Propagate exceptions from httpx_client_factory

[thsmale]

Question

I want to customize the behavior to connect to an MCP gateway via streamable http. This MCP gateway will return a 401 when the JWT Bearer token I have provided is invalid. I have been referencing the Custom HTTP Client Factory Example to handle this. Using httpx transports or event_hooks seems like the best way to catch the 401 from the MCP gateway. While I can add logic to detect a 401 response, I am unable to raise an exception that I can catch after calling connect.

Below is an example transport and event hook. While I am raising the error, it will not be propagated. Instead, a asyncio.CancelledError exception is raised after connect fails.

Transport Example

class CustomTransport(AsyncBaseTransport):
    def __init__(self, **kwargs):
        self._wrapper = AsyncHTTPTransport(**kwargs)

    async def handle_async_request(self, request):
        response = await self._wrapper.handle_async_request(request)
        if (response.status_code == 401):
            raise Exception("This will be lost forever...")

event_hooks example

async def event_hook_raise_on_unauthorized(response):
    if (response.status_code == 401):
        raise Exception("This will be lost forever...")

My workaround using event_hooks

I have provided a x-transaction-id to MCPServerStreamableHttp. In the transport or event_hook I can retrieve this header and use it to set the key in a global dict. Then when I catch the asyncio.CancelledError from connect I can check this global dict for further error details.

error_tracker = {}

async def event_hook_raise_on_unauthorized(response):
    if (response.status_code == 401):
        response.raise_for_status()
        transaction_id = response.request.headers.get('x-transaction-id')
        error_tracker[transaction_id] = "Unauthorized"

async def connect():
    try:
        await mcp_server.connect()
    except (asyncio.CancelledError, Exception) as e:
        transaction_id = mcp_server.params["headers"]['x-transaction-id']
        err_msg = error_tracker[transaction_id]
        del error_tracker[transaction_id]
        raise Exception(err_msg)

Do you have any alternative suggestions or guidance for handling this?