Meta is excited to announce the alpha version of Pyrefly, an open source Python type checker and IDE extension developed in Rust. This project aims to improve the developer experience by providing a static type checker that analyzes Python code for consistency and helps catch errors before they run.
Pyrefly is designed to work seamlessly with popular IDEs like VSCode and offers CLI usage for flexibility in incorporating it into workflows. The open source community plays a vital role in shaping the project, and Meta invites contributions to improve Pyrefly’s performance, inference, and user experience.
Key principles guiding Pyrefly include performance, IDE-first design, and inference, which enable users to benefit from types even if they haven’t annotated their code. As an open source project, Pyrefly is available on GitHub under the MIT license and encourages pull requests and issue reports.
This alpha version marks the beginning of a journey to improve Python developers’ experience with types. Meta plans to share learnings and tooling through blogs and language enhancements, aiming to build a community around Pyrefly. The project’s success relies on user feedback, so please try it out and report any bugs or suggestions for improvement.
Source: https://engineering.fb.com/2025/05/15/developer-tools