Have apps request display mode instead

Author: martinalong

specification/draft/apps.mdx

@@ -537,6 +537,39 @@ Host behavior:
 * Host SHOULD add the message to the conversation context, preserving the specified role.
 * Host MAY request user consent.
 
+`ui/request-display-mode` - Request host to change display mode
+
+```typescript
+// Request
+{
+  jsonrpc: "2.0",
+  id: 3,
+  method: "ui/request-display-mode",
+  params: {
+    mode: "inline" | "fullscreen" | "pip"  // Requested display mode
+  }
+}
+
+// Success Response
+{
+  jsonrpc: "2.0",
+  id: 3,
+  result: {
+    mode: "inline" | "fullscreen" | "pip"  // Actual display mode set
+  }
+}
+```
+
+Host behavior:
+* Host SHOULD check if the requested mode is in `availableDisplayModes` from host context.
+* If the requested mode is available, Host SHOULD switch to that mode and return it in the response.
+* If the requested mode is not available, Host SHOULD return the current display mode in the response.
+* Host MAY coerce modes on certain platforms (e.g., "pip" to "fullscreen" on mobile).
+
+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.
+
 #### 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-bridge.ts

@@ -71,6 +71,9 @@ import {
   McpUiSandboxProxyReadyNotification,
   McpUiSandboxProxyReadyNotificationSchema,
   McpUiSizeChangedNotificationSchema,
+  McpUiRequestDisplayModeRequest,
+  McpUiRequestDisplayModeRequestSchema,
+  McpUiRequestDisplayModeResult,
 } from "./types";
 export * from "./types";
 export { RESOURCE_URI_META_KEY, RESOURCE_MIME_TYPE } from "./app";
@@ -222,6 +225,13 @@ export class AppBridge extends Protocol<
       this.onping?.(request.params, extra);
       return {};
     });
+
+    // Default handler for requestDisplayMode - returns current mode from host context.
+    // Hosts can override this by setting bridge.onrequestdisplaymode = ...
+    this.setRequestHandler(McpUiRequestDisplayModeRequestSchema, (request) => {
+      const currentMode = this._hostContext.displayMode ?? "inline";
+      return { mode: currentMode };
+    });
   }
 
   /**
@@ -494,6 +504,52 @@ export class AppBridge extends Protocol<
     );
   }
 
+  /**
+   * Register a handler for display mode change requests from the Guest UI.
+   *
+   * The Guest UI sends `ui/request-display-mode` requests when it wants to change
+   * its display mode (e.g., from "inline" to "fullscreen"). The handler should
+   * check if the requested mode is in `availableDisplayModes` from the host context,
+   * update the display mode if supported, and return the actual mode that was set.
+   *
+   * If the requested mode is not available, the handler should return the current
+   * display mode instead.
+   *
+   * @param callback - Handler that receives the requested mode and returns the actual mode set
+   *   - params.mode - The display mode being requested ("inline" | "fullscreen" | "pip")
+   *   - extra - Request metadata (abort signal, session info)
+   *   - Returns: Promise<McpUiRequestDisplayModeResult> with the actual mode set
+   *
+   * @example
+   * ```typescript
+   * bridge.onrequestdisplaymode = async ({ mode }, extra) => {
+   *   const availableModes = hostContext.availableDisplayModes ?? ["inline"];
+   *   if (availableModes.includes(mode)) {
+   *     setDisplayMode(mode);
+   *     return { mode };
+   *   }
+   *   // Return current mode if requested mode not available
+   *   return { mode: currentDisplayMode };
+   * };
+   * ```
+   *
+   * @see {@link McpUiRequestDisplayModeRequest} for the request type
+   * @see {@link McpUiRequestDisplayModeResult} for the result type
+   */
+  set onrequestdisplaymode(
+    callback: (
+      params: McpUiRequestDisplayModeRequest["params"],
+      extra: RequestHandlerExtra,
+    ) => Promise<McpUiRequestDisplayModeResult>,
+  ) {
+    this.setRequestHandler(
+      McpUiRequestDisplayModeRequestSchema,
+      async (request, extra) => {
+        return callback(request.params, extra);
+      },
+    );
+  }
+
   /**
    * Register a handler for logging messages from the Guest UI.
    *

src/app.ts

@@ -42,6 +42,8 @@ import {
   McpUiToolInputPartialNotificationSchema,
   McpUiToolResultNotification,
   McpUiToolResultNotificationSchema,
+  McpUiRequestDisplayModeRequest,
+  McpUiRequestDisplayModeResultSchema,
 } from "./types";
 import { Transport } from "@modelcontextprotocol/sdk/shared/transport.js";
 
@@ -839,6 +841,44 @@ export class App extends Protocol<AppRequest, AppNotification, AppResult> {
     );
   }
 
