MCP Apps: No shared session identity between AppBridge and Model sub-connections

[hwennnn]

MCP Apps: AppBridge and Model get separate session IDs — no way to correlate them

I'm building an MCP server with an MCP App (AppBridge iframe). The App UI needs to know what the Model is doing - e.g. when the Model kicks off a tool call, the App should show live progress.

The problem: Claude Desktop calls initialize twice, once for the AppBridge and once for the Model - so they end up with different Mcp-Session-Id values. My server generates a session ID on each initialize:

const headerSessionId = request.headers.get("Mcp-Session-Id");
const mcpSessionId =
  headerSessionId ||
  (jsonRpcRequest.method === "initialize" ? crypto.randomUUID() : null);

Since initialize fires twice, I get two UUIDs. Here's what the request log looks like:

#	Method	Session ID	Origin
1	initialize	eceb9a34…	AppBridge
2–6	resources/list, tools/list, etc.	eceb9a34…	AppBridge
7	initialize	913a52df…	Model
8	tools/call	913a52df…	Model

So when the Model calls a tool, the server stores state under 913a52df…. But the AppBridge queries using eceb9a34… - it never finds it.

I don't see a way in the spec to link these two sessions together. Is the dual-initialize intentional? If so, is there a planned way to share a conversation-level or connection-level ID across both sub-connections?

A previously working prototype also broke with this behavior, so this might be a recent change in Claude Desktop.

Environment: Claude Desktop/Claude Web