`*.svelte.(ts|js)` files are not checked for Svelte compiler warnings (svelte-check / language tools)

[ampled]

Describe the bug

I have been trying to get "Svelte for VS Code" to emit Svelte warnings for *.svelte.(ts|js)-files with no luck.

Example:

// somefile.svelte.ts
let current = $state('')

// Expecting state_referenced_locally warning here
if (current) {
	//
}

The equivalent works in a normal .svelte-file.

Compiler warnings are emitted by vite-plugin-svelte for files like these when running build, but not when running svelte-check.

I noticed this works fine in the Svelte REPL, so I figured I'd missed something simple to get this working in VS Code, but to me it seems like only .svelte-files are picked up by the extension and svelte-check.

I could not find another issue or documentation about this, so sorry if this is a duplicate.

Reproduction

• Create new / blank VSCode profile
• Install "Svelte for VS Code" extension
• Enable TypeScript plugin when extension prompts for it
• Specify project should use workspace TypeScript version (VS Code setting)
• Initialize new SvelteKit Project (add TypeScript, ESLint, Prettier)
• Create file with .svelte.ts extension and add sample code

Sample code in REPL: https://svelte.dev/playground/be495e6d6e6c4c19a200ccead58f2e49?version=latest

Expected behaviour

*.svelte.(js|ts)-files should be checked for compiler warnings in IDE and when running svelte-check.

System Info

• OS: macOS 15.6.1
• IDE: VSCode

Which package is the issue about?

Svelte for VS Code extension

Additional Information, eg. Screenshots