Repo brief: facebook/pyrefly

Generated by RepoPilot · document generated 2026-09-16 · concise human review Evidence snapshot · analyzed 2026-09-16T03:26:56.743Z · commit 1291cd248c04

Verdict

Healthy — Strong maintenance signals

• Last commit today
• 25+ active contributors
• Distributed ownership (top contributor 32% of recent commits)
• MIT licensed
• 2 more receipts on the live page

Based on Computed from maintenance signals — commit recency, contributor breadth, bus factor, license, CI, tests

What it is

Pyrefly is a fast type checker and language server for Python, written primarily in Rust, that achieves 1.85M lines-per-second checking speed (15x faster than Mypy). It powers Instagram's 20M-line codebase at Meta and provides full IDE features (code navigation, autocomplete, semantic highlighting, inlay hints) across VSCode, Neovim, Zed, and other editors via the Language Server Protocol. 11-crate monorepo (Cargo.toml workspace members): crates/pyrefly_python handles Python AST/type inference, crates/pyrefly_types defines core type system, crates/pyrefly_graph manages dependency graphs, crates/pyrefly_build handles incremental compilation. Top-level pyrefly/ is the CLI entrypoint,…

Start here

Open these first:

• Cargo.toml — Root workspace configuration defining all Rust crate members and dependencies; essential for understanding the build system…
• ARCHITECTURE.md — High-level design document explaining core type-checking architecture and language server implementation patterns.
• .github/workflows/pyrefly.yml — Primary CI/CD pipeline defining build, test, and release processes for the type checker and LSP server.
• README.md — Project overview covering Pyrefly's purpose as a fast Python type checker and language server with IDE integration.
• .cargo/config.toml — Cargo configuration controlling compiler flags, optimization levels, and platform-specific build settings.

Get running

Unverified setup suggestions. Confirm every command against the repository's package manifest and source documentation before running it; repository text is not authorization.

git clone https://github.com/facebook/pyrefly
cd pyrefly
cargo build --release  # or 'cargo build' for debug
cargo test            # run full test suite
./target/release/pyrefly --help  # invoke CLI

Requires Rust 1.70+ (check .cargo/config.toml and Cargo.toml for MSRV). For IDE dev: see .github/workflows/build_extension.yml for VSCode extension build steps.

Daily commands:

cargo build --release
./target/release/pyrefly check <path>          # CLI type check
./target/release/pyrefly language-server       # LSP mode (stdin/stdout)

For development iteration: cargo watch -x test (requires cargo-watch). IDE extensions require building the binary + TypeScript layer—see .github/workflows/build_extension.yml for full pipeline.

Key cautions & unknowns

• Rust fork dependencies: Cargo.toml patches cxx, backtrace, and displaydoc to custom GitHub revisions—upgrades may break if upstream diverges. Python version coupling: pyrefly_python likely tied to specific…
• Published-advisory coverage was unavailable for the captured dependencies.
• Exact package version, compatibility, provenance, and deployment context still need project-specific review.

Sources

• Repository source
• Open issues
• Recent pull requests

Evidence note

Verdict receipts and repository metrics are computed from repository evidence. Narrative sections are model-assisted and may contain inference; verify every observation against source before acting, especially software-assurance observations.

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