MCP: Fails to decode json rpc message

[Hritik003]

Description:

Had observed that aigateway extproc fails to decode the json rpc messages for mcp remote servers like slack ( https://mcp.slack.com/mcp ). Apparently Slack’s MCP (and similar servers) often responds with SSE, e.g. lines like data: {"jsonrpc":"2.0",...}, not raw JSON starting with{.

Repro steps:
Configure a mcpRoute with slack mcp remote server, and hit tools/list

Environment:

latest

Logs:

time=2026-03-29T16:52:13.752Z level=ERROR msg="failed to collect messages from MCP backend" 

component=mcp-proxy backend=slack-remote session_id=4c5bfda2-9132-40e8-9634-723176bcd485 

error="failed to decode jsonrpc message from MCP response body: unmarshaling jsonrpc message: json: invalid 

character '\x13' looking for beginning of value: \x13ep\x8cHM\xea\x01\xd0\b\x1d>\xe7\xed\xff\xfb\xa6\xda\xd7\xef\x1d\xe8tDf@Jv\xd6\ "\xab\xbc..."
...

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cc: @nacx

[nacx]

Is this an open source MCP? If that's the case, can you share the link to the upstream issue so we can track it?
The main cause here is that the upstream is not properly sending the content-type, and we want to track that upstream as well.

[Hritik003]

Yes, faced this issue with the official slack remote mcp server https://docs.slack.dev/ai/slack-mcp-server/. But i dont think they open sourced the code for the same