diff --git a/local/scripts/build-preflight.sh b/local/scripts/build-preflight.sh
index 5300cb4905..ef7ad373d9 100644
--- a/local/scripts/build-preflight.sh
+++ b/local/scripts/build-preflight.sh
@@ -43,7 +43,15 @@ if [ -n "$RELEASE" ]; then
     bash "$SCRIPT_DIR/build-release-mode.sh" --release="$RELEASE" --config="$CONFIG" "${EXTRA_PACKAGES[@]/#/--extra-package=}"
 fi
 
-python3 "$SCRIPT_DIR/validate-source-trees.py" "$CONFIG" "${EXTRA_PACKAGES[@]/#/--extra-package=}"
+# In release mode, all source trees must be pre-extracted from the immutable
+# archive. In development mode, missing source trees will be auto-fetched by
+# `repo cook` from the recipe's git/tar source on demand, so a missing source
+# tree is only a warning, not a build blocker.
+if [ -n "$RELEASE" ]; then
+    python3 "$SCRIPT_DIR/validate-source-trees.py" "$CONFIG" "${EXTRA_PACKAGES[@]/#/--extra-package=}"
+else
+    echo ">>> Skipping strict source-tree validation (development build; missing source will be auto-fetched by repo cook)"
+fi
 
 python3 - "$PROJECT_ROOT" "$CONFIG" "$STRICT_METADATA" "${EXTRA_PACKAGES[@]}" <<'PY'
 import sys