diff --git a/.jules/bolt.md b/.jules/bolt.md
new file mode 100644
index 0000000000..c88abaa3af
--- /dev/null
+++ b/.jules/bolt.md
@@ -0,0 +1,3 @@
+## 2024-05-24 - os.path.commonpath Bottleneck
+**Learning:** `os.path.commonpath` is a significant performance bottleneck for path containment checks due to internal list allocations and path splitting.
+**Action:** Use `os.path.abspath` combined with `str.startswith()` and a conditional trailing `os.sep` to safely and efficiently check path containment without traversal bugs.
diff --git a/helpers/files.py b/helpers/files.py
index c77ab54cf1..eb52105c86 100644
--- a/helpers/files.py
+++ b/helpers/files.py
@@ -653,7 +653,8 @@ def is_in_dir(path: str, dir: str):
     # check if the given path is within the directory
     abs_path = os.path.abspath(path)
     abs_dir = os.path.abspath(dir)
-    return os.path.commonpath([abs_path, abs_dir]) == abs_dir
+    # ⚡ Bolt: Avoid os.path.commonpath overhead for significant speedup
+    return abs_path == abs_dir or abs_path.startswith(abs_dir + ('' if abs_dir.endswith(os.sep) else os.sep))
 
 
 def get_subdirectories(