Diff
A list of exactly what changed between two versions: the lines removed, the lines added, and a few around them for context. Removed lines start with a minus sign and added lines with a plus.
An agent's summary is a story it tells afterwards. The diff is the thing itself, and it shows what the summary left out: a deleted test, a changed setting, a file you never asked it to touch. Learning to read one is the first skill of supervising an agent.
Say it to your agent
“Show me git diff --stat main...HEAD first, then the full diff. If you reformatted anything, split it out: I want one commit with spacing changes only, and another with the real change.”