pyrefly

The Problem with Python Type Checking

Python's dynamic nature is a double-edged sword. Flexibility accelerates early development but bites back at scale: type errors surface only at runtime, silent mismatches propagate across large codebases, and existing tools struggle to keep up. Running mypy on a 120,000-line project takes roughly nine minutes. Pyright, while faster, consumes over 3 GB of RAM on large packages like NumPy and still clocks in at 70 seconds for a single dependency. At Instagram's scale — 20 million lines of Python — neither tool is viable in a tight feedback loop.

The Solution: Pyrefly

Pyrefly is an open-source Python type checker and language server written in Rust, developed by Meta and battle-tested on the Instagram codebase. It checks over 1.8 million lines of code per second, completing a full pass on NumPy in 4.8 seconds compared to mypy's 8 seconds and pyright's 70 seconds. It reaches over 90% conformance with the Python typing specification and ships as a single binary that handles both static analysis and LSP-based editor features — no separate daemon required.

Unlike Microsoft's Pylance, which is a closed-source extension limited to official VS Code builds, Pyrefly is MIT-licensed and works in any editor: VS Code, Neovim, Zed, and Emacs. Teams using PyCharm Professional for its deep type inference can replace that paid dependency with Pyrefly's LSP, which delivers equivalent in-editor type checking, hover types, go-to-definition, and autocomplete at no cost.

Self-Hosting and Installation

Install with a single command: pip install pyrefly. It also supports uv, poetry, conda, and cargo. Run pyrefly init to generate a config file and pyrefly check to analyze your project. The VS Code extension is available on the marketplace (). Neovim users can wire it up via . No cloud account, no telemetry required — fully self-hosted from day one.