[MCP Apps] Set widget state

specification/draft/apps.mdx

@@ -800,6 +800,60 @@ Guest UI behavior:
 * Guest UI SHOULD check `availableDisplayModes` in host context before requesting a mode change.
 * Guest UI MUST handle the response mode differing from the requested mode.
 
+`ui/set-widget-state` - Persist widget state for inclusion in future prompts
+
+```typescript
+// Request
+{
+  jsonrpc: "2.0",
+  id: 4,
+  method: "ui/set-widget-state",
+  params: {
+    toolId: string,    // Unique identifier for this widget/tool instance
+    toolName: string,  // Human-readable name of the tool (shown in prompts)
+    content: string    // JSON string containing the widget's current state
+  }
+}
+
+// Success Response
+{
+  jsonrpc: "2.0",
+  id: 4,
+  result: {}  // Empty result on success
+}
+
+// Error Response (if failed)
+{
+  jsonrpc: "2.0",
+  id: 4,
+  error: {
+    code: -32000,  // Implementation-defined error
+    message: "Failed to store widget state"
+  }
+}
+```
+
+This request allows apps to persist state that the model can see in subsequent messages. The state is stored per-conversation and sent with each completion request, enabling the model to be aware of the current widget configuration and user selections.
+
+**Important:** Widget state is ephemeral and not persisted across browser sessions or page reloads. Apps SHOULD send their initial state after receiving the `ui/notifications/tool-input` notification to ensure the model has context from the start of each session.
+
+Host behavior:
+* Host MUST store the widget state associated with the current conversation.
+* Host MUST include all stored widget states in subsequent completion requests to the model.
+* Host SHOULD treat widget state content as untrusted third-party data and wrap it appropriately when presenting to the model.
+* Host MAY implement size limits on widget state content.
+
+Guest UI behavior:
+* Guest UI SHOULD call this method whenever its state changes meaningfully in a way that the model should be aware of.
+* Guest UI SHOULD use a consistent `toolId` for the same widget instance.
+* Guest UI SHOULD serialize state as a JSON string in the `content` field.
+* Guest UI SHOULD include relevant context such as selected items, current view configuration, and user choices.
+
+Example use cases:
+* A chart widget reports which data point the user has selected
+* A form widget reports current field values before submission
+* A data explorer reports active filters and sort order
+
 #### Notifications (Host → UI)
 
 `ui/notifications/tool-input` - Host MUST send this notification with the complete tool arguments after the Guest UI's initialize request completes.

src/app.ts

@@ -45,6 +45,8 @@ import {
   McpUiToolResultNotificationSchema,
   McpUiRequestDisplayModeRequest,
   McpUiRequestDisplayModeResultSchema,
+  McpUiSetWidgetStateRequest,
+  McpUiSetWidgetStateResultSchema,
 } from "./types";
 import { Transport } from "@modelcontextprotocol/sdk/shared/transport.js";
 
@@ -886,6 +888,53 @@ export class App extends Protocol<AppRequest, AppNotification, AppResult> {
     );
   }
 
+  /**
+   * Set widget state that will be included in future prompts.
+   *
+   * This allows apps to persist state that Claude can see in subsequent messages.
+   * The state is treated as untrusted third-party content and will be wrapped
+   * in a document block with explicit trust metadata on the backend.
+   *
+   * Call this method whenever your widget's state changes meaningfully in a way
+   * that Claude should be aware of. The state is stored per-conversation and
+   * sent with each completion request.
+   *
+   * @param params - Widget state parameters
+   * @param params.toolId - Unique identifier for this widget/tool instance
+   * @param params.toolName - Human-readable name of the tool (shown in prompts)
+   * @param params.content - JSON string containing the widget's current state
+   * @param options - Request options (timeout, etc.)
+   * @returns Result indicating success or error
+   *
+   * @example Persist chart configuration
+   * ```typescript
+   * await app.setWidgetState({
+   *   toolId: "customer-segmentation-chart",
+   *   toolName: "Customer Segmentation",
+   *   content: JSON.stringify({
+   *     xAxis: "revenue",
+   *     yAxis: "engagement",
+   *     hiddenSegments: ["inactive"]
+   *   })
+   * });
+   * ```
+   *
+   * @see {@link McpUiSetWidgetStateRequest} for request structure
+   */
+  setWidgetState(
+    params: McpUiSetWidgetStateRequest["params"],
+    options?: RequestOptions,
+  ) {
+    return this.request(
+      <McpUiSetWidgetStateRequest>{
+        method: "ui/set-widget-state",
+        params,
+      },
+      McpUiSetWidgetStateResultSchema,
+      options,
+    );
+  }
+
   /**
    * Notify the host of UI size changes.
    *

