Microsoft's PRAssistant number is real, and it's a floor, not a promise

Microsoft published the rare thing in this space: a first-party, attributed throughput number for AI code review. PRAssistant ran across 5,000 internal repos, covered 90%+ of PRs and 600K+ pull requests a month, and measured a 10–20% median improvement in PR completion time. Then it shipped externally as GitHub Copilot code review.

That's worth slowing down on, because "median PR completion time" does real work in that sentence and people keep reading past it.

What that metric captures is a throughput proxy. Code review is a queue: reviewers pick up a PR, read it, and merge it. Automation that screens for the obvious problems and unblocks trivial merges moves a strict majority of the queue much faster, which is exactly what a median improvement shows. It does not measure that the code is better. A reviewer who trusts the bot and merges faster, on code no human actually reasoned about, gets a faster merge and a worse product. The number can't tell the difference.

That's not a critique of Microsoft. It's a caution for anyone who quotes the 10–20% as an argument that the tool catches more bugs. It changes when work starts, not necessarily what lands.

The other part worth keeping is the scale language. 90%+ of PRs and 600K a month means they ran this only where it could keep up. PRAssistant is free to be picky about what it reviews because Microsoft's review volume is gigantic and the automation is boring; it wins by being available on almost everything. Then the rollout became Copilot code review, so the number is best read as a floor. What it did at Microsoft's volume is not a ceiling for what a smaller team should expect.

The practical habit, if your team is about to adopt one of these: track review throughput and signal quality separately. Watch median time-to-merge on your side, sure. But also watch how many merged PRs came back with a regression or a follow-up fix six weeks later. One tracks speed, the other tracks whether the speed is real. Both are easy to measure, and nobody should sell you a number that keeps only one of them.