Can AI Check a VAT Return Before You File? Coding Error Checks

Yes, as a second reader. Export the quarter's transactions from your accounting software, then have AI flag lines where the VAT amount, tax code and description disagree: wrong rates, zero-rated sales that should not be, and reverse-charge purchases coded as plain no-VAT. A person must review each flag and sign off before the return is submitted.

AI does not know your business, and it does not file the return. What it does well is read hundreds of lines looking for patterns that do not fit, and write the spreadsheet formulas that catch arithmetic and coding conflicts. The people who get most from it use it on the export from their software, not on the return's nine totals, because the totals only show a mistake after it has happened.

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Digital records: why the export is the place to check

Under Making Tax Digital for VAT, all VAT-registered businesses, apart from those granted an exemption, keep their VAT records digitally and submit returns from software through the tax authority's API rather than keying figures into a web form. The authority's notice allows information to be input manually, but data moved between programs must travel through a digital link (a linked spreadsheet cell, a file import, an API), and it does not count copying and pasting as one. A spreadsheet can be part of the setup when it passes its figures through a digital link to bridging software that submits the return.

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That has a practical consequence for AI checking. The review happens on a copy: you export, check and fix the errors in your accounting software at source, then submit from there. Never retype corrected figures into a different program and file from that. Fix the source transaction, re-export, and re-run the checks until the flags clear.

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What to export, and what to remove first

Most accounting packages produce a transaction-level VAT report or a general ledger export. You want one row per transaction line, with these columns: date, supplier or customer, description, account code, net amount, VAT amount, gross amount, tax code or rate, and country of the counterparty if the system holds it. Exclude bank-only lines with no VAT effect.

• Remove or replace personal customer names for anyone who is an individual, because a VAT check does not need them.
• Drop bank account numbers and any free-text notes that contain personal data.
• Keep supplier names, since patterns by supplier are the most useful signal.
• Use a business-tier assistant whose terms say your content is not used for training by default, and delete the chat afterwards.

A 640-line quarter at a ten-person marketing agency

Say a marketing agency has a quarter's export of 640 lines. Its revenue is mostly standard-rated retainers, with a few clients based overseas, and its costs include software subscriptions from overseas suppliers, freelancers, client entertainment and a few small purchases. The bookkeeper will submit the return on Friday. Illustrative return totals before checking: VAT due on sales 38,420.00, VAT reclaimed on purchases 14,960.00, net payable 23,460.00.

Layer one: let a spreadsheet catch the arithmetic

Before any AI judgement, run deterministic checks. Ask the assistant to write Excel formulas for the columns of your export, then check the formulas yourself. For rate mismatches, assuming net in column E, VAT in column F and a rate in column H as a number (0.2, 0.05, 0):

=IF(ABS(F2-ROUND(E2*H2,2))>0.02,"RATE MISMATCH","")
=IF(AND(H2=0,F2<>0),"VAT ON ZERO CODE","")
=IF(ABS(G2-(E2+F2))>0.01,"GROSS DOES NOT ADD","")

Run these across all 640 lines and filter the flagged rows. In the illustration the formulas flag 14 lines: six rate mismatches (mostly mixed-rate invoices posted as one line), five VAT-on-zero-code lines and three gross-does-not-add lines (rounding differences of a few pence, left alone). This layer needs no AI judgement and cannot hallucinate. Excel's own Copilot side pane can help write the formulas. Do not build the check around the COPILOT worksheet function: Microsoft's support page says it is no longer available in Excel from 14 September 2026, and recalculated cells that used it return an error.

Layer two: ask AI to read the descriptions against the codes

Arithmetic cannot tell that "Team lunch with client" was coded as recoverable VAT. Judgement about descriptions is where a language model earns its place. Paste the sanitised export (or chunks of 150 to 200 lines) with this prompt:

You are reviewing a VAT transaction export for coding errors
before a return is filed. Do not give legal or tax advice and do
not change any figures. For each line that looks wrong, return:
Row number | Description | Current tax code | What looks wrong
| Why (one sentence) | Confidence: high/medium/low.
Check specifically for:
1. Overseas suppliers of services with no reverse charge applied.
2. Sales to customers outside the home market coded standard-rated.
3. Client entertainment or gifts with VAT reclaimed.
4. Anything described as books, food, childcare, insurance,
   training, postage or financial services with a standard rate.
5. Identical supplier, date and amount appearing twice.
6. Transactions dated outside the return period.
7. Credit notes with the wrong sign.
List only lines you would query. Do not list lines that look fine.

