Remote MCP servers with Bearer headers fail on Linux (SSE 400, no Streamable HTTP fallback) #24946

Description

Remote MCP servers configured with static Bearer token headers (e.g., GitHub's mcp.github.tools.sap/mcp and api.githubcopilot.com/mcp/) fail to connect on Linux with SSE error: Non-200 status code (400). The same config works on macOS.

Related Issues

• Unauthenticated streamable HTTP MCP servers are reported as SSE errors instead of missing auth #16247 — Root cause: Streamable HTTP servers reported as SSE errors
• config Feishu MCP server for open code mmet an Error message：SSE error: Non-200 status code (404) #18724 — Same symptom with Feishu MCP

Environment

• OpenCode version: 1.14.29
• OS: Linux (Ubuntu 24.04 arm64, running in Lima VM)
• macOS host: works fine with identical config and version

Config

{
  "mcp": {
    "github.com": {
      "type": "remote",
      "url": "https://api.githubcopilot.com/mcp/",
      "headers": {
        "Authorization": "Bearer {env:GITHUB_COM_TOKEN}"
      },
      "enabled": true
    }
  }
}

Investigation

1. Token is valid — curl -s -H "Authorization: Bearer $TOKEN" https://api.github.com/user returns 200
2. Endpoint is reachable — POST with MCP initialize message returns valid response from inside the VM
3. SSE GET returns 405/400 — The server only supports POST (Streamable HTTP), not GET (SSE)
4. macOS works, Linux doesn't — Same binary version, same config. On macOS, opencode falls back to Streamable HTTP. On Linux, it stops at the SSE error.
5. oauth: false doesn't help — Tried disabling OAuth auto-detection, no change
6. Other remote MCP servers work — context7, jira, hyperspace portal all connect fine (they use OAuth via mcp-auth.json, not static headers)

Steps to Reproduce

1. Run opencode 1.14.29 on Linux (e.g., Lima VM with Ubuntu 24.04)
2. Configure a remote MCP server with Bearer headers pointing to a Streamable HTTP endpoint
3. Run opencode mcp list
4. Observe: SSE error: Non-200 status code (400)

Expected Behavior

opencode should fall back from SSE to Streamable HTTP when the SSE probe gets a non-200 response, the same way it does on macOS.

Workaround

None available. The stdio approach (github-mcp-server binary) also fails due to a protocol framing mismatch (Content-Length vs raw JSON lines).