[MCP Apps] Send follow up message

specification/draft/apps.mdx

@@ -767,6 +767,43 @@ Host behavior:
 * Host SHOULD add the message to the conversation context, preserving the specified role.
 * Host MAY request user consent.
 
+`ui/follow-up-message` - Send a follow-up message to continue the conversation
+
+```typescript
+// Request
+{
+  jsonrpc: "2.0",
+  id: 3,
+  method: "ui/follow-up-message",
+  params: {
+    content: ContentBlock[]  // Message content blocks (text, image, etc.)
+  }
+}
+
+// Success Response
+{
+  jsonrpc: "2.0",
+  id: 3,
+  result: {}  // Empty result on success
+}
+
+// Error Response (if denied or failed)
+{
+  jsonrpc: "2.0",
+  id: 3,
+  result: {
+    isError: true
+  }
+}
+```
+
+Use this to continue the conversation based on user interaction with the app or prompt a follow up app. For example, when a user clicks on a data point, the app can send a follow-up message asking for an analysis of that data point, or to display a new app with more details about that data point.
+
+Host behavior:
+* Host SHOULD add the message to the conversation context as a user message.
+* Host MAY request user consent before sending.
+* Host MAY allow user to review and edit the message before sending.
+
 `ui/request-display-mode` - Request host to change display mode
 
 ```typescript

src/app.ts

@@ -20,6 +20,8 @@ import { PostMessageTransport } from "./message-transport";
 import {
   LATEST_PROTOCOL_VERSION,
   McpUiAppCapabilities,
+  McpUiFollowUpMessageRequest,
+  McpUiFollowUpMessageResultSchema,
   McpUiHostCapabilities,
   McpUiHostContext,
   McpUiHostContextChangedNotification,
@@ -783,6 +785,48 @@ export class App extends Protocol<AppRequest, AppNotification, AppResult> {
     );
   }
 
+  /**
+   * Send a follow-up message to the host's chat.
+   *
+   * Use this to continue the conversation based on user interaction with the app.
+   * For example, when a user clicks on a data point, the app can send a follow-up
+   * message asking for more details about that item.
+   *
+   * @param params - Message role and content
+   * @param options - Request options (timeout, etc.)
+   * @returns Result indicating success or error
+   *
+   * @throws {Error} If the host rejects the request
+   * @throws {Error} If the request times out or the connection is lost
+   *
+   * @example Send a follow-up question based on user interaction
+   * ```typescript
+   * try {
+   *   await app.sendFollowUpMessage({
+   *     role: "user",
+   *     content: [{ type: "text", text: "Tell me more about this item" }]
+   *   });
+   * } catch (error) {
+   *   console.error("Failed to send message:", error);
+   * }
+   * ```
+   *
+   * @see {@link McpUiFollowUpMessageRequest} for request structure
+   */
+  sendFollowUpMessage(
+    params: McpUiFollowUpMessageRequest["params"],
+    options?: RequestOptions,
+  ) {
+    return this.request(
+      <McpUiFollowUpMessageRequest>{
+        method: "ui/follow-up-message",
+        params,
+      },
+      McpUiFollowUpMessageResultSchema,
+      options,
+    );
+  }
+
   /**
    * Send log messages to the host for debugging and telemetry.
    *