Unable To Pass Custom Headers #78
Description
I would like to pass custom headers, which seems possible according to the documentation. When I test this in Claude Desktop I get the following error with this configuration.
This is on a Macbook M1 and using mcp-proxy v0.8.0.
It appears that customer headers are illegal.
httpx.LocalProtocolError: Illegal header name b'x-username: testuser'
Configuration
{
"mcpServers": {
"mcp-server": {
"command": "/Users/user/.local/bin/mcp-proxy",
"args": [
"http://localhost:8000/mcp",
"--headers",
"x-username: testuser",
"--transport",
"sse"
]
}
}
}
Error
2025-06-06T17:59:18.190Z [mcp-server] [info] Initializing server...
2025-06-06T17:59:18.231Z [mcp-server] [info] Server started and connected successfully
2025-06-06T17:59:18.232Z [mcp-server] [info] Message from client: {"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"claude-ai","version":"0.1.0"}},"jsonrpc":"2.0","id":0}
+ Exception Group Traceback (most recent call last):
| File "/Users/user/.local/bin/mcp-proxy", line 10, in <module>
| sys.exit(main())
| ^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/mcp_proxy/__main__.py", line 351, in main
| _handle_sse_client_mode(args_parsed, logger)
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/mcp_proxy/__main__.py", line 221, in _handle_sse_client_mode
| asyncio.run(run_sse_client(args_parsed.command_or_url, headers=headers))
| File "/Users/user/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/asyncio/runners.py", line 194, in run
| return runner.run(main)
| ^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/asyncio/runners.py", line 118, in run
| return self._loop.run_until_complete(task)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/asyncio/base_events.py", line 687, in run_until_complete
| return future.result()
| ^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/mcp_proxy/sse_client.py", line 20, in run_sse_client
| async with sse_client(url=url, headers=headers) as streams, ClientSession(*streams) as session:
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/contextlib.py", line 210, in __aenter__
| return await anext(self.gen)
| ^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/mcp/client/sse.py", line 54, in sse_client
| async with anyio.create_task_group() as tg:
| ^^^^^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/anyio/_backends/_asyncio.py", line 772, in __aexit__
| raise BaseExceptionGroup(
| ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
+-+---------------- 1 ----------------
| Traceback (most recent call last):
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpx/_transports/default.py", line 101, in map_httpcore_exceptions
| yield
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpx/_transports/default.py", line 394, in handle_async_request
| resp = await self._pool.handle_async_request(req)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpcore/_async/connection_pool.py", line 256, in handle_async_request
| raise exc from None
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpcore/_async/connection_pool.py", line 236, in handle_async_request
| response = await connection.handle_async_request(
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpcore/_async/connection.py", line 103, in handle_async_request
| return await self._connection.handle_async_request(request)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpcore/_async/http11.py", line 136, in handle_async_request
| raise exc
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpcore/_async/http11.py", line 86, in handle_async_request
| await self._send_request_headers(**kwargs)
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpcore/_async/http11.py", line 144, in _send_request_headers
| with map_exceptions({h11.LocalProtocolError: LocalProtocolError}):
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/contextlib.py", line 158, in __exit__
| self.gen.throw(value)
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpcore/_exceptions.py", line 14, in map_exceptions
| raise to_exc(exc) from exc
| httpcore.LocalProtocolError: Illegal header name b'x-username: testuser'
|
| The above exception was the direct cause of the following exception:
|
| Traceback (most recent call last):
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/mcp/client/sse.py", line 58, in sse_client
| async with aconnect_sse(
| ^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/contextlib.py", line 210, in __aenter__
| return await anext(self.gen)
| ^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpx_sse/_api.py", line 69, in aconnect_sse
| async with client.stream(method, url, headers=headers, **kwargs) as response:
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/contextlib.py", line 210, in __aenter__
| return await anext(self.gen)
| ^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpx/_client.py", line 1583, in stream
| response = await self.send(
| ^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpx/_client.py", line 1629, in send
| response = await self._send_handling_auth(
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpx/_client.py", line 1657, in _send_handling_auth
| response = await self._send_handling_redirects(
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpx/_client.py", line 1694, in _send_handling_redirects
| response = await self._send_single_request(request)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpx/_client.py", line 1730, in _send_single_request
| response = await transport.handle_async_request(request)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpx/_transports/default.py", line 393, in handle_async_request
| with map_httpcore_exceptions():
| ^^^^^^^^^^^^^^^^^^^^^^^^^
| File "/Users/user/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/contextlib.py", line 158, in __exit__
| self.gen.throw(value)
| File "/Users/user/.local/share/uv/tools/mcp-proxy/lib/python3.12/site-packages/httpx/_transports/default.py", line 118, in map_httpcore_exceptions
| raise mapped_exc(message) from exc
| httpx.LocalProtocolError: Illegal header name b'x-username: testuser'
+------------------------------------
2025-06-06T17:59:18.643Z [mcp-server] [info] Server transport closed
2025-06-06T17:59:18.643Z [mcp-server] [info] Client transport closed
2025-06-06T17:59:18.643Z [mcp-server] [info] Server transport closed unexpectedly, this is likely due to the process exiting early. If you are developing this MCP server you can add output to stderr (i.e. `console.error('...')` in JavaScript, `print('...', file=sys.stderr)` in python) and it will appear in this log.
2025-06-06T17:59:18.643Z [mcp-server] [error] Server disconnected. For troubleshooting guidance, please visit our [debugging documentation](https://modelcontextprotocol.io/docs/tools/debugging) {"context":"connection"}
2025-06-06T17:59:18.644Z [mcp-server] [info] Client transport closed