Custom webpack builder v20.0.0 breaks with Angular 20.3.x: “Cannot read properties of undefined (reading 'tap')”

[ZeeshanC]

After upgrading Angular CLI/build tooling to 20.3.x, builds and dev server fail when using @angular-builders/custom-webpack v20.0.0. The error appears during the builder’s integration with the Angular CLI pipeline:

Error: Cannot read properties of undefined (reading 'tap')

This same setup worked on Angular 20.1.x (e.g., 20.1.6). The break only occurs after upgrading to 20.3.x while still using the custom-webpack builder 20.0.0. There does not appear to be a 20.3.x-compatible release of @angular-builders/custom-webpack.

Environment

Angular packages (selected):
@angular-devkit/build-angular: ^20.3.8
@angular/cli: ^20.3.8
@angular/compiler-cli: ^20.3.9
@angular/core: ^20.3.9
Builder:
@angular-builders/custom-webpack: 20.0.0
TypeScript: ~5.9.3
Webpack: ^5.102.1

package.json snippets

Dependencies (Angular 20.3.x line): package.json "@angular/common": "^20.3.9", "@angular/compiler": "^20.3.9", "@angular/core": "^20.3.9", "@angular/forms": "^20.3.9", "@angular/material": "^20.2.11", "@angular/platform-browser": "^20.3.9", "@angular/platform-browser-dynamic": "^20.3.9", "@angular/router": "^20.3.9", "rxjs": "~7.8.2", "zone.js": "^0.15.1"

DevDependencies: package.json "devDependencies": { "@angular-builders/custom-webpack": "20.0.0", "@angular-devkit/build-angular": "^20.3.8", "@angular/cli": "^20.3.8", "@angular/compiler-cli": "^20.3.9", "typescript": "~5.9.3", "webpack": "^5.102.1", "webpack-cli": "^6.0.1", "webpack-obfuscator": "^3.5.1" }

Steps to reproduce

Start with Angular 20.1.x + @angular-builders/custom-webpack 20.0.0 (works)
Upgrade Angular tooling to 20.3.x:
@angular-devkit/build-angular: ^20.3.8
@angular/cli: ^20.3.8
@angular/compiler-cli: ^20.3.9
Keep @angular-builders/custom-webpack at 20.0.0 (latest available)
Run:
ng serve
or ng build (any configuration)
Observe the failure with error similar to:
“Cannot read properties of undefined (reading 'tap')”
Observed behavior

Build/dev-server fail with “Cannot read properties of undefined (reading 'tap')” during the builder/CLI integration phase.
Reverting Angular CLI/build back to 20.1.x fixes the problem without changing any app code or webpack config.
There is no published @angular-builders/custom-webpack 20.3.x to align with the Angular 20.3.x changes.

Request

Please confirm whether Angular 20.3.x is supported by @angular-builders/custom-webpack, and if not, whether a 20.3-compatible release is planned.
If there’s a known workaround (e.g., a specific pinned version of build-angular/cli or a beta/next tag of custom-webpack), please share.
If migration off custom-webpack is the recommended approach for 20.3.x, guidance/recipes for replicating typical use cases (plugins, aliases, defines) using supported Angular options would be very helpful.

Thank you!