diff --git a/scripts/review.sh b/scripts/review.sh
index 41ca4bf..fda5e42 100755
--- a/scripts/review.sh
+++ b/scripts/review.sh
@@ -73,15 +73,35 @@ echo "::endgroup::"
 # ─── Phase 2: Generate diff ───────────────────────────────────────────────────
 echo "::group::Generate diff"
 
-# Ensure we have full history (runner may have done a shallow checkout)
-git fetch --unshallow --filter=blob:none origin 2>/dev/null || true
-git fetch origin main 2>/dev/null || git fetch origin master 2>/dev/null || true
+# actions/checkout for PRs only fetches the PR ref (refs/pull/N/head).
+# It does NOT create remote tracking branches like origin/main.
+# We must explicitly fetch the base branch.
 
-BASE="origin/main"
-if ! git rev-parse --verify "$BASE" >/dev/null 2>&1; then
+# Unshallow if needed (fetch-depth: 0 already does this, but be safe)
+git fetch --unshallow origin 2>/dev/null || true
+
+# Fetch base branch with explicit refspec to ensure origin/main exists
+if git fetch origin refs/heads/main:refs/remotes/origin/main 2>/dev/null; then
+  BASE="origin/main"
+elif git fetch origin refs/heads/master:refs/remotes/origin/master 2>/dev/null; then
   BASE="origin/master"
+else
+  # Fallback: try Gitea/GitHub event context for the target branch
+  TARGET_BRANCH="${GITEA_BASE_REF:-${GITHUB_BASE_REF:-}}"
+  if [ -n "${TARGET_BRANCH}" ] && git fetch origin "refs/heads/${TARGET_BRANCH}:refs/remotes/origin/${TARGET_BRANCH}" 2>/dev/null; then
+    BASE="origin/${TARGET_BRANCH}"
+  else
+    echo "::warning::Could not fetch base branch. Trying origin/HEAD."
+    git fetch origin 2>/dev/null || true
+    BASE="origin/HEAD"
+  fi
 fi
 
+echo "Base ref: ${BASE} -> $(git rev-parse --short "${BASE}" 2>/dev/null || echo 'NOT FOUND')"
+echo "HEAD:     $(git rev-parse --short HEAD)"
+echo "Files changed:"
+git diff --stat "${BASE}...HEAD" 2>/dev/null | tail -3 || echo "(could not stat diff)"
+
 # Build exclude pathspecs
 EXCLUDE_ARGS=""
 for pattern in $PI_EXCLUDE; do