A realistic, illustrative output on the agency's file:

You then work through the flags. The AI's confidence column is a sorting aid, not a verdict. In this example, the dinner and the duplicate were real errors, the software purchase needed a reverse charge, the retainer needed a place-of-supply decision from the accountant, and the stationery line was fine on inspection (the "books" were notebooks).

Layer three: totals sense-check

Last, compare the return's totals with the previous four quarters. Ask: "Here are the nine box totals for five quarters. Which box moved most against the trend, and which transactions in the current export could explain it?" This works as a final reasonableness pass only. A box that moves for a good reason, such as a big project, is not an error.

The VAT-specific error checklist

These are the coding errors that matter most, with how each shows up in an export and what to do.

Rate and arithmetic errors

• VAT does not equal net times the rate. Shows up as a difference over a few pence. Often a mixed-rate invoice posted as one line. Split the line.
• Wrong code on a recurring supplier. The same supplier appears with two different codes across the quarter. Check the supplier record and set a default.
• Credit notes with the wrong sign. A credit note posted as a positive purchase adds VAT to your reclaim rather than subtracting it.

Zero-rated, exempt and outside-scope mistakes

• Exempt coded as zero-rated. Zero-rated supplies allow you to reclaim related input VAT, while exempt ones may restrict it. The distinction can change your recovery, so ask your accountant when your sales include exempt items.
• Zero-rated sales without evidence. Goods exported to overseas customers can be zero-rated only if you hold valid evidence of export, and the authority's export notice sets a normal limit of three months to obtain it. A flag here means "check the paperwork", not "the code is wrong".
• Standard-rated sales that should be outside the scope or zero-rated. Services to business customers abroad are often treated differently under the place-of-supply rules. Ask for a decision per customer type, then set the code at customer level.

Reverse-charge mistakes

• Services from overseas suppliers coded as no VAT. You usually account for the VAT yourself: the VAT appears in the box for VAT due on sales and the matching box for VAT reclaimed, so for a business that can reclaim in full, the net payable is unchanged, but the total sales and total purchases boxes are understated if you skip it. The authority's place-of-supply notice puts the VAT in boxes 1 and 4 and the full value of the service in boxes 6 and 7.
• Domestic reverse charge in construction. Where it applies, the customer accounts for the VAT. The supplier charges none, but its invoice must say the reverse charge applies and show the VAT due under it. Check that both sides code it the same way.
• Reverse charge applied wrongly to goods. Goods imported have a separate treatment. If the description says "hardware" or "equipment", query it.

Recovery errors

• Entertainment. Input VAT on client entertainment cannot be recovered. Entertainment for employees generally can, but not events for directors or partners alone, and the share for any non-employee guests is blocked. Check the description and the guest list.
• Personal or mixed-use items. A phone bill, car costs or home-office items may need a business-use split.
• Reclaims without a valid VAT invoice. The authority treats the VAT invoice as the primary evidence for a reclaim, and a full invoice shows the supplier's VAT registration number. Flag reclaim lines where the supplier's VAT number is blank, and find the invoice before filing.

Timing and period errors

• Transactions dated outside the period. A sale dated one day after quarter end usually belongs in the next return, depending on its tax point (the date the supply counts as made).
• Scheme-specific timing. Businesses on cash accounting or flat-rate schemes follow different rules. Tell the tool which scheme you use, or exclude the checks that do not apply.

Working through the agency's flags: what changed in the figures

In the illustration, 23 lines were flagged across the two layers, and 9 turned out to be real errors. Four reconciled to specific changes.

