Wondy.
KO|EN
·AI Tools·19 min read·Wondy

Why Do I Owe Taxes When Nothing Changed? A ChatGPT W-4 Audit

I ran one 2026 pay stub and my W-4 through Claude. It found a stale dependent credit and a $2,910 shortfall, with eight paychecks left to fix it.

Same job, same salary, same everything. So why is there a bill?

Because the number on your pay stub is a forecast, and the form that made it may describe a household you no longer have. Withholding does not read your life. It reads a W-4 you signed once, possibly years ago, and then divides.

The IRS had processed 143,925,000 returns by May 8, 2026 and issued 99,138,000 refunds against them. The other 44.8 million, about 31%, produced no refund. That group is not identical to "people who owed," since a return can land at zero, but it is the closest published figure to it. Meanwhile the average refund climbed 11.5% to $3,276.

What you need in front of you
  1. 01
    Your latest pay stub

    The YTD column, not just this check

  2. 02
    The W-4 as filed

    Step 3 dollar amount, 4(a), 4(c)

  3. 03
    Income with no withholding

    1099-INT, 1099-DIV, side work

  4. 04
    Paychecks left, counted

    This is the divisor at the end

The fourth one is why this works in August and stops working in December.

The file, and why it is not mine

Running my own stub would make the exercise unverifiable, since nobody but my payroll department could grade it. So I built one, with a specific error inside it, and handed it over without saying what the error was.

PAY STUB  ·  semi-monthly, period 16 of 24, pay date 2026-08-05
  Gross this period            $3,250.00     YTD  $52,000.00
  Federal income tax withheld    $255.42     YTD   $4,086.72
  Social Security                $201.50     YTD   $3,224.00
  Medicare                        $47.13     YTD     $754.08
  State income tax                 $0.00   (no state income tax)
  Net pay                      $2,745.95
  Annual salary $78,000.00. No pre-tax deductions.

FORM W-4 ON FILE  ·  signed 2024-02-11
  Step 1(c)  Single
  Step 2     unchecked
  Step 3     $2,200
  Step 4(a)  blank    4(b) blank    4(c) blank

OTHER 2026 INCOME  ·  no withholding on either
  1099-INT  savings interest        $4,300.00
  1099-DIV  ordinary dividends (1a) $1,200.00,  qualified (1b) $0

HOUSEHOLD
  Single. One dependent, son, born 2009-03-04, lives with me all year,
  no income of his own. No other jobs. No tips, no overtime, no vehicle loan.

The birth date is the whole test. It sits in the household block looking like paperwork, and it means the child turned 17 on March 4, 2026. Under the 2026 Form W-4, line 3(a) is worth $2,200 for each qualifying child under 17 and line 3(b) is worth $500 for each other dependent. The $2,200 on that form stopped being right in March. Payroll has no way to know.

The prompt

I am a single filer doing a mid-year withholding check. Below is my
latest 2026 pay stub, the Form W-4 my employer has on file, my other
2026 income, and my household facts.

Using 2026 federal figures, work out:
1. my total 2026 federal income tax
2. what my current W-4 will withhold by December 31
3. the difference
4. exactly what to put on a new W-4, given how many paychecks I have left

Show the bracket math so I can check it.

[paste the four blocks]

Nothing in there says "find my mistake." Asking a model to flag mismatches hands it the answer, and then the run tells you nothing about whether it would have noticed. The one instruction worth keeping is the last line, because bracket math printed line by line is math you can check against a published table in about ninety seconds.

The run

AI 답변 (실제 실행)Claude (Opus 5)2026-08-07 실행

Working from the four blocks only, using the 2026 federal figures.

1. Total 2026 federal income tax

Wages $78,000 (24 × $3,250) + interest $4,300 + ordinary dividends $1,200 = AGI $83,500. Standard deduction, single, 2026: $16,100. Taxable income: $67,400.

