seendiff

seendiff is a local code diff viewer that helps you review large diffs without losing track of what you've already seen. It runs a lightweight web server on your machine, so nothing leaves your computer. The tool is designed for developers who work with big changes—files with thousands of lines scroll smoothly thanks to virtual rendering. You get a minimap on the side of each file that shows you at a glance which blocks are seen, reviewed, or still waiting.

What makes seendiff different is how it handles review progress. As you scroll through a diff, code blocks are automatically marked as 'seen'. You can then explicitly mark blocks as 'reviewed' using keyboard shortcuts (R for a block, f for a whole file). The review state survives a rebase, an amend, or even closing the browser window—so you can pick up where you left off the next day. The tool is also fully keyboard-driven: arrow keys jump between changes, n and N navigate to the next unreviewed block, and s toggles between side-by-side and unified views. There's also a cross-file search that shows matches in all changed files at once.

seendiff is especially useful for teams that use AI coding agents. It ships with a walkthrough mode where an AI agent can explain the changes it made, step by step, directly on the diff. This makes it easier to understand and trust AI-generated code. The tool is free, open source, and installs in seconds with a single pip command. If you review code regularly—whether human-written or AI-generated—seendiff gives you a clean, focused environment to do it.