---
id: pyrefly
version: "1.2.0"
license: unclear
license_treatment: permissive
maintenance: active
---

# pyrefly — A fast type checker and language server for Python with powerful IDE features

License: permissive · Maintenance: active · Downloads: 6.6M/mo

## What it is and what it does

Pyrefly is a production-grade type checker and language server for Python developed by Meta, designed to handle large codebases at scale. It combines fast type checking with IDE features like code navigation, autocomplete, hover information, and semantic highlighting, available both as a command-line tool and as extensions for VSCode, Neovim, Zed, and other editors. The package includes built-in support for popular frameworks like Pydantic, Django, and pytest.

For teams migrating from other type checkers, Pyrefly provides adoption tools: `pyrefly init` for migration setup, `pyrefly suppress` to silence existing errors during rollout, and `pyrefly infer` to generate type annotations. It has no runtime dependencies, making it straightforward to add to a project. The tool is actively maintained with monthly minor releases and patches for critical fixes.

Use it for:

- Migrate an existing codebase to Pyrefly using `pyrefly init` and suppress errors with `pyrefly suppress` during rollout
- Set up IDE type checking and code completion in VSCode, Neovim, or Zed with consistent results across CLI and editor
- Type-check large Python projects where type-checking speed is a bottleneck in development workflow
- Validate Pydantic models, Django ORM fields, and pytest fixtures with built-in framework support
- Generate initial type annotations for an untyped codebase using `pyrefly infer` to bootstrap type safety

## Worth the install?

AI-flagged interpretation of the facts above — verify before relying.

Pyrefly is a fast type checker and language server for Python that provides IDE features including code navigation, semantic highlighting, and code completion across multiple editors.

Yes. Pyrefly is production-stable, actively maintained, has no dependencies, and offers a permissive license. It is worth installing if you need fast type checking for a large codebase or want IDE integration across multiple editors. The adoption tools make it practical to migrate incrementally. Minor version releases may introduce breaking changes, but the `suppress` command mitigates migration friction.

## Install

pip install pyrefly
uv add pyrefly
poetry add pyrefly

## Installing pyrefly

Before you install: Medium install friction due to platform-specific wheels across multiple architectures (macOS, Linux, Windows, including ARM variants). Active maintenance with a release 13 days ago and no runtime dependencies simplifies deployment.

License in practice: MIT license (permissive) means you can use, modify, and distribute Pyrefly freely with minimal restrictions, making it suitable for both open-source and commercial projects.

Quickstart:

pip install pyrefly
pyrefly --help
# For IDE integration, install the appropriate extension (VSCode, Neovim, Zed, etc.) from your editor's marketplace

Requires Python 3.8 or later. IDE features require a compatible editor extension; the CLI tool works standalone.

Verify before relying:

- Whether type-checking speed claims (1.85 million lines/second, 15x faster) are independently verified
- Compatibility matrix details for each supported IDE extension and minimum version requirements
- Whether monthly release cycle and non-strict semantic versioning means breaking changes occur frequently

## Package facts

- License: not declared (permissive)
- Python support: supports_current
- Install friction: medium
- Maintenance: active
- Downloads: 6.6M/month (top 5,000 on PyPI)
- Known vulnerabilities: none known

## Tags

python type checker, language server protocol python, fast type checking, python ide features, code completion python, semantic highlighting python, python type annotations, type-checking, language-server, ide-integration

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