Bracket stack:

  • 10% on the first $12,400 → $1,240.00
  • 12% on $12,400 to $50,400, i.e. $38,000 → $4,560.00
  • 22% on $50,400 to $67,400, i.e. $17,000 → $3,740.00
  • Tax before credits: $9,540.00

Credits: your dependent was born 2009-03-04, so he turned 17 on 2026-03-04. The child tax credit requires the child to be under 17 at the end of the tax year, so for 2026 he is not a qualifying child. He is a dependent worth the $500 credit for other dependents instead, assuming the support and residency tests are met.

Tax after credits: $9,540.00 − $500 = $9,040.00

2. What the current W-4 will withhold

Your employer withholds on wages only. It does not know about the interest or the dividends. With Step 2 unchecked on a post-2020 W-4, the annual percentage method applies the standard deduction, then subtracts the Step 3 amount.

$78,000 − $16,100 = $61,900 withholding basis.

  • 10% on $12,400 → $1,240.00
  • 12% on $38,000 → $4,560.00
  • 22% on $11,500 → $2,530.00
  • Tentative annual withholding: $8,330.00
  • Less Step 3 credit of $2,200: $6,130.00
  • ÷ 24 periods: $255.42 per check

That matches your stub exactly, and your YTD of $4,086.72 is 16 × $255.42. The stub and the form agree with each other. They are both wrong about your tax.

3. The difference

$9,040.00 − $6,130.00 = $2,910.00 short, in two pieces:

  • $1,700 because Step 3 still claims $2,200 for a dependent now worth $500
  • $1,210 because $5,500 of interest and dividends arrives with no withholding and stacks on top of wages inside the 22% bracket ($5,500 × 22%)

4. What to put on the new W-4

The trap is dividing $2,910 by 8 remaining checks and entering $363.75 on line 4(c). That double counts, because correcting Step 3 raises your withholding on its own.

With Step 3 corrected to $500:

  • tentative annual withholding $8,330 − $500 = $7,830, or $326.25 per check
  • 16 checks already gone at $255.42 = $4,086.72
  • 8 checks at $326.25 = $2,610.00 → running total $6,696.72
  • still short: $9,040.00 − $6,696.72 = $2,343.28, over 8 checks = $292.91

So: Step 3 = $500, Step 4(c) = $292.91. Filing $363.75 alongside the Step 3 fix over-withholds by $566.72. That comes back as a refund rather than a problem, but it is $566.72 you did not need to advance.

What I cannot answer from these blocks

Whether you owe a penalty turns on the prior-year safe harbor, and your 2025 total tax is not here. 90% of your 2026 tax is $8,136, and $6,130 of withholding does not reach it, so that prong fails. The other prong is 100% of your 2025 tax, and 100% rather than 110% since your AGI is under $150,000. If your 2025 total tax was $6,130 or less, no penalty applies, though you still owe the $2,910 in April.

Separately, your employer may produce a figure a dollar or two off $255.42 if it uses the Pub 15-T wage bracket tables instead of the percentage method, since those round.

Grading it

I rebuilt every figure in Python with decimal arithmetic and checked the brackets against IRS IR-2025-103, which publishes the Rev. Proc. 2025-32 tables. Standard deduction $16,100, thresholds at $12,400, $50,400 and $105,700, all correct. The stale-credit catch is correct. The $255.42 reconciles.

The part I did not expect is that the run beat my own answer key. I built the file expecting $363.75, which is the gap divided by the paychecks left, and that is the number every mid-year-adjustment article gives you. It is wrong whenever you are also fixing Step 3, which in this file you are.

Piece of the gapAmountWhere it comes from
Dependent aged out of the $2,200 credit$1,700.002026 W-4 line 3(a) vs 3(b)
Interest and dividends, no withholding$1,210.00$5,500 stacked at 22%
Total owed in April$2,910.00tax $9,040.00 less withholding $6,130.00
Naive fix: gap ÷ 8 checks$363.75ignores that Step 3 also changes
Correct line 4(c)$292.91after Step 3 correction adds $566.72 by itself

Two failure modes the fact-checking was watching for never appeared: the 2025 standard deduction of $15,750, and the pre-2025 $2,000 child credit. Both are the kind of stale figure a model trained through last year reaches for, and neither turned up here.

