ResourcesUpdatedNotification support is missing on the client side

[vinsguru]

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Discussion Topic

According to the MCP protocol, servers can send:

{
  "jsonrpc": "2.0",
  "method": "notifications/resources/updated",
  "params": {
    "uri": "file:///project/src/main.rs"
  }
}

We do have ResourcesUpdatedNotification.

McpClient.SyncSpec is missing the support for reacting to this notification.

Is this intentional?

[ryux1]

It was an omission in the synchronous client, not an intentional protocol limitation. resourcesUpdateConsumer() was added to McpClient.SyncSpec by #735 and is present from v0.17.1 onward (as well as the current 1.x/2.x releases).

With the current API, register the consumer when building the client and then subscribe to the resource URI:

McpSyncClient client = McpClient.sync(transport)
    .resourcesUpdateConsumer(contents -> {
        // Updated contents for the subscribed URI.
        contents.forEach(System.out::println);
    })
    .build();

client.initialize();
client.subscribeResource(
    McpSchema.SubscribeRequest.builder("file:///project/src/main.rs").build()
);

The callback receives List<ResourceContents>, rather than the raw ResourcesUpdatedNotification. When the notification arrives, the SDK deserializes its URI, calls resources/read for that URI, and passes the returned contents to each registered consumer. The server must advertise the resources.subscribe capability, and it should only send updates for URIs to which that client subscribed.

The async equivalent is:

McpAsyncClient client = McpClient.async(transport)
    .resourcesUpdateConsumer(contents -> Mono.fromRunnable(() -> {
        contents.forEach(System.out::println);
    }))
    .build();

client.initialize()
    .then(client.subscribeResource(
        McpSchema.SubscribeRequest.builder("file:///project/src/main.rs").build()
    ))
    .subscribe();

See the current resource-subscription documentation and the client notification handler, which performs the automatic readResource call.