Modifications to the server.proxy in the plugin API configureServer did not take effect

[heggria]

Describe the bug

I tried writing a plugin to switch server.proxy in the development environment for dynamically changing the proxy, but it didn't seem to actually switch during runtime. I tested switching other configurations of the server, such as server.port, and they worked fine.

Reproduction

https://stackblitz.com/edit/vitejs-vite-mqmnh4?file=vite-plugin-proxy-switch.ts

Steps to reproduce

Clicking the button at the bottom right of the preview switches the server.proxy configuration, and clicking the count button triggers a fetch request. By checking the request domain and the identifier in the request headers in the console, you can see that the server.proxy configuration switch did not take effect. However, directly modifying the useFirstProxy variable in the code allows the switch to work as expected.

System Info

System:
    OS: macOS 14.4
    CPU: (8) arm64 Apple M3
    Memory: 79.53 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.20.4 - ~/.nvm/versions/node/v18.20.4/bin/node
    Yarn: 1.22.22 - ~/.nvm/versions/node/v18.20.4/bin/yarn
    npm: 10.7.0 - ~/.nvm/versions/node/v18.20.4/bin/npm
    pnpm: 9.7.0 - ~/.nvm/versions/node/v18.20.4/bin/pnpm
  Browsers:
    Chrome: 128.0.6613.120
    Safari: 17.4

Used Package Manager

pnpm

Logs

No response

Validations

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• Read the Contributing Guidelines.
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• The provided reproduction is a minimal reproducible example of the bug.

[stackblitz]

Start a new pull request in StackBlitz Codeflow.

• vitejs-vite-mqmnh4 (Open reproduction)

[sapphi-red]

We don't support modifying the ResolvedConfig.