SvelteKit Remote Functions Sourcemap Bug

Minimal reproduction for a bug where vite-plugin-sveltekit-remote produces broken sourcemaps.

The plugin's transform hook modifies .remote.* files but doesn't return a map property, causing Vite to lose the sourcemap chain entirely. The resulting .map files have sources: [], empty sourcesContent, and semicolon-only mappings.

Reproduce

npm install

1. See the bug

npm run build:bug

You'll see warnings like:

[plugin vite-plugin-sveltekit-remote] Sourcemap is likely to be incorrect: a]plugin (vite-plugin-sveltekit-remote)
failed to generate a sourcemap for src/lib/example.remote.ts with id src/lib/example.remote.ts.

Inspect the broken sourcemap:

cat .svelte-kit/output/server/chunks/example.remote.js.map

You'll see "sources":[], "sourcesContent":[], and only semicolons in "mappings".

2. See the fix

npm run build:fix

No warnings. Inspect the fixed sourcemap:

cat .svelte-kit/output/server/chunks/example.remote.js.map

Now has proper sources, sourcesContent, and real VLQ mappings.

What the patch does

See patches/@sveltejs+kit+2.53.4.patch:

• SSR path: Uses MagicString (already a dependency) instead of bare string concatenation, returns { code, map } with a proper sourcemap
• Client path: Adds map: { mappings: '' } to signal an intentional full replacement (no original code preserved)

Key files

• src/lib/example.remote.ts — Remote function file (the file that triggers the bug)
• src/routes/+page.svelte — Page that imports the remote function
• svelte.config.js — Enables experimental.remoteFunctions
• vite.config.js — Enables build.sourcemap (required to generate .map files)
• patches/@sveltejs+kit+2.53.4.patch — The fix