mcp: Figma Desktop fails with Error POSTing — ?codemode= causes 404 #45499

Summary

Remote MCP connections to Figma Desktop Dev Mode MCP (http://127.0.0.1:3845/mcp) fail with Streamable HTTP error: Error POSTing to endpoint: because OpenCode appends ?codemode=false (or true) to the MCP URL. Figma’s desktop server returns 404 with an empty body for any non-empty query string. The same endpoint works when called without a query string (curl / official MCP SDK).

Environment

• opencode version: 0.0.0-beta-18371
• OS: Darwin 25.6.0 (darwin arm64)
• Terminal: TERM_PROGRAM=ghostty, TERM=xterm-256color, COLORTERM=truecolor
• Shell: /bin/zsh
• Install/channel: beta (@opencode-ai/cli)
• Active plugins: nexos-models.ts, herdr-agent-state.js, (workaround) figma-mcp-desktop.ts

Reproduction

1. Open Figma desktop app → Dev Mode → enable desktop MCP server (listens on http://127.0.0.1:3845/mcp).
2. Configure a remote MCP server in opencode.json(c):

{
  "mcp": {
    "servers": {
      "figma": {
        "type": "remote",
        "url": "http://127.0.0.1:3845/mcp",
        "oauth": false
      }
    }
  }
}

3. Run opencode2 mcp list (or start a session so MCP connects).

Expected Behavior

OpenCode connects to the Figma Desktop MCP over Streamable HTTP and catalogs tools (e.g. get_design_context, get_screenshot, …). codemode should only affect how OpenCode exposes MCP tools to the model, not the upstream HTTP URL path/query.

Actual Behavior

✗ figma  failed: Streamable HTTP error: Error POSTing to endpoint: 

Server log:

mcp connect failed server=figma status.error="Streamable HTTP error: Error POSTing to endpoint: "

A local reverse-proxy capture of OpenCode’s request shows:

POST /mcp?codemode=false HTTP/1.1
Host: 127.0.0.1:3845
Accept: application/json, text/event-stream
Content-Type: application/json
User-Agent: opencode2/0.0.0-beta-18371

{"method":"initialize","params":{"protocolVersion":"2025-11-25","capabilities":{...},"clientInfo":{"name":"cli","version":"0.0.0-beta-18371"}},"jsonrpc":"2.0","id":0}

Figma responds with 404, empty body (hence the empty suffix after Error POSTing to endpoint: ).

Additional Context

Control experiments (same machine, Figma listening on :3845)

Config notes

• V1-shaped mcp.figma is normalized correctly to mcp.servers.figma (not a config-shape bug).
• Setting "oauth": false alone does not fix this; the failure is the query string, not OAuth discovery.
• When Figma desktop is closed, errors are Unable to connect… instead — distinct from this 404.

Suggested fix

Do not append codemode (or any OpenCode-only flag) to the remote MCP HTTP URL. Keep codemode as internal connection metadata when grouping/exposing tools. Alternatively, strip unknown query params only for known-local desktop endpoints — but not putting them on the wire is cleaner.

Workaround

Strip the query string via a tiny local proxy (127.0.0.1:3846 → 127.0.0.1:3845/mcp without ?…), or use Figma’s hosted remote MCP (https://mcp.figma.com/mcp) with OAuth (different feature set).