diff --git a/python/packages/core/agent_framework/_types.py b/python/packages/core/agent_framework/_types.py
index 736474de60..879f54fcf7 100644
--- a/python/packages/core/agent_framework/_types.py
+++ b/python/packages/core/agent_framework/_types.py
@@ -121,6 +121,8 @@ def detect_media_type_from_base64(
         if data is not None:
             raise ValueError("Provide exactly one of data_bytes, data_str, or data_uri.")
         # Remove data URI prefix if present
+        if ";base64," not in data_uri:
+            raise ValueError("Data URI must use base64 encoding.")
         data_str = data_uri.split(";base64,", 1)[1]
     if data_str is not None:
         if data is not None:
diff --git a/python/packages/core/tests/core/test_types.py b/python/packages/core/tests/core/test_types.py
index 23c7dd8cd6..1a1549ad9e 100644
--- a/python/packages/core/tests/core/test_types.py
+++ b/python/packages/core/tests/core/test_types.py
@@ -204,6 +204,11 @@ def test_data_content_detect_image_format_from_base64():
         detect_media_type_from_base64(data_bytes=b"data", data_uri="data:application/octet-stream;base64,AAA")
         detect_media_type_from_base64(data_str="data", data_uri="data:application/octet-stream;base64,AAA")
 
+    with pytest.raises(ValueError, match="Data URI must use base64 encoding"):
+        detect_media_type_from_base64(data_uri="data:text/plain,Hello%20World")
+    with pytest.raises(ValueError, match="Data URI must use base64 encoding"):
+        detect_media_type_from_base64(data_uri="http://example.com/image.png")
+
 
 def test_data_content_create_data_uri_from_base64():
     """Test the create_data_uri_from_base64 class method."""