feat: add ui/update-model-context (#125)

---------

Co-authored-by: Olivier Chafik <ochafik@anthropic.com>

src/app.ts

@@ -9,6 +9,7 @@ import {
   CallToolRequestSchema,
   CallToolResult,
   CallToolResultSchema,
+  EmptyResultSchema,
   Implementation,
   ListToolsRequest,
   ListToolsRequestSchema,
@@ -20,6 +21,7 @@ import { PostMessageTransport } from "./message-transport";
 import {
   LATEST_PROTOCOL_VERSION,
   McpUiAppCapabilities,
+  McpUiUpdateModelContextRequest,
   McpUiHostCapabilities,
   McpUiHostContext,
   McpUiHostContextChangedNotification,
@@ -809,6 +811,52 @@ export class App extends Protocol<AppRequest, AppNotification, AppResult> {
     });
   }
 
+  /**
+   * Update the host's model context with app state.
+   *
+   * Unlike `sendLog`, which is for debugging/telemetry, context updates
+   * are intended to be available to the model in future reasoning,
+   * without requiring a follow-up action (like `sendMessage`).
+   *
+   * The host will typically defer sending the context to the model until the
+   * next user message (including `ui/message`), and will only send the last
+   * update received. Each call overwrites any previous context update.
+   *
+   * @param params - Context content and/or structured content
+   * @param options - Request options (timeout, etc.)
+   *
+   * @throws {Error} If the host rejects the context update (e.g., unsupported content type)
+   *
+   * @example Update model context with current app state
+   * ```typescript
+   * await app.updateModelContext({
+   *   content: [{ type: "text", text: "User selected 3 items totaling $150.00" }]
+   * });
+   * ```
+   *
+   * @example Update with structured content
+   * ```typescript
+   * await app.updateModelContext({
+   *   structuredContent: { selectedItems: 3, total: 150.00, currency: "USD" }
+   * });
+   * ```
+   *
+   * @returns Promise that resolves when the context update is acknowledged
+   */
+  updateModelContext(
+    params: McpUiUpdateModelContextRequest["params"],
+    options?: RequestOptions,
+  ) {
+    return this.request(
+      <McpUiUpdateModelContextRequest>{
+        method: "ui/update-model-context",
+        params,
+      },
+      EmptyResultSchema,
+      options,
+    );
+  }
+
   /**
    * Request the host to open an external URL in the default browser.
    *