Introduction

Basilisk is a complete Python language server and VS Code extension that replaces Pylance and Pyright. Everything Pylance does — autocomplete, go-to-definition, hover information, refactoring, diagnostics — Basilisk does too, fully open source and strict by default.

It is not just a type checker. It is a feature-complete LSP that works in VS Code, Neovim, Emacs, and any editor that speaks the Language Server Protocol. No proprietary extensions. No Node.js. A single Rust binary.

The problem Basilisk solves

Pylance is the most-used Python extension in VS Code. It is also proprietary — you cannot inspect, modify, or redistribute it. Pyright, the open-source type checker underneath, is powerful but is only a type checker — it does not provide completions, hover, go-to-definition, or refactoring without the proprietary Pylance wrapper.

Every other Python type checker (mypy, ty, Pyrefly) defaults to gradual typing. Untyped code passes silently. Any spreads through type inference without warning. Strictness is something you must deliberately opt into, configure, remember to enforce in CI, and re-explain to every new team member.

Basilisk takes a different position. It replaces the entire Pylance stack — type checking, language features, and VS Code integration — with an open-source alternative that is strict by default. Type annotations are contracts, not documentation.

What Basilisk is

• A full-featured language server (LSP) — autocomplete, go-to-definition, hover, find references, rename, code actions, inlay hints
• A VS Code extension that replaces Pylance — install it, disable Pylance, and everything works
• An integrated debugger — press F5 to debug Python with breakpoints, stepping, variable inspection, and watch expressions, all brokered through the Basilisk LSP
• A strict-by-default type checker — no --strict flag, no gradual mode, no opt-in
• A CLI tool for CI integration — exits with code 1 when errors are found
• A migration assistant that reads your existing pyrightconfig.json or mypy.ini
• Written in Rust — ships as a single binary with no runtime dependencies

What Basilisk is not

• Not a compiler — your Python code runs on CPython as normal
• Not a runtime type checker — analysis happens statically at development time
• Not a Mojo dependency — Basilisk's ownership annotations work with standard Python today

One mode only

Basilisk has a single operating mode. There is no --basic, --standard, or --permissive flag. This is intentional.

When strictness is opt-in, teams drift toward permissive defaults. Deadlines arrive. Technical debt accumulates. The --strict flag never gets added to the CI script. Basilisk removes that possibility entirely.

Opting out is still possible — for legacy directories, with per-path configuration and an optional deadline after which the relaxation expires:

[tool.basilisk.per-path-overrides."legacy/**"]
strict = false
deadline = "2026-12-31"

This acknowledges that large codebases cannot be fully typed overnight, while ensuring that the permissive period has an expiration date.

Mojo-inspired safety

Basilisk adds Mojo-inspired ownership semantics as static analysis annotations over standard Python syntax. Using Annotated from the typing module, you can declare that a parameter is:

• Borrowed — a read-only reference; mutation is a type error
• InOut — a mutable reference; must be explicitly declared
• Owned — ownership is transferred; use after transfer is a type error

These are not runtime constructs. They are statically checked annotations. Code that passes Basilisk's ownership checks is structurally compatible with Mojo's type expectations.

Project status

Basilisk is currently at v0.1.0 — the core checker, LSP server, and VS Code extension are all working. Autocomplete, go-to-definition, hover, diagnostics, and inlay hints are shipping today.

Next steps

• Install Basilisk — build from source or install via cargo
• Quick Start — your first type check in under 5 minutes
• Debugging — set breakpoints, step through code, inspect variables
• All Rules — browse every BSK-E and BSK-W diagnostic code