How do I migrate from Mypy?

Run `pyrefly init` at your project root. It reads your `mypy.ini` or `setup.cfg` configuration and generates a Pyrefly config. Then run `pyrefly suppress` to add inline suppressions for existing errors so you start clean without touching legacy code.

Is the CLI type checker the same binary as the language server?

Yes. The language server and CLI share the same type-checking engine. You get consistent results whether you are running a check in CI or getting hover information in your editor.

pyrefly: Lightning-fast type checking for Python

Python type checking with Mypy or Pyright becomes painfully slow on large codebases: a full check of a project like PyTorch can take minutes, breaking the development feedback loop. Language servers with separate type checker and IDE backends produce inconsistent results between the CLI and the editor. Teams adopting type checking incrementally get blocked by the difficulty of migrating existing untyped code without triggering thousands of new errors in code they are not currently working on.

pyrefly is an MIT-licensed Python type checker and language server that competes with proprietary IDE typing experiences such as Pylance in VS Code and PyCharm inspections.

vs Pylance: Pylance gives VS Code users a polished proprietary language server with Python analysis settings built into the Microsoft Python workflow. pyrefly is the better fit when you want the same checker available in CI, the terminal, VS Code, Neovim, Zed, and other editors, with one command-line tool and one set of diagnostics.

vs PyCharm: PyCharm is stronger when a team wants a full commercial Python IDE with inspections, navigation, refactoring, debugger, and JetBrains support in one product. pyrefly is narrower: it focuses on fast type checking, IDE diagnostics, language-server features, and migration from mypy or pyright with the pyrefly init command.

vs mypy or Pyright workflows: pyrefly is most compelling for large Python codebases that need faster checks and consistent CLI plus editor behavior. Existing mypy or Pyright setups may still be safer when plugin compatibility, long-established behavior, or exact diagnostic stability matters more than speed.

No. Pyrefly targets Python 3 and the modern type annotation syntax introduced in PEP 484 and subsequent PEPs. Python 2 is not supported.

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pyrefly

Who pyrefly is for#

Team migrating from Mypy

Developer wanting fast editor feedback

Data science team with Pydantic pipelines

Library author releasing to PyPI

What it's built on#

FAQ#

Similar open-source tools#

deer-flow

Django developer

The problem it solves#

How it solves it#

1.85M lines/second, 15x faster than Mypy

Sub-10ms IDE recheck on save

Full language server for VSCode, Neovim, Zed

Incremental migration from Mypy/Pyright

Built-in Pydantic and Django support

Production-proven at Meta scale

Strengths and trade-offs#

Strengths

Trade-offs

pyrefly vs alternatives#

Install and self-host#

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iroh

orca

CLI-Anything

oh-my-pi