Python: [Bug]: Incorrect exception raised while MCP server is unreachable

[jjwong0915]

Description

In the description of #5687, it mentioned that the error is wrapped in a ToolException after the fix. However, it raised the following error instead in my test: Error: ('Failed to enter context manager.', ExceptionGroup('unhandled errors in a TaskGroup', [ConnectError('All connection attempts failed')]))”.

Code Sample

This is the sample code example as #5667.

import asyncio
import os

import dotenv
from agent_framework import Agent, MCPStreamableHTTPTool
from agent_framework.openai import OpenAIChatCompletionClient
from httpx import AsyncClient

dotenv.load_dotenv()


async def http_mcp_example():
    """Example using an HTTP-based MCP server."""
    client = OpenAIChatCompletionClient(
        azure_endpoint=os.environ["AI_PLATFORM_ENDPOINT"],
        api_key=os.environ["AI_PLATFORM_API_KEY"],
        model="gpt-5.4-2026-03-05",
    )
    try:
        async with (
            MCPStreamableHTTPTool(
                name="Microsoft Learn MCP",
                url="https://127.0.0.1:12345/mcp",  # Simulate downed MCP server.
                http_client=AsyncClient(verify=False),
            ) as mcp_server,
            Agent(
                client=client,
                name="DocsAgent",
                instructions="You help with Microsoft documentation questions.",
            ) as agent,
        ):
            result = await agent.run(
                "How to create an Azure storage account using az cli?", tools=mcp_server
            )
            print(result)
    except Exception as e: # The error is not handled due the bug.
        print(f"Error: {e}")


if __name__ == "__main__":
    asyncio.run(http_mcp_example())

Error Messages / Stack Traces

Error: ('Failed to enter context manager.', ExceptionGroup('unhandled errors in a TaskGroup', [ConnectError('All connection attempts failed')]))

Package Versions

agent-framework: 1.3.0

Python Version

Python: 3.14

Additional Context

No response

[github-actions]

🤖 Automated triage reproduction notes (agent-authored — trust but verify)

Agent analysis

Repro: MCPTool._safe_close_exit_stack() in packages/core/agent_framework/_mcp.py (line 631-646) only catches RuntimeError and asyncio.CancelledError, but the anyio task group in the MCP streamable HTTP client raises ExceptionGroup during cleanup when a child task crashed (e.g., ConnectError from unreachable server). This escapes _connect_on_owner's except handler (line 687-703), preventing the error from being wrapped in ToolException. Minimal repro: construct MCPStreamableHTTPTool, mock get_mcp_client().aexit to raise ExceptionGroup and session.initialize to raise ConnectionRefusedError, then call async with tool: — observe ToolExecutionException("Failed to enter context manager.") instead of ToolException.

• Failing test: python/packages/core/tests/core/test_mcp_unreachable_exception_bug.py
• Files examined: python/packages/core/agent_framework/_mcp.py, python/packages/core/agent_framework/exceptions.py, python/.venv/lib/python3.13/site-packages/mcp/client/streamable_http.py, python/packages/core/tests/core/test_mcp.py
• Tests run: python/packages/core/tests/core/test_mcp_unreachable_exception_bug.py::test_unreachable_mcp_server_raises_tool_exception_not_execution_exception, python/packages/core/tests/core/test_mcp_unreachable_exception_bug.py::test_safe_close_exit_stack_does_not_catch_exception_group
• Reported version: 1.3.0
• Current version: 1.4.0