diff --git a/pyproject.toml b/pyproject.toml
index edff1091e..e13aec773 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -14,6 +14,7 @@ dependencies = [
     "rich>=14.0.0",
     "python-dotenv>=1.1.0",
     "watchfiles>=1.1.0",
+    "numpy>=1.23.2",
 ]
 license = "Apache-2.0"
 urls = { Homepage = "https://cocoindex.io/" }
diff --git a/python/cocoindex/functions.py b/python/cocoindex/functions.py
index 58f41eb24..2b9ae1802 100644
--- a/python/cocoindex/functions.py
+++ b/python/cocoindex/functions.py
@@ -1,20 +1,17 @@
 """All builtin functions."""
 
-from typing import Annotated, Any, TYPE_CHECKING, Literal
-import numpy as np
-from numpy.typing import NDArray
 import dataclasses
+from typing import Annotated, Any, Literal
 
-from .typing import Float32, Vector, TypeAttr
-from . import op, llm
+import numpy as np
+from numpy.typing import NDArray
 
-# Libraries that are heavy to import. Lazily import them later.
-if TYPE_CHECKING:
-    import sentence_transformers
+from . import llm, op
+from .typing import TypeAttr, Vector
 
 # Check if sentence_transformers is available
 try:
-    import sentence_transformers
+    import sentence_transformers  # type: ignore
 
     _SENTENCE_TRANSFORMERS_AVAILABLE = True
 except ImportError: