The setup: 50 invoices, two models, one spreadsheet of mistakes
I sell vintage prints and greeting cards—half my orders ship within the UK, the other half to Ireland. Every month I generate about fifty invoices: standard sales, the occasional refund credit, and a handful of cross-border transactions where I need to show the correct VAT treatment or explain why there's no VAT at all. I'd been doing this in a spreadsheet with a formula I copied from a forum in 2019, and it worked until I issued a refund credit in euros, forgot to reverse the sign on the VAT line, and spent twenty minutes on the phone with my accountant.
So I decided to test Claude Sonnet 4.5 and Gemini Pro. I gave each model the same brief: generate a VAT-compliant invoice for a UK customer, then an Irish customer, then a refund credit for each. I wanted to see which one got the rates right (20% UK, 23% Ireland), formatted the document so it would pass a Companies House spot check, and handled the edge cases—goods under £135 shipped to Ireland, services that flip to reverse charge, credits that needed negative VAT.
I ran the test over two weeks in January, using real transaction data with names swapped out. By invoice thirty I knew which model I'd use for which job. By invoice fifty I had a checklist I still follow.
1. Claude gets UK VAT invoices right the first time, including the registration number placement
Why it matters: A compliant UK invoice needs your VAT number, the customer's address, a unique invoice number, the rate applied, and the VAT amount separated from the net total. Miss one and you're not compliant; format it badly and your accountant sends it back.
I prompted Claude: "Generate a VAT invoice for a £40 sale to a UK customer. My VAT number is GB123456789. Item: vintage map print. Invoice number 2025-001. Date 8 January 2025."
Claude returned a clean invoice with the VAT number under my business name, the customer address in the right place, a line for the net amount (£33.33), a line for VAT at 20% (£6.67), and a total of £40.00. It added the note "VAT included in total" and formatted everything in a table. I've used that template for thirty invoices since; my accountant has never questioned it.
Gemini Pro gave me the same information but put the VAT number at the bottom, after the payment terms, and formatted the amounts in a paragraph instead of a table. Technically compliant, but harder to read and not the format Companies House expects if you're ever spot-checked.
2. Gemini handles Irish cross-border VAT better when you're under the €10,000 threshold
Why it matters: If you're a UK seller shipping goods to Ireland and your total Irish sales stay under €10,000 a year, you charge UK VAT (20%) and don't register for Irish VAT. Cross that threshold and you need to charge Irish VAT (23%) or register. Most sole traders don't know where they stand until they add it up in March.
I asked both models: "I sold a €50 print to a customer in Dublin. My total sales to Ireland this year are €8,000. Generate the invoice."
Gemini Pro replied with a clean invoice showing €50 total, UK VAT at 20% (€8.33 VAT, €41.67 net), and a note: "UK VAT applied. Seller has not exceeded the €10,000 distance-selling threshold for Ireland." That note saved me a follow-up email from the customer asking why they were charged UK VAT.
Claude gave me the same numbers but didn't include the threshold note. When I asked it to explain, it did—but I had to prompt twice. For cross-border sales where the customer might query the rate, Gemini's default is more helpful.
3. Both models generate refund credits, but Claude formats the negative VAT line correctly
Why it matters: A refund credit is a negative invoice. The net, VAT, and total all carry minus signs, and the document needs to reference the original invoice number. Format it wrong and your bookkeeping software (or your accountant) rejects it.
I prompted Claude: "Generate a VAT credit note for invoice 2025-001. Full refund of £40. Original sale was a vintage map print to a UK customer."
Claude returned a document titled "Credit Note 2025-001C" with a reference line ("Original Invoice: 2025-001"), a net of –£33.33, VAT of –£6.67, and a total of –£40.00. It included the note "This credit note reduces your VAT liability for the period." I've issued eight refund credits using that format; all of them imported cleanly into my accounting spreadsheet.
Gemini Pro gave me a credit note with the correct numbers but formatted the VAT as +£6.67 and the total as –£40.00. Technically the total was right, but the VAT line confused my accountant, who thought I'd issued a partial refund. I had to regenerate it and manually flip the sign.
4. Claude explains reverse-charge VAT for services; Gemini assumes you already know
Why it matters: If you sell a service (not goods) to a business customer in Ireland, you don't charge VAT—the customer accounts for it under reverse charge. If you sell to a consumer, you charge UK VAT. The distinction trips up most sole traders at least once.
I asked both models: "I sold a £200 design consultation to a business in Cork. Do I charge VAT? Generate the invoice."
Claude replied: "No VAT charged. This is a B2B service sale to an Irish business customer. Under reverse charge rules, the customer accounts for VAT in Ireland. Include the note 'Reverse charge applies' on the invoice." It then generated an invoice with £200 net, no VAT line, and the reverse-charge note at the bottom.
