Formatting an entire 25 million line codebase overnight: the rubyfmt story

Summary

Stripe’s 25-million-line Ruby codebase needed a true autoformatter; before rubyfmt, no industry-wide Ruby formatter existed, and earlier tools had crashed on its files. rubyfmt began as a zero-configuration open-source project motivated by formatting disputes, Ruby’s slow startup, and the uncertainty faced by engineers arriving from other languages. To meet a 100ms target, it initially used a hand-authored C launcher and later moved formatting into Rust, with ripper parse trees first passing through JSON and eventually being deserialized directly from Ruby VALUE objects. Stripe adopted per-file opt-in, expanded coverage cautiously, and formatted the codebase on a Saturday in 2024; the excerpt reports that all 42-million-line Ruby codebase is now formatted. In 2025, migrating from ripper to Prism removed the need to link a Ruby VM and made rubyfmt smaller and dramatically faster.

Takeaways

1. 1

   rubyfmt targeted formatting latency below 100ms for all but the very largest files, avoiding normal Ruby and Bundler startup paths.

2. 2

   Stripe reduced rollout risk with per-file opt-in, gradual expansion, ripper-tree diffs, and a Saturday-wide formatting change intended to avoid merge conflicts.

3. 3

   Prism’s native parse trees removed rubyfmt’s serde and object-deserialization layers, shrinking the binary by megabytes and making it dramatically faster.