diff --git a/src/google/adk/tools/_function_parameter_parse_util.py b/src/google/adk/tools/_function_parameter_parse_util.py
index 9eda7a078c..108b07a120 100644
--- a/src/google/adk/tools/_function_parameter_parse_util.py
+++ b/src/google/adk/tools/_function_parameter_parse_util.py
@@ -303,8 +303,9 @@ def _parse_schema_from_parameter(
       schema.default = param.default
     schema.type = types.Type.OBJECT
     schema.properties = {}
+    required_fields = []
     for field_name, field_info in param.annotation.model_fields.items():
-      schema.properties[field_name] = _parse_schema_from_parameter(
+      field_schema = _parse_schema_from_parameter(
           variant,
           inspect.Parameter(
               field_name,
@@ -313,6 +314,17 @@ def _parse_schema_from_parameter(
           ),
           func_name,
       )
+
+      if field_info.description:
+        field_schema.description = field_info.description
+
+      if field_info.is_required():
+        required_fields.append(field_name)
+
+      schema.properties[field_name] = field_schema
+
+    schema.required = required_fields
+
     _raise_if_schema_unsupported(variant, schema)
     return schema
   if param.annotation is None:
diff --git a/tests/unittests/tools/test_build_function_declaration.py b/tests/unittests/tools/test_build_function_declaration.py
index 8be1f86520..8b526378a8 100644
--- a/tests/unittests/tools/test_build_function_declaration.py
+++ b/tests/unittests/tools/test_build_function_declaration.py
@@ -15,6 +15,7 @@
 from enum import Enum
 from typing import Dict
 from typing import List
+from typing import Optional
 
 from google.adk.tools import _automatic_function_calling_util
 from google.adk.tools.tool_context import ToolContext
@@ -23,6 +24,7 @@
 # TODO: crewai requires python 3.10 as minimum
 # from crewai_tools import FileReadTool
 from pydantic import BaseModel
+from pydantic import Field
 import pytest
 
 
@@ -154,33 +156,82 @@ class SimpleFunction(BaseModel):
 
 
 def test_nested_basemodel_input():
-  class ChildInput(BaseModel):
-    input_str: str
+  """Test nested Pydantic models with and without Field annotations."""
 
-  class CustomInput(BaseModel):
-    child: ChildInput
+  class ChildInput(BaseModel):
+    name: str = Field(description='The name of the child')
+    age: int  # No Field annotation
+    nickname: Optional[str] = Field(
+        default=None, description='Optional nickname'
+    )
+    email: Optional[str] = None  # No Field annotation, Optional with default
+
+  class ParentInput(BaseModel):
+    title: str = Field(description='The title of the parent')
+    basic_field: str  # No Field annotation
+    child: ChildInput = Field(description='Child information')
+    optional_field: Optional[str] = Field(
+        default='default_value', description='An optional field with default'
+    )
+    status: Optional[str] = None  # No Field annotation, Optional with default
 
-  def simple_function(input: CustomInput) -> str:
+  def simple_function(input: ParentInput) -> str:
     return {'result': input}
 
   function_decl = _automatic_function_calling_util.build_function_declaration(
       func=simple_function
   )
 
+  # Check top-level structure
   assert function_decl.name == 'simple_function'
   assert function_decl.parameters.type == 'OBJECT'
   assert function_decl.parameters.properties['input'].type == 'OBJECT'
+
+  # Check ParentInput properties with and without Field annotations
+  parent_props = function_decl.parameters.properties['input'].properties
+  assert parent_props['title'].type == 'STRING'
+  assert parent_props['title'].description == 'The title of the parent'
+  assert parent_props['basic_field'].type == 'STRING'
+  assert parent_props['basic_field'].description is None  # No Field annotation
+  assert parent_props['child'].type == 'OBJECT'
+  assert parent_props['child'].description == 'Child information'
+  assert parent_props['optional_field'].type == 'STRING'
   assert (
-      function_decl.parameters.properties['input'].properties['child'].type
-      == 'OBJECT'
+      parent_props['optional_field'].description
+      == 'An optional field with default'
   )
+  assert parent_props['status'].type == 'STRING'
+  assert parent_props['status'].description is None  # No Field annotation
+
+  # Check ParentInput required fields
+  parent_required = function_decl.parameters.properties['input'].required
+  assert 'title' in parent_required
+  assert 'basic_field' in parent_required
+  assert 'child' in parent_required
+  assert 'optional_field' not in parent_required  # Has default value
   assert (
-      function_decl.parameters.properties['input']
-      .properties['child']
-      .properties['input_str']
-      .type
-      == 'STRING'
-  )
+      'status' not in parent_required
+  )  # No Field annotation, Optional with default
+
+  # Check ChildInput properties with and without Field annotations
+  child_props = parent_props['child'].properties
+  assert child_props['name'].type == 'STRING'
+  assert child_props['name'].description == 'The name of the child'
+  assert child_props['age'].type == 'INTEGER'
+  assert child_props['age'].description is None  # No Field annotation
+  assert child_props['nickname'].type == 'STRING'
+  assert child_props['nickname'].description == 'Optional nickname'
+  assert child_props['email'].type == 'STRING'
+  assert child_props['email'].description is None  # No Field annotation
+
+  # Check ChildInput required fields
+  child_required = parent_props['child'].required
+  assert 'name' in child_required
+  assert 'age' in child_required
+  assert 'nickname' not in child_required  # Optional with default None
+  assert (
+      'email' not in child_required
+  )  # No Field annotation, Optional with default
 
 
 def test_basemodel_with_nested_basemodel():
@@ -223,7 +274,6 @@ def simple_function(
 
 
 def test_enums():
-
   class InputEnum(Enum):
     AGENT = 'agent'
     TOOL = 'tool'