diff --git a/src/google/adk/tools/function_tool.py b/src/google/adk/tools/function_tool.py
index 15b9c6b..96ffe67 100644
--- a/src/google/adk/tools/function_tool.py
+++ b/src/google/adk/tools/function_tool.py
@@ -59,6 +59,23 @@ class FunctionTool(BaseTool):
     if 'tool_context' in signature.parameters:
       args_to_call['tool_context'] = tool_context
 
+    # Before invoking the function, we check for if the list of args passed in
+    # has all the mandatory arguments or not.
+    # If the check fails, then we don't invoke the tool and let the Agent know
+    # that there was a missing a input parameter. This will basically help
+    # the underlying model fix the issue and retry.
+    mandatory_args = self._get_mandatory_args()
+    missing_mandatory_args = [
+        arg for arg in mandatory_args if arg not in args_to_call
+    ]
+
+    if missing_mandatory_args:
+      missing_mandatory_args_str = '\n'.join(missing_mandatory_args)
+      error_str = f"""Invoking `{self.name}()` failed as the following mandatory input parameters are not present:
+{missing_mandatory_args_str}
+You could retry calling this tool, but it is IMPORTANT for you to provide all the mandatory parameters."""
+      return {'error': error_str}
+
     if inspect.iscoroutinefunction(self.func):
       return await self.func(**args_to_call) or {}
     else:
@@ -85,3 +102,28 @@ class FunctionTool(BaseTool):
       args_to_call['tool_context'] = tool_context
     async for item in self.func(**args_to_call):
       yield item
+
+  def _get_mandatory_args(
+      self,
+  ) -> list[str]:
+    """Identifies mandatory parameters (those without default values) for a function.
+
+    Returns:
+      A list of strings, where each string is the name of a mandatory parameter.
+    """
+    signature = inspect.signature(self.func)
+    mandatory_params = []
+
+    for name, param in signature.parameters.items():
+      # A parameter is mandatory if:
+      # 1. It has no default value (param.default is inspect.Parameter.empty)
+      # 2. It's not a variable positional (*args) or variable keyword (**kwargs) parameter
+      #
+      # For more refer to: https://docs.python.org/3/library/inspect.html#inspect.Parameter.kind
+      if param.default == inspect.Parameter.empty and param.kind not in (
+          inspect.Parameter.VAR_POSITIONAL,
+          inspect.Parameter.VAR_KEYWORD,
+      ):
+        mandatory_params.append(name)
+
+    return mandatory_params
diff --git a/tests/unittests/tools/test_function_tool.py b/tests/unittests/tools/test_function_tool.py
new file mode 100644
index 0000000..60c432f
--- /dev/null
+++ b/tests/unittests/tools/test_function_tool.py
@@ -0,0 +1,238 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from unittest.mock import MagicMock
+
+from google.adk.tools.function_tool import FunctionTool
+import pytest
+
+
+def function_for_testing_with_no_args():
+  """Function for testing with no args."""
+  pass
+
+
+async def async_function_for_testing_with_1_arg_and_tool_context(
+    arg1, tool_context
+):
+  """Async function for testing with 1 arge and tool context."""
+  assert arg1
+  assert tool_context
+  return arg1
+
+
+async def async_function_for_testing_with_2_arg_and_no_tool_context(arg1, arg2):
+  """Async function for testing with 2 arge and no tool context."""
+  assert arg1
+  assert arg2
+  return arg1
+
+
+def function_for_testing_with_1_arg_and_tool_context(arg1, tool_context):
+  """Function for testing with 1 arge and tool context."""
+  assert arg1
+  assert tool_context
+  return arg1
+
+
+def function_for_testing_with_2_arg_and_no_tool_context(arg1, arg2):
+  """Function for testing with 2 arge and no tool context."""
+  assert arg1
+  assert arg2
+  return arg1
+
+
+async def async_function_for_testing_with_4_arg_and_no_tool_context(
+    arg1, arg2, arg3, arg4
+):
+  """Async function for testing with 4 args."""
