Symptom
The agent wrote it, the agent reviewed it, both agreed
You noticed
A coding agent wrote the change. A second pass, by the same model or a differently prompted one, approved it. The bug both of them missed was in an assumption both of them share.
The doubt
correlated blind spot. The sensor checks the code against something else: a second implementation, a mock, the old version, another AI model. If that other thing has the same bug, the two agree and the sensor reads green.
Commonly misread as closing it: Differential Testing.
Point this at it
Independent review by one person who did not write the change and did not prompt the agent that wrote it. Choose the reviewer for distance from the author rather than for seniority. Ask for one thing in writing: what does this change do, and what does it break elsewhere? A review that quotes the caller, the schema, or the spec is doing the work; one that reads only the diff is checking the change against itself.
Keep the change small enough that a human can read all of it. A second model, session, or prompt reviewing the same code is a different sensor and does not close this doubt, because the thing it checks against can share the original’s blind spot.
Reading it
- Red: specific objections that name functions, inputs, and failure modes. Reproduce blocking findings before arguing. When author and reviewer disagree about what the code does, at least one of them holds a wrong model, and that disagreement is what you paid for.
- Green: a reviewer who opened the callers found nothing to object to. Judge it by the specificity of what they wrote; “looks good” is not a reading.
- It does not prove anything if the approval came in ninety seconds on a forty-file diff. Time from request to approval, per reviewer, tells you whether a review happened.
If the doubt survives
If a careful stranger reads the change against its callers and agrees, and it is still wrong, the shared belief is probably about what the system should do. That is wrong specification, and the same reviewer can reach it only by checking the change against the requirement rather than against the author’s description of it.