feat: expose mcps streamable http custom httpx factory parameter

Merge https://github.com/google/adk-python/pull/3634

Reopen of https://github.com/google/adk-python/pull/2997 after the accepted changes in that PR were reverted by https://github.com/google/adk-python/commit/e15e19da05ee1b763228467e83f6f73e0eced4b5

---

This PR addresses https://github.com/google/adk-python/issues/3005 and https://github.com/google/adk-python/issues/2963 to allow control over the ssl cert used when connecting to an mcp server by exposing the `httpx_client_factory` parameter exposed when creating a `MCPSessionManager` in adk. Overlaps with https://github.com/google/adk-python/pull/2966 but I don't believe that that PR's implementation will work. `streamablehttp_client` needs a client factory, not a client.

## testing plan

Adds test checking that `streamablehttp_client` uses the custom httpx factory. Could also test that a factory which obeys the `McpHttpClientFactory` protocol produces valid behavior when the session is opened?

## related issues

#2227 and #2881 both request _slightly different_ options to control the ssl certs used internally by adk. I think exposing a httpx factory is a good pattern which could be followed for those issues too.

Co-authored-by: Kathy Wu <wukathy@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/3634 from mikeedjones:feat/expose-mcps-streamable-http-custom-httpx-factory-parameter 1017a9875c697b28b96a7a8d1311494ef89af65a
PiperOrigin-RevId: 852443631

src/google/adk/tools/mcp_tool/mcp_session_manager.py

@@ -25,6 +25,8 @@ import sys
 from typing import Any
 from typing import Dict
 from typing import Optional
+from typing import Protocol
+from typing import runtime_checkable
 from typing import TextIO
 from typing import Union
 
@@ -33,8 +35,11 @@ from mcp import ClientSession
 from mcp import StdioServerParameters
 from mcp.client.sse import sse_client
 from mcp.client.stdio import stdio_client
+from mcp.client.streamable_http import create_mcp_http_client
+from mcp.client.streamable_http import McpHttpClientFactory
 from mcp.client.streamable_http import streamablehttp_client
 from pydantic import BaseModel
+from pydantic import ConfigDict
 
 logger = logging.getLogger('google_adk.' + __name__)
 
@@ -73,6 +78,11 @@ class SseConnectionParams(BaseModel):
   sse_read_timeout: float = 60 * 5.0
 
 
+@runtime_checkable
+class CheckableMcpHttpClientFactory(McpHttpClientFactory, Protocol):
+  pass
+
+
 class StreamableHTTPConnectionParams(BaseModel):
   """Parameters for the MCP Streamable HTTP connection.
 
@@ -88,13 +98,18 @@ class StreamableHTTPConnectionParams(BaseModel):
         Streamable HTTP server.
       terminate_on_close: Whether to terminate the MCP Streamable HTTP server
         when the connection is closed.
+      httpx_client_factory: Factory function to create a custom HTTPX client. If
+        not provided, a default factory will be used.
   """
 
+  model_config = ConfigDict(arbitrary_types_allowed=True)
+
   url: str
   headers: dict[str, Any] | None = None
   timeout: float = 5.0
   sse_read_timeout: float = 60 * 5.0
   terminate_on_close: bool = True
+  httpx_client_factory: CheckableMcpHttpClientFactory = create_mcp_http_client
 
 
 def retry_on_errors(func):
@@ -275,6 +290,7 @@ class MCPSessionManager:
               seconds=self._connection_params.sse_read_timeout
           ),
           terminate_on_close=self._connection_params.terminate_on_close,
+          httpx_client_factory=self._connection_params.httpx_client_factory,
       )
     else:
       raise ValueError(

tests/unittests/tools/mcp_tool/test_mcp_session_manager.py

@@ -114,6 +114,54 @@ class TestMCPSessionManager:
 
     assert manager._connection_params == http_params
 
+  @patch("google.adk.tools.mcp_tool.mcp_session_manager.streamablehttp_client")
+  def test_init_with_streamable_http_custom_httpx_factory(
+      self, mock_streamablehttp_client
+  ):
+    """Test that streamablehttp_client is called with custom httpx_client_factory."""
+    custom_httpx_factory = Mock()
+
+    http_params = StreamableHTTPConnectionParams(
+        url="https://example.com/mcp",
+        timeout=15.0,
+        httpx_client_factory=custom_httpx_factory,
+    )
+    manager = MCPSessionManager(http_params)
+
+    manager._create_client()
+
+    mock_streamablehttp_client.assert_called_once_with(
+        url="https://example.com/mcp",
+        headers=None,
+        timeout=timedelta(seconds=15.0),
+        sse_read_timeout=timedelta(seconds=300.0),
+        terminate_on_close=True,
+        httpx_client_factory=custom_httpx_factory,
+    )
+
+  @patch("google.adk.tools.mcp_tool.mcp_session_manager.streamablehttp_client")
+  def test_init_with_streamable_http_default_httpx_factory(
+      self, mock_streamablehttp_client
+  ):
+    """Test that streamablehttp_client is called with default httpx_client_factory."""
+    http_params = StreamableHTTPConnectionParams(
+        url="https://example.com/mcp", timeout=15.0
+    )
+    manager = MCPSessionManager(http_params)
+
+    manager._create_client()
+
+    mock_streamablehttp_client.assert_called_once_with(
+        url="https://example.com/mcp",
+        headers=None,
+        timeout=timedelta(seconds=15.0),
+        sse_read_timeout=timedelta(seconds=300.0),
+        terminate_on_close=True,
+        httpx_client_factory=StreamableHTTPConnectionParams.model_fields[
+            "httpx_client_factory"
+        ].get_default(),
+    )
+
   def test_generate_session_key_stdio(self):
     """Test session key generation for stdio connections."""
     manager = MCPSessionManager(self.mock_stdio_connection_params)