Text Diff Viewer

A diff shows precisely what changed between two versions of text. This viewer performs a line-by-line comparison and labels every line as unchanged, added, or removed, using a longest-common-subsequence alignment so that insertions and deletions line up sensibly instead of showing every line as different.

How you'll use it. Paste an original and a changed version to see the delta. It is the fast way to compare two config files, two API responses, two policy documents, or a before/after of any text without setting up a repository. When you are diffing JSON, run both sides through the JSON formatter first so consistent indentation reduces the change to the fields that actually differ. Because it runs locally, you can safely diff sensitive material.

How the alignment works. Rather than comparing line 1 to line 1, line 2 to line 2, the tool finds the longest sequence of lines the two versions share and treats everything else as inserted or deleted. That is why adding a single line at the top does not mark the whole file as changed — the common lines still align. The summary counts of added, removed, and unchanged lines give you the shape of the change at a glance. This is the same longest-common-subsequence idea behind git diff and diff -u; the difference here is that it is line-level and self-contained, with no repository, staging, or tooling to set up — paste, read, done.

Common mistakes when interpreting a diff. Whitespace and line-ending differences (spaces vs tabs, CRLF vs LF) show up as changes even when the visible text looks identical — a frequent surprise when comparing files created on Windows and Unix. And a line that was modified appears as one removed line plus one added line, since a line-level diff has no concept of an in-place edit.

Defensive angle. Diffing is a core security-review technique. Reviewing the exact diff of a dependency update, an infrastructure-as-code change, or a configuration deployment is how you catch a malicious or accidental modification — a slipped-in backdoor, a loosened firewall rule, a disabled check — before it ships. In incident response, diffing a known-good baseline against a current file reveals tampering. Make "read the diff, not just the result" a habit for anything security-relevant.