diff --git a/services/ml/app/main.py b/services/ml/app/main.py
index 82d4c6b..a7f2cde 100644
--- a/services/ml/app/main.py
+++ b/services/ml/app/main.py
@@ -58,6 +58,18 @@ async def verify_api_key(x_api_key: str = Header(default="")):
         raise HTTPException(status_code=401, detail="Unauthorized")
 
 
+# --- User ID Header Dependency ---
+
+async def get_user_id(x_user_id: str = Header(default="")) -> Optional[str]:
+    """
+    Extract X-User-ID header from incoming requests.
+
+    Returns the user ID if present, or None if not provided.
+    This is optional — callers decide whether to enforce it.
+    """
+    return x_user_id if x_user_id else None
+
+
 # --- Lifespan ---
 
 @asynccontextmanager