Commenting a pull request in a GitHub action

A canonical way is using the official Github Script actions. Don’t get confused, issues and PRs are the same for the GitHub API.

2020:

on:
  # Trigger the workflow on push or pull request
  pull_request:
    branches:
      - master
      - develop

jobs:
  comment:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/github-script@v3
        with:
          github-token: ${{secrets.GITHUB_TOKEN}}
          script: |
            github.issues.createComment({
              issue_number: context.issue.number,
              owner: context.repo.owner,
              repo: context.repo.repo,
              body: '👋 Thanks for reporting!'
            })

Seen on: https://github.com/actions/github-script#comment-on-an-issue

EDIT 2021:

Adapting to GH’s newer versions, one has to reference the rest client directly. See note as of writing.

on:
  # Trigger the workflow on push or pull request
  pull_request:
    branches:
      - master
      - develop

jobs:
  comment:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/github-script@v5
        with:
          github-token: ${{secrets.GITHUB_TOKEN}}
          script: |
            github.rest.issues.createComment({
              issue_number: context.issue.number,
              owner: context.repo.owner,
              repo: context.repo.repo,
              body: '👋 Thanks for reporting!'
            })

EDIT 2023:

V6 is out, here’s how to use it. No breaking changes, just the script version has changed.

on:
  # Trigger the workflow on push or pull request
  pull_request:
    branches:
      - master
      - develop

jobs:
  comment:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/github-script@v6
        with:
          script: |
            github.rest.issues.createComment({
              issue_number: context.issue.number,
              owner: context.repo.owner,
              repo: context.repo.repo,
              body: '👋 Thanks for reporting!'
            })