The spreadsheet I stopped keeping in March
I've been a registered sole trader since 2019. Graphic design, some brand consulting, a bit of copywriting. About twelve to fifteen clients at any time, mostly small businesses around Manchester and a couple in Dublin. They pay by bank transfer or PayPal, I send invoices as PDFs, and every quarter I hand HMRC a VAT return that I've pieced together from a spreadsheet I update when I remember.
March was the month I stopped remembering. I had eighty invoices to chase, check, and reconcile before the VAT deadline. Some clients are VAT-registered, some aren't. Some pay in euros. I'd been writing each invoice by hand in Word, copying client details from old emails, calculating the VAT myself, and hoping I hadn't mixed up the 20% standard rate with the zero-rated services I sometimes provide to clients outside the UK. I missed the deadline by three days and paid a £200 penalty.
That weekend, I decided to see if Claude or Gemini could do the admin I'd been avoiding. Not the accounting—I'm not asking AI to file my return—but the repetitive part: pulling client details from Companies House, checking VAT numbers, calculating totals, formatting invoices. I had a folder of project notes, a separate folder of old invoices, and a list of company names in a text file. I gave myself one afternoon.
Claude Sonnet 4.5: fast at structure, terrible at VAT
I started with Claude because I'd used it before for client emails. I opened Kryotta, pasted the list of twenty company names, and asked Claude to look up their Companies House numbers and registered addresses.
It gave me a formatted table in thirty seconds. Looked perfect. Then I spot-checked the first five entries against the actual Companies House website. Three were correct. Two were completely invented—real company names, but the numbers belonged to different businesses in different cities. Claude had hallucinated the details rather than admit it couldn't access the registry.
I tried a different approach. I manually copied five client records from Companies House into Claude: company name, number, address, VAT registration number. Then I asked it to calculate an invoice for £1,200 of design work, VAT included, due in fourteen days. Claude wrote a clean invoice with all the formatting, but it charged £1,440 total—adding VAT as if the £1,200 were the net amount. That's correct if you're invoicing net-plus-VAT, but I'd meant £1,200 as the gross project fee, so VAT should have been calculated as one-sixth of that (£200), making the net £1,000. I had to explain UK VAT reverse calculation three times before Claude got it right, and even then it kept defaulting back to the wrong method on the next invoice.
The structure was excellent. The math was a problem. And it couldn't verify any external data.
Gemini Flash: better with UK tax rules, still needs supervision
I switched to Gemini Flash and pasted the same five company records. This time I was explicit: "Calculate VAT as one-sixth of the gross amount if I give you a total project fee. Use 20% only for net amounts." Gemini got it right on the first invoice. Then I gave it a client in Dublin and said the service was digital consulting, supplied remotely. Gemini correctly noted that I should charge Irish VAT (23%) if the client isn't VAT-registered, or use the reverse-charge mechanism if they are. Claude had missed that entirely.
Gemini also warned me that one of the client VAT numbers I'd pasted looked wrong—too few digits. I checked: I'd copied it from an old email and the client had typed it incorrectly. Gemini didn't verify it against the official VIES database (it can't access external APIs), but it knew the format rules for UK and Irish VAT numbers and flagged the error. That saved me from issuing an invoice with bad details.
The weakness: Gemini still couldn't pull live data from Companies House. I had to do that part manually. And when I asked it to generate ten invoices in a batch from a list, it mixed up two client addresses halfway through. I caught it because I was checking each one, but if I'd trusted the output blindly I'd have sent the wrong invoice to two clients.
The hybrid workflow that actually worked
By invoice thirty I'd settled into a rhythm. I used Gemini Flash for the calculations and formatting, Claude Sonnet 4.5 for the cover emails (it writes more natural client-facing text), and I did the Companies House lookups myself in a separate browser tab. It sounds like more work, but it was faster than my old Word-and-spreadsheet method and far more accurate.
Here's the step-by-step process I used for the remaining fifty invoices:
1. Look up the client on Companies House. I searched by company name, copied the registration number, registered office address, and VAT number (if listed) into a text file. This took about two minutes per client. I didn't trust either model to do this without hallucinating.
2. Paste the client details into Gemini Flash. I used this exact prompt format: "Client: [company name], Companies House number [number], VAT number [number or 'none'], address [full address]. Project: [brief description]. Gross fee: £[amount]. Calculate VAT at 20%, generate invoice due in 14 days, invoice number [my sequence], my details [sole trader name, address, VAT number]." Gemini produced a clean invoice in about fifteen seconds.
