ty does not respect cell-local private variable scope

[Antyos]

Describe the bug

I can't think of a better name for this.

There are two cases I've noticed:

1. Variables in cells placed before they are defined will have red squiggles under them until the cell where they are defined is run. However, I would expect that Marimo would know about the variable and not require running the cell for the error to go away.
2. Undefined private variables will not show an undefined name error if they are declared in a different cell (which is not how Marimo works).

See, for example:

Environment

Details

marimo VS Code Extension Diagnostics
=====================================

Language Server (LSP):
	UV Bin: Bundled (c:\Users\avg170002\.vscode\extensions\marimo-team.vscode-marimo-0.13.3-win32-x64\bundled\libs\bin\uv.exe)
	UV: 0.11.3 (45da18ac3 2026-04-01) ✓
	Using bundled marimo-lsp via uvx

Python Extension:
	Interpreter: c:\Users\avg170002\Code\project\code\.venv\Scripts\python.exe
	Version: 3.13.9.final.0
	Environment: VirtualEnvironment (biaxtic)

Python Language Server (ty):
	Status: running ✓
	Version: 0.0.26 (940305127 2026-03-26)
	Binary: UvInstalled (c:\Users\avg170002\AppData\Roaming\Code\User\globalStorage\marimo-team.vscode-marimo\libs\bin\ty.exe)
	Python: c:\Users\avg170002\Code\project\code\.venv\Scripts\python.exe (3.13.9.final.0)

Ruff Language Server:
	Status: running ✓
	Version: 0.15.16
	Binary: CompanionExtension/bundled (charliermarsh.ruff, c:\Users\avg170002\.vscode\extensions\charliermarsh.ruff-2026.48.0-win32-x64\bundled\libs\bin\ruff.exe)

Extension Configuration:
	Version: 0.13.3 
	UV integration disabled: false 

System Information:
	Host: desktop 
	IDE: Visual Studio Code 
	IDE version: 1.124.0 
	Platform: win32 
	Architecture: x64 
	Node version: v24.15.0 

Common Issues:
	1. If notebooks won't open:
		- Check Python interpreter is selected
		- Ensure marimo is installed
		- Check 'marimo-lsp' output channel for errors
	2. If features are missing:
		 - Ensure marimo version is >= 0.23.3
		- Try reloading the window

Steps to reproduce

Example notebook

import marimo

__generated_with = "0.23.6"
app = marimo.App()

with app.setup:
    import marimo as mo


@app.cell
def _(x):
    print(x)  # <- Has red squiggles until next cell is run
    #         # ty(unresolved-reference): Name `x` used when not defined
    #         # ruff(F821): undefined name 'x'
    return


@app.cell
def _():
    _x = 3
    x = 3
    return (x,)


@app.cell
def _():
    print(_x)  # <- Should have red squiggles
    print(_y)  # <- Has red squiggles
    return


if __name__ == "__main__":
    app.run()

[manzt]

Thanks for the report! Two things here:

• print(x) looks like you're on an older bundled ty; the forward-reference (public var defined in a later cell) is already fixed upstream. I bumped in Upgrade ty and ruff binaries to latest versions #605, so it'll clear up next release.
• print(_x) / print(_y) the underscore names are cell-local in marimo, but ty sees the whole notebook as one scope, s_x resolves (no squiggle, though it shouldn't be in scope) while undefined _y gets flagged.

That same whole-notebook view is what makes the x case work, so I don't have a clean fix for the underscore case yet. Leaving it open as a known limitation.