fix: emit additional_properties in dict branch of _parse_schema_from_parameter

The dict origin branch in _parse_schema_from_parameter previously returned
Schema(type=OBJECT) without any value type information for parameterized
dict types like dict[str, str]. This caused Gemini to receive a bare
{"type": "object"} with no constraints, leading to empty tool arguments
and infinite retry loops when combined with output_schema.

Now the dict branch parses the value type argument and sets
additional_properties with the corresponding schema, so dict[str, V]
correctly emits {"type": "object", "additionalProperties": <V schema>}.

Fixes #4868

Author: weiguangli-io

src/google/adk/tools/_function_parameter_parse_util.py

@@ -294,6 +294,17 @@ def _parse_schema_from_parameter(
     args = get_args(param.annotation)
     if origin is dict:
       schema.type = types.Type.OBJECT
+      if len(args) == 2:
+        value_type = args[1]
+        schema.additional_properties = _parse_schema_from_parameter(
+            variant,
+            inspect.Parameter(
+                'value',
+                inspect.Parameter.POSITIONAL_OR_KEYWORD,
+                annotation=value_type,
+            ),
+            func_name,
+        )
       if param.default is not inspect.Parameter.empty:
         if not _is_default_value_compatible(param.default, param.annotation):
           raise ValueError(default_value_error_msg)

tests/unittests/tools/test_build_function_declaration.py

@@ -102,6 +102,53 @@ def simple_function(input_str: dict[str, str]) -> str:
   assert function_decl.name == 'simple_function'
   assert function_decl.parameters.type == 'OBJECT'
   assert function_decl.parameters.properties['input_str'].type == 'OBJECT'
+  assert (
+      function_decl.parameters.properties[
+          'input_str'
+      ].additional_properties.type
+      == 'STRING'
+  )
+
+
+def test_dict_input_with_int_values():
+  def simple_function(input_str: dict[str, int]) -> str:
+    return {'result': input_str}
+
+  function_decl = _automatic_function_calling_util.build_function_declaration(
+      func=simple_function
+  )
+
+  assert function_decl.name == 'simple_function'
+  assert function_decl.parameters.type == 'OBJECT'
+  assert function_decl.parameters.properties['input_str'].type == 'OBJECT'
+  assert (
+      function_decl.parameters.properties[
+          'input_str'
+      ].additional_properties.type
+      == 'INTEGER'
+  )
+
+
+def test_list_of_dict_input():
+  """Test list[dict[str, str]] emits proper schema with additional_properties."""
+
+  def simple_function(fruits: list[dict[str, str]]) -> str:
+    return str(fruits)
+
+  function_decl = _automatic_function_calling_util.build_function_declaration(
+      func=simple_function
+  )
+
+  assert function_decl.name == 'simple_function'
+  assert function_decl.parameters.type == 'OBJECT'
+  assert function_decl.parameters.properties['fruits'].type == 'ARRAY'
+  assert function_decl.parameters.properties['fruits'].items.type == 'OBJECT'
+  assert (
+      function_decl.parameters.properties[
+          'fruits'
+      ].items.additional_properties.type
+      == 'STRING'
+  )
 
 
 def test_basemodel_input():
@@ -279,6 +326,12 @@ def simple_function(
   assert function_decl.parameters.properties['input_str'].items.type == 'STRING'
   assert function_decl.parameters.properties['input_dir'].type == 'ARRAY'
   assert function_decl.parameters.properties['input_dir'].items.type == 'OBJECT'
+  assert (
+      function_decl.parameters.properties[
+          'input_dir'
+      ].items.additional_properties.type
+      == 'STRING'
+  )
 
 
 def test_enums():