Migrating Copilot code review to shared CLI tools made review quality worse until instructions were rewritten

GitHub (Copilot code review users)·GitHub Copilot code reviewFixed

GitHub swapped Copilot code review's purpose-built code-exploration tools for the shared grep/glob/view tools used by the Copilot CLI harness, expecting a clean upgrade, but benchmarks showed reviews got more expensive and caught fewer real issues because the surrounding instructions weren't rewritten for the new tools.

Blast radius

Benchmarked regression in review cost and issue-catch rate during the migration period, caught before broad rollout via internal evaluation rather than by customers.

Root cause

The tool layer was swapped without rewriting the workflow instructions around it, so instructions tuned for the old, more verbose tools didn't match how the new Unix-style tools should be driven.

How it was caught

Internal benchmarking comparing old vs. new tool sets flagged higher cost and lower issue detection before the regression reached general availability.

What changed

GitHub rewrote the tool-use instructions for how a reviewer actually reads a pull request, cutting average review cost by roughly 20% while restoring review quality.

Take-awayWhen migrating an agent to a shared tool harness, the prompts and workflow around the tools are part of the interface — swap them together, not the tools alone.