feat(tools): expose httpx_client_factory on RestApiTool and OpenAPIToolset

Mirror the pattern merged for MCP in #2997 (StreamableHTTPConnectionParams.
httpx_client_factory) on the OpenAPI tool surface.

Adds an optional httpx_client_factory parameter to:

  - RestApiTool.__init__
  - RestApiTool.from_parsed_operation
  - OpenAPIToolset.__init__

OpenAPIToolset forwards the factory to every generated RestApiTool the same
way ssl_verify and header_provider are already forwarded. When provided, the
factory's client is used to issue each API call; when None (default), the
existing httpx.AsyncClient(verify=..., timeout=None) construction is
preserved exactly. This unlocks httpx.AsyncClient knobs that the narrower
ssl_verify parameter can't reach: proxies, HTTP/2, custom transports
(e.g. request signing), and shared connection pools.

Closes #5681

Author: darshil3011

src/google/adk/tools/openapi_tool/openapi_spec_parser/openapi_toolset.py

@@ -36,6 +36,7 @@
 from ...base_toolset import BaseToolset
 from ...base_toolset import ToolPredicate
 from .openapi_spec_parser import OpenApiSpecParser
+from .rest_api_tool import HttpxClientFactory
 from .rest_api_tool import RestApiTool
 
 logger = logging.getLogger("google_adk." + __name__)
@@ -77,6 +78,7 @@ def __init__(
       header_provider: Optional[
           Callable[[ReadonlyContext], Dict[str, str]]
       ] = None,
+      httpx_client_factory: Optional[HttpxClientFactory] = None,
       preserve_property_names: bool = False,
   ):
     """Initializes the OpenAPIToolset.
@@ -130,6 +132,14 @@ def __init__(
         an argument, allowing dynamic header generation based on the current
         context. Useful for adding custom headers like correlation IDs,
         authentication tokens, or other request metadata.
+      httpx_client_factory: Optional zero-argument callable returning an
+        ``httpx.AsyncClient`` to use for every generated tool's API calls.
+        When provided, it takes precedence over the per-tool default client
+        construction and unlocks ``httpx.AsyncClient`` options that
+        ``ssl_verify`` can't reach (proxies, HTTP/2, custom transports such as
+        request signing, shared connection pools). Defaults to ``None``, which
+        preserves today's behaviour. Mirrors the pattern exposed for MCP by
+        ``StreamableHTTPConnectionParams.httpx_client_factory``.
       preserve_property_names: If True, preserve the original property names
         from the OpenAPI spec instead of converting them to snake_case. This
         is useful when calling APIs that expect camelCase or other
@@ -155,6 +165,7 @@ def __init__(
     if not spec_dict:
       spec_dict = self._load_spec(spec_str, spec_str_type)
     self._ssl_verify = ssl_verify
+    self._httpx_client_factory = httpx_client_factory
     self._tools: Final[List[RestApiTool]] = list(self._parse(spec_dict))
     if auth_scheme or auth_credential:
       self._configure_auth_all(auth_scheme, auth_credential)
@@ -237,6 +248,7 @@ def _parse(self, openapi_spec_dict: Dict[str, Any]) -> List[RestApiTool]:
           o,
           ssl_verify=self._ssl_verify,
           header_provider=self._header_provider,
+          httpx_client_factory=self._httpx_client_factory,
       )
       logger.info("Parsed tool: %s", tool.name)
       tools.append(tool)

src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py

@@ -75,6 +75,17 @@ def snake_to_lower_camel(snake_case_string: str):
 
 AuthPreparationState = Literal["pending", "done"]
 
+HttpxClientFactory = Callable[..., httpx.AsyncClient]
+"""Type alias for a factory returning an ``httpx.AsyncClient``.
+
+When supplied to ``RestApiTool`` or ``OpenAPIToolset``, the factory is invoked
+once per API call and its returned client is used (as an async context
+manager) to issue the request, in place of the default
+``httpx.AsyncClient(verify=..., timeout=None)``. This unlocks knobs that the
+narrower ``ssl_verify`` parameter can't reach: proxies, HTTP/2, custom
+transports (e.g. request-signing), shared connection pools, and so on.
+"""
+
 
