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home / developersection / interviews / what is the new “builder api” in angular cli 21?
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Anubhav Sharma
Anubhav Sharma

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The Anubhav portal was launched in March 2015 at the behest of the Hon'ble Prime Minister for retiring government officials to leave a record of their experiences while in Govt service .

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Anubhav Sharma
Anubhav Sharma 23 Dec 2025
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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 processes
  • Integrate alternative toolchains
  • Automate 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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