April 30, 2025

Pyrefly: Meta's New Type Checker for Python

Meta has announced Pyrefly, a new static type checker built in Rust to deliver fast performance and scalability for large Python codebases.

While inspired by Meta’s earlier tool Pyre, Pyrefly is not a rewrite: it’s a clean-slate implementation with a new type inference engine, a custom incremental computation model, and cross-platform support.

Despite the maturity of tools like Mypy, Pyright, and Pyre, Meta found that scaling static analysis across massive codebases required a faster, more flexible foundation. Pyrefly is designed to deliver instant IDE feedback with strong type inference.

This comes right as Astral, the team behind uv and ruff, prepare to release Red Knot, their fast typechecker also written in Rust.