Avoid blocking SSL context creation in the MCP SSE transport (#180367)

homeassistant/components/mcp/coordinator.py

@@ -27,6 +27,7 @@ from homeassistant.helpers import llm
 from homeassistant.helpers.httpx_client import create_async_httpx_client
 from homeassistant.helpers.update_coordinator import DataUpdateCoordinator, UpdateFailed
 from homeassistant.util.json import JsonObjectType
+from homeassistant.util.ssl import SSL_ALPN_HTTP11, SSLCipherList, client_context
 
 from .auth import AuthenticateHeader
 from .const import DOMAIN
@@ -39,6 +40,26 @@ TIMEOUT = 10
 type TokenManager = Callable[[], Awaitable[str]]
 
 
+def _create_sse_httpx_client(
+    headers: dict[str, str] | None = None,
+    timeout: httpx.Timeout | None = None,
+    auth: httpx.Auth | None = None,
+) -> httpx.AsyncClient:
+    """Create the httpx client used by the SSE transport.
+
+    The SSE transport closes the client itself, so it cannot be handed one of
+    the Home Assistant managed clients. Building it here keeps it off the SDK
+    default, which reads the CA bundle from disk inside the event loop.
+    """
+    return httpx.AsyncClient(
+        verify=client_context(SSLCipherList.PYTHON_DEFAULT, SSL_ALPN_HTTP11),
+        follow_redirects=True,
+        headers=headers,
+        timeout=timeout,
+        auth=auth,
+    )
+
+
 @asynccontextmanager
 async def mcp_client(
     hass: HomeAssistant,
@@ -81,7 +102,11 @@ async def mcp_client(
             )
             try:
                 async with (
-                    sse_client(url=url, headers=headers) as streams,
+                    sse_client(
+                        url=url,
+                        headers=headers,
+                        httpx_client_factory=_create_sse_httpx_client,
+                    ) as streams,
                     ClientSession(*streams) as session,
                 ):
                     await session.initialize()

tests/components/mcp/test_init.py

@@ -1,6 +1,7 @@
 """Tests for the Model Context Protocol component."""
 
 import re
+import ssl
 from unittest.mock import AsyncMock, Mock, patch
 
 import httpx
@@ -705,3 +706,31 @@ async def test_tool_call_http_error(
             ),
             create_llm_context(),
         )
+
+
+async def test_sse_client_does_not_build_ssl_context(
+    hass: HomeAssistant,
+    config_entry: MockConfigEntry,
+    mock_http_streamable_client: AsyncMock,
+    mock_sse_client: AsyncMock,
+) -> None:
+    """Test the SSE transport does not load certificates in the event loop."""
+    http_405 = httpx.HTTPStatusError(
+        "Method not allowed", request=None, response=httpx.Response(405)
+    )
+    mock_http_streamable_client.side_effect = ExceptionGroup(
+        "Method not allowed", [http_405]
+    )
+    mock_sse_client.side_effect = ExceptionGroup(
+        "Connection error", [httpx.ConnectError("Connection failed")]
+    )
+
+    await hass.config_entries.async_setup(config_entry.entry_id)
+    assert config_entry.state is ConfigEntryState.SETUP_RETRY
+
+    client_factory = mock_sse_client.call_args.kwargs["httpx_client_factory"]
+    with patch.object(ssl.SSLContext, "load_verify_locations") as mock_load_certs:
+        client = client_factory(headers={}, timeout=httpx.Timeout(5))
+
+    assert not mock_load_certs.called
+    await client.aclose()