diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 23fb0d4b..73e5a7a1 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,10 +21,25 @@ jobs:
     steps:
       - uses: actions/checkout@v4
         with:
-          fetch-depth: 2
+          fetch-depth: 0
       - id: check
         run: |
-          DIFF=$(git diff --name-only HEAD~1 HEAD 2>/dev/null || echo ".github/workflows/ci.yml")
+          # For push events: diff against previous commit (handles merge commits)
+          # For PR events: diff against the base branch
+          if [ "${{ github.event_name }}" = "pull_request" ]; then
+            BASE="${{ github.event.pull_request.base.sha }}"
+          else
+            BASE="${{ github.event.before }}"
+          fi
+          # Fallback: if BASE is empty or all zeros (new branch), run everything
+          if [ -z "$BASE" ] || echo "$BASE" | grep -qE '^0+$'; then
+            echo "platform=true" >> "$GITHUB_OUTPUT"
+            echo "canvas=true" >> "$GITHUB_OUTPUT"
+            echo "python=true" >> "$GITHUB_OUTPUT"
+            echo "scripts=true" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+          DIFF=$(git diff --name-only "$BASE" HEAD 2>/dev/null || echo ".github/workflows/ci.yml")
           echo "platform=$(echo "$DIFF" | grep -qE '^platform/|^\.github/workflows/ci\.yml$' && echo true || echo false)" >> "$GITHUB_OUTPUT"
           echo "canvas=$(echo "$DIFF" | grep -qE '^canvas/|^\.github/workflows/ci\.yml$' && echo true || echo false)" >> "$GITHUB_OUTPUT"
           echo "python=$(echo "$DIFF" | grep -qE '^workspace-template/|^\.github/workflows/ci\.yml$' && echo true || echo false)" >> "$GITHUB_OUTPUT"