pull request diff

A pull request diff is a comparison screen that lays the old code next to the new code so you can see exactly what changed. Think of the "track changes" feature in a word processor: deleted lines show up in red, added lines show up in green, so you can spot what's different without rereading the whole file.

In a shared codebase where multiple people edit code together, this comparison screen automatically comes along whenever someone proposes a change through a pull request. Instead of scanning the entire codebase again, other developers or automated tools can look at just this diff to figure out what changed, why, and whether anything still needs fixing.

More recently, AI tools have started treating this diff itself as a self-contained unit of work — using it to write new tests or check for issues based on exactly what just changed.

The article explains that "on 15 tasks targeting a specific pull request diff, the agent passed all of them while stock Copilot passed none." Here, diff doesn't mean the entire new codebase — it refers to the comparison screen that highlights just the part that changed, and the point is that the agent accurately read and responded to that narrow scope.

You can ask a coding agent or AI chatbot: "Summarize what changed in this diff, and tell me if any new tests are needed because of this change." Then paste in a few lines of the actual before/after code on the next line.