What is a diff?
A diff (short for difference) is a comparison of two pieces of text that shows exactly what changed between them. It highlights additions, deletions, and unchanged lines, making it immediately obvious what was modified without having to read both versions in full. The concept comes from the Unix diff command, invented in the early 1970s, and is now fundamental to version control, code review, and document editing.
How to use the ToolHive diff checker
Paste the original text in the left panel and the modified version in the right panel. The comparison appears instantly below — there is no submit button. Lines that were added are shown in green, lines that were removed are shown in red, and unchanged lines appear in the normal text colour. Each changed line also shows the specific characters that changed within the line, not just the whole line.
Line-by-line vs character-by-character
The diff checker uses a two-level comparison. First, it identifies which lines are different between the two texts. Then, within changed lines, it highlights the specific characters that differ. This is the same approach used by GitHub's pull request view and most professional code review tools, and it makes it easy to spot small changes within long lines.
Common uses for the diff checker
Code review is the most developer-oriented use: compare a function before and after refactoring, or check what actually changed in a code snippet copied from two different sources. Legal document revision is another major use case — contract versions, policy documents, and terms of service updates need careful comparison where every changed word matters. Academic editing is similar: compare a draft with a revised version to see every change an editor or peer reviewer made.
Configuration file comparison is extremely practical — comparing two versions of a config file (nginx.conf, docker-compose.yml, package.json) to understand what changed between environments or deploys. Email thread cleanup is an unexpected but common use: comparing two versions of a document that were edited by different people and merged manually.
Privacy
Both texts stay entirely in your browser. Nothing is sent to any server. You can safely compare confidential contracts, proprietary code, or sensitive documents.
What the diff checker does not do
The diff checker compares plain text. It does not understand semantic meaning — it will flag a comment change and a logic change with equal prominence. For code, a dedicated code review tool in your IDE will give you syntax-aware comparison. For Microsoft Word documents, the built-in Track Changes feature is better for rich-text comparison with formatting. The diff checker excels at quick, no-friction plain text comparison when you need an answer in seconds.