+  pass
+
+
+def function_for_testing_with_4_arg_and_no_tool_context(arg1, arg2, arg3, arg4):
+  """Function for testing with 4 args."""
+  pass
+
+
+def test_init():
+  """Test that the FunctionTool is initialized correctly."""
+  tool = FunctionTool(function_for_testing_with_no_args)
+  assert tool.name == "function_for_testing_with_no_args"
+  assert tool.description == "Function for testing with no args."
+  assert tool.func == function_for_testing_with_no_args
+
+
+@pytest.mark.asyncio
+async def test_run_async_with_tool_context_async_func():
+  """Test that run_async calls the function with tool_context when tool_context is in signature (async function)."""
+
+  tool = FunctionTool(async_function_for_testing_with_1_arg_and_tool_context)
+  args = {"arg1": "test_value_1"}
+  result = await tool.run_async(args=args, tool_context=MagicMock())
+  assert result == "test_value_1"
+
+
+@pytest.mark.asyncio
+async def test_run_async_without_tool_context_async_func():
+  """Test that run_async calls the function without tool_context when tool_context is not in signature (async function)."""
+  tool = FunctionTool(async_function_for_testing_with_2_arg_and_no_tool_context)
+  args = {"arg1": "test_value_1", "arg2": "test_value_2"}
+  result = await tool.run_async(args=args, tool_context=MagicMock())
+  assert result == "test_value_1"
+
+
+@pytest.mark.asyncio
+async def test_run_async_with_tool_context_sync_func():
+  """Test that run_async calls the function with tool_context when tool_context is in signature (synchronous function)."""
+  tool = FunctionTool(function_for_testing_with_1_arg_and_tool_context)
+  args = {"arg1": "test_value_1"}
+  result = await tool.run_async(args=args, tool_context=MagicMock())
+  assert result == "test_value_1"
+
+
+@pytest.mark.asyncio
+async def test_run_async_without_tool_context_sync_func():
+  """Test that run_async calls the function without tool_context when tool_context is not in signature (synchronous function)."""
+  tool = FunctionTool(function_for_testing_with_2_arg_and_no_tool_context)
+  args = {"arg1": "test_value_1", "arg2": "test_value_2"}
+  result = await tool.run_async(args=args, tool_context=MagicMock())
+  assert result == "test_value_1"
+
+
+@pytest.mark.asyncio
+async def test_run_async_1_missing_arg_sync_func():
+  """Test that run_async calls the function with 1 missing arg in signature (synchronous function)."""
+  tool = FunctionTool(function_for_testing_with_2_arg_and_no_tool_context)
+  args = {"arg1": "test_value_1"}
+  result = await tool.run_async(args=args, tool_context=MagicMock())
+  assert result == {
+      "error": (
+          """Invoking `function_for_testing_with_2_arg_and_no_tool_context()` failed as the following mandatory input parameters are not present:
+arg2
+You could retry calling this tool, but it is IMPORTANT for you to provide all the mandatory parameters."""
+      )
+  }
+
+
+@pytest.mark.asyncio
+async def test_run_async_1_missing_arg_async_func():
+  """Test that run_async calls the function with 1 missing arg in signature (async function)."""
+  tool = FunctionTool(async_function_for_testing_with_2_arg_and_no_tool_context)
+  args = {"arg2": "test_value_1"}
+  result = await tool.run_async(args=args, tool_context=MagicMock())
+  assert result == {
+      "error": (
+          """Invoking `async_function_for_testing_with_2_arg_and_no_tool_context()` failed as the following mandatory input parameters are not present:
+arg1
+You could retry calling this tool, but it is IMPORTANT for you to provide all the mandatory parameters."""
+      )
+  }
+
+
+@pytest.mark.asyncio
+async def test_run_async_3_missing_arg_sync_func():
+  """Test that run_async calls the function with 3 missing args in signature (synchronous function)."""
