AI Invoice Automation (Gemini) — Waldo Vanderhaeghen

A self-learning automated pipeline that processes German tax invoices for €0/month. It replaces manual data entry by connecting Google Drive directly to Gemini’s multimodal AI.

Drop a PDF into an inbox folder. A few seconds later, the vendor, date, amount, and category are in your tax spreadsheet. The file is archived to the right year folder. You never touch it.

Key features

Zero-cost architecture — runs entirely on Google Apps Script (serverless) and Gemini Flash (free-tier compatible). No infrastructure, no monthly bill.

Self-healing categorization — if it encounters a new vendor, it attempts to categorize based on context, then learns that rule by reading your historical sheet data on the next run.

Smart year routing — automatically detects the fiscal year from the invoice date and archives to the correct regulatory folder.

Vendor split logic — handles complex edge cases: mixed personal/business bills, invoices where only a percentage is deductible. A single PDF can produce multiple rows with different categories and amounts.

MD5 duplicate detection — every processed file’s hash is written to a log tab. Duplicates are quarantined, not reprocessed.

Full audit trail — every decision the script made is in the script_log tab: timestamp, vendor, category, invoice number, MD5, error reason. Fully inspectable without opening a single file.

Drive verification — after every run, the script checks that every row in the spreadsheet has a corresponding file in the archive folder. Mismatches are flagged red.

graph LR
    Inbox[("Google Drive\nInbox")] -->|Trigger on file| Script("Apps Script\nOrchestrator")
    Script -->|Send image| Gemini("Gemini 2.5 Flash\nIntelligence")
    Gemini -->|Return JSON| Script
    Script -->|Normalize data| Logic("Logic Layer\nRouting & Rules")
    Logic -->|Check history| Sheets[("Google Sheet\nDatabase")]
    Logic -->|Write data| Sheets
    Script -->|Move file| Archive[("Archive Folder\nStorage")]

Configuration

The script needs a CONFIG object at the top with your environment:

Variable	Description
`API_KEY`	Your Gemini API key from Google AI Studio
`SHEET_ID`	ID from your Google Spreadsheet URL
`INBOX_ID`	Drive folder ID where you upload raw receipts
`ARCHIVE_XXXX_ID`	IDs of the year-specific archive folders

Set up a Google Apps Script trigger to run processInvoices every 10 minutes. It only does real work when there are files in the inbox — idle runs are instant.

The full write-up of how this evolved from v1 to v7 is in this post.