src/spec.types.ts

@@ -548,3 +548,35 @@ export interface McpUiToolMeta {
    */
   visibility?: McpUiToolVisibility[];
 }
+
+/**
+ * @description Request to set widget state that will be included in future prompts.
+ * The state is treated as untrusted third-party content and will be wrapped
+ * in a document block with explicit trust metadata.
+ * @see {@link app.App.setWidgetState} for the method that sends this request
+ */
+export interface McpUiSetWidgetStateRequest {
+  method: "ui/set-widget-state";
+  params: {
+    /** @description Unique identifier for this widget/tool instance. Typically the app_id or tool name. */
+    toolId: string;
+    /** @description Human-readable name of the tool (shown in prompts). */
+    toolName: string;
+    /** @description JSON string containing the widget's current state. This will be included in future completion requests. */
+    content: string;
+  };
+}
+
+/**
+ * @description Result from setting widget state.
+ * @see {@link McpUiSetWidgetStateRequest}
+ */
+export interface McpUiSetWidgetStateResult {
+  /** @description True if the host failed to store the widget state. */
+  isError?: boolean;
+  /**
+   * Index signature required for MCP SDK `Protocol` class compatibility.
+   * Note: The schema intentionally omits this to enforce strict validation.
+   */
+  [key: string]: unknown;
+}

src/types.ts

@@ -44,6 +44,8 @@ export {
   type McpUiRequestDisplayModeResult,
   type McpUiToolVisibility,
   type McpUiToolMeta,
+  type McpUiSetWidgetStateRequest,
+  type McpUiSetWidgetStateResult,
 } from "./spec.types.js";
 
 // Import types needed for protocol type unions (not re-exported, just used internally)
@@ -53,6 +55,7 @@ import type {
   McpUiMessageRequest,
   McpUiResourceTeardownRequest,
   McpUiRequestDisplayModeRequest,
+  McpUiSetWidgetStateRequest,
   McpUiHostContextChangedNotification,
   McpUiToolInputNotification,
   McpUiToolInputPartialNotification,
@@ -67,6 +70,7 @@ import type {
   McpUiMessageResult,
   McpUiResourceTeardownResult,
   McpUiRequestDisplayModeResult,
+  McpUiSetWidgetStateResult,
 } from "./spec.types.js";
 
 // Re-export all schemas from generated/schema.ts (already PascalCase)
@@ -101,6 +105,8 @@ export {
   McpUiRequestDisplayModeResultSchema,
   McpUiToolVisibilitySchema,
   McpUiToolMetaSchema,
+  McpUiSetWidgetStateRequestSchema,
+  McpUiSetWidgetStateResultSchema,
 } from "./generated/schema.js";
 
 // Re-export SDK types used in protocol type unions
@@ -129,7 +135,7 @@ import {
  * All request types in the MCP Apps protocol.
  *
  * Includes:
- * - MCP UI requests (initialize, open-link, message, resource-teardown, request-display-mode)
+ * - MCP UI requests (initialize, open-link, message, resource-teardown, request-display-mode, set-widget-state)
  * - MCP server requests forwarded from the app (tools/call, resources/*, prompts/list)
  * - Protocol requests (ping)
  */
@@ -139,6 +145,7 @@ export type AppRequest =
   | McpUiMessageRequest
   | McpUiResourceTeardownRequest
   | McpUiRequestDisplayModeRequest
+  | McpUiSetWidgetStateRequest
   | CallToolRequest
   | ListToolsRequest
   | ListResourcesRequest
@@ -186,6 +193,7 @@ export type AppResult =
   | McpUiMessageResult
   | McpUiResourceTeardownResult
   | McpUiRequestDisplayModeResult
+  | McpUiSetWidgetStateResult
   | CallToolResult
   | ListToolsResult
   | ListResourcesResult