FaulltLine

FaultLine helps developers and coding agents prove a fix actually works before shipping it. Instead of relying on basic unit tests, a real headless browser explores a preview build, catches runtime failures, and outputs reproduction steps alongside a deployment-blocking exit code. From there, Meta’s model proposes a source patch that we run in an isolated sandbox against independent browser and business-logic checks. Our demo walks through a broken checkout flow that gets patched and verified against 44 assertions, showing why a fix that returns a 200 OK can still violate business rules. Developers can inspect the exact diff, screenshots, logs, and verification results before merging. Right now, the prototype handles single-file repairs for this checkout scenario.