 class RestApiTool(BaseTool):
   """A generic tool that interacts with a REST API.
@@ -103,6 +114,7 @@ def __init__(
       header_provider: Optional[
           Callable[[ReadonlyContext], Dict[str, str]]
       ] = None,
+      httpx_client_factory: Optional[HttpxClientFactory] = None,
       *,
       credential_key: Optional[str] = None,
   ):
@@ -142,6 +154,15 @@ def __init__(
           an argument, allowing dynamic header generation based on the current
           context. Useful for adding custom headers like correlation IDs,
           authentication tokens, or other request metadata.
+        httpx_client_factory: Optional zero-argument callable returning an
+          ``httpx.AsyncClient``. When provided, the returned client is used to
+          issue the request, allowing callers to configure proxies, HTTP/2,
+          custom transports (e.g. request signing), shared connection pools,
+          or any other ``httpx.AsyncClient`` option that ``ssl_verify`` can't
+          reach. When ``None`` (default), behaviour is unchanged: a fresh
+          ``httpx.AsyncClient(verify=..., timeout=None)`` is created per
+          request. Mirrors the pattern exposed for MCP by
+          ``StreamableHTTPConnectionParams.httpx_client_factory``.
         credential_key: Optional stable key used for interactive auth and
           credential caching.
     """
@@ -169,6 +190,7 @@ def __init__(
     self._default_headers: Dict[str, str] = {}
     self._ssl_verify = ssl_verify
     self._header_provider = header_provider
+    self._httpx_client_factory = httpx_client_factory
     self._logger = logger
     if should_parse_operation:
       self._operation_parser = OperationParser(self.operation)
@@ -181,6 +203,7 @@ def from_parsed_operation(
       header_provider: Optional[
           Callable[[ReadonlyContext], Dict[str, str]]
       ] = None,
+      httpx_client_factory: Optional[HttpxClientFactory] = None,
   ) -> "RestApiTool":
     """Initializes the RestApiTool from a ParsedOperation object.
 
@@ -192,6 +215,9 @@ def from_parsed_operation(
           an argument, allowing dynamic header generation based on the current
           context. Useful for adding custom headers like correlation IDs,
           authentication tokens, or other request metadata.
+        httpx_client_factory: Optional zero-argument callable returning an
+          ``httpx.AsyncClient`` to be used for the API call. See
+          ``RestApiTool.__init__`` for details.
 
     Returns:
         A RestApiTool object.
@@ -212,6 +238,7 @@ def from_parsed_operation(
         auth_credential=parsed.auth_credential,
         ssl_verify=ssl_verify,
         header_provider=header_provider,
+        httpx_client_factory=httpx_client_factory,
     )
     generated._operation_parser = operation_parser
     return generated
@@ -520,7 +547,9 @@ async def call(
       if provider_headers:
         request_params.setdefault("headers", {}).update(provider_headers)
 
-    response = await _request(**request_params)
+    response = await _request(
+        httpx_client_factory=self._httpx_client_factory, **request_params
+    )
 
     # Log the API response
     self._logger.debug(
@@ -569,9 +598,14 @@ def __repr__(self):
     )
 
 
-async def _request(**request_params) -> httpx.Response:
-  async with httpx.AsyncClient(
-      verify=request_params.pop("verify", True),
-      timeout=None,
-  ) as client:
+async def _request(
+    *,
+    httpx_client_factory: Optional[HttpxClientFactory] = None,
+    **request_params,
+) -> httpx.Response:
+  verify = request_params.pop("verify", True)
+  if httpx_client_factory is not None:
+    async with httpx_client_factory() as client:
+      return await client.request(**request_params)
+  async with httpx.AsyncClient(verify=verify, timeout=None) as client:
     return await client.request(**request_params)

tests/unittests/tools/openapi_tool/openapi_spec_parser/test_openapi_toolset.py

@@ -153,6 +153,29 @@ def test_openapi_toolset_verify_on_init(
   assert all(tool._ssl_verify == verify_value for tool in toolset._tools)
 
 
+def test_openapi_toolset_httpx_client_factory_on_init(
+    openapi_spec: Dict[str, Any],
+):
+  """The httpx_client_factory is forwarded to every generated tool."""
+  custom_factory = lambda: None  # noqa: E731 - placeholder, never invoked here
+  toolset = OpenAPIToolset(
+      spec_dict=openapi_spec, httpx_client_factory=custom_factory
+  )
+  assert toolset._httpx_client_factory is custom_factory
+  assert all(
+      tool._httpx_client_factory is custom_factory for tool in toolset._tools
+  )
+
+
+def test_openapi_toolset_httpx_client_factory_none_by_default(
+    openapi_spec: Dict[str, Any],
+):
+  """httpx_client_factory is None on the toolset and each tool by default."""
+  toolset = OpenAPIToolset(spec_dict=openapi_spec)
+  assert toolset._httpx_client_factory is None
+  assert all(tool._httpx_client_factory is None for tool in toolset._tools)
+
+
 def test_openapi_toolset_configure_verify_all(openapi_spec: Dict[str, Any]):
   """Test configure_verify_all method."""
   toolset = OpenAPIToolset(spec_dict=openapi_spec)