Gemini Pro gave me an invoice with no VAT and no explanation. When I asked why, it explained reverse charge in a follow-up—but if I'd been generating ten invoices in a row, I'd have missed the reasoning. For edge cases where the rule isn't obvious, Claude's upfront explanation is worth the extra sentence.
5. Use Gemini for batch invoice generation; use Claude to check one complex invoice
Why it matters: If you're generating twenty invoices for straightforward UK sales, you want speed and consistency. If you're handling a refund credit for a cross-border sale with a partial return, you want a model that explains its working.
I tested batch generation by giving each model a list of ten transactions (mix of UK and Irish sales, all standard rates, no refunds). I prompted: "Generate invoices for these ten sales. Use the format from earlier. Number them 2025-010 through 2025-019."
Gemini Pro returned all ten in under fifteen seconds, formatted consistently, with correct VAT calculations. I spot-checked three and found no errors.
Claude took twenty-two seconds and returned nine invoices. The tenth failed because the prompt listed a sale in euros and Claude asked me to confirm whether I wanted UK or Irish VAT before proceeding. Correct behaviour, but slower.
For batch work where the transactions are clean, Gemini's speed wins. For anything that needs judgment—refund credits, cross-border services, partial returns—I use Claude and review its explanation before I send the invoice.
6. Both models miss the Northern Ireland VAT edge case unless you prompt for it
Why it matters: Goods sold from Northern Ireland to Ireland are treated as domestic Irish sales (no customs, but Irish VAT applies if you're registered). Goods sold from Great Britain to Northern Ireland follow UK VAT rules. Most sole traders don't ship enough to NI to worry about this, but if you do, you need to prompt explicitly.
I asked both models: "I sold a £30 print to a customer in Belfast. I'm shipping from Manchester. Generate the invoice."
Both returned a standard UK invoice with 20% VAT. Correct.
Then I asked: "I sold a €35 print to a customer in Dundalk. I'm shipping from Belfast. I'm registered for Irish VAT. Generate the invoice."
Both models paused. Claude asked if I wanted to apply Irish VAT (23%) or UK VAT (20%). Gemini applied UK VAT by default and added a note: "Confirm VAT treatment for NI-to-Ireland sales with your accountant."
Neither model got it right without clarification. If you ship from Northern Ireland, you need to tell the model your VAT registration status and the destination. Don't assume it knows the protocol.
7. Claude's invoice template exports to PDF cleanly; Gemini's needs formatting tweaks
Why it matters: Most sole traders send invoices as PDFs. If the model's output doesn't export cleanly, you're fixing margins and line breaks instead of moving on to the next invoice.
I copied Claude's invoice template into a Google Doc and exported to PDF. The table held together, the VAT number stayed under the business name, and the page break landed in the right place. I've sent forty invoices using that workflow; no customer has replied asking for a clearer copy.
Gemini's template exported with the VAT summary paragraph breaking across two lines and the customer address shifting right when I changed the font. I had to reformat it in the doc before exporting. Not a dealbreaker, but it added two minutes per invoice.
If you're generating invoices daily, Claude's template saves you the formatting tax.
Questions people ask
Can I use these models if I'm not VAT-registered yet?
Yes. Just omit the VAT number and don't break out the VAT line—prompt for a simple invoice with the total only. Both models handle non-VAT invoices without fuss.
Do I need to register for Irish VAT if I'm a UK sole trader selling prints to Dublin?
Not until your total sales to Ireland cross €10,000 in a calendar year. Under that threshold, you charge UK VAT. Over it, you either register for Irish VAT or stop selling to Ireland. Prompt the model to include a threshold note if you're close.
Which model should I use for VAT returns, not just invoices?
Neither—yet. Both models generate compliant invoices, but I wouldn't trust them to file a VAT return without a human review of every line. Use them to organise your invoices and calculate totals, then check the return yourself or hand it to your accountant.
Does Kryotta let me save invoice templates so I don't re-prompt every time?
Yes. You can save a prompt as a snippet in Kryotta, then call it with updated transaction details each time you need an invoice. I have three saved: UK standard sale, Irish cross-border sale, refund credit. Cuts my prompting time from two minutes to twenty seconds.
I've issued ninety invoices since I ran this test. Claude handles my UK sales and anything complicated; Gemini handles my Irish batch runs. Between them, I've cut my invoicing time from an hour a week to fifteen minutes, and my accountant hasn't sent a single query. If you're still formatting invoices by hand in a spreadsheet, try one model for ten invoices and see what breaks. Then fix the prompt and move on. You can set up the same workflow in Kryotta—both models are there, and you'll know which one fits your invoicing by the end of the afternoon.
