In October 2026 an AI that marks handwritten math the way a teacher does drew a crowd on Hacker News (a Show HN). This one marks typed working - and checks every line itself first, before any model sees it.
Where did your working first slip?
Type the problem and your working, one step per line. The page checks each line against the one before it, for free. One run marks it in red: right or wrong on every line, what went wrong and why, a nudge instead of the answer, and one practice problem like it.
1 The problem and your working
Type math as you would write it: 3(x - 2), x^2 or x², 2/3 or ½, 1 1/2, 3 × 4, 8 ÷ 2, sqrt(9), 20%. One letter as the unknown. Lines with words are read as notes. A line may start with "=" to carry on from the line before.
2 The page
- = follows from the line before
- ≠ does not follow
- ? not checked
- → first line, nothing before it to compare
- – words, read as a note
3 Mark it
The verdict follows a fixed rule: rework with two or more slips, one slip with exactly one, not finished with none but no answer reached, full marks otherwise. A line the page found does not follow is always a slip; a line that follows correctly from a slip is marked "follows on", not wrong.
4 Kept pages
How the page checks a line
Each line is read with a small calculator built into the page (brackets, powers, square roots, fractions, mixed numbers, percent, and one letter as the unknown). When you solve an equation, two lines match when they have the same solutions - the page finds them numerically between -10000 and 10000 - so a line that loses or gains a solution is caught. When you simplify, expand or work out, two expressions match when they give the same value at ten different values of the letter. The problem itself counts as the line before line 1, when the page can find its math. After a wrong line, the next line is compared with the wrong one, so correct work that carries a mistake forward reads as following on. Lines with words, more than one letter or anything the calculator cannot read are left for the marker. It is a check of equality, not of method: a line can be equal and still be a long way round.