A text diff checker compares two blocks of text and shows exactly what changed between them: what was added, what was removed, and what stayed the same. It is a genuinely old idea, the same concept behind the diff command line tool that has existed since the 1970s, but the way people actually use a diff checker day to day has shifted as more of what gets compared is prose, prompts and generated content rather than only source code.
Word mode versus line mode
The single most important setting in a diff checker is whether it compares word by word or line by line, because the two modes are useful for genuinely different kinds of content:
- Word mode highlights the specific words that changed within a sentence or paragraph, which is what you want for prose: a contract clause, an email draft, a blog post revision, or a prompt, where the interesting change is usually a handful of words inside an otherwise unchanged sentence. Line-by-line comparison on prose tends to mark an entire paragraph as changed just because one word in it moved, which is much less useful.
- Line mode compares whole lines against each other, which matches how code, configuration files, and other line-structured documents are normally diffed. A one-word change inside a long line still marks that whole line as changed in line mode, which is the right behavior for most code review contexts, where you generally want to know which lines were touched.
Picking the wrong mode for the content produces a technically correct but practically useless diff: line mode on a rewritten paragraph marks the entire thing red and green with no insight into what actually changed inside it, while word mode on heavily restructured code produces noisy, hard-to-follow output.
Comparing two versions of a prompt
A prompt is, at its core, just text, and small wording changes to a prompt can have a disproportionate effect on a language model's output, sometimes changing tone, structure, or correctness in ways that are hard to predict without testing. When a prompt gets revised, whether by a teammate, by trial and error, or by an automated prompt optimization process, diffing the old and new versions in word mode makes the actual change immediately visible instead of relying on remembering what was edited or manually reading two versions side by side looking for differences. This is especially useful when a prompt stops working as well after a change and you need to quickly isolate exactly what was different about the version that worked.
Comparing two versions of a document
Beyond prompts, word-mode diffing is useful for any prose document that goes through revisions: comparing a contract draft against a redline, checking what an editor actually changed in a piece of content, or verifying that an automated rewrite (including one done by an AI tool) only touched the parts it was supposed to touch and did not quietly alter something elsewhere in the document.
Ignoring noise
Two options matter for keeping a diff focused on real changes rather than formatting noise:
- Ignore case treats uppercase and lowercase as equal, useful when a change is purely stylistic capitalization rather than a substantive edit.
- Ignore extra whitespace collapses repeated spaces and blank lines before comparing, useful when two versions of a document differ only in how they were formatted or pasted, not in their actual content.
Our free text diff checker compares two texts word by word or line by line, with options to ignore case and extra whitespace, and runs entirely in your browser. Nothing you paste is uploaded or saved.