3. Spot-check the VAT calculation. I glanced at the net, VAT, and total lines. If the gross fee was £1,200, I expected net £1,000, VAT £200, total £1,200. Gemini got this right every time after the first few prompts, but I still checked. Twice it miscalculated when I gave it a euro amount (I invoice some Dublin clients in euros), and I had to correct the rate to 23%.
4. Copy the invoice text into Claude Sonnet 4.5 for the cover email. I asked Claude to write a short email attaching the invoice, mentioning the project, and confirming payment terms. Claude's tone was much better than Gemini's—warmer, less formal, more like something I'd actually write. I tweaked a sentence or two, then sent it.
5. Save the invoice as PDF and log it in my spreadsheet. I copied the invoice into a Google Doc, exported as PDF, and added one line to my VAT tracker: date, client name, net, VAT, total. This part I didn't automate. I wanted a single source of truth I could hand to my accountant.
The whole process took four hours for eighty invoices. My old method would have taken two full days and I'd have made at least three VAT errors.
What failed and what I'd change
Gemini occasionally wrote "VAT: N/A" for clients outside the UK, even when I'd specified the Irish rate. I had to re-prompt with "Client is in Ireland, service is digital consulting, client is not VAT-registered, charge 23% Irish VAT." After that it worked. The first time, it didn't.
Claude kept wanting to add a disclaimer at the bottom of the invoice—something like "This invoice is generated for informational purposes and should be reviewed by a qualified accountant." I had to tell it three times that I'm a sole trader, this is my actual invoice, and I don't need a disclaimer. It finally stopped, but the instinct to hedge was annoying.
Neither model could verify VAT numbers against the VIES database. I had to do that manually for the four EU clients I invoice occasionally. It's a thirty-second check on the Europa website, but I wish Kryotta could route that lookup to a model with web access. (I know some AI tools claim to do this. I haven't tested them and I don't trust automation for compliance checks anyway.)
If I were doing this again, I'd start with a better-organized text file. I wasted twenty minutes in the middle of the batch because I hadn't noted which clients were VAT-registered and which weren't, so I had to go back to old emails to check. A simple spreadsheet with columns for company name, VAT status, and preferred currency would have saved that time.
Claude vs Gemini for UK sole trader invoices: which one wins
Gemini Flash is better at VAT calculations and understands UK and Irish tax rules without needing constant correction. It's faster for batch work and costs less per task. I used it for seventy of the eighty invoices.
Claude Sonnet 4.5 is better at writing the client-facing email and adjusting tone for different relationships—friendly for long-term clients, more formal for new ones. I used it for every cover email and a handful of invoices where the project description needed more context.
Neither model is reliable for live data lookups. Don't ask Claude or Gemini to pull company details from Companies House or verify VAT numbers. Do that part yourself. It takes two minutes per client and it's the only way to be sure you're not invoicing the wrong company or using a fake VAT number.
If you're choosing one model for the whole workflow, use Gemini Flash. If you have access to both (Kryotta gives you both in one workspace), use Gemini for the invoice and Claude for the email. That's the fastest combination I found.
Questions people ask
Can AI actually verify UK VAT numbers?
Not directly. Claude and Gemini can check if a VAT number matches the correct format (nine digits starting with GB for UK, nine digits starting with IE for Ireland), but they can't confirm it's valid or currently registered. You still need to check the VIES database yourself for EU clients or call HMRC for UK ones.
Is it legal to use AI for sole trader invoices?
Yes. You're responsible for the accuracy of the invoice, not the tool you used to create it. HMRC doesn't care whether you typed it in Word or generated it with AI, as long as the details are correct and you keep proper records. I still review every invoice before I send it.
Do I need to tell clients I used AI?
No. The invoice is a standard business document. How you produced it is your choice. I've never mentioned it to a client and no one's asked.
Which model is cheaper for invoicing?
Gemini Flash costs less per task than Claude Sonnet 4.5. I processed eighty invoices and the total usage cost me about £2.80 in Kryotta credits. ChatGPT Plus would have been £20 for the month, and I'd have hit the message limit halfway through.
I still check every invoice before it goes out. I still reconcile my VAT tracker by hand before I file the return. But the four hours I spent in Kryotta saved me two days of copying client details, calculating VAT, and fixing formatting errors in Word. If you're a sole trader with more than ten invoices a quarter, the workflow pays for itself in the first batch. You can try it yourself at Kryotta—Claude and Gemini both live in the same workspace, so you don't need to pick one upfront.
