diff --git a/astrbot/core/agent/tool.py b/astrbot/core/agent/tool.py
index 4cee6ba6d1..5083aac4f2 100644
--- a/astrbot/core/agent/tool.py
+++ b/astrbot/core/agent/tool.py
@@ -250,9 +250,44 @@ def convert_schema(schema: dict) -> dict:
                 "integer": {"int32", "int64"},
                 "number": {"float", "double"},
             }
+            support_fields = {
+                "title",
+                "description",
+                "enum",
+                "minimum",
+                "maximum",
+                "maxItems",
+                "minItems",
+                "nullable",
+                "required",
+            }
 
-            if "anyOf" in schema:
-                return {"anyOf": [convert_schema(s) for s in schema["anyOf"]]}
+            def apply_supported_fields(result: dict, source: dict) -> None:
+                for key in support_fields:
+                    if key in source and key not in result:
+                        result[key] = source[key]
+
+            for union_key in ("anyOf", "oneOf"):
+                union_value = schema.get(union_key)
+                if isinstance(union_value, list):
+                    converted_branches = [
+                        convert_schema(item) if isinstance(item, dict) else item
+                        for item in union_value
+                    ]
+                    non_null_branches = [
+                        item
+                        for item in converted_branches
+                        if not (isinstance(item, dict) and item.get("type") == "null")
+                    ]
+                    if len(non_null_branches) == 1 and isinstance(
+                        non_null_branches[0], dict
+                    ):
+                        result = non_null_branches[0].copy()
+                        if len(converted_branches) > 1:
+                            result["nullable"] = True
+                        apply_supported_fields(result, schema)
+                        return result
+                    return {union_key: converted_branches}
 
             result = {}
 
@@ -268,6 +303,12 @@ def convert_schema(schema: dict) -> dict:
 
             if target_type in supported_types:
                 result["type"] = target_type
+                if (
+                    isinstance(origin_type, list)
+                    and "null" in origin_type
+                    and target_type != "null"
+                ):
+                    result["nullable"] = True
                 if "format" in schema and schema["format"] in supported_formats.get(
                     result["type"],
                     set(),
@@ -276,18 +317,7 @@ def convert_schema(schema: dict) -> dict:
             else:
                 result["type"] = "null"
 
-            support_fields = {
-                "title",
-                "description",
-                "enum",
-                "minimum",
-                "maximum",
-                "maxItems",
-                "minItems",
-                "nullable",
-                "required",
-            }
-            result.update({k: schema[k] for k in support_fields if k in schema})
+            apply_supported_fields(result, schema)
 
             if "properties" in schema:
                 properties = {}
diff --git a/tests/unit/test_tool_google_schema.py b/tests/unit/test_tool_google_schema.py
index f1046e6af3..a2be6c2fb9 100644
--- a/tests/unit/test_tool_google_schema.py
+++ b/tests/unit/test_tool_google_schema.py
@@ -75,3 +75,131 @@ def test_google_schema_fills_missing_array_items_with_string_schema():
 
     assert source_uuids["type"] == "array"
     assert source_uuids["items"] == {"type": "string"}
+
+
+def test_google_schema_collapses_nullable_anyof_property():
+    tool_module = load_tool_module()
+    FunctionTool = tool_module.FunctionTool
+    ToolSet = tool_module.ToolSet
+
+    tool = FunctionTool(
+        name="search_sources",
+        description="Search sources by recency.",
+        parameters={
+            "type": "object",
+            "properties": {
+                "time_range": {
+                    "description": "Optional recency filter.",
+                    "anyOf": [
+                        {
+                            "type": "string",
+                            "enum": ["day", "week", "month", "year"],
+                        },
+                        {"type": "null"},
+                    ],
+                    "default": None,
+                }
+            },
+        },
+    )
+
+    schema = ToolSet([tool]).google_schema()
+    time_range = schema["function_declarations"][0]["parameters"]["properties"][
+        "time_range"
+    ]
+
+    assert time_range["type"] == "string"
+    assert time_range["description"] == "Optional recency filter."
+    assert time_range["enum"] == ["day", "week", "month", "year"]
+    assert time_range["nullable"] is True
+    assert "anyOf" not in time_range
+    assert "default" not in time_range
+
+
+def test_google_schema_collapses_single_branch_anyof_property():
+    tool_module = load_tool_module()
+    FunctionTool = tool_module.FunctionTool
+    ToolSet = tool_module.ToolSet
+
+    tool = FunctionTool(
+        name="search_sources",
+        description="Search sources by query.",
+        parameters={
+            "type": "object",
+            "properties": {
+                "query": {
+                    "description": "Search query.",
+                    "anyOf": [
+                        {
+                            "type": "string",
+                        }
+                    ],
+                }
+            },
+        },
+    )
+
+    schema = ToolSet([tool]).google_schema()
+    query = schema["function_declarations"][0]["parameters"]["properties"]["query"]
+
+    assert query["type"] == "string"
+    assert query["description"] == "Search query."
+    assert "nullable" not in query
+    assert "anyOf" not in query
+
+
+def test_google_schema_preserves_non_dict_union_branches():
+    tool_module = load_tool_module()
+    FunctionTool = tool_module.FunctionTool
+    ToolSet = tool_module.ToolSet
+
+    tool = FunctionTool(
+        name="search_sources",
+        description="Search sources by literal value.",
+        parameters={
+            "type": "object",
+            "properties": {
+                "value": {
+                    "anyOf": [
+                        {"type": "string"},
+                        False,
+                    ],
+                }
+            },
+        },
+    )
+
+    schema = ToolSet([tool]).google_schema()
+    value = schema["function_declarations"][0]["parameters"]["properties"]["value"]
+
+    assert value["anyOf"] == [
+        {"type": "string"},
+        False,
+    ]
+
+
+def test_google_schema_marks_type_list_with_null_as_nullable():
+    tool_module = load_tool_module()
+    FunctionTool = tool_module.FunctionTool
+    ToolSet = tool_module.ToolSet
+
+    tool = FunctionTool(
+        name="search_sources",
+        description="Search sources by recency.",
+        parameters={
+            "type": "object",
+            "properties": {
+                "query": {
+                    "type": ["string", "null"],
+                    "description": "Optional query.",
+                }
+            },
+        },
+    )
+
+    schema = ToolSet([tool]).google_schema()
+    query = schema["function_declarations"][0]["parameters"]["properties"]["query"]
+
+    assert query["type"] == "string"
+    assert query["description"] == "Optional query."
+    assert query["nullable"] is True