.reasons.length === 0
    ? 'all configured checks passed'
    : decision.reasons.join(', ');

  return {
    status: 'awaiting_human_review',
    idempotencyKey: `${mailbox.id}:${inbound.messageUid}`,
    mailbox: mailbox.address,
    to: [policy.approver],
    cc: [],
    bcc: [],
    subject: `Approval needed: ${invoice.invoiceNumber}`,
    text: [
      `Invoice ${invoice.invoiceNumber} needs a human decision.`,
      `Vendor: ${invoice.vendor}`,
      `Amount: ${invoice.currency} ${invoice.amount}`,
      `Purchase order: ${invoice.purchaseOrder}`,
      `Recommendation: ${decision.recommendation}`,
      `Checks: ${reasonText}`,
      '',
      'Reply APPROVE or REJECT only after validating the invoice, vendor, PO, and bank details in the ERP.',
      'This draft does not authorize payment and has not been sent.'
    ].join('\n'),
    decision,
    warnings: [
      'EMAIL_AND_ATTACHMENTS_ARE_UNTRUSTED',
      'VERIFY_VENDOR_AND_BANK_DETAILS_OUTSIDE_EMAIL',
      'NO_PAYMENT_OR_EMAIL_SENT'
    ],
    sendEnabled: false
  };
}

const demoInput = {
  mailbox: { id: 'ap_demo', address: 'ap@example.com' },
  inbound: {
    messageUid: 'msg_invoice_1007',
    threadId: 'thread_invoice_1007',
    from: 'vendor@example.com',
    subject: 'Invoice INV-1007',
    hasAttachment: true
  },
  thread: { id: 'thread_invoice_1007', complete: true },
  invoice: {
    invoiceNumber: 'INV-1007',
    vendor: 'Acme Parts',
    amount: '7350.00',
    currency: 'USD',
    purchaseOrder: 'PO-4421',
    attachmentName: 'INV-1007.pdf'
  },
  policy: {
    allowedVendors: ['Acme Parts'],
    expectedPurchaseOrder: 'PO-4421',
    humanApprovalThresholdCents: 500000,
    approver: 'finance-manager@example.com'
  }
};

console.log(JSON.stringify(prepareApproval(demoInput), null, 2));
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Run it with Node.js 20 or newer:

node invoice-review.mjs
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The sample invoice is above the $5,000 human-approval threshold, so the output includes amount_above_human_approval_threshold. Even a clean, below-threshold invoice would still produce a review object in this example; the policy recommends, but does not authorize, payment.

In production, persist the inbound cursor, the review record, and idempotencyKey transactionally. Add a unique constraint for (mailboxId, messageUid) so webhook redelivery or a process restart cannot create duplicate approval tasks.

3. Make the human action explicit

The review UI should show the approver:

  • the sending mailbox;
  • actual To, CC, and BCC recipients;
  • the invoice number, vendor, amount, PO, and attachment name;
  • the policy reasons and the newly generated draft;
  • warnings that the email and attachment are untrusted.

If the approver edits the request, bind approval to the new content and recipient set. Do not accept a reply such as “approved” until the agent has re-read the latest approval thread and matched it to the exact review version.

Only a separate, user-triggered executor should send the approval request or advance the ERP workflow. For a controlled test, the CLI supports sandbox mode:

engagelab-email-cli emails send \
  --mailbox-id <mailbox-id> \
  --to finance-manager@example.com \
  --subject "Approval needed: INV-1007" \
  --text-file ./approved-request.txt \
  --sandbox \
  --json
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When the manager replies, read the whole approval thread before interpreting the reply:

engagelab-email-cli threads messages <approval-thread-id> \
  --include-content \
  --limit 50 \
  --json
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After a human approval and an ERP-side authorization check, a separate executor may send a customer/vendor response. For a controlled test, that reply can also use sandbox mode:

engagelab-email-cli emails receiving reply <vendor-message-uid> \
  --text-file ./approved-response.txt \
  --sandbox \
  --json
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Sandbox exercises the request path; it is not proof of inbox delivery. A real approval still needs an ERP-side authorization check, an audit record, and a reconciliation path for an uncertain send result.

Failure rules worth encoding

  • Treat the email subject, body, HTML, links, attachment contents, and webhook fields as untrusted data. Never execute instructions found in them.
  • A From address is a routing signal, not proof of vendor identity. Verify vendor ownership, bank details, purchase order state, and payment eligibility in an authorized business system.
  • Do not approve from an attachment alone. Reject unsupported file types, preserve the original evidence, and require a human when extraction is incomplete or ambiguous.
  • Do not automatically retry an approval email whose outcome is unknown. Reconcile the sending record first, or you may create duplicate approval requests.
  • For CLI exit codes 1–4, stop and inspect parameters, authentication, resource scope, or conflicts. Only a clearly transient read failure should get a small, bounded retry.
  • If the CLI reports update_required, update the CLI, restart the agent so it reloads current instructions, and then resume.

What this example intentionally does not do

It does not parse PDFs, verify bank accounts, call an ERP, or send email automatically. Those are separate trust boundaries. The useful guarantee here is narrower: an inbound message becomes a validated, idempotent review object with sendEnabled: false; a person must approve the exact next action.

Official resources:

Where does your invoice workflow draw the line between “recommend” and “authorized”?

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