+  /**
+   * Request a change to the display mode.
+   *
+   * Requests the host to change the UI container to the specified display mode
+   * (e.g., "inline", "fullscreen", "pip"). The host will respond with the actual
+   * display mode that was set, which may differ from the requested mode if
+   * the requested mode is not available (check `availableDisplayModes` in host context).
+   *
+   * @param params - The display mode being requested
+   * @param options - Request options (timeout, etc.)
+   * @returns Result containing the actual display mode that was set
+   *
+   * @example Request fullscreen mode
+   * ```typescript
+   * const context = app.getHostContext();
+   * if (context?.availableDisplayModes?.includes("fullscreen")) {
+   *   const result = await app.requestDisplayMode({ mode: "fullscreen" });
+   *   console.log("Display mode set to:", result.mode);
+   * }
+   * ```
+   *
+   * @see {@link McpUiRequestDisplayModeRequest} for request structure
+   * @see {@link McpUiHostContext} for checking availableDisplayModes
+   */
+  requestDisplayMode(
+    params: McpUiRequestDisplayModeRequest["params"],
+    options?: RequestOptions,
+  ) {
+    return this.request(
+      <McpUiRequestDisplayModeRequest>{
+        method: "ui/request-display-mode",
+        params,
+      },
+      McpUiRequestDisplayModeResultSchema,
+      options,
+    );
+  }
+
   /**
    * Notify the host of UI size changes.
    *

src/generated/schema.json

@@ -1545,6 +1545,67 @@
       },
       "additionalProperties": {}
     },
+    "McpUiRequestDisplayModeRequest": {
+      "$schema": "https://json-schema.org/draft/2020-12/schema",
+      "type": "object",
+      "properties": {
+        "method": {
+          "type": "string",
+          "const": "ui/request-display-mode"
+        },
+        "params": {
+          "type": "object",
+          "properties": {
+            "mode": {
+              "description": "The display mode being requested.",
+              "anyOf": [
+                {
+                  "type": "string",
+                  "const": "inline"
+                },
+                {
+                  "type": "string",
+                  "const": "fullscreen"
+                },
+                {
+                  "type": "string",
+                  "const": "pip"
+                }
+              ]
+            }
+          },
+          "required": ["mode"],
+          "additionalProperties": false
+        }
+      },
+      "required": ["method", "params"],
+      "additionalProperties": false
+    },
+    "McpUiRequestDisplayModeResult": {
+      "$schema": "https://json-schema.org/draft/2020-12/schema",
+      "type": "object",
+      "properties": {
+        "mode": {
+          "description": "The display mode that was actually set. May differ from requested if not supported.",
+          "anyOf": [
+            {
+              "type": "string",
+              "const": "inline"
+            },
+            {
+              "type": "string",
+              "const": "fullscreen"
+            },
+            {
+              "type": "string",
+              "const": "pip"
+            }
+          ]
+        }
+      },
+      "required": ["mode"],
+      "additionalProperties": {}
+    },
     "McpUiResourceCsp": {
       "$schema": "https://json-schema.org/draft/2020-12/schema",
       "type": "object",

src/generated/schema.test.ts

@@ -83,6 +83,14 @@ export type McpUiResourceMetaSchemaInferredType = z.infer<
   typeof generated.McpUiResourceMetaSchema
 >;
 
+export type McpUiRequestDisplayModeRequestSchemaInferredType = z.infer<
+  typeof generated.McpUiRequestDisplayModeRequestSchema
+>;
+
+export type McpUiRequestDisplayModeResultSchemaInferredType = z.infer<
+  typeof generated.McpUiRequestDisplayModeResultSchema
+>;
+
 export type McpUiMessageRequestSchemaInferredType = z.infer<
   typeof generated.McpUiMessageRequestSchema
 >;
@@ -195,6 +203,18 @@ expectType<spec.McpUiResourceCsp>({} as McpUiResourceCspSchemaInferredType);
 expectType<McpUiResourceCspSchemaInferredType>({} as spec.McpUiResourceCsp);
 expectType<spec.McpUiResourceMeta>({} as McpUiResourceMetaSchemaInferredType);
 expectType<McpUiResourceMetaSchemaInferredType>({} as spec.McpUiResourceMeta);
+expectType<spec.McpUiRequestDisplayModeRequest>(
+  {} as McpUiRequestDisplayModeRequestSchemaInferredType,
+);
+expectType<McpUiRequestDisplayModeRequestSchemaInferredType>(
+  {} as spec.McpUiRequestDisplayModeRequest,
+);
+expectType<spec.McpUiRequestDisplayModeResult>(
+  {} as McpUiRequestDisplayModeResultSchemaInferredType,
+);
+expectType<McpUiRequestDisplayModeResultSchemaInferredType>(
+  {} as spec.McpUiRequestDisplayModeResult,
+);
 expectType<spec.McpUiMessageRequest>(
   {} as McpUiMessageRequestSchemaInferredType,
 );

src/generated/schema.ts

@@ -307,6 +307,31 @@ export const McpUiResourceMetaSchema = z.object({
     ),
 });
 