A margin note on checking AI against primary sources. For the 37% bracket on a joint return, the IRS newsroom release says the threshold is $768,700. The Tax Foundation's write-up of the same revenue procedure prints $768,600. One of them has a typo, and the IRS release is the one to use. It changes nothing for the filer in this post, who is single and nowhere near it, which is exactly why it is worth mentioning: the discrepancies you catch are usually in the rows you did not need.

Which side of the safe harbor are you on

Every article on this topic converges on the same instruction: a big refund is an interest-free loan to the government, so tune your W-4 until you break even. I think that advice is aimed at the wrong number.

Start with the evidence that people ignore it on purpose. Damon Jones, in NBER working paper 15963, found that households do not adjust their withholding even when handed a clear reason to, in a pattern consistent with using over-withholding as forced savings rather than with not understanding the form. A Federal Reserve staff paper reached the same place from the other direction, documenting an explicit preference for over-withholding among low- and moderate-income filers. Whatever the float is costing them, they are buying something with it.

Then look at the payoff. Over-withholding costs you whatever your savings account pays on the money while the IRS holds it. Under-withholding costs you a lump sum in April plus underpayment interest, which ran 7% in the third quarter of 2026. Per dollar, those two prices are not the same, and unless your savings rate is above 7% the downside sits entirely on one side.

So the target is not zero. The target is the safe harbor: pay in the smaller of 90% of this year's tax or 100% of last year's, with that second number rising to 110% if last year's AGI topped $150,000. Under $1,000 owed and no penalty applies at all. In the file above, 90% of the tax was $8,136 against $6,130 withheld, so the 90% prong failed and everything hinged on the prior-year figure the pay stub could not supply.

That is the one number this whole exercise needs and cannot generate: the total tax line on your 2025 Form 1040. Not the refund line. Pull it before you run anything, or the answer stops one step short of the question you asked.

What the 2026 W-4 added for tips, overtime and car loan interest

Worth two minutes if any of them apply to you, and skippable if none do. They all live on the Step 4(b) Deductions Worksheet, which lowers withholding.

Qualified tips, up to $25,000, if total income is under $150,000 ($300,000 joint). Qualified overtime, up to $12,500 ($25,000 joint), counting only the premium half of time-and-a-half rather than the whole overtime check. Passenger vehicle loan interest, up to $10,000, under $100,000 of income ($200,000 joint). There is a senior line too, $6,000 each below $75,000 ($150,000 joint). Step 4 also carries an explicit "exempt from withholding" checkbox, and the SALT cap line reads $40,400.

Your employer cannot fill any of this in for you. It does not know your tip total, and it cannot tell which half of your overtime was the premium. Leave the worksheet blank and you get the deduction back as a refund in April instead of in your paycheck. One caution, since it runs against the direction of this post: entering these lines reduces withholding. If you are already short, they widen the gap.

What I would file

Send one corrected W-4 that does both jobs at once, rather than fixing Step 3 now and revisiting 4(c) later. A W-4 filed today typically lands on the second payroll run after it, so count the checks the change will actually reach, not the ones left on the wall calendar. Then divide.

The alternative route for the untaxed $5,500 is a Q3 estimated payment by September 15. I would still use the W-4, because withholding is treated as paid evenly across the year regardless of when it was actually taken, while an estimated payment is credited when it arrives. That timing rule is why a December correction can still repair a January problem, and it is the single most useful thing in Publication 505.

If you would rather not hand any of this to a model, the IRS Tax Withholding Estimator does the same job in about 25 minutes and its arithmetic is the IRS's own. It will not read your pay stub for you, it does not work for nonresident aliens, and it needs a W-2 or pension to function. What the model added in my run was the connection nobody asks it to make: a birth date in one block and a dollar amount in another, three years and one form apart.

Quick O/X quiz
  1. 01

    The $2,200 child tax credit still applies for the year in which your child turns 17.

  2. 02

    If you owe less than $1,000 after withholding and credits, there is no underpayment penalty.

FAQ

Why do I owe taxes this year when nothing changed?

