Build your own invoicing tool with a VAT return section
The owner of the small company Lammetje built the company's invoicing tool with Claude Opus 5.5 and, while he was at it, added a section that prepares the VAT return too.
Source
@lammetje_nl on X · 2026-10-09
Written in our own words. Credit to the original author. Link checked 2026-10-21.
What they did
The owner of the small company Lammetje built the company's invoicing tool with Claude Opus 5.5 and, while he was at it, added a section that prepares the VAT return too.
Result (as claimed)
In use for his own company; he offers to help others build the same.
Tools they used
Cost: Claude Opus 5.5: from $4 in / $20 out per 1M tokens.
Set it up
Gather the documents
Collect statements, bills, invoices or tax documents for the period you want reviewed.
Ask for an audit, not actions
Have Claude Opus 5.5 list findings, amounts and suggested next steps before it does anything.
Approve each action
Approve cancellations, disputes or payments one at a time. Never give an agent open-ended access to move money.
Track the result
Record what changed and what you saved so you can check it next month. This is not financial advice.
Copy the full setup prompt
Paste into your agent and it will walk you through the setup, asking before anything that sends, posts or spends.
You are helping me set up this AI use case end to end in Claude Code. Goal: Build your own invoicing tool with a VAT return section What the original author did: The owner of the small company Lammetje built the company's invoicing tool with Claude Opus 5.5 and, while he was at it, added a section that prepares the VAT return too. Reported result (as claimed by the author): In use for his own company; he offers to help others build the same. Source: https://x.com/lammetje_nl/status/2108585805963018343 Tools they used: Claude Opus 5.5 Category: Money Please: 1. Ask me 3 short questions about my situation (accounts I have, budget, what I want the output to look like). 2. Propose a step-by-step plan using these tools or cheaper alternatives, with the time and cost of each step. Use official pricing pages; say "check pricing" if unsure. 3. Then do the setup with me one step at a time, waiting for my OK before anything that sends messages, posts publicly, spends money or deletes data. 4. Finish with a checklist to run it again next time.
You are helping me set up this AI use case end to end in Codex. Goal: Build your own invoicing tool with a VAT return section What the original author did: The owner of the small company Lammetje built the company's invoicing tool with Claude Opus 5.5 and, while he was at it, added a section that prepares the VAT return too. Reported result (as claimed by the author): In use for his own company; he offers to help others build the same. Source: https://x.com/lammetje_nl/status/2108585805963018343 Tools they used: Claude Opus 5.5 Category: Money Please: 1. Ask me 3 short questions about my situation (accounts I have, budget, what I want the output to look like). 2. Propose a step-by-step plan using these tools or cheaper alternatives, with the time and cost of each step. Use official pricing pages; say "check pricing" if unsure. 3. Then do the setup with me one step at a time, waiting for my OK before anything that sends messages, posts publicly, spends money or deletes data. 4. Finish with a checklist to run it again next time.
You are helping me set up this AI use case end to end in Grok Bot. Goal: Build your own invoicing tool with a VAT return section What the original author did: The owner of the small company Lammetje built the company's invoicing tool with Claude Opus 5.5 and, while he was at it, added a section that prepares the VAT return too. Reported result (as claimed by the author): In use for his own company; he offers to help others build the same. Source: https://x.com/lammetje_nl/status/2108585805963018343 Tools they used: Claude Opus 5.5 Category: Money Please: 1. Ask me 3 short questions about my situation (accounts I have, budget, what I want the output to look like). 2. Propose a step-by-step plan using these tools or cheaper alternatives, with the time and cost of each step. Use official pricing pages; say "check pricing" if unsure. 3. Then do the setup with me one step at a time, waiting for my OK before anything that sends messages, posts publicly, spends money or deletes data. 4. Finish with a checklist to run it again next time.
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