Invoice Checker

Before: Re-adding an invoice by eye with a calculator, and sending it with the due date before the invoice date.

After (skill output, from a real test run): Test run on a sample invoice we wrote with planted errors. Result, unedited: "2 differences found. Line 2's total is wrong, and that error carries into every total below it." It showed 3 x 18.25 is 54.75 not 55.75, the corrected chain of subtotal, discount, VAT and total, the due date a month before the invoice date, and the missing invoice number, customer address, payment details and VAT number. It changed nothing and said which tax questions to ask an accountant.

Give it an invoice, yours or one you received. A small script (Python 3 standard library, exact decimal arithmetic) recomputes every line, the subtotal, the discount, the tax and the total, and lists every difference. It lists common fields that are missing and flags date, currency and rounding problems. It gives no tax advice.

1 run on a sample invoice we wrote with planted errors (Claude Sonnet).

Tested on a real example before release. The example is shown in the Before and after section above.

How we test, what we do not do, and which assistants we ran it on.

• No tax advice. It does not say what rate applies or what a field requires in your country.
• Without code execution it shows the sums by hand and says they need checking.
• It cannot read an image or a scanned PDF. Give it the text or type in the figures.
• It checks arithmetic and completeness, not whether the work was done or the price is fair.

Works with any assistant that reads SKILL.md skills (Claude, Codex, Gemini CLI, Cursor and others), plus a paste-in prompt version for ChatGPT or any chatbot. The recompute script needs Python 3 (standard library only). Without code execution it shows the sums by hand.

1. Run npx skills add proskillpacks/skills in your project and pick this skill. Or copy the invoice-checker folder from GitHub (https://github.com/proskillpacks/skills/tree/main/skills/finance/invoice-checker).
2. Put the invoice-checker folder where your assistant looks for skills. The generic path is .agents/skills/ (or npx skills add from a folder). Tool-specific paths:
   • Claude Code: ~/.claude/skills/ (or .claude/skills/ inside one project)
   • OpenAI Codex, Gemini CLI, Cursor and others: .agents/skills/ or the skills folder their docs name
   • claude.ai: zip the skill folder yourself, then Customize, Skills, the plus button, Create skill, Upload a skill (code execution must be on in Settings, Capabilities)
   • ChatGPT or any chatbot: open the skill's prompt.md, paste it into the chat, then paste your own text or link
   • Steps for each tool, with links to its official docs: how to install Agent Skills
3. Ask for the job in plain words, for example: "Check this invoice before I send it: [invoice text].". The skill loads by itself.