MCP — Model Context Protocol

Kern connects to Model Context Protocol servers and exposes their tools to the agent. Each server is defined in the agent's config and connected at startup.

Configuring servers

Add mcpServers to .kern/config.json:

{
  "mcpServers": {
    "github": {
      "transport": "http",
      "url": "https://api.githubcopilot.com/mcp",
      "headers": {
        "Authorization": "Bearer ${GITHUB_TOKEN}"
      }
    },
    "filesystem": {
      "transport": "stdio",
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-filesystem", "/tmp"]
    },
    "zapier": {
      "transport": "sse",
      "url": "https://actions.zapier.com/mcp/..."
    }
  }
}

Each server needs a transport field:

Transport	Fields	Notes
http	url, optional headers	Recommended for remote servers
sse	url, optional headers	Alternative HTTP transport
stdio	command, optional args, optional env	Spawns a local process

Environment variable substitution

${VAR} references in any string field (url, headers, command, args, env) are resolved from .kern/.env at startup. Names follow shell convention (letters, digits, underscore; first char not a digit; case-sensitive). Missing vars are left as literal ${VAR} and logged as a warning — the server will return its real auth error when it fails.

Use this for tokens and secrets. Don't commit resolved values to config.

Tool naming

MCP tools are namespaced with the server name: <server>__<tool>. So a create_issue tool from a server named github appears to the agent as github__create_issue. This prevents collisions when multiple servers expose tools with the same name.

Failure handling

• Servers connect in parallel at startup. One server failing doesn't block others or the agent.
• Failed connections log an error and are dropped. Fix the config and restart to retry.
• The /status endpoint reports mcp: <connected>/<configured> server(s), <M> tool(s). If <connected> is less than <configured>, check logs for the connection error.

Scope

Kern's MCP integration is intentionally minimal:

• Tools only — resources, prompts, and elicitation are not exposed.
• No OAuth flow — authorize tokens out-of-band and pass via headers or env.
• All-or-nothing — all tools from a connected server are available to the agent. There's no per-tool filter yet.

File an issue if you need something beyond this.