Better tools made Copilot code review worse. Here's how we actually improved it.

Give an agent better tools and it should do better work. That’s the instinct, anyway.

When you open a pull request, Copilot code review reads the diff and explores the surrounding code to find the problems that matter before they ship. To do that, it used its own code exploration tools. So when we swapped in the better-maintained, shared tools that power the Copilot CLI, grep, glob, and view, we expected a clean upgrade.

Instead, in our benchmarks, we found that the cost of reviews was higher and fewer issues were being caught.

But the tools weren’t the problem. The instructions were. Once we rewrote them for the way a reviewer actually reads a pull request, the regression flipped into a win: roughly 20% lower average review cost, while maintaining the same review quality.

This is the story of how adjusting the workflows around the tools led us to a fix.