What a diff can and cannot see
A diff matches lines, so a paragraph rewritten as one long line shows as entirely removed and entirely added, even if only a word changed. Text that has been reflowed to a different width shows as changed throughout.
It also cannot tell a move from a delete plus an insert. A section relocated from the top to the bottom of a document appears twice — once red, once green — which is correct but rarely what you meant to see.
What it is genuinely good for
Two versions of a contract, where a single altered clause matters and reading both in full is unreliable. Two API responses that should be identical. A configuration file before and after a change nobody documented. A draft returned by someone who said they "made a couple of edits".
For structured data, comparing formatted JSON rather than raw gives a far more readable result — one key per line means the diff lands on the key that changed.
Why it runs here
The obvious use for a diff tool is comparing two versions of something confidential. Pasting an unsigned contract or an internal document into a site that uploads it defeats the purpose, so both sides stay in the page and nothing is transmitted.
Frequently asked questions
Why is my whole paragraph marked as changed?
Comparison is line by line. If the text was reflowed or is one long line, a single-word edit changes the whole line. Break long paragraphs onto separate lines first for a finer result.
Can it compare Word or PDF files?
Paste the text rather than the file. For a PDF, extract the text first — what gets compared is the words, not the formatting.
Does it show moved blocks?
No — a move appears as a deletion and an addition. That is technically accurate and often not what you wanted to see; there is no way to distinguish the two from the text alone.
Is my text uploaded?
No. Both sides stay in the page, which is rather the point when what you are comparing is a contract or an unreleased draft.
Something wrong with this tool, or an idea for it? Tell us