---
title: "Formatting an entire 25 million line codebase overnight: the rubyfmt story"
description: "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."
---

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

[Stripe](https://yomu.fyi/company/stripe) · Fable Tales · Apr 28, 2026

**Type:** Problem & solution

## 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.

## Context

Ruby lacked a true autoformatter, while existing approaches could crash on Stripe’s files. Configurable linting also left room for style disagreements, and Ruby’s startup and Bundler overhead made format-on-save workflows too slow. Stripe additionally needed to manage merge-conflict and correctness risks when applying formatting across a very large codebase.

## Approach / What changed

rubyfmt pursued zero configuration and a strict 100ms formatting budget. It used a hand-authored C launcher, later rewrote the formatter in Rust, optimized ripper parse-tree handling from JSON serialization to direct Ruby VALUE deserialization, and introduced per-file opt-in before broad rollout. The project later migrated from ripper to Prism, which can build parse trees without linking a Ruby VM.

## Takeaways

- rubyfmt targeted formatting latency below 100ms for all but the very largest files, avoiding normal Ruby and Bundler startup paths.
- 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.
- Prism’s native parse trees removed rubyfmt’s serde and object-deserialization layers, shrinking the binary by megabytes and making it dramatically faster.

**Tags:** [Developer Experience](https://yomu.fyi/topic/developer-experience), [Open Source](https://yomu.fyi/topic/open-source), [Performance](https://yomu.fyi/topic/performance), [Refactoring](https://yomu.fyi/topic/refactoring), [Rust](https://yomu.fyi/topic/rust)

- Source: [Stripe](https://stripe.dev/blog/formatting-an-entire-25-million-line-codebase-overnight-the-rubyfmt-story)
- Source URL: https://stripe.dev/blog/formatting-an-entire-25-million-line-codebase-overnight-the-rubyfmt-story
- Ingested by Yomu: 2026-08-28T20:00:48.210Z

[Read original post](https://stripe.dev/blog/formatting-an-entire-25-million-line-codebase-overnight-the-rubyfmt-story)