Dev server SSR do not load ESM version of externalized package #6125

Which project does this relate to?

Start

Describe the bug

If a package has following package.json and no exports is defined:

{
    "name": "my-lib",
    "main": "src/index.cjs",
    "module": "src/index.mjs"
}

Then if it was externalized in vite.config.ts:

// vite.config.ts
const config = defineConfig({
  ssr: {
    external: true,
  },
  // rest...
});

Then when launch the dev server with vite, the SSR resolution will pick the CJS version of my-lib from main field, although we are importing it.

Your Example Website or App

https://github.com/guyutongxue/vite-ssr-cjs-issue

Steps to Reproduce the Bug or Issue

1. bun i and cd packages/my-app and bun dev
2. Launch the App, and you will see the my-lib was imported as CJS and hydrated later to ESM.

The my-lib just export a simple function returning which module type it is; and the app code just import it and render the return value to <App>.

Expected behavior

Expected always ESM with no hydration mistmatch.

Screenshots or Videos

Platform

• Router / Start Version: 1.141.5
• OS: Windows, also reproduced on Mac
• Browser: Edge
• Browser Version: 143.0.3650.80
• Bundler: Vite
• Bundler Version: 7.3.0

Additional context

I'm not sure whether this is a Vite bug because I cannot make a reproducible example code on pure Vite SSR (too low level sorry).

I figured out that TanStack Start run into the code in Vite here:

https://github.com/vitejs/vite/blob/ef3d59648fd9dd3f9b3118d09d216dc0afcb8c33/packages/vite/src/node/ssr/fetchModule.ts#L52

that won't consider ESM module at all.