Fix SSE transport invalid parameter issue

Remove terminate_on_close parameter from sse_client call in SSETransport.
This parameter is only valid for streamable HTTP transport, not for SSE transport.

Fixes #3938

Changes:
- Remove terminate_on_close=True from sse_client() call in SSETransport.connect()
- Add comprehensive unit tests for SSETransport that mock sse_client and verify correct parameters
- Add error-path tests to ensure Agent._get_native_mcp_tools raises proper RuntimeError instead of UnboundLocalError when connection fails

Co-Authored-By: João <joao@crewai.com>

lib/crewai/src/crewai/mcp/transports/sse.py

@@ -66,7 +66,6 @@ class SSETransport(BaseTransport):
             self._transport_context = sse_client(
                 self.url,
                 headers=self.headers if self.headers else None,
-                terminate_on_close=True,
             )
 
             read, write = await self._transport_context.__aenter__()

lib/crewai/tests/mcp/test_agent_mcp_error_handling.py

@@ -0,0 +1,78 @@
+import sys
+from types import ModuleType
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from crewai.agent.core import Agent
+from crewai.mcp import MCPServerSSE
+
+
+class FakeSSEClientError:
+    def __init__(self, url, headers=None):
+        self.url = url
+        self.headers = headers
+
+    async def __aenter__(self):
+        raise Exception("SSE connection failed")
+
+    async def __aexit__(self, exc_type, exc_val, exc_tb):
+        pass
+
+
+@pytest.fixture
+def mock_mcp_sse_error():
+    fake_mcp = ModuleType("mcp")
+    fake_mcp_client = ModuleType("mcp.client")
+    fake_mcp_client_sse = ModuleType("mcp.client.sse")
+
+    sys.modules["mcp"] = fake_mcp
+    sys.modules["mcp.client"] = fake_mcp_client
+    sys.modules["mcp.client.sse"] = fake_mcp_client_sse
+
+    mock_sse_client = MagicMock(side_effect=FakeSSEClientError)
+    fake_mcp_client_sse.sse_client = mock_sse_client
+
+    yield mock_sse_client
+
+    del sys.modules["mcp.client.sse"]
+    del sys.modules["mcp.client"]
+    del sys.modules["mcp"]
+
+
+def test_agent_get_native_mcp_tools_raises_runtime_error_not_unbound_local_error(
+    mock_mcp_sse_error,
+):
+    agent = Agent(
+        role="Test Agent",
+        goal="Test goal",
+        backstory="Test backstory",
+    )
+
+    mcp_config = MCPServerSSE(
+        url="https://example.com/sse",
+        headers={"Authorization": "Bearer token"},
+    )
+
+    with pytest.raises(RuntimeError, match="Failed to get native MCP tools"):
+        agent._get_native_mcp_tools(mcp_config)
+
+
+def test_agent_get_native_mcp_tools_error_message_contains_original_error(
+    mock_mcp_sse_error,
+):
+    agent = Agent(
+        role="Test Agent",
+        goal="Test goal",
+        backstory="Test backstory",
+    )
+
+    mcp_config = MCPServerSSE(
+        url="https://example.com/sse",
+    )
+
+    with pytest.raises(RuntimeError) as exc_info:
+        agent._get_native_mcp_tools(mcp_config)
+
+    assert "Failed to get native MCP tools" in str(exc_info.value)
+    assert exc_info.value.__cause__ is not None

lib/crewai/tests/mcp/transports/__init__.py

@@ -0,0 +1 @@
+

lib/crewai/tests/mcp/transports/test_sse_transport.py

@@ -0,0 +1,112 @@
+import sys
+from types import ModuleType
+from unittest.mock import AsyncMock, MagicMock, call
+
+import pytest
+
+from crewai.mcp.transports.sse import SSETransport
+
+
+class FakeSSEClient:
+    def __init__(self, url, headers=None):
+        self.url = url
+        self.headers = headers
+        self._read = AsyncMock()
+        self._write = AsyncMock()
+
+    async def __aenter__(self):
+        return (self._read, self._write)
+
+    async def __aexit__(self, exc_type, exc_val, exc_tb):
+        pass
+
+
+@pytest.fixture
+def mock_mcp_sse():
+    fake_mcp = ModuleType("mcp")
+    fake_mcp_client = ModuleType("mcp.client")
+    fake_mcp_client_sse = ModuleType("mcp.client.sse")
+
+    sys.modules["mcp"] = fake_mcp
+    sys.modules["mcp.client"] = fake_mcp_client
+    sys.modules["mcp.client.sse"] = fake_mcp_client_sse
+
+    mock_sse_client = MagicMock(side_effect=FakeSSEClient)
+    fake_mcp_client_sse.sse_client = mock_sse_client
+
+    yield mock_sse_client
+
+    del sys.modules["mcp.client.sse"]
+    del sys.modules["mcp.client"]
+    del sys.modules["mcp"]
+
+
+@pytest.mark.asyncio
+async def test_sse_transport_connect_without_terminate_on_close(mock_mcp_sse):
+    transport = SSETransport(
+        url="https://example.com/sse",
+        headers={"Authorization": "Bearer token"},
+    )
+
+    await transport.connect()
+
+    mock_mcp_sse.assert_called_once_with(
+        "https://example.com/sse",
+        headers={"Authorization": "Bearer token"},
+    )
+
+    call_kwargs = mock_mcp_sse.call_args[1]
+    assert "terminate_on_close" not in call_kwargs
+
+    assert transport._connected is True
+
+
+@pytest.mark.asyncio
+async def test_sse_transport_connect_without_headers(mock_mcp_sse):
+    transport = SSETransport(url="https://example.com/sse")
+
+    await transport.connect()
+
+    mock_mcp_sse.assert_called_once_with(
+        "https://example.com/sse",
+        headers=None,
+    )
+
+    call_kwargs = mock_mcp_sse.call_args[1]
+    assert "terminate_on_close" not in call_kwargs
+
+
+@pytest.mark.asyncio
+async def test_sse_transport_connect_sets_streams(mock_mcp_sse):
+    transport = SSETransport(url="https://example.com/sse")
+
+    await transport.connect()
+
+    assert transport._read_stream is not None
+    assert transport._write_stream is not None
+    assert transport._connected is True
+
+
+@pytest.mark.asyncio
+async def test_sse_transport_context_manager(mock_mcp_sse):
+    async with SSETransport(url="https://example.com/sse") as transport:
+        assert transport._connected is True
+
+    assert transport._connected is False
+
+
+@pytest.mark.asyncio
+async def test_sse_transport_connect_failure_raises_connection_error(mock_mcp_sse):
+    mock_sse_client_error = MagicMock(
+        side_effect=Exception("Connection failed")
+    )
+
+    fake_mcp_client_sse = sys.modules["mcp.client.sse"]
+    fake_mcp_client_sse.sse_client = mock_sse_client_error
+
+    transport = SSETransport(url="https://example.com/sse")
+
+    with pytest.raises(ConnectionError, match="Failed to connect to SSE MCP server"):
+        await transport.connect()
+
+    assert transport._connected is False