Refactoring Codebases at Scale

MerciYanis engineers report rewriting about 250,000 lines across 10 repositories in one day, using roughly 30 parallel agents and 1.6 billion tokens. Their method was to constrain the output rather than review it: contract-style tests through public APIs, a Git check that rejects any modified test file, end-to-end tests, and a handbook of rules. Only two small regressions reached QA. All figures come from the company itself, and the approach relied on unreleased-sounding model versions.