Designing a Python Language Server: Lessons from Pyre that Shaped Pyrefly

Pyrefly is a next-generation Python type checker and language server built in Rust, designed as a spiritual successor to Pyre (originally written in OCaml). The post shares architectural lessons learned from running Pyre on massive Python codebases, covering topics like language-server-first design, AST lowering, soundness vs. usability trade-offs, and caching cyclic dependencies.