diff --git a/compiler/fory_compiler/generators/cpp.py b/compiler/fory_compiler/generators/cpp.py
index f7302dd150..c5d4de1a34 100644
--- a/compiler/fory_compiler/generators/cpp.py
+++ b/compiler/fory_compiler/generators/cpp.py
@@ -1063,9 +1063,19 @@ def generate_union_definition(
         body_indent = f"{indent}  "
 
         case_enum = f"{class_name}Case"
-        case_types = [
+        raw_case_types = [
             self.get_union_case_type(field, parent_stack) for field in union.fields
         ]
+        case_aliases = [
+            f"ForyCase{self.to_pascal_case(field.name)}Type"
+            if "," in case_type
+            else None
+            for field, case_type in zip(union.fields, raw_case_types)
+        ]
+        case_types = [
+            alias if alias is not None else case_type
+            for alias, case_type in zip(case_aliases, raw_case_types)
+        ]
         variant_type = f"std::variant<{', '.join(case_types)}>"
 
         comment = self.format_type_id_comment(union, f"{indent}//")
@@ -1080,6 +1090,12 @@ def generate_union_definition(
         lines.append(f"{body_indent}  }};")
         lines.append("")
 
+        for alias, case_type in zip(case_aliases, raw_case_types):
+            if alias is not None:
+                lines.append(f"{body_indent}  using {alias} = {case_type};")
+        if any(alias is not None for alias in case_aliases):
+            lines.append("")
+
         lines.append(f"{body_indent}  {class_name}() = default;")
         lines.append("")
 
@@ -1204,15 +1220,8 @@ def generate_union_macros(
             union_type = self.get_namespaced_type_name(union.name, parent_stack)
             lines.append(f"FORY_UNION({union_type},")
             for index, field in enumerate(union.fields):
-                case_type = self.generate_namespaced_type(
-                    field.field_type,
-                    False,
-                    field.ref,
-                    field.element_optional,
-                    field.element_ref,
-                    False,
-                    False,
-                    parent_stack,
+                case_type = self.get_union_case_macro_type(
+                    field, union_type, parent_stack
                 )
                 case_ctor = self.to_snake_case(field.name)
                 meta = self.get_union_field_meta(field)
@@ -1225,16 +1234,7 @@ def generate_union_macros(
         case_ids = ", ".join(str(field.number) for field in union.fields)
         lines.append(f"FORY_UNION_IDS({union_type}, {case_ids});")
         for field in union.fields:
-            case_type = self.generate_namespaced_type(
-                field.field_type,
-                False,
-                field.ref,
-                field.element_optional,
-                field.element_ref,
-                False,
-                False,
-                parent_stack,
-            )
+            case_type = self.get_union_case_macro_type(field, union_type, parent_stack)
             case_ctor = self.to_snake_case(field.name)
             meta = self.get_union_field_meta(field)
             lines.append(
@@ -1243,6 +1243,29 @@ def generate_union_macros(
 
         return lines
 
+    def get_union_case_macro_type(
+        self,
+        field: Field,
+        union_type: str,
+        parent_stack: List[Message],
+    ) -> str:
+        """Return the C++ type name used in FORY_UNION and FORY_UNION_CASE macros."""
+        case_type = self.generate_namespaced_type(
+            field.field_type,
+            False,
+            field.ref,
+            field.element_optional,
+            field.element_ref,
+            False,
+            False,
+            parent_stack,
+        )
+        # FORY_UNION and FORY_UNION_CASE split macro arguments on commas,
+        # so raw template types such as std::unordered_map<K, V> need an alias.
+        if "," in case_type:
+            return f"{union_type}::ForyCase{self.to_pascal_case(field.name)}Type"
+        return case_type
+
     def get_union_case_type(self, field: Field, parent_stack: List[Message]) -> str:
         """Return the C++ type for a union case."""
         return self.generate_type(
diff --git a/compiler/fory_compiler/tests/test_generated_code.py b/compiler/fory_compiler/tests/test_generated_code.py
index 16e8fca31e..c61c9c2ceb 100644
--- a/compiler/fory_compiler/tests/test_generated_code.py
+++ b/compiler/fory_compiler/tests/test_generated_code.py
@@ -1126,6 +1126,71 @@ def test_cpp_generator_supports_decimal_fields_and_unions():
     assert "(amount, fory::serialization::Decimal, fory::F(1))" in cpp_output
 
 
+def test_cpp_union_aliases_comma_payload_types():
+    schema = parse_fdl(
+        dedent(
+            """
+            package gen;
+
+            union MapChoice {
+                map<string, any> by_name = 1;
+                map<string, int32> counts = 2;
+                list<any> values = 3;
+                string name = 4;
+            }
+
+            union LargeChoice {
+                map<string, int32> counts = 1;
+                bool enabled = 2;
+                int8 i8 = 3;
+                int16 i16 = 4;
+                int32 i32 = 5;
+                int64 i64 = 6;
+                uint8 u8 = 7;
+                uint16 u16 = 8;
+                uint32 u32 = 9;
+                uint64 u64 = 10;
+                float32 f32 = 11;
+                float64 f64 = 12;
+                string name = 13;
+                bytes blob = 14;
+                decimal amount = 15;
+                date day = 16;
+                timestamp ts = 17;
+            }
+            """
+        )
+    )
+
+    cpp_output = render_files(generate_files(schema, CppGenerator))
+    assert (
+        "using ForyCaseByNameType = std::unordered_map<std::string, std::any>;"
+        in cpp_output
+    )
+    assert (
+        "using ForyCaseCountsType = std::unordered_map<std::string, int32_t>;"
+        in cpp_output
+    )
+    assert (
+        "(by_name, gen::MapChoice::ForyCaseByNameType, "
+        "fory::F(1).map(fory::T::string(), fory::FieldNodeSpec{}))" in cpp_output
+    )
+    assert (
+        "(counts, gen::MapChoice::ForyCaseCountsType, "
+        "fory::F(2).map(fory::T::string(), fory::T::int32().varint()))" in cpp_output
+    )
+    assert (
+        "(values, std::vector<std::any>, "
+        "fory::F(3).list(fory::FieldNodeSpec{}))" in cpp_output
+    )
+    assert "(name, std::string, fory::F(4))" in cpp_output
+    assert (
+        "FORY_UNION_CASE(gen::LargeChoice, 1, "
+        "gen::LargeChoice::ForyCaseCountsType, gen::LargeChoice::counts, "
+        "fory::F(1).map(fory::T::string(), fory::T::int32().varint()));" in cpp_output
+    )
+
+
 def test_cpp_omits_equality_for_any_types():
     schema = parse_fdl(
         dedent(