dev-server: bypass + [vite] http proxy error: Must provide a proper URL as target · Issue #26198

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The previous version in which this bug was not present was

works fine with the webpack-builder

Description

When using bypass in the proxy config, no (proxy) target should be needed. However, if no target is specified, the viteProxyMiddleware reports an error on the command line. Besides this, everything works as expected. However, such an unnecessary error is very confusing and distracts from other console outputs.

Also, the webpack-based dev-server does not show such an error.

Minimal Reproduction

Assume this proxy-config in a file proxy.mjs :

export default {
  '/': {
    "bypass": function (req, res, proxyOptions) {
      if (req.url === '/ping') {
        res.setHeader('Content-Type', "text/plain");
        res.end("pong");
        return true;
      else {
        return req.url;
There is no need for a target. In such cases, there might not even be a target.
Starting ng serve --proxy-config proxy.mjs with the webpack-based dev-server works fine. When starting it with the esbuild-based dev-server an exception occurs on the command line:
[vite] http proxy error: Must provide a proper URL as target
Exception or Error
Error: Must provide a proper URL as target
    at ProxyServer.<anonymous> (file:///C:/temp/ping/node_modules/vite/dist/node/chunks/dep-3b8eb186.js:63692:36)
    at viteProxyMiddleware (file:///C:/temp/ping/node_modules/vite/dist/node/chunks/dep-3b8eb186.js:64005:23)
    at call (file:///C:/temp/ping/node_modules/vite/dist/node/chunks/dep-3b8eb186.js:50559:7)
    at next (file:///C:/temp/ping/node_modules/vite/dist/node/chunks/dep-3b8eb186.js:50503:5)
    at cors (file:///C:/temp/ping/node_modules/vite/dist/node/chunks/dep-3b8eb186.js:51031:7)
    at file:///C:/temp/ping/node_modules/vite/dist/node/chunks/dep-3b8eb186.js:51067:17
    at originCallback (file:///C:/temp/ping/node_modules/vite/dist/node/chunks/dep-3b8eb186.js:51057:15)
    at file:///C:/temp/ping/node_modules/vite/dist/node/chunks/dep-3b8eb186.js:51062:13
    at optionsCallback (file:///C:/temp/ping/node_modules/vite/dist/node/chunks/dep-3b8eb186.js:51042:9)
    at corsMiddleware (file:///C:/temp/ping/node_modules/vite/dist/node/chunks/dep-3b8eb186.js:51047:7)
Your Environment
Angular CLI: 16.2.9
Node: 20.9.0 (Unsupported)
Package Manager: npm 10.1.0
OS: win32 x64
Angular: 16.2.11
... animations, common, compiler, compiler-cli, core, forms
... platform-browser, platform-browser-dynamic, router
Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1602.9
@angular-devkit/build-angular   16.2.9
@angular-devkit/core            16.2.9
@angular-devkit/schematics      16.2.9
@angular/cli                    16.2.9
@schematics/angular             16.2.9
rxjs                            7.8.1
typescript                      5.1.6
zone.js                         0.13.3
Anything else relevant?
I've seen the same behavior in the current RC of Angular 17.
          This is work as webpack-dev-server specific implementation for bypass. https://github.com/webpack/webpack-dev-server/blob/master/lib/Server.js#L2234
The proxy library that is used by both vite and webpack-dev-server always requires a target including when using bypass.
          As mentioned, the bypass method is a Webpack extension to the underlying proxy package.  While Vite does provide some support for this, it does not support the use of a true return value.  Returning a string url replacement or false appears to be implemented but all other values result in the underlying proxy package being used.

Resolving this would most likely require a feature request for Vite itself to handle the return true case.
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