`@volar/typescript` mistakenly processes VS Code's untitled files as TypeScript code

[mizdra]

I am developing a TypeScript Language Service Plugin that loads CSS files using @volar/typescript.

In VS Code, you can create a new file via "New File...". Immediately after creation, it is not yet saved to the file system, and this state is called "untitled".

@volar/typescript appears unable to handle these untitled files correctly.

How to reproduce

minimum reproduction: https://github.com/mizdra/repro-volar-untitled-non-ts-file

1. Run commands in your terminal

   git clone git@github.com:mizdra/repro-volar-untitled-non-ts-file.git
   cd repro-volar-untitled-non-ts-file
   npm i

2. Open the workspace in VS Code
3. Run "Start extension in VS Code" debug task (F5)
4. Create untitle file by Command+N
5. Type .foo {} in the file
6. Change language mode CSS

2025-08-26.22.42.14.mov

Additional information

• It appears that tsserver is mistakenly processing the code as TypeScript.
  • If you write TypeScript code, no errors will be reported.
  • 
• This issue occurs in both css-modules-kit and Vue (Official).
  • css-modules-kit: Errors are reported in an untitled CSS file in VS Code mizdra/css-modules-kit#237
  • Vue (Official):
    • 
• The languageId not found warning was output to tsserver.log. It might be related to this issue.
  • The warning comes from Volar.js.

  • volar.js/packages/language-core/index.ts

    Line 57 in 762323e

    console.warn(`languageId not found for ${id}`);

tsserver.log:

...
Info 112  [22:51:56.697] Loading typescript-plugin-css-modules from /Users/mizdra/src/localhost/gomi/repro-volar-untitled-non-ts-file/packages/vscode (resolved to /Users/mizdra/src/localhost/gomi/repro-volar-untitled-non-ts-file/packages/vscode/node_modules)
Info 113  [22:51:56.697] Plugin validation succeeded
Info 114  [22:51:56.698] Starting updateGraphWorker: Project: /dev/null/inferredProject1*
Err 115   [22:51:56.698] languageId not found for ^/untitled/ts-nul-authority/Untitled-1
...