A diff is the list of differences between two versions of a file or a project: which lines were added, which were removed and where. Added lines are usually marked with a plus sign and removed ones with a minus sign. Tools like git use it to show what really changed, without relying on anyone's summary.
A question from the app. Pick an answer.
You ask to rename a function and the diff touches many files. What do you do first?
Renaming touches every place the function is used. Size alone does not condemn it.
Being expected does not tell you nothing else slipped in among so many changes.
A big diff calls for checking its scope. If every change is the rename, fine; if there is something else, that gets discussed.
With made up amounts, in USD: an online store raises the minimum for free shipping. The diff shows "- freeShippingFrom = 50" and right below "+ freeShippingFrom = 60". There are not two new lines: changing a line shows up as removing the old one and adding the new one.
Around it you see a few lines with no sign: they did not change, they are there so you can place the change. Each block starts with a line saying where in the file it is, and many tools color added lines green and removed ones red.
A coding agent finishes with a summary: "I translated the error messages". The summary is what it says it did; the diff is what it did. Checking the red lines in the tests is a good habit: if a test was failing, the agent may have deleted or loosened it so it passes.
A rule in your CLAUDE.md like "do not delete tests to make them pass" helps, but the agent can forget it. The diff does not forget: if a test was removed, there it is.
A big diff is not a bad diff: updating a library or running a formatter can touch dozens of files. The risk is a real change hiding among 2,000 lines of spaces and commas. Start with the file list: if one shows up that has nothing to do with the request, like the payments config, open it first.
Two tricks: ask the agent to keep formatting in a separate change, and use the hide whitespace option that many tools have. That way what stays visible is what matters.
"If the tests pass, I do not need to read it." Tests do not warn you about a change nobody asked for. "The diff tells me it works." No: it tells you what changed. "Everything is in the diff." Almost: a new file git does not track yet may not show up, and a binary one, like an image, shows up with no detail.