Unauthenticated streamable HTTP MCP servers are reported as SSE errors instead of missing auth

[jonahsnider]

Description

If you connect to a streamable HTTP MCP server that doesn't support SSE, running /status in the TUI will show you that OpenCode encountered an error when connecting. This is kinda true, but it should then try using streamable HTTP, which would then get a 401 and prompt the user to authenticate.

Plugins

N/A

OpenCode version

1.2.18

Steps to reproduce

1. Add an MCP server that supports streamable HTTP, but not SSE (ex. the Sanity MCP)

   {
     "$schema": "https://opencode.ai/config.json",
     "mcp": {
       "sanity": {
         "type": "remote",
         "url": "https://mcp.sanity.io",
         "oauth": {}
       }
     }
   }

2. Run opencode and then /status
3. Observe that the sanity MCP server says "SSE error: Non-200 status code (405)", instead of saying the server needs to be authenticated via OAuth

Screenshot and/or share link

Operating System

macOS 26.3

Terminal

Ghostty

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• HTTP 405 on Figma MCP #5636: Same symptom (HTTP 405 shown as SSE error) on a different streamable HTTP MCP server (Figma MCP)
• MCP client doesn't follow specification for Authentication #1492: MCP client not following the authentication spec — 401 responses not triggering OAuth flow

If your issue is distinct from the above, feel free to clarify the differences.