Every diff has a blind spot: the one who made it sees it worst. This is true both for you and for the agent that wrote the code - both have already made a set of decisions and tend to justify those same decisions. A fresh look, tied to nothing, catches what the author rationalizes and skips.
The naive move is to read your own diff once and commit, and if an agent wrote the code, to rely on it having written the tests too. The double work seems unnecessary: the changes are in plain sight, the tests are green, what else is there to look at. It is on exactly this complacency that regressions ride into the repository.
It breaks on the nature of author blindness. The same context that produced the error also reviews it: the agent that wrote the code judges it by the same assumptions the bug grew from. Tests written together with the code check what the author considered important - and stay silent precisely where he did not think. A check from inside one's own context systematically misses a certain class of slips.
The professional mechanism is Quick Review: a dedicated review subagent available for the Devin Local agent and not supported in legacy Cascade. It runs an agentic review of local changes, analyzes the diff and gives feedback right in the editor. It is worth running after your own check and before a commit or PR. Give it the context of the goal and separately ask it to look for regressions, security issues, missing tests and extra scope - then the look is targeted rather than abstract. The feedback lands right in the editor next to the changes, so each item is tied to a concrete place in the diff rather than hanging as a separate list.
Why a separate agent rather than one more pass by the same one. The review subagent has fresh context and no stake in this code: it does not defend the decisions taken, because it did not take them. On top of that, you can choose a different model for the review than the one that wrote the code - the second look turns out to be a second opinion, not an echo of the first. This is a cheap form of diversity: two different mechanisms err differently, and what one context missed the other is more likely to notice.
The choice of review model is a live source, and it should be treated as a snapshot. At the date of this snapshot the documentation listed three options in the dropdown: SWE-check - a fast, lightweight model for common problems, free; GPT 5.5 and Opus 4.7 - frontier models for deep agentic review, billed by tokens. The catalog and billing may change, so the real list is always read in the selector itself, not in this line. And the billing of a specific model is checked before the run, not assumed known from memory.
Hence the cost. A light free model suits frequent small checks and catches common issues; a frontier one costs tokens and is justified on a complex or risky diff, where depth matters more than speed. Running an expensive model on a one-line edit is paying for depth where there is nothing to apply it to. A reasonable default is to lean on the light model and switch on a frontier one pointwise, when the cost of a mistake in this diff outweighs the cost of tokens.
Quick Review is justified on a non-trivial diff before a commit or PR, especially if the changes touched a public contract, security, or edges that are easy to forget. On a one-line edit with an obvious effect it is overkill - it is cheaper to read the diff by eye. As with the choice of surface, the power of the review is matched to what actually changed. A special point of the review appears where the diff was written by an agent: a fresh look compensates for the author and the checker here being one and the same process.
You must check not only the code but the status of the review itself, because a review is not a gate by itself. Any finding is a hypothesis confirmed by code or a test, not a verdict; and conversely, the absence of findings does not prove correctness but only says this look noticed nothing. Quick Review adds a lens, it does not issue a green light, and the readiness decision stays with the human and the checks. In practice this means: a finding is first reproduced, then fixed, and a green run after the fix is the proof the review itself does not provide.
The typical failures grow from here. The first is taking "no findings" for proof of correctness and committing the unverified. The second is rushing to fix by a finding without reproducing the problem, and spending time on a false alarm or breaking what worked. The third is choosing an expensive model on a trivial diff and paying for depth with nowhere to apply it. The sign of all three is the same: the review result was taken for a verdict rather than a hint still to be grounded.