Scaling code health with always on rewrites and AI

Spotify runs more than 2,900 microservices across 190,000 repositories, and manual migrations stopped working years ago. Jonatan Dahl and Sanjana Seetharam walk through how their platform team applies Java conventions, dependency updates, and security patches across the entire fleet automatically: OpenRewrite for structural refactoring, Error Prone for compile-time checks, and Fleet Shift, Spotify’s homegrown large-scale code change platform, whose Auto Merger and Fire Watch systems build enough trust to auto-merge thousands of PRs. They cover remediating Log4j across 80% of services in a week, and introduce Honk, Spotify’s background coding agent built on Claude, which handles the migrations deterministic recipes can’t reach with a verification loop that runs full CI before a PR ever opens.