[MCP Apps] Update viewport type

specification/draft/apps.mdx

@@ -461,12 +461,20 @@ interface HostContext {
   displayMode?: "inline" | "fullscreen" | "pip";
   /** Display modes the host supports */
   availableDisplayModes?: string[];
-  /** Current and maximum dimensions available to the UI */
+  /** Container dimensions for the iframe. Specify either width or maxWidth, and either height or maxHeight. */
+  containerDimensions?: (
+    | { height: number }      // If specified, container is fixed at this height
+    | { maxHeight?: number }  // Otherwise, container height is determined by the UI height, up to this maximum height (if defined)
+  ) & (
+    | { width: number }       // If specified, container is fixed at this width
+    | { maxWidth?: number }   // Otherwise, container width is determined by the UI width, up to this maximum width (if defined)
+  );
+  /** Host window viewport dimensions */
   viewport?: {
-    width: number;
-    height: number;
-    maxHeight?: number;
-    maxWidth?: number;
+    /** Window viewport width in pixels. */
+    width?: number;
+    /** Window viewport height in pixels. */
+    height?: number;
   };
   /** User's language/region preference (BCP 47, e.g., "en-US") */
   locale?: string;
@@ -516,12 +524,87 @@ Example:
         }
       },
       "displayMode": "inline",
-      "viewport": { "width": 400, "height": 300 }
+      "containerDimensions": { "width": 400, "maxHeight": 600 }
+      "viewport": { "width": 1920, "height": 1080 },
     }
   }
 }
 ```
 
+### Viewport and Dimensions
+
+The `HostContext` provides two separate fields for sizing information:
+
+- **`containerDimensions`**: The dimensions of the container that holds the app. This controls the actual space the app occupies within the host. Each dimension (height and width) operates independently and can be either **fixed** or **flexible**.
+
+- **`viewport`**: The host window's dimensions (e.g., `window.innerWidth` and `window.innerHeight`). Apps can use this to make responsive layout decisions based on the overall screen size.
+
+#### Dimension Modes
+
+| Mode | Dimensions Field | Meaning |
+|------|-----------------|---------|
+| Fixed | `height` or `width` | Host controls the size. App should fill the available space. |
+| Flexible | `maxHeight` or `maxWidth` | App controls the size, up to the specified maximum. |
+| Unbounded | Field omitted | App controls the size with no limit. |
+
+These modes can be combined independently. For example, a host might specify a fixed width but flexible height, allowing the app to grow vertically based on content.
+
+#### App Behavior
+
+Apps should check the containerDimensions configuration and apply appropriate CSS:
+
+```typescript
+// In the app's initialization
+const containerDimensions = hostContext.containerDimensions;
+
+if (containerDimensions) {
+  // Handle height
+  if ("height" in containerDimensions) {
+    // Fixed height: fill the container
+    document.documentElement.style.height = "100vh";
+  } else if ("maxHeight" in containerDimensions && containerDimensions.maxHeight) {
+    // Flexible with max: let content determine size, up to max
+    document.documentElement.style.maxHeight = `${containerDimensions.maxHeight}px`;
+  }
+  // If neither, height is unbounded
+
+  // Handle width
+  if ("width" in containerDimensions) {
+    // Fixed width: fill the container
+    document.documentElement.style.width = "100vw";
+  } else if ("maxWidth" in containerDimensions && containerDimensions.maxWidth) {
+    // Flexible with max: let content determine size, up to max
+    document.documentElement.style.maxWidth = `${containerDimensions.maxWidth}px`;
+  }
+  // If neither, width is unbounded
+}
+
+// Apps can also use viewport for additional data to make responsive layout decisions
+const viewport = hostContext.viewport;
+if (viewport?.width && viewport.width < 768) {
+  // Apply mobile-friendly layout
+}
+```
+
+#### Host Behavior
+
+When using flexible dimensions (no fixed `height` or `width`), hosts MUST listen for `ui/notifications/size-changed` notifications from the app and update the iframe dimensions accordingly:
+
+```typescript
+// Host listens for size changes from the app
+bridge.onsizechange = ({ width, height }) => {
+  // Update iframe to match app's content size
+  if (width != null) {
+    iframe.style.width = `${width}px`;
+  }
+  if (height != null) {
+    iframe.style.height = `${height}px`;
+  }
+};
+```
+
+Apps using the SDK automatically send size-changed notifications via ResizeObserver when `autoResize` is enabled (the default). The notifications are debounced and only sent when dimensions actually change.
+
 ### Theming
 
 Hosts can optionally pass CSS custom properties via `HostContext.styles.variables` for visual cohesion with the host environment.

src/app-bridge.test.ts

@@ -114,6 +114,7 @@ describe("App <-> AppBridge integration", () => {
         theme: "dark" as const,
         locale: "en-US",
         viewport: { width: 800, height: 600 },
+        containerDimensions: { width: 800, maxHeight: 600 },
       };
       const newBridge = new AppBridge(
         createMockClient() as Client,
@@ -338,6 +339,7 @@ describe("App <-> AppBridge integration", () => {
         theme: "light" as const,
         locale: "en-US",
         viewport: { width: 800, height: 600 },
+        containerDimensions: { width: 800, maxHeight: 600 },
       };
       const newBridge = new AppBridge(
         createMockClient() as Client,
@@ -354,20 +356,28 @@ describe("App <-> AppBridge integration", () => {
       newBridge.sendHostContextChange({ theme: "dark" });
       await flush();
 
-      // Send another partial update: only viewport changes
+      // Send another partial update: only viewport and containerDimensions change
       newBridge.sendHostContextChange({
         viewport: { width: 1024, height: 768 },
+        containerDimensions: { width: 1024, maxHeight: 768 },
       });
       await flush();
 
       // getHostContext should have accumulated all updates:
       // - locale from initial (unchanged)
       // - theme from first partