How to write a composite action in GitHub Actions

Composite Actions vs Reusable Workflows

Composite actions bundle multiple steps into a single reusable step. They run within a job, unlike reusable workflows which define entire jobs. Use composite actions for setup sequences; use reusable workflows for full pipelines.

Creating a Composite Action

# .github/actions/setup-app/action.yml
name: 'Setup Application'
description: 'Checks out code, sets up Node, and installs deps'
inputs:
  node-version:
    description: 'Node.js version'
    default: '20'

runs:
  using: 'composite'
  steps:
    - uses: actions/checkout@v4
    - uses: actions/setup-node@v4
      with:
        node-version: ${{ inputs.node-version }}
        cache: 'npm'
    - run: npm ci
      shell: bash

Using the Composite Action

steps:
  - uses: ./.github/actions/setup-app
    with:
      node-version: '20'

  - run: npm test

The using: composite key is required. Every run step inside a composite action must have a shell key. The action lives in .github/actions/ and is referenced with a relative path.