tests/unittests/tools/openapi_tool/openapi_spec_parser/test_rest_api_tool.py

@@ -1339,6 +1339,113 @@ async def test_call_without_header_provider(
 
       assert result == {"result": "success"}
 
+  def test_init_httpx_client_factory_none_by_default(
+      self,
+      sample_endpoint,
+      sample_operation,
+  ):
+    """httpx_client_factory is None by default."""
+    tool = RestApiTool(
+        name="test_tool",
+        description="Test Tool",
+        endpoint=sample_endpoint,
+        operation=sample_operation,
+    )
+    assert tool._httpx_client_factory is None
+
+  def test_init_with_httpx_client_factory(
+      self,
+      sample_endpoint,
+      sample_operation,
+  ):
+    """A user-supplied httpx_client_factory is stored on the tool."""
+    custom_factory = MagicMock()
+    tool = RestApiTool(
+        name="test_tool",
+        description="Test Tool",
+        endpoint=sample_endpoint,
+        operation=sample_operation,
+        httpx_client_factory=custom_factory,
+    )
+    assert tool._httpx_client_factory is custom_factory
+
+  @pytest.mark.asyncio
+  async def test_call_uses_custom_httpx_client_factory(
+      self,
+      mock_tool_context,
+      sample_endpoint,
+      sample_operation,
+      sample_auth_scheme,
+      sample_auth_credential,
+  ):
+    """When a factory is provided, its client is used to issue the request."""
+    mock_response = mock.create_autospec(requests.Response, instance=True)
+    mock_response.json.return_value = {"result": "success"}
+    mock_response.configure_mock(status_code=200)
+
+    mock_client = mock.create_autospec(
+        httpx.AsyncClient, instance=True, spec_set=True
+    )
+    mock_client.request = AsyncMock(return_value=mock_response)
+    # Make the mock client work as an async context manager.
+    mock_client.__aenter__ = AsyncMock(return_value=mock_client)
+    mock_client.__aexit__ = AsyncMock(return_value=None)
+
+    custom_factory = MagicMock(return_value=mock_client)
+
+    tool = RestApiTool(
+        name="test_tool",
+        description="Test Tool",
+        endpoint=sample_endpoint,
+        operation=sample_operation,
+        auth_scheme=sample_auth_scheme,
+        auth_credential=sample_auth_credential,
+        httpx_client_factory=custom_factory,
+    )
+
+    with patch.object(httpx, "AsyncClient", autospec=True) as mock_default:
+      result = await tool.call(args={}, tool_context=mock_tool_context)
+
+    # Factory must be invoked once and the default client must not be built.
+    custom_factory.assert_called_once_with()
+    mock_default.assert_not_called()
+    mock_client.request.assert_awaited_once()
+    assert result == {"result": "success"}
+
+  @pytest.mark.asyncio
+  async def test_call_without_httpx_client_factory_uses_default_client(
+      self,
+      mock_tool_context,
+      sample_endpoint,
+      sample_operation,
+      sample_auth_scheme,
+      sample_auth_credential,
+  ):
+    """When no factory is provided, the default httpx.AsyncClient is used."""
+    mock_response = mock.create_autospec(requests.Response, instance=True)
+    mock_response.json.return_value = {"result": "success"}
+    mock_response.configure_mock(status_code=200)
+
+    mock_client = mock.create_autospec(
+        httpx.AsyncClient, instance=True, spec_set=True
+    )
+    mock_client.request = AsyncMock(return_value=mock_response)
+
+    tool = RestApiTool(
+        name="test_tool",
+        description="Test Tool",
+        endpoint=sample_endpoint,
+        operation=sample_operation,
+        auth_scheme=sample_auth_scheme,
+        auth_credential=sample_auth_credential,
+    )
+
+    with patch.object(
+        httpx, "AsyncClient", return_value=mock_client, autospec=True
+    ) as mock_async_client:
+      await tool.call(args={}, tool_context=mock_tool_context)
+      assert mock_async_client.called
+
   def test_prepare_request_params_extracts_embedded_query_params(
       self, sample_auth_credential, sample_auth_scheme
   ):