Ruff errors incorrectly reported on unsaved Markdown files

[OmeGak]

Ruff produces Python linting diagnostics on untitled (unsaved) tabs whose language is set to Markdown. Any text content triggers errors since Ruff parses the entire document as Python source code. This appears to be a regression introduced by #950, which added onLanguage:markdown to the activation events.

Steps to reproduce

1. Open a new untitled tab (Cmd+N)
2. Set the language mode to Markdown (via the language selector in the status bar)
3. Type any plain text, e.g. hello
4. Observe Python diagnostics reported on line 1 (e.g. F821 Undefined name 'hello', syntax errors, etc.)

Expected behavior

No diagnostics. The file is a Markdown document, not a Python file. Ruff correctly handles saved .md files (only linting inside ```python fences) and the same should apply to unsaved Markdown tabs.

Actual behavior

Ruff parses the entire document as Python source and reports errors on arbitrary prose content.

Root cause hypothesis

For untitled: URIs there is no file extension to infer the language from. Ruff's native server appears to fall back to treating the document as Python rather than reading the languageId (markdown) from the LSP textDocument/didOpen notification. Saved .md files work correctly because the .md extension provides a reliable signal.

Environment

• Extension version: 2026.38.0
• Ruff version: v0.15.7 (bundled)
• VS Code version: 1.115.0
• OS: macOS

[MichaReiser]

I tried to reproduce this with Ruff 0.15.7 and main but failed. I did see syntax errors when changing the document type to Python, but they disappear as soon as I select Markdown. This is an unsaved text document.

I created a pyproject.toml file and set preview = true.

Would you mind sharing the output from Output Panel, in the drop down select Ruff as well as the output when selecting Ruff language server? see https://github.com/astral-sh/ruff-vscode#troubleshooting

[OmeGak]

It turns out that this was happening only outside of VSCode workspaces. The Ruff extension was falling back to a globally installed ruff which was very outdated (v0.9). I upgraded the globally installed ruff and the problem is gone.