1. Client dinner 203: reclaimed VAT 96.00 removed. Box for VAT reclaimed falls by 96.00.
2. Duplicate supplier invoice 533: VAT reclaimed on 2,400.00 net at 20 per cent, 480.00, removed. Reclaimed falls by a further 480.00.
3. Design software 112: reverse charge of 240.00 on a 1,200.00 net service added to both boxes. Net payable is unchanged, but total sales and total purchases each rise by 1,200.00.
4. Mixed-rate invoice split into 20 per cent and 5 per cent lines: a 31.50 difference in VAT due.

The corrected totals: VAT reclaimed 14,960.00 minus 96.00 minus 480.00 plus 240.00 equals 14,624.00, and VAT due 38,420.00 plus 240.00 plus 31.50 equals 38,691.50. Net payable becomes 24,067.50, which is 607.50 higher than the first draft. Without the check, the business would have under-declared. With it, the correction costs an hour. (All figures are illustrative.)

A second business: a dental practice with mixed supplies

Mixed supplies are where a language model's description reading helps most. Say a four-dentist practice sells mainly exempt clinical treatment, plus a smaller amount of standard-rated retail items such as toothbrushes and toothpaste sold separately from treatment. Input VAT on costs used only for exempt work generally cannot be recovered, VAT on costs used only for the standard-rated sales can be recovered in full, and shared costs such as rent are apportioned using the standard method unless the authority has approved a special one. Whether the practice falls under partial exemption rules, and which method applies, is the accountant's decision.

What the AI check can do is separate the lines that need a decision. Ask: "Group these purchases into: clearly for clinical (exempt) work, clearly for retail sales, shared overheads, and unclear. Do not decide recoverability." In an illustrative quarter of 410 lines, the groups come out at 142 clinical, 31 retail, 198 shared and 39 unclear. The 39 unclear lines are the ones the accountant looks at, and the reclaim on the clinical group is checked against the code used. Anything coded as fully recoverable that sits in the clinical group is a candidate error. The grouping saves the accountant sorting 410 lines, but the percentage and the method remain theirs.

Finding inconsistent supplier codes with a formula

A recurring supplier miscoded some quarters and not others is one of the commonest patterns, and it is easy to find without any AI at all. Add a helper column that counts how many different tax codes each supplier has in the export. If your spreadsheet supports dynamic arrays, you can list unique codes per supplier directly; otherwise sort by supplier and filter where adjacent rows with the same supplier differ in code. Ask the assistant to explain the formula to you in plain words before you run it, and test it on five rows where you know the answer.

A before-and-after, illustrative: a cloud storage supplier appears 12 times in the quarter, nine times as "reverse charge services" and three times as "no VAT". The three odd lines are all payments made by a different card. Fixing the three transactions corrects one return. Setting the supplier's default tax code in the software fixes the next ones. That second step is where the time saving compounds, because errors that recur by supplier are fixed once.

A "known and approved" list so the check stops crying wolf

After one or two quarters, you will have decisions your accountant has already made: the mobile network's VAT treatment, a particular supplier's zero rating, a customer abroad treated as outside the scope. Keep them in a short list and paste it into each run so the tool does not re-flag them:

Known and approved treatments (do not flag):
- Supplier "Hosting Co": reverse charge services, approved by
  the accountant on [date].
- Customer group "Overseas retainers": outside the scope,
  approved on [date].
- Staff team events: VAT reclaimed on the venue only.
Flag anything that conflicts with the list.

The last line matters. A treatment that conflicts with an approved decision is more likely to be a slip than a judgement, so it deserves a flag.

Cadence: check monthly, not just the week before filing

A single quarter-end review of 640 lines takes a few hours and feels like a chore. A monthly run of 200 lines takes about 45 minutes and catches a miscoded supplier before it repeats. A sensible rhythm for a small business is a light check at each month-end (formulas plus the AI description pass), a fuller review before filing that includes the trend check against previous quarters, and a quarterly update of the known-and-approved list. Put a date in the diary three working days before the filing deadline, so there is room to correct source transactions and re-export. A late discovery on the last day leaves you choosing between filing a known error and missing a deadline.

