--- title: "What is the new “builder API” in Angular CLI 21?" description: "What is the new “builder API” in Angular CLI 21?" author: "Anubhav Sharma" published: 2025-12-23 updated: 2025-12-23 canonical: https://www.mindstick.com/interview/34428/what-is-the-new-builder-api-in-angular-cli-21 category: "angular js" tags: ["angular js"] reading_time: 4 minutes --- # What is the new “builder API” in Angular CLI 21? ### What the Builder API *Is* The *Builder API* (often just called [**Angular CLI**](https://www.mindstick.com/interview/34419/what-is-the-difference-between-angular-cli-14-15-16-and-cli-21) **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. ## Answers ### Answer by Anubhav Sharma ### What the Builder API *Is* The *Builder API* (often just called [**Angular CLI**](https://www.mindstick.com/interview/34419/what-is-the-difference-between-angular-cli-14-15-16-and-cli-21) **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. --- Original Source: https://www.mindstick.com/interview/34428/what-is-the-new-builder-api-in-angular-cli-21 Copyright © MindStick Software Pvt. Ltd. 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