issue: SSE Connection Failure with Localhost in MCP Proxy Setup​，502 Bad Gateway Error

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Check Existing Issues

• I have searched the existing issues and discussions.
• I am using the latest version of mcpo.

mcpo Version

mcpo-0.0.12

Open WebUI Version (if applicable)

No response

Operating System

win11

Browser (if applicable)

No response

Confirmation

• I have read and followed all instructions in README.md.
• I am using the latest version of both MCPO and Open WebUI.
• I have included the browser console logs.
• I have included the Docker container logs.
• I have listed steps to reproduce the bug in detail.

Expected Behavior

In SSE mode, using the command mcpo --server-type "sse" -- http://localhost:3333/sse should normally start the MCP proxy server.

Actual Behavior

502 Bad Gateway Error

Steps to Reproduce

1.​​MCP Server Initialization​​
Successfully launched the Figma-Context-MCP server with the following terminal logs:
Initializing Figma MCP Server in HTTP mode on port 3333...
HTTP server listening on port 3333
SSE endpoint available at http://localhost:3333/sse
Message endpoint available at http://localhost:3333/messages
(: Server logs explicitly reference localhost for SSE endpoints.)

​​2.Proxy Setup Failure​​
Attempted to start the proxy using:
mcpo --server-type "sse" -- http://localhost:3333/sse
Resulted in ​​502 Bad Gateway Error​​.
​​
3.Workaround Validation​​
Modified the URL to use explicit addresses:
-127.0.0.1: Proxy started successfully.like mcpo --server-type "sse" -- http://127.0.0.1:3333/sse
-[::1] (IPv6 loopback): Proxy also started successfully.
​​

Logs & Screenshots

D:>mcpo --port 5566 --server-type "sse" -- http://localhost:3333/sse
Starting MCP OpenAPI Proxy on 0.0.0.0:5566 with command: http://localhost:3333/sse
INFO: Started server process [7572]
INFO: Waiting for application startup.
ERROR: Traceback (most recent call last):
File "D:\Miniconda\Lib\site-packages\mcp\client\sse.py", line 53, in sse_client
event_source.response.raise_for_status()
File "D:\Miniconda\Lib\site-packages\httpx_models.py", line 829, in raise_for_status
raise HTTPStatusError(message, request=request, response=self)
httpx.HTTPStatusError: Server error '502 Bad Gateway' for url 'http://localhost:3333/sse'
For more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/502

During handling of the above exception, another exception occurred:

• Exception Group Traceback (most recent call last):
  | File "D:\Miniconda\Lib\site-packages\starlette\routing.py", line 692, in lifespan
  | async with self.lifespan_context(app) as maybe_state:
  | File "D:\Miniconda\Lib\contextlib.py", line 210, in aenter
  | return await anext(self.gen)
  | ^^^^^^^^^^^^^^^^^^^^^
  | File "D:\Miniconda\Lib\site-packages\mcpo\main.py", line 101, in lifespan
  | async with sse_client(url=args[0], sse_read_timeout=None) as (reader, writer):
  | File "D:\Miniconda\Lib\contextlib.py", line 210, in aenter
  | return await anext(self.gen)
  | ^^^^^^^^^^^^^^^^^^^^^
  | File "D:\Miniconda\Lib\site-packages\mcp\client\sse.py", line 43, in sse_client
  | async with anyio.create_task_group() as tg:
  | File "D:\Miniconda\Lib\site-packages\anyio_backends_asyncio.py", line 767, in aexit
  | raise BaseExceptionGroup(
  | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
  +-+---------------- 1 ----------------
  | Traceback (most recent call last):
  | File "D:\Miniconda\Lib\site-packages\mcp\client\sse.py", line 53, in sse_client
  | event_source.response.raise_for_status()
  | File "D:\Miniconda\Lib\site-packages\httpx_models.py", line 829, in raise_for_status
  | raise HTTPStatusError(message, request=request, response=self)
  | httpx.HTTPStatusError: Server error '502 Bad Gateway' for url 'http://localhost:3333/sse'
  | For more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/502
  +------------------------------------

ERROR: Application startup failed. Exiting.

Additional Information

The failure appears to stem from ​​localhost resolution inconsistencies​​ in the underlying SSE URL parser.