Running two ng serve instances (same project, different configurations) causes “504 Outdated Optimize Dep” errors in Angular CLI 20.3.8

Start with the package.json start:both reproduction and inspect the shared .angular/cache/.../vite optimizer directory while running both ng serve processes. The fix is complete when parallel servers for different configurations load successfully without 504 Outdated Optimize Dep errors.

Command

serve

Is this a regression?

• Yes, this behavior used to work in the previous version

The previous version in which this bug was not present was

20.3.6

Description

Since updating to Angular v20.3.9 / CLI v20.3.8, it’s no longer possible to run two ng serve instances for the same project (e.g. localized builds) in parallel.
The second instance fails to load with a 504 (Outdated Optimize Dep) error for one or more chunks (e.g. chunk-5V7QYGNA.js).

This setup worked correctly in previous Angular 20.3.6 versions (before the CLI migrated the dev-server to the Vite-based implementation).

The issue appears to be caused by both ng serve processes sharing the same Vite optimizer cache directory inside .angular/cache/.../vite.
When one process updates or invalidates the cache, the other process detects stale metadata and refuses to serve pre-bundled chunks.

Minimal Reproduction

package.json scripts
"scripts": {
"start": "ng serve --host 0.0.0.0",
"startfr": "ng serve -c fr --host 0.0.0.0",
"startfr-alt": "ng serve -c fr --host 0.0.0.0 --port 4201",
"start:both": "concurrently "npm run start" "npm run startfr-alt""
}

Steps to reproduce
1. Run npm run start:both.
2. Wait for both dev servers to start.
3. Observe that the second instance fails to load in the browser console with:
Failed to load resource: the server responded with a status of 504 (Outdated Optimize Dep)

Exception or Error

Failed to load resource: the server responded with a status of 504 (Outdated Optimize Dep)

Your Environment

Angular CLI: 20.3.8
Node: 24.1.0
Package Manager: yarn 4.9.2
OS: darwin arm64
    

Angular: 20.3.9
... animations, common, compiler, compiler-cli, core, forms
... language-service, localize, platform-browser
... platform-browser-dynamic, router

Package                         Version
---------------------------------------
@angular-devkit/architect       0.2003.8
@angular-devkit/build-angular   20.3.8
@angular-devkit/core            20.3.8
@angular-devkit/schematics      20.3.8
@angular/cdk                    20.2.11
@angular/cli                    20.3.8
@angular/material               20.2.11
@schematics/angular             20.3.8
rxjs                            7.8.2
typescript                      5.8.3
zone.js                         0.15.1

Anything else relevant?

No response