Introduction

Basilisk is a complete, open-source Python language server. Everything you rely on a modern Python extension for — autocomplete, go-to-definition, hover information, refactoring, diagnostics, integrated debugging, profiling — Basilisk does too, fully open source and strict by default.

It is not just a type checker. It is a feature-complete LSP with first-class extensions for VS Code, Zed, and Neovim — plus any other editor that speaks the Language Server Protocol. Cursor and Windsurf (via Open VSX) are coming very soon, and JetBrains is on the way. No proprietary extension, no Node.js — a single Rust binary, the same experience in every editor.

The problem Basilisk solves

Pylance is the default Python language 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 brings the whole stack — type checking, language features, debugging, and profiling — into a single open-source tool that is strict by default and runs the same in every editor, not just VS Code. Type annotations are contracts, not documentation.

What Basilisk is

• A full-featured language server (LSP) — autocomplete, go-to-definition, hover, find references, rename, a full refactoring suite, code actions, inlay hints
• Editor extensions for every major IDE — VS Code, Neovim (0.10+), and Zed today; Cursor and Windsurf (via Open VSX) coming very soon, and JetBrains (IntelliJ / PyCharm) on the way
• Enrichment fixes — one-click code actions that add the missing type annotations for you
• An integrated debugger — press F5 to debug Python with breakpoints, stepping, variable inspection, and watch expressions, all brokered through the Basilisk LSP
• An integrated profiler — CPU profiling via py-spy with inline heatmap annotations, flamegraphs, memory leak detection, and reference graph visualization, all inside your editor
• 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
• uv integration — workspace detection, lock file parsing, and package management commands
• 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 tied to one editor — the same server powers VS Code, Cursor, Windsurf, Zed, and Neovim

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, you can disable or soften specific rules per path:

[tool.basilisk.per-path-overrides."legacy/**"]
disabled = ["BSK-E0011"]        # turn a rule off for legacy code
rules."BSK-E0010" = "warning"   # or just soften its severity

This acknowledges that large codebases cannot be fully typed overnight, while keeping the relaxation explicit and scoped to the paths that need it.

Project status

Basilisk is currently in alpha — the core checker, LSP server, and editor extensions are all working. Autocomplete, go-to-definition, hover, diagnostics, inlay hints, refactoring, debugging, and profiling are shipping today.

Architecture

Basilisk is a Cargo workspace with 16 Rust crates, each owning one layer of the system:

Next steps

• Install Basilisk — Homebrew, Scoop, your editor's marketplace, or build from source
• Quick Start — your first type check in under 5 minutes
• Refactoring — the full refactoring suite (extract, inline, move, rename, convert)
• Debugging — set breakpoints, step through code, inspect variables
• Profiler — CPU heatmaps, flamegraphs, memory leak detection, and reference graphs
• All Rules — browse every BSK-E and BSK-W diagnostic code