Because withholding is a forecast, and the form making the forecast was filled out in a year that no longer describes you. Three things move a withholding number without ever touching your salary. A dependent ages out: the child tax credit requires the child to be under 17 at the end of the year, so the year your kid turns 17 the credit drops from $2,200 to $500, and nothing on your pay stub announces it. Income arrives with no withholding attached, which is what savings interest, dividends and side work all have in common, and it stacks on top of your wages at your top bracket rather than at some average rate. Or a bracket threshold moved and your raise did not. In the file I ran, the first two happened together on a salary that had not changed in two years, and the result was $2,910 owed in April on $78,000 of wages. The pay stub looked completely normal the whole time.

How much extra should I put on line 4(c) if I am $2,900 short with eight paychecks left?

Not the gap divided by eight, if you are also fixing Step 3 at the same time. That is the mistake I planted in my own test file and the run caught it. Step 3 is an annual credit that your employer spreads across every remaining check, so correcting a stale $2,200 down to $500 raises your withholding on its own before line 4(c) does anything. In my file the gap was $2,910 and the naive answer was $363.75 per check. With Step 3 corrected, per-check withholding rises from $255.42 to $326.25 by itself, which covers $566.72 of the gap over eight checks, and the correct line 4(c) entry is $292.91. Filing both changes with $363.75 would over-withhold by $566.72. The order matters: work out what the corrected Steps 1 through 3 will withhold on their own, subtract that from your total tax, and only then divide by the checks you have left.

What is the safe harbor, and how do I tell whether I have cleared it?

The safe harbor is the point past which the IRS stops charging you interest for underpaying during the year. You clear it by paying, through withholding and estimated payments, the smaller of 90% of your 2026 tax or 100% of your 2025 tax. That second figure becomes 110% if your 2025 AGI was above $150,000, or $75,000 filing separately. There is also a floor: no penalty if you owe under $1,000 after withholding and credits. You cannot read any of this off a pay stub, because the prior-year prong needs the total tax line from your 2025 Form 1040, not the refund line. In my file, 90% of the 2026 tax was $8,136 against $6,130 of projected withholding, so that prong failed, and whether a penalty applied came down entirely to a number that was not in the pay stub. The underpayment interest rate was 7% for the third quarter of 2026.

Does the new tips and overtime deduction on the 2026 W-4 change my paycheck withholding?

Only if you put it on the form. The 2026 Form W-4 added lines to the Step 4(b) Deductions Worksheet for qualified tips up to $25,000, qualified overtime up to $12,500 ($25,000 filing jointly, counting only the premium half of time-and-a-half), and passenger vehicle loan interest up to $10,000. Each carries an income ceiling: $150,000 for tips ($300,000 joint), $100,000 for vehicle loan interest ($200,000 joint). There is also a senior line worth $6,000 each below $75,000 ($150,000 joint). Your employer does not know your tip total or how much of your overtime was the premium half, so none of this reaches your paycheck automatically. Entering it lowers your withholding, which means a smaller refund and a larger paycheck for the same annual tax. If you are already short on withholding, entering these lines makes the shortfall bigger, not smaller.

Disclaimer

This article is an educational explainer, not tax advice, and it does not account for your state, your credits, or anything in your return beyond the figures shown. The pay stub, Form W-4, 1099 totals and household facts are synthetic. I wrote them in code and planted the stale credit inside them, so no number describes a real household, mine or anyone else's. The AI output is a real run on August 7, 2026 against that file; a different model, prompt, or day will produce different output, and every figure in it should be checked against the IRS tables before you act on it. Withholding figures assume the annual percentage method under Publication 15-T; your employer may use the wage bracket tables and land a few dollars away. Underpayment interest rates are set quarterly and the 7% figure is the third quarter of 2026. Speak with a tax professional before changing your withholding if your return has moving parts this file does not.

For adjacent versions of this exercise, How to Audit Your Bank Fees With ChatGPT runs the same planted-error method on a checking statement, Mid-Year 401(k) Checkup With AI covers the other August correction worth making, and Calculate Your Personal Inflation Rate With AI builds a household number out of public data the same way.

Sources