When AI gets the VAT answer wrong

• It guesses a rate with confidence. A model may say a product is zero-rated because a similar one is. Treat every rate claim as a lead and check against the tax authority's guidance.
• It ignores the scheme you are on. Flat-rate and cash-accounting users get false flags. Say so in the prompt.
• It misses things that are not in the export. A missing invoice, a sale not yet booked, or a bad debt relief claim will not appear. A reconciliation to the bank and the sales ledger is still your job.
• It cannot see the paperwork. Export evidence, VAT numbers and place-of-supply facts live in files and emails. The checker only reads what you give it.
• It treats a pattern as an error. An unusual but correct treatment, agreed with your accountant, will be flagged every quarter. Add a short "known and approved" list to the prompt.

A sign-off block to keep with each return

VAT return: quarter ending [date]
Export taken: [date], [software], [number of lines]
Checks run: arithmetic formulas / AI description review / trend check
Flags raised: [number] | Real errors: [number] | Corrected in source: yes
Judgement calls referred to accountant: [list]
Totals before / after checks: [figures]
Reviewed by: [name] | Approved for filing: [date]
Submitted from: [software] | Digital link used: yes

The block takes five minutes to complete and records that a human reviewed the AI's flags. If the authority ever asks questions, you have evidence of a considered process, which is better than a screenshot of a chatbot.

Capture and filing tools versus a coding review

Most VAT software sold to small businesses does one of two other jobs: capturing invoices into your books, or submitting the return. Both are different from reviewing the coding. If you want the supply side, Dext and Making Tax Digital covers capture, and AI bookkeeping with MTD and VAT compares the tools around filing. A reviewer who does this for clients may prefer the approach in AI tax return prep: extract, review, sign off, and the difference between AI review and the diagnostics inside software is set out in AI return review versus software diagnostics. For the spreadsheet half, the general clean messy data tutorial helps with exports that arrive in a state.

Keep the division of labour clear. AI reads and flags. Your software holds the records and files the return. You decide what is correct and sign it off, and a qualified accountant answers the judgement calls about rates, place of supply and reverse charge that no assistant should settle for you.

VAT return checking questions

Will AI file my VAT return for me?

A general assistant will not and should not. Filing under Making Tax Digital goes through compatible accounting software that submits through the tax authority's interface. Use AI before that step to review the transaction export, then approve and submit from your software yourself.

Is it safe to upload my transaction list?

Only if you remove personal customer names and bank details first and use a business plan whose terms say your content is not used for training. A supplier name and an amount are commercially sensitive but usually low risk. Check your accountant's and your own data policies before uploading.

Can AI tell me the right VAT rate for a product?

It can suggest, but it cannot be your authority. Rates and exemptions depend on the exact supply and change over time. Treat any answer as a lead, then confirm on your tax authority's guidance or ask your accountant before you recode a transaction.

How many errors should I expect to find?

In a clean set of books, a handful of coding slips per quarter is normal. A pattern, such as the same supplier miscoded every time, matters more than the count. Fix the rule in your software so the error stops recurring.

Further reads

• Can AI Do My Bookkeeping? What Still Needs an Accountant — What AI bookkeeping can and cannot settle.
• AI Invoice Processing: Stop Typing Supplier Bills by Hand — Stop retyping supplier bills before they reach VAT.
• Claude for Excel for Accountants: Reconciliations a Manager Can Sign Off — Review spreadsheets with Claude for Excel.
• Can AI Draft Journal Entries? Accruals and Prepayments, Then Review — Review accruals and prepayments, which affect VAT timing.
• How to Match Supplier Invoices to Purchase Orders Automatically — Matching bills to orders to catch duplicates.
• AI Receipt Capture and Bank Categorisation: A Bookkeeper's Setup — Capturing receipts correctly at the start.
• Tradesman Invoice Template: VAT, CIS and Reminder Wording Drafted With AI — Nine invoice fields, three filled-in invoices (no VAT, VAT, reverse charge with CIS) and AI prompts for terms and polite reminders.
• AI Tools and AI Development: The Complete 2026 Guide — the AI hub, including every tutorial in the AI-for-business series.

Sources: the tax authority's notices on Making Tax Digital for VAT (700/22), place of supply of services (741A), business entertainment (700/65), record keeping (700/21), goods exported (703), health professionals (701/57) and partial exemption (706); its reverse charge technical guide and VAT rates page; Microsoft Support, COPILOT function (no longer available from 14 September 2026). Transaction figures are illustrative.