What is compare?

Compare is a free tool for comparing two versions of a Word document and producing a redline, the kind of colored markup lawyers use to see exactly what changed between drafts. You upload the old and new version, and it shows the document rendered as it actually looks, with deleted text struck through in red, inserted text underlined in blue, and paragraphs that moved shown in green. You can then download a real Word file with genuine tracked changes that anyone can accept or reject in their own copy of Word, or save the marked-up view as a PDF. What sets this apart is a built-in correctness check. Before showing you any result, the tool simulates clicking accept all changes and checks that this exactly reproduces the new draft, then simulates reject all changes and checks that it exactly reproduces the old draft. If either check fails, it refuses to show a redline rather than risk giving a wrong one, and this verification runs on every single comparison, not just in testing. It tells you it stores nothing: documents are compared only in memory and then discarded, with no database or upload storage involved. You can run it entirely on your own machine with a single Python dependency, deploy your own copy to Vercel, or use a command line version that works without a browser at all. There is also a simple API that accepts two base64-encoded Word files and returns the tracked-changes file, a colored blackline version, statistics on how much changed, the verification results, and separate reports for changes in footnotes, headers, footers, and formatting-only differences that are not marked inside the Word file itself. The tool compares body text and tables word by word, matching table rows by similarity first. It has some limits: files must be under 1.5 MB each, both drafts need any earlier tracked changes already accepted or rejected before comparing, and documents using Word content controls are refused rather than handled incorrectly. Under the hood it aligns paragraphs by text similarity, does a word-level diff on the parts that changed, and writes the differences as native Word revision marks. The whole comparison engine lives in one lightweight Python file, and the project is released under the MIT license.