The diff is a summary. The session trace is what actually happened.

Task: refactor the auth module. Agent ran 34 minutes, 51 logged actions. PR diff showed 46. Hence 5 actions never appeared in the review surface. The five: a file modification adding an undeclared key, a package version bump, a debug log written to a folder, reads on three files outside the task scope including two from the billing module, and a shell command not in the original task plan. The PR looked clean, but nobody pulled the session trace before merging. This is not a "broken agent" problem. The agent completed the task. But the diff is the output surface, not the evidence layer. The session trace is the evidence. Most review workflows review the output. Almost nobody looks at the full session record. From a Sonar survey this year: 96% of developers don't fully trust AI output, but only 48% always verify before committing. The other 52% are trusting the diff. The uncomfortable part is not that 5 actions went unreviewed. It is that you cannot be certain it was the first time. You just happened to look this once.