preview-docs.yml

name: Preview Docs

on:
  pull_request_target:
    types: [opened, synchronize, ready_for_review]
    branches:
      - main

jobs:
  run:
    runs-on: ubuntu-latest
    permissions:
      pull-requests: write # Only for commenting
      contents: read # For checking out code
    steps:
      - name: Checkout repository
        uses: actions/checkout@v5
        with:
          fetch-depth: 0  # Fetch full history for git diff

      - name: Checkout PR
        run: |
          git fetch origin pull/${{ github.event.pull_request.number }}/head:pr-${{ github.event.pull_request.number }}
          git checkout pr-${{ github.event.pull_request.number }}

      - name: Setup Fern CLI
        uses: fern-api/setup-fern-cli@v1

      - name: Generate preview URL
        id: generate-docs
        env:
          FERN_TOKEN: ${{ secrets.FERN_TOKEN }}
        run: |
          # Omit --id so each run gets a fresh preview hash. Pinning the id to the
          # branch name makes the CLI reuse the same preview deploy across pushes,
          # which masks staleness when reviewers re-test after a force-push or a
          # CDN cache pin to old content. A throwaway preview per push is cheap
          # and lines up 1:1 with the comment posted by this workflow.
          OUTPUT=$(fern generate --docs --preview --instance fern-api.docs.buildwithfern.com/learn 2>&1) || true
          echo "$OUTPUT"
          URL=$(echo "$OUTPUT" | grep -oP 'Published docs to \K.*(?= \()')
          echo "preview_url=$URL" >> $GITHUB_OUTPUT
          echo "Preview URL: $URL"

      - name: Get page links for changed MDX files
        id: page-links
        env:
          FERN_TOKEN: ${{ secrets.FERN_TOKEN }}
        run: |
          PREVIEW_URL="${{ steps.generate-docs.outputs.preview_url }}"
          CHANGED_FILES=$(git diff --name-only origin/main...HEAD -- '*.mdx' 2>/dev/null || echo "")
          
          if [ -z "$CHANGED_FILES" ] || [ -z "$PREVIEW_URL" ]; then
            echo "page_links=" >> $GITHUB_OUTPUT; exit 0
          fi
          
          BASE_URL=$(echo "$PREVIEW_URL" | grep -oP 'https?://[^/]+')
          
          FILES_PARAM=$(echo "$CHANGED_FILES" | tr '\n' ',' | sed 's/,$//')
          RESPONSE=$(curl -sf -H "FERN_TOKEN: $FERN_TOKEN" "${PREVIEW_URL}/api/fern-docs/get-slug-for-file?files=${FILES_PARAM}" 2>/dev/null) || {
            echo "page_links=" >> $GITHUB_OUTPUT; exit 0
          }
          
          PAGE_LINKS=$(echo "$RESPONSE" | jq -r --arg url "$BASE_URL" \
            '.mappings[] | select(.slug != null) | "- [\(.slug)](\($url)/\(.slug))"')
          
          if [ -n "$PAGE_LINKS" ]; then
            { echo "page_links<<EOF"; echo "$PAGE_LINKS"; echo "EOF"; } >> $GITHUB_OUTPUT
          else
            echo "page_links=" >> $GITHUB_OUTPUT
          fi

      - name: Create comment content
        run: |
          echo ":herb: **Preview your docs:** <${{ steps.generate-docs.outputs.preview_url }}>" > comment.md
          
          if [ -n "${{ steps.page-links.outputs.page_links }}" ]; then
            echo "" >> comment.md
            echo "Here are the markdown pages you've updated:" >> comment.md
            echo "${{ steps.page-links.outputs.page_links }}" >> comment.md
          fi

      - name: Post PR comment
        uses: thollander/actions-comment-pull-request@v2.4.3
        with:
          filePath: comment.md
          mode: create