Meta launches Pyrefly, a fast, Rust-based Python type checker with IDE integration

Meta has announced the alpha release of Pyrefly, a new open-source Python type checker and IDE extension developed in Rust. Pyrefly aims to deliver high performance and usability, capable of analyzing 1.8 million lines of code per second, and is designed to support both large-scale codebases and responsive development environments. It provides IDE integration (currently with VSCode), a command-line interface, and features like type inference for unannotated code, with support for inserting inferred types directly in the IDE.

The project builds on Meta’s prior experience with the Pyre type checker, originally written in OCaml to support Instagram’s vast codebase. Pyrefly reimagines type checking from the ground up, focusing on speed, extensibility, and a consistent developer experience across tools. Key design principles include performance, an IDE-first philosophy, and support for partially typed Python code.

The team behind Pyrefly encourages community involvement through GitHub and Discord, and the tool is licensed under MIT. Meta sees Pyrefly as a way to further support the Python ecosystem, improve developer productivity, and drive forward innovations in the language’s type system. The alpha release is available now, with plans to exit alpha later this summer.