What is the new “builder API” in Angular CLI 21?

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Anubhav Sharma · Answer

What the Builder API Is The Builder API (often just called Angular CLI Builders) isn’t brand-new in Angular 21—it’s been part of the Angular CLI for years as the mechanism that executes CLI tasks like ng build, ng serve, and ng test by delegating to “builders.” A builder is a function that performs a task (build, test, serve, etc.) based on options and context provided by the CLI. You can also write custom builders to extend the CLI’s capabilities. In Angular CLI: Builders replace older internal tooling like direct Webpack configs.They are configured in angular.json under "architect" (or "targets" in newer formats).Each builder has: An options schema that defines input.A handler function that performs the work.Access to a BuilderContext for reporting and scheduling tasks. Why It Matters in Angular CLI 21 While the underlying Builder API hasn’t fundamentally changed just for v21, Angular 21 makes greater use of it by shifting default tooling toward new, modern builders: Default Builders Evolved Angular CLI 21 now uses more modern builders out of the box: Vitest-based test builder@angular/build:unit-test as the default for ng test instead of the older Karma/Jasmine builders. The modern application builder (often based on @angular/build:application) that uses esbuild + Vite for faster builds and dev server tasks. These builders are implemented using the Builder API and configured in angular.json, so the CLI commands now delegate to new underlying implementations. The Builder API makes this extensible and configurable. Custom Builders Still Supported You can still create your own builders to: Customize build/test/serve processesIntegrate alternative toolchainsAutomate custom workflows This is done using the Builder API utilities like createBuilder() and implementing a builder handler that returns a BuilderOutput. TL;DR CLI Builder API is the core extensibility layer that Angular CLI uses to run commands like build, serve, and test.Angular CLI 21 leverages this API to provide modern builders (e.g., esbuild/Vite-based application builder and Vitest test builder) for better performance and defaults.You can create and use custom builders by implementing and registering them in your workspace.

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What is the new “builder API” in Angular CLI 21?

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What the Builder API Is

Why It Matters in Angular CLI 21

Default Builders Evolved

Custom Builders Still Supported

TL;DR