From 7c282973ea193841fee79f90b8a91c5e02627ccc Mon Sep 17 00:00:00 2001
From: "Xiang (Sean) Zhou" <seanzhougoogle@google.com>
Date: Wed, 14 Jan 2026 19:56:13 -0800
Subject: [PATCH] fix: Support Generator and AsyncGenerator tool declaration

use yield type as return type

Co-authored-by: Xiang (Sean) Zhou <seanzhougoogle@google.com>
PiperOrigin-RevId: 856459995
---
 .../tools/_automatic_function_calling_util.py | 17 ++++
 .../tools/test_from_function_with_options.py  | 77 +++++++++++++++++++
 2 files changed, 94 insertions(+)

diff --git a/src/google/adk/tools/_automatic_function_calling_util.py b/src/google/adk/tools/_automatic_function_calling_util.py
index 92df8871..2b00c799 100644
--- a/src/google/adk/tools/_automatic_function_calling_util.py
+++ b/src/google/adk/tools/_automatic_function_calling_util.py
@@ -14,12 +14,15 @@
 
 from __future__ import annotations
 
+import collections.abc
 import inspect
 from types import FunctionType
 import typing
 from typing import Any
 from typing import Callable
 from typing import Dict
+from typing import get_args
+from typing import get_origin
 from typing import Optional
 from typing import Union
 
@@ -391,6 +394,20 @@ def from_function_with_options(
 
   return_annotation = inspect.signature(func).return_annotation
 
+  # Handle AsyncGenerator and Generator return types (streaming tools)
+  # AsyncGenerator[YieldType, SendType] -> use YieldType as response schema
+  # Generator[YieldType, SendType, ReturnType] -> use YieldType as response schema
+  origin = get_origin(return_annotation)
+  if origin is not None and (
+      origin is collections.abc.AsyncGenerator
+      or origin is collections.abc.Generator
+  ):
+    type_args = get_args(return_annotation)
+    if type_args:
+      # First type argument is the yield type
+      yield_type = type_args[0]
+      return_annotation = yield_type
+
   # Handle functions with no return annotation
   if return_annotation is inspect._empty:
     # Functions with no return annotation can return any type
diff --git a/tests/unittests/tools/test_from_function_with_options.py b/tests/unittests/tools/test_from_function_with_options.py
index 61670a26..eae16453 100644
--- a/tests/unittests/tools/test_from_function_with_options.py
+++ b/tests/unittests/tools/test_from_function_with_options.py
@@ -14,7 +14,9 @@
 
 from collections.abc import Sequence
 from typing import Any
+from typing import AsyncGenerator
 from typing import Dict
+from typing import Generator
 
 from google.adk.tools import _automatic_function_calling_util
 from google.adk.utils.variant_utils import GoogleLLMVariant
@@ -242,3 +244,78 @@ def test_from_function_with_collections_return_type():
   assert declaration.name == 'test_function'
   assert declaration.response.type == types.Type.ARRAY
   assert declaration.response.items.type == types.Type.STRING
+
+
+def test_from_function_with_async_generator_return_vertex():
+  """Test from_function_with_options with AsyncGenerator return for VERTEX_AI."""
+
+  async def test_function(param: str) -> AsyncGenerator[str, None]:
+    """A streaming function that yields strings."""
+    yield param
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert declaration.name == 'test_function'
+  assert declaration.parameters.type == 'OBJECT'
+  assert declaration.parameters.properties['param'].type == 'STRING'
+  # VERTEX_AI should extract yield type (str) from AsyncGenerator[str, None]
+  assert declaration.response is not None
+  assert declaration.response.type == types.Type.STRING
+
+
+def test_from_function_with_async_generator_return_gemini():
+  """Test from_function_with_options with AsyncGenerator return for GEMINI_API."""
+
+  async def test_function(param: str) -> AsyncGenerator[str, None]:
+    """A streaming function that yields strings."""
+    yield param
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.GEMINI_API
+  )
+
+  assert declaration.name == 'test_function'
+  assert declaration.parameters.type == 'OBJECT'
+  assert declaration.parameters.properties['param'].type == 'STRING'
+  # GEMINI_API should not have response schema
+  assert declaration.response is None
+
+
+def test_from_function_with_generator_return_vertex():
+  """Test from_function_with_options with Generator return for VERTEX_AI."""
+
+  def test_function(param: str) -> Generator[int, None, None]:
+    """A streaming function that yields integers."""
+    yield 42
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert declaration.name == 'test_function'
+  assert declaration.parameters.type == 'OBJECT'
+  assert declaration.parameters.properties['param'].type == 'STRING'
+  # VERTEX_AI should extract yield type (int) from Generator[int, None, None]
+  assert declaration.response is not None
+  assert declaration.response.type == types.Type.INTEGER
+
+
+def test_from_function_with_async_generator_complex_yield_type_vertex():
+  """Test from_function_with_options with AsyncGenerator yielding dict."""
+
+  async def test_function(param: str) -> AsyncGenerator[Dict[str, str], None]:
+    """A streaming function that yields dicts."""
+    yield {'result': param}
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert declaration.name == 'test_function'
+  assert declaration.parameters.type == 'OBJECT'
+  assert declaration.parameters.properties['param'].type == 'STRING'
+  # VERTEX_AI should extract yield type (Dict[str, str]) from AsyncGenerator
+  assert declaration.response is not None
+  assert declaration.response.type == types.Type.OBJECT