+  tool = FunctionTool(function_for_testing_with_4_arg_and_no_tool_context)
+  args = {"arg2": "test_value_1"}
+  result = await tool.run_async(args=args, tool_context=MagicMock())
+  assert result == {
+      "error": (
+          """Invoking `function_for_testing_with_4_arg_and_no_tool_context()` failed as the following mandatory input parameters are not present:
+arg1
+arg3
+arg4
+You could retry calling this tool, but it is IMPORTANT for you to provide all the mandatory parameters."""
+      )
+  }
+
+
+@pytest.mark.asyncio
+async def test_run_async_3_missing_arg_async_func():
+  """Test that run_async calls the function with 3 missing args in signature (async function)."""
+  tool = FunctionTool(async_function_for_testing_with_4_arg_and_no_tool_context)
+  args = {"arg3": "test_value_1"}
+  result = await tool.run_async(args=args, tool_context=MagicMock())
+  assert result == {
+      "error": (
+          """Invoking `async_function_for_testing_with_4_arg_and_no_tool_context()` failed as the following mandatory input parameters are not present:
+arg1
+arg2
+arg4
+You could retry calling this tool, but it is IMPORTANT for you to provide all the mandatory parameters."""
+      )
+  }
+
+
+@pytest.mark.asyncio
+async def test_run_async_missing_all_arg_sync_func():
+  """Test that run_async calls the function with all missing args in signature (synchronous function)."""
+  tool = FunctionTool(function_for_testing_with_4_arg_and_no_tool_context)
+  args = {}
+  result = await tool.run_async(args=args, tool_context=MagicMock())
+  assert result == {
+      "error": (
+          """Invoking `function_for_testing_with_4_arg_and_no_tool_context()` failed as the following mandatory input parameters are not present:
+arg1
+arg2
+arg3
+arg4
+You could retry calling this tool, but it is IMPORTANT for you to provide all the mandatory parameters."""
+      )
+  }
+
+
+@pytest.mark.asyncio
+async def test_run_async_missing_all_arg_async_func():
+  """Test that run_async calls the function with all missing args in signature (async function)."""
+  tool = FunctionTool(async_function_for_testing_with_4_arg_and_no_tool_context)
+  args = {}
+  result = await tool.run_async(args=args, tool_context=MagicMock())
+  assert result == {
+      "error": (
+          """Invoking `async_function_for_testing_with_4_arg_and_no_tool_context()` failed as the following mandatory input parameters are not present:
+arg1
+arg2
+arg3
+arg4
+You could retry calling this tool, but it is IMPORTANT for you to provide all the mandatory parameters."""
+      )
+  }
+
+
+@pytest.mark.asyncio
+async def test_run_async_with_optional_args_not_set_sync_func():
+  """Test that run_async calls the function for sync funciton with optional args not set."""
+
+  def func_with_optional_args(arg1, arg2=None, *, arg3, arg4=None, **kwargs):
+    return f"{arg1},{arg3}"
+
+  tool = FunctionTool(func_with_optional_args)
+  args = {"arg1": "test_value_1", "arg3": "test_value_3"}
+  result = await tool.run_async(args=args, tool_context=MagicMock())
+  assert result == "test_value_1,test_value_3"
+
+
+@pytest.mark.asyncio
+async def test_run_async_with_optional_args_not_set_async_func():
+  """Test that run_async calls the function for async funciton with optional args not set."""
+
+  async def async_func_with_optional_args(
+      arg1, arg2=None, *, arg3, arg4=None, **kwargs
+  ):
+    return f"{arg1},{arg3}"
+
+  tool = FunctionTool(async_func_with_optional_args)
+  args = {"arg1": "test_value_1", "arg3": "test_value_3"}
+  result = await tool.run_async(args=args, tool_context=MagicMock())
+  assert result == "test_value_1,test_value_3"