Anthropic rebuilt code review
The team behind Claude Code, which an AI mostly writes, explains how it checks a flood of changes: machines do most of the reading, and people step in where the risk is.
Claude writes about 90% of Claude Code's own code. Merged pull requests more than doubled from June to September, while the share later reverted fell by over 40%. Across Anthropic's monorepo, daily changes went from about 2,500 in May to over 30,000 in September.
Every pull request now gets an AI review: several agents hunt for bugs in parallel, separate verifier agents vote on each finding, and only agreed findings are posted, sorted as important, nit or pre-existing. A separate security pass and required CI follow. Then a classifier sets the risk tier: low risk needs no human approval, medium needs one person and high needs two. Sensitive paths such as billing, the auto-updater and CI workflows always need a person.
The reviewer is itself tested. Past incidents and reverts form an evaluation set that runs daily, and every change to the reviewer is scored against it. Noise is measured by how authors respond to comments: fixed, dismissed or intentional.
54%of internal pull requests now get substantive review comments, up from 16%
Technical details
Convergence numbers after the team tuned the reviewer: pull requests passing after one review-and-fix round rose from 65% to 78%, those needing five or more rounds fell from 8.9% to 1.5%, and mean reviews to pass dropped from 2.08 to 1.32. Safeguards include a three-round cap, diff-only re-review after early passes, flagging bug classes instead of single lines, and never repeating a finding.
About 59% of what the reviewer kept flagging was old code that predated its first pass. Agent approvals never count as human approvals, and approvals apply to one specific commit.
Two files steer it. CLAUDE.md describes the project's stack, conventions and goals and is loaded on every review, so it should stay short. REVIEW.md is written by code owners and says what matters in their area, what failure looks like, what to skip, and to cite file and line as evidence.
Why it mattersAny team using coding agents hits the same wall: more code than people can read. This is a worked answer with numbers, and most of it (a verifier step, risk tiers, a REVIEW.md per area) can be copied at small scale.
For youYour own repos already run an AI review pass; the parts to borrow are the risk tier (which plugin modules always need your eyes) and a REVIEW.md per area. Also the eval set built from your past reverts.
- pull requests
- Proposed changes to a codebase, reviewed before they are merged into the main version.
- reverted
- Undone after merging, usually because the change caused a problem.
- monorepo
- One shared code repository holding many projects.
- CI
- Continuous integration: automatic builds and tests that run on every proposed change.
- classifier
- A model that sorts each item into one of a few categories, here low, medium or high risk.
- evaluation set
- A fixed collection of test cases used to score a system the same way every time.