From a0af35f2dccff8c5b820ddffb9b63061346cb35c Mon Sep 17 00:00:00 2001
From: Allen Porter <allen.porter@gmail.com>
Date: Fri, 13 Feb 2026 10:39:34 -0800
Subject: [PATCH] Improve MCP SSE fallback error handling (#162655)

---
 homeassistant/components/mcp/coordinator.py |  8 +++-
 tests/components/mcp/test_init.py           | 41 ++++++++++++++-------
 2 files changed, 35 insertions(+), 14 deletions(-)

diff --git a/homeassistant/components/mcp/coordinator.py b/homeassistant/components/mcp/coordinator.py
index 6c3303c647d..2e299a06605 100644
--- a/homeassistant/components/mcp/coordinator.py
+++ b/homeassistant/components/mcp/coordinator.py
@@ -7,6 +7,7 @@ import datetime
 import logging
 
 import httpx
+from mcp import McpError
 from mcp.client.session import ClientSession
 from mcp.client.sse import sse_client
 from mcp.client.streamable_http import streamable_http_client
@@ -63,10 +64,15 @@ async def mcp_client(
         # Method not Allowed likely means this is not a streamable HTTP server,
         # but it may be an SSE server. This is part of the MCP Transport
         # backwards compatibility specification.
+        # We also handle other generic McpErrors since proxies may not respond
+        # consistently with a 405.
         if (
             isinstance(main_error, httpx.HTTPStatusError)
             and main_error.response.status_code == 405
-        ):
+        ) or isinstance(main_error, McpError):
+            _LOGGER.debug(
+                "Streamable HTTP client failed, attempting SSE client: %s", main_error
+            )
             try:
                 async with (
                     sse_client(url=url, headers=headers) as streams,
diff --git a/tests/components/mcp/test_init.py b/tests/components/mcp/test_init.py
index 24aebe0101f..1f063e445ee 100644
--- a/tests/components/mcp/test_init.py
+++ b/tests/components/mcp/test_init.py
@@ -4,7 +4,8 @@ import re
 from unittest.mock import AsyncMock, Mock, patch
 
 import httpx
-from mcp.types import CallToolResult, ListToolsResult, TextContent, Tool
+from mcp import McpError
+from mcp.types import CallToolResult, ErrorData, ListToolsResult, TextContent, Tool
 import pytest
 import voluptuous as vol
 
@@ -136,30 +137,44 @@ async def test_mcp_server_sse_transport_failure(
         "Connection error", [httpx.ConnectError("Connection failed")]
     )
 
-    await hass.config_entries.async_setup(config_entry.entry_id)
-    assert config_entry.state is ConfigEntryState.SETUP_RETRY
-
 
+@pytest.mark.parametrize(
+    ("side_effect"),
+    [
+        (
+            ExceptionGroup(
+                "Method not allowed",
+                [
+                    httpx.HTTPStatusError(
+                        "Method not allowed",
+                        request=None,
+                        response=httpx.Response(405),
+                    )
+                ],
+            ),
+        ),
+        (
+            ExceptionGroup(
+                "Some exception group",
+                [McpError(ErrorData(code=500, message="Session terminated"))],
+            )
+        ),
+    ],
+)
 async def test_mcp_client_fallback_to_sse_success(
     hass: HomeAssistant,
     config_entry: MockConfigEntry,
     mock_http_streamable_client: AsyncMock,
     mock_sse_client: AsyncMock,
     mock_mcp_client: Mock,
+    side_effect: Exception,
 ) -> None:
-    """Test mcp_client falls back to SSE on method not allowed error.
+    """Test mcp_client falls back to SSE on some errors.
 
     This exercises the backwards compatibility part of the MCP Transport
     specification.
     """
-    http_405 = httpx.HTTPStatusError(
-        "Method not allowed",
-        request=None,  # type: ignore[arg-type]
-        response=httpx.Response(405),
-    )
-    mock_http_streamable_client.side_effect = ExceptionGroup(
-        "Method not allowed", [http_405]
-    )
+    mock_http_streamable_client.side_effect = side_effect
 
     # Setup mocks for SSE fallback
     mock_sse_client.return_value.__aenter__.return_value = ("read", "write")