Problem

Some tests in our CI are expensive (long-running, resource-heavy, or
require special secrets).
Running them for every PR is inefficient and unnecessary.

Goal

Enable opt‑in expensive test runs by commenting a slash-command on a
PR, for example:

/run-expensive-tests

This should:

• Trigger a GitHub Actions workflow via issue_comment
• Detect that the comment is on a PR
• Verify the comment text starts with /run-expensive-tests
• Check out the PR merge ref
• Run the expensive test suite only on demand

Proposed Workflow (GitHub Actions)

Create .github/workflows/expensive-tests-on-comment.yml:

name: Expensive Tests (On Comment)

on:
  issue_comment:
    types: [created]

jobs:
  expensive-tests:
    # Must be a PR comment AND contain the trigger command
    if: >
      github.event.issue.pull_request &&
      startsWith(github.event.comment.body, '/run-expensive-tests')
    runs-on: ubuntu-latest

    steps:
      - name: Extract PR number
        run: echo "PR_NUMBER=${{ github.event.issue.number }}" >> $GITHUB_ENV

      - name: Checkout PR merge commit
        uses: actions/checkout@v4
        with:
          ref: refs/pull/${{ env.PR_NUMBER }}/merge

      - name: Run expensive tests
        run: ./scripts/run_expensive_tests.sh

Notes / Best Practices

• issue_comment does fire for PR comments (PRs are issues
  internally)
• github.event.issue.pull_request filters out non‑PR comments
• This avoids running the expensive suite on every PR automatically
• For repos needing extra safety:

  • restrict execution to team members:

    if: github.event.comment.author_association == 'MEMBER'

• If the PR is from a fork:
  • secrets will not be available (GitHub restriction)
  • but compute-only tests still run fine

Acceptance Criteria

• Team members can trigger expensive CI by commenting
  /run-expensive-tests
• The workflow runs only when explicitly invoked
• Regular fast CI remains unchanged
• Documentation added to CONTRIBUTING.md

Next Steps

• Add the workflow file
• Add label/documentation for contributors
• (Optional) Add bot reaction or status comment to PR