What are composite actions and when should you choose them over reusable workflows?

A composite action is a custom action that groups multiple run: and uses: steps into a single reusable unit referenced with uses: inside a step — not as a job. It is defined by an action.yml file in a repository and runs within the calling job's runner, sharing its environment and filesystem.

# .github/actions/setup-env/action.yml
name: 'Setup Build Environment'
description: 'Install tools and restore cache'
inputs:
  node-version:
    required: true
    default: '20'
runs:
  using: 'composite'
  steps:
    - uses: actions/setup-node@v4
      with:
        node-version: ${{ inputs.node-version }}
    - uses: actions/cache@v4
      with:
        path: ~/.npm
        key: npm-${{ runner.os }}-${{ hashFiles('**/package-lock.json') }}
    - run: npm ci
      shell: bash

Usage in a workflow:

steps:
  - uses: actions/checkout@v4
  - uses: ./.github/actions/setup-env
    with:
      node-version: '22'
  - run: npm test

Choose a composite action when you want to extract a few repeated setup steps within a job. Choose a reusable workflow when you want to share a complete, self-contained pipeline job (with its own runner, concurrency, and environment) across multiple repositories.

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