ci: diff explicit base...head shas to match files-changed

.github/workflows/pr-size.yml

@@ -29,4 +29,30 @@ jobs:
             lib/crewai/src/crewai/cli/templates/**
             **/*.json
             **/test_durations/**
-            **/cassettes/**
+            **/cassettes/**
+
+  python-diff-size:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+        with:
+          fetch-depth: 0
+      - name: Enforce Python diff size limit
+        env:
+          MAX: "1500"
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+          HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+        run: |
+          # Three-dot base...head == merge-base(base, head)..head: matches GitHub's
+          # "Files changed" diff and ignores the synthetic merge commit at HEAD.
+          # Sum added + deleted lines across changed .py files; skip binaries ("-").
+          total=$(git diff --numstat "$BASE_SHA...$HEAD_SHA" -- '*.py' \
+            | awk '$1 != "-" && $2 != "-" { sum += $1 + $2 } END { print sum + 0 }')
+          echo "Python churn: $total lines (limit $MAX)"
+          if [ "$total" -gt "$MAX" ]; then
+            echo "::error::Python changes total $total lines, over the $MAX-line limit. Split into smaller PRs."
+            git diff --numstat "$BASE_SHA...$HEAD_SHA" -- '*.py' | sort -rn
+            exit 1
+          fi