fernando-brain MCP SSE transport: tools/call returns -32602 after successful handshake

Description

The fernando-brain MCP server over SSE transport returns -32602 on all tools/call requests from the opencode client (v1.18.5). The SSE handshake succeeds (HTTP 200, correct endpoint event), but every tool invocation fails.

Steps to reproduce

1. Configure a remote MCP server with SSE transport:

   {
     "mcp": {
       "fernando-brain": {
         "type": "remote",
         "url": "https://srv1300387.hstgr.cloud/sse",
         "enabled": true
       }
     }
   }
   

2. Start opencode. The server connects (endpoint event received, tools list populated).
3. Call any MCP tool (e.g., retrieve_context). Result: -32602 (Invalid params).

Evidence

• The SSE handshake succeeds (HTTP 200, endpoint event for message?sessionId=...).
• tools/list returns correctly (tools are visible in the client).
• tools/call fails with -32602.
• Using curl directly against the SSE endpoint works fine — the server responds correctly.
• The issue is specifically in the opencode client SSE transport layer.

Root cause hypothesis

Two possibilities:

1. Missing initialize handshake: opencode may skip the JSON-RPC initialize/initialized handshake for SSE transport (as opposed to Streamable HTTP). The server may reject calls without prior initialization.
2. Parameter encoding mismatch: opencode may encode the arguments field differently than the server expects over SSE.

Related upstream issues

• #25287
• #37626

Workaround

Test manually with curl, or switch to streamable-http transport if the server supports it.

Environment

• opencode version: 1.18.5
• Platform: macOS
• MCP transport: SSE (remote)