How to Run a Matrix Only on Certain Events in GitHub Actions

Compute the matrix in a setup job keyed on github.event_name so pull requests run a subset and main runs the full fan-out.

A setup job branches on github.event_name, emits a small or large JSON list, and the build job consumes it with fromJSON.

Steps

• In a setup job, choose the version list based on the event.
• Emit it as JSON to $GITHUB_OUTPUT.
• Consume it with fromJSON in the build matrix.

Workflow

jobs:
  setup:
    runs-on: ubuntu-latest
    outputs:
      versions: ${{ steps.pick.outputs.versions }}
    steps:
      - id: pick
        run: |
          if [ "${{ github.event_name }}" = "pull_request" ]; then
            echo 'versions=[20]' >> "$GITHUB_OUTPUT"
          else
            echo 'versions=[18,20,22]' >> "$GITHUB_OUTPUT"
          fi
  test:
    needs: setup
    runs-on: ubuntu-latest
    strategy:
      matrix:
        node: ${{ fromJSON(needs.setup.outputs.versions) }}
    steps:
      - run: echo "Testing on ${{ matrix.node }}"

Gotchas

• Keep required-check names stable across events so branch protection still matches.
• The emitted string must be valid JSON, including the brackets.

Related guides