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You ran vite build on a CRXJS extension project and got: 
[crx:manifest-post] Content script fileName is undefined: "src/content.ts"
The build fails before the manifest is written, and the path in the error points to a file that exists in your project. Here is what it means and how to fix it.

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What the error means

@crxjs/vite-plugin resolves each content script to its emitted output file during the crx:manifest-post step (in generateBundle). If the plugin cannot match your content-script source to an output chunk โ€” because of how that chunk is named or keyed โ€” the filename comes back undefined and the plugin throws. It is not โ€œyour file does not existโ€; it is the plugin and the bundler disagreeing about the emitted chunk for your content script.

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Is this a Vite 8 / Rolldown problem?

Mostly historical. Vite 8 switched its default bundler from Rollup to Rolldown, and CRXJS needed compatibility work for that transition (see #1042 and #1067). That work landed: @crxjs/vite-plugin now supports Vite 8 (#1132). On current versions (@crxjs/vite-plugin@2.7.0 with Vite 8) this error does not reproduce in common setups โ€” including a content script that is also imported with ?script. The original report (#883) was on Vite 5 with SolidStart/vinxi, not Rolldown.

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How to fix it

  1. Update @crxjs/vite-plugin to the latest release (2.7.0 or newer), which supports Vite 8 / Rolldown: 
    npm install @crxjs/vite-plugin@latest --save-dev
  2. Check how the manifest references the script. Use a root-relative path that matches your project, and confirm the same file is not also imported elsewhere with a ?script suffix (double-registration is the most common remaining edge case).
  3. If you cannot upgrade yet, pinning vite@^7 (still Rollup-based) is a temporary unblock until you can move to a current @crxjs/vite-plugin.
If you still hit this on current versions, the maintainers are responsive โ€” open an issue with a minimal reproduction at crxjs/chrome-extension-tools.