A fresh agent
A new session with none of the conversation that produced the code, handed the diff and the spec and asked to judge it. It has no history to defend and no memory of why any of this seemed like a good idea.
This has no real equivalent before agents. You could never make a colleague forget a week of context on demand. Now a stranger who has read nothing is one command away and costs a few cents, so there is little excuse for letting an author mark its own work.
Say it to your agent
“Open a new session and run: claude -p "Read docs/spec.md and the output of git diff main...HEAD. You did not write this. For each defect, quote the line and give the input that breaks it. If you find none, say so."”