Approve Vendor Invoices from Gmail with Claude, Sheets & Slack

Approve vendor invoices from Gmail in minutes—using Claude, Google Sheets, and Slack

This n8n workflow automatically reads PDF vendor invoices arriving in Gmail, validates them against your Google Sheets rules, and either auto-approves them or sends an approval request to Slack—with every decision logged for audit-ready traceability.

What this workflow does

• Monitors Gmail for invoice emails: Triggers every minute when a message arrives with a PDF attachment, running a separate execution per invoice email.
• Extracts structured invoice data with Claude: Sends the attached PDF (plus email context) to the Anthropic Messages API to extract vendor, invoice number, dates, totals, PO number, and bank details.
• Checks Vendors, Purchase Orders, and invoice log entries: Loads the Vendors, Purchase orders, and Invoice log tabs from Google Sheets and verifies vendor status, sender domain, bank-detail matches, arithmetic consistency, duplicates, and PO validity/tolerance.
• Auto-approves or routes to approvers: Invoices passing all checks, matching an open PO, and staying under your configured auto-approve limit are approved automatically. Anything else is routed for approval—or held if blocking issues are found.
• Requests approval in Slack: Sends a Slack DM to the designated approver (and optionally a second approver above the threshold) with a time limit for response.
• Logs outcomes in Google Sheets: Writes the final decision and reasons to the Invoice log, and posts a status summary to a configured Slack channel.

Use cases

• AP teams that want faster approvals while preventing PO mismatches and duplicate invoices.
• SaaS operators needing audit-friendly invoice decision logs without manual spreadsheet updates.
• Automation engineers implementing controlled, rules-based invoice processing using n8n.

Technical details

• n8n nodes/logic: if, set, code, slack, switch, sticky note.
• Integrations: Gmail (PDF invoice trigger), Anthropic Messages API (Claude extraction), Google Sheets (Vendors, Purchase orders, Invoice log), Slack (approval DM + channel summary).

Setup

Create a Google Sheets document with tabs named Vendors, Purchase orders, and Invoice log, and add the expected columns for your vendor reconciliation and approval workflow.