RPC transport should support bindings to another worker

[kamal]

The current implementation of #565 implements an RPC transport to another Durable Object binding in the same worker. I would like to be able to use the RPC transport to point to another worker entirely. In wrangler, this would look like

"services": [
    {
        "binding": "MyMCP",
        "service": "my-mcp-worker"
    }
 ]

Related: #551

[mattzcarey]

Hey @kamal, thanks for this issue. what would you expect the worker of the mcp server to look like.

export class MyMcpAgent extends McpAgent {
  //declare server stuff
}

export class MyMcpWorker extends WorkerEntrypoint {
 //serve via rpc
 handleMcpMessage(message) {
    const agent = getAgentByName()
    return agent.handleMcpMessage(message)
  }

  //serve externally
  fetch(request, env, ctx) {
    return MyMcpAgent.serve("/mcp")
  }
}

this was not the usecase I originally thought of so just trying to understand a bit more. I see a bunch of problems since we set props and the name of the DO explicitly in the transport.

[kamal]

Sorry, let me flesh this out a bit more. The use-case I'm looking is to spin up a separate worker that effectively hosts our AI agent. It has separate concerns than that of an MCP server. For quick iteration of the agent, we would present a chat UI or just a /chat endpoint ala Mastra to integrate into an in-product chat widget.

In the current implementation, co-locating the agent and MCP server in one worker is counter-intuitive to me. Why would I go through the trouble of exposing tools to my agent via an MCP server versus just inlining it directly?

The MCP server worker is already serving external clients by default exporting new OAuthProvider from workers-oauth-providers. I've never exposed a service binding before, but reading the documentation, the need to expose a WorkerEntrypoint seems reasonable.

Additionally, the MCP server expects quite a bit of Props that would be populated by the oauth callback, which we get to rehydrate automatically when an external client presents the bearer token. Since the RPC route effectively bypasses authentication, we will have to find a way to enrich the props before handing it off to the agent.

export class MyMcpWorker extends WorkerEntrypoint {
 handleMcpMessage(message, props) {
    const agent = getAgentByName()
    const fullProps = buildPropsFromPartialProps(props)
    // set fullProps somewhere?
    agent.updateProps(fullProps) // maybe?
    return agent.handleMcpMessage(message)
  }
}