Basilisk

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The open-source Python language server.
The only Python type checker with a perfect 100% score on the official python/typing conformance results.
Complete language server, type checker, debugger, and profiler — strict by default.
VS Code, Cursor & Windsurf (Open VSX) • Zed • Neovim. Built in Rust — single binary, no runtime.

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100.0% PEP conformance — 141 of 141 tests in the official python/typing conformance suite (commit f4f2952), scored on the wheel-installed CLI in its default config by the real upstream harness. We target python/typing@main and ratchet the score up only.

The only 100% checker — and the fastest

Basilisk is the only Python type checker with a perfect score on the official python/typing conformance suite: 100.0% (141/141 files, 970 required errors caught, 0 false positives), measured by the real upstream harness on the wheel-installed CLI in its default config.

And it is the fastest checker we’ve measured — median cold full-file check, from scratch:

Type checker	Median cold check
⚡ Basilisk	12 ms
zuban	27 ms
ty	37 ms
Pyrefly	145 ms
Pyright	568 ms
mypy	588 ms

Median cold full-file check across 26 single-construct typing-spec stress fixtures on an Apple M4 Max — lower is better. Basilisk’s warm re-check drops to ~5 ms. Every figure is produced by hyperfine and committed per machine, so nothing here is hand-typed. Clone the repo, run make bench on your own hardware, and send us the CSV — independent audits are welcome. Full benchmarks & methodology →

Try it

The examples/ folder has ready-to-go Python files:

basilisk check examples/bad.py    # everything flagged
basilisk check examples/good.py   # clean
basilisk check examples/mixed.py  # some errors, some clean
basilisk check examples/          # all three at once

Editors

One extension, the whole workflow: strict-by-default diagnostics, autocomplete, hover, go-to-definition, refactoring code actions, debugging, and profiling. No Node.js or Python runtime — a single Rust binary drives it all.

• VS Code, Cursor & Windsurf — install from Open VSX
• Zed • Neovim 0.10+

Every diagnostic teaches: rustc-style output with a help, a note, and a link to a per-rule explainer. See the full diagnostic reference and the install guide.

Development

cargo build          # build all crates
cargo test           # run all tests
cargo clippy         # lint (zero warnings policy)
cargo fmt            # format

Rust 1.87+ required.

Contributing

Basilisk is built by a human + AI partnership, with the work split on purpose. See CONTRIBUTING.md — For Humans (testing, code-quality review, conformance/security audits, IDE feature parity, sharpening the AI instructions) and For AI (the technical execution, under the standing rules in CLAUDE.md).

Acknowledgments

Basilisk builds on the open-source community — with thanks to:

• Astral — Ruff, whose parser, AST, and formatter crates Basilisk embeds (MIT). The foundation we rely on most.
• typeshed — standard-library type stubs (Apache-2.0).
• Salsa — incremental query engine.
• Rayon — data parallelism.
• tower-lsp — LSP scaffolding.
• debugpy — debug adapter (bundled in the VS Code extension).
• The python/typing conformance suite.

Full component list and licenses: NOTICES. All dependencies are permissively licensed.

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License

MIT.

Built by NIMBLESITE PTY LTD.