OpenStellar MCP Adapter

Connect any MCP server to OpenCode. One config entry. Zero guesswork.

You already have MCP servers for web search, databases, file systems, design tools — the ecosystem is growing fast. The problem is getting them all working inside OpenCode.

The adapter is the bridge. Add a server, it appears as tools. Handles schema conversion, response flattening, timeouts, and error messages automatically.

What You Get

| Feature | What it does | Why it matters | | ----------------------- | --------------------------------------------------------------------------------------- | ------------------------------------------------------------------ | | Plug & play | Add servers to opencode.jsonc, tools appear automatically | No manual tool definitions. No boilerplate. | | Schema conversion | Converts JSON Schema to Zod — handles anyOf, unions, nullable, $ref, merged objects | OpenCode needs Zod. MCP sends JSON Schema. One bridge, no crashes. | | Response flattening | Turns images, audio, files, resource links into plain text | The LLM reads text, not base64. No silent data loss. | | Token savings | [image: image/png] (12KB) instead of 600+ tokens of raw base64 | Fewer tokens per response. Cheaper conversations. | | Timeout guard | 60s timeout, resets on server progress | No frozen conversations. Long-running tools still work. | | Clean errors | "Tool X on server Y failed: reason" instead of stack traces | The LLM tells you what happened instead of throwing noise. | | Smart naming | notion + notion_search → notion_search, not notion_notion_search | Clean tool names. No repetition. | | Auto-update | Checks npm registry on first session, invalidates stale cache, notifies you by toast | Always runs the latest version. No manual cleanup. |

Get Started

npm install -g @openstellar/mcp-adapter

Add to opencode.jsonc:

{
  "plugin": [
    [
      "@openstellar/mcp-adapter@latest",
      {
        "mcp": {
          "example-remote": {
            "type": "remote",
            "url": "https://mcp.example.com/mcp",
            "headers": {
              "Authorization": "Bearer YOUR_TOKEN"
            }
          },
          "example-local": {
            "type": "local",
            "command": ["npx", "-y", "mcp-server-package"]
          }
        }
      }
    ]
  ]
}

Restart OpenCode. Tools appear. Done.

Local MCP servers keep stderr out of OpenCode's CLI by default while retaining a bounded startup trace for the debug log (OPENSTELLAR_MCP_DEBUG=true). Their stdout remains connected to the MCP JSON-RPC protocol. To opt into live diagnostics for a specific local server, add "stderr": "inherit".

Config

| Field | Required | Description | | --------- | ------------ | ------------------------------------------ | | type | ✅ | "local" (stdio) or "remote" (HTTP SSE) | | command | for local | Child process args | | url | for remote | SSE endpoint | | headers | optional | Auth tokens, etc. | | env | optional | Environment for child process | | stderr | optional | Set to "inherit" for live child diagnostics; omitted by default (stderr is captured off-terminal) | | timeout | optional | Per-tool timeout in ms (default: 60000) |

How It Works

MCP Server ──→ Adapter ──→ OpenCode Tool
                  ├─ JSON Schema → Zod
                  ├─ Response flattening
                  ├─ Timeout guard
                  └─ Error wrapping

Related Repositories

• openstellar-tool-search
• opencode-tool-search
• opencode-mcp-adapter

License

MIT © 2026 OpenStellar