+/**
+ * @description Request to change the display mode of the UI.
+ * The host will respond with the actual display mode that was set,
+ * which may differ from the requested mode if not supported.
+ * @see {@link app.App.requestDisplayMode} for the method that sends this request
+ */
+export const McpUiRequestDisplayModeRequestSchema = z.object({
+  method: z.literal("ui/request-display-mode"),
+  params: z.object({
+    /** @description The display mode being requested. */
+    mode: McpUiDisplayModeSchema.describe("The display mode being requested."),
+  }),
+});
+
+/**
+ * @description Result from requesting a display mode change.
+ * @see {@link McpUiRequestDisplayModeRequest}
+ */
+export const McpUiRequestDisplayModeResultSchema = z.looseObject({
+  /** @description The display mode that was actually set. May differ from requested if not supported. */
+  mode: McpUiDisplayModeSchema.describe(
+    "The display mode that was actually set. May differ from requested if not supported.",
+  ),
+});
+
 /**
  * @description Request to send a message to the host's chat interface.
  * @see {@link app.App.sendMessage} for the method that sends this request

src/spec.types.ts

@@ -371,3 +371,31 @@ export interface McpUiResourceMeta {
   /** @description Visual boundary preference - true if UI prefers a visible border. */
   prefersBorder?: boolean;
 }
+
+/**
+ * @description Request to change the display mode of the UI.
+ * The host will respond with the actual display mode that was set,
+ * which may differ from the requested mode if not supported.
+ * @see {@link app.App.requestDisplayMode} for the method that sends this request
+ */
+export interface McpUiRequestDisplayModeRequest {
+  method: "ui/request-display-mode";
+  params: {
+    /** @description The display mode being requested. */
+    mode: McpUiDisplayMode;
+  };
+}
+
+/**
+ * @description Result from requesting a display mode change.
+ * @see {@link McpUiRequestDisplayModeRequest}
+ */
+export interface McpUiRequestDisplayModeResult {
+  /** @description The display mode that was actually set. May differ from requested if not supported. */
+  mode: McpUiDisplayMode;
+  /**
+   * 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

@@ -36,6 +36,8 @@ export {
   type McpUiInitializedNotification,
   type McpUiResourceCsp,
   type McpUiResourceMeta,
+  type McpUiRequestDisplayModeRequest,
+  type McpUiRequestDisplayModeResult,
 } from "./spec.types.js";
 
 // Import types needed for protocol type unions (not re-exported, just used internally)
@@ -44,6 +46,7 @@ import type {
   McpUiOpenLinkRequest,
   McpUiMessageRequest,
   McpUiResourceTeardownRequest,
+  McpUiRequestDisplayModeRequest,
   McpUiHostContextChangedNotification,
   McpUiToolInputNotification,
   McpUiToolInputPartialNotification,
@@ -57,6 +60,7 @@ import type {
   McpUiOpenLinkResult,
   McpUiMessageResult,
   McpUiResourceTeardownResult,
+  McpUiRequestDisplayModeResult,
 } from "./spec.types.js";
 
 // Re-export all schemas from generated/schema.ts (already PascalCase)
@@ -85,6 +89,8 @@ export {
   McpUiInitializedNotificationSchema,
   McpUiResourceCspSchema,
   McpUiResourceMetaSchema,
+  McpUiRequestDisplayModeRequestSchema,
+  McpUiRequestDisplayModeResultSchema,
 } from "./generated/schema.js";
 
 // Re-export SDK types used in protocol type unions
@@ -113,7 +119,7 @@ import {
  * All request types in the MCP Apps protocol.
  *
  * Includes:
- * - MCP UI requests (initialize, open-link, message, resource-teardown)
+ * - MCP UI requests (initialize, open-link, message, resource-teardown, request-display-mode)
  * - MCP server requests forwarded from the app (tools/call, resources/*, prompts/list)
  * - Protocol requests (ping)
  */
@@ -122,6 +128,7 @@ export type AppRequest =
   | McpUiOpenLinkRequest
   | McpUiMessageRequest
   | McpUiResourceTeardownRequest
+  | McpUiRequestDisplayModeRequest
   | CallToolRequest
   | ListToolsRequest
   | ListResourcesRequest
@@ -168,6 +175,7 @@ export type AppResult =
   | McpUiOpenLinkResult
   | McpUiMessageResult
   | McpUiResourceTeardownResult
+  | McpUiRequestDisplayModeResult
   | CallToolResult
   | ListToolsResult
   | ListResourcesResult