Dev server will return index when file not exists

[tocque]

Describe the bug

request a not existed file will receive /index.html.

Reproduction

https://stackblitz.com/edit/vitejs-vite-vlvqcb?file=index.html&terminal=dev

Steps to reproduce

just visit https://vitejsvitevlvqcb-s5sf--5173--28a8ce1b.local-corp.webcontainer.io/aaa.js

System Info

System:
    OS: Windows 11 10.0.22635
    CPU: (8) x64 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
    Memory: 2.30 GB / 15.84 GB
  Binaries:
    Node: 22.3.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.19 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 10.8.1 - C:\Program Files\nodejs\npm.CMD
    pnpm: 9.7.1 - ~\AppData\Roaming\npm\pnpm.CMD
  Browsers:
    Edge: Chromium (127.0.2651.8)
    Internet Explorer: 11.0.22621.3566
  npmPackages:
    @vitejs/plugin-react: ^4.3.1 => 4.3.1 
    vite: ^5.4.5 => 5.4.5

Used Package Manager

pnpm

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to vuejs/core instead.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

[stackblitz]

Start a new pull request in StackBlitz Codeflow.

• vitejs-vite-vlvqcb (Open reproduction)

[hi-ogawa]

I would assume this is how SPA fallback works normally https://vitejs.dev/config/shared-options.html#apptype
For example, you can configure appType: "mpa" to avoid such fallback https://stackblitz.com/edit/vitejs-vite-gfcunr?file=vite.config.ts

[sapphi-red]

Yep, this is an expected behavior.