diff --git a/docs/docs/ai/llm.mdx b/docs/docs/ai/llm.mdx
index b3073d8e0..7966b0ab0 100644
--- a/docs/docs/ai/llm.mdx
+++ b/docs/docs/ai/llm.mdx
@@ -309,7 +309,7 @@ cocoindex.LlmSpec(
 
 You can find the full list of models supported by OpenRouter [here](https://openrouter.ai/models).
 
-### vLLM 
+### vLLM
 
 Install vLLM:
 
@@ -338,4 +338,4 @@ cocoindex.LlmSpec(
 ```
 
 </TabItem>
-</Tabs>
\ No newline at end of file
+</Tabs>
diff --git a/python/cocoindex/convert.py b/python/cocoindex/convert.py
index 648cf83f3..3d3dca8c1 100644
--- a/python/cocoindex/convert.py
+++ b/python/cocoindex/convert.py
@@ -5,7 +5,6 @@
 import dataclasses
 import datetime
 import inspect
-import uuid
 from enum import Enum
 from typing import Any, Callable, Mapping, get_origin
 
@@ -14,7 +13,6 @@
 from .typing import (
     KEY_FIELD_NAME,
     TABLE_TYPES,
-    AnalyzedTypeInfo,
     DtypeRegistry,
     analyze_type_info,
     encode_enriched_type,
@@ -74,23 +72,16 @@ def make_engine_value_decoder(
     Returns:
         A decoder from an engine value to a Python value.
     """
-
     src_type_kind = src_type["kind"]
 
-    dst_type_info: AnalyzedTypeInfo | None = None
-    if (
-        dst_annotation is not None
-        and dst_annotation is not inspect.Parameter.empty
-        and dst_annotation is not Any
-    ):
-        dst_type_info = analyze_type_info(dst_annotation)
-        if not _is_type_kind_convertible_to(src_type_kind, dst_type_info.kind):
-            raise ValueError(
-                f"Type mismatch for `{''.join(field_path)}`: "
-                f"passed in {src_type_kind}, declared {dst_annotation} ({dst_type_info.kind})"
-            )
-
-    if dst_type_info is None:
+    dst_is_any = (
+        dst_annotation is None
+        or dst_annotation is inspect.Parameter.empty
+        or dst_annotation is Any
+    )
+    if dst_is_any:
+        if src_type_kind == "Union":
+            return lambda value: value[1]
         if src_type_kind == "Struct" or src_type_kind in TABLE_TYPES:
             raise ValueError(
                 f"Missing type annotation for `{''.join(field_path)}`."
@@ -98,6 +89,41 @@ def make_engine_value_decoder(
             )
         return lambda value: value
 
+    dst_type_info = analyze_type_info(dst_annotation)
+
+    if src_type_kind == "Union":
+        dst_type_variants = (
+            dst_type_info.union_variant_types
+            if dst_type_info.union_variant_types is not None
+            else [dst_annotation]
+        )
+        src_type_variants = src_type["types"]
+        decoders = []
+        for i, src_type_variant in enumerate(src_type_variants):
+            src_field_path = field_path + [f"[{i}]"]
+            decoder = None
+            for dst_type_variant in dst_type_variants:
+                try:
+                    decoder = make_engine_value_decoder(
+                        src_field_path, src_type_variant, dst_type_variant
+                    )
+                    break
+                except ValueError:
+                    pass
+            if decoder is None:
+                raise ValueError(
+                    f"Type mismatch for `{''.join(field_path)}`: "
+                    f"cannot find matched target type for source type variant {src_type_variant}"
+                )
+            decoders.append(decoder)
+        return lambda value: decoders[value[0]](value[1])
+
+    if not _is_type_kind_convertible_to(src_type_kind, dst_type_info.kind):
+        raise ValueError(
+            f"Type mismatch for `{''.join(field_path)}`: "
+            f"passed in {src_type_kind}, declared {dst_annotation} ({dst_type_info.kind})"
+        )
+
     if dst_type_info.kind in ("Float32", "Float64", "Int64"):
         dst_core_type = dst_type_info.core_type
 
@@ -196,9 +222,6 @@ def decode(value: Any) -> Any | None:
         field_path.pop()
         return decode
 
-    if src_type_kind == "Union":
-        return lambda value: value[1]
-
     return lambda value: value
 
 
diff --git a/python/cocoindex/tests/test_convert.py b/python/cocoindex/tests/test_convert.py
index acc19e779..ee9d200da 100644
--- a/python/cocoindex/tests/test_convert.py
+++ b/python/cocoindex/tests/test_convert.py
@@ -104,7 +104,9 @@ def eq(a: Any, b: Any) -> bool:
     )
     decoder = make_engine_value_decoder([], encoded_output_type, value_type)
     decoded_value = decoder(value_from_engine)
-    assert eq(decoded_value, value)
+    assert eq(decoded_value, value), (
+        f"{decoded_value} != {value}; {encoded_value}; {value_type}; {encoded_output_type}"
+    )
 
     if other_decoded_values is not None:
         for other_value, other_type in other_decoded_values:
@@ -613,6 +615,18 @@ def test_roundtrip_union_timedelta() -> None:
     validate_full_roundtrip(value, t)
 
 
+def test_roundtrip_vector_of_union() -> None:
+    t = list[str | int]
+    value = ["a", 1]
+    validate_full_roundtrip(value, t)
+
+
+def test_roundtrip_union_with_vector() -> None:
+    t = NDArray[np.float32] | str
+    value = np.array([1.0, 2.0, 3.0], dtype=np.float32)
+    validate_full_roundtrip(value, t, ([1.0, 2.0, 3.0], list[float] | str))
+
+
 def test_roundtrip_ltable() -> None:
     t = list[Order]
     value = [Order("O1", "item1", 10.0), Order("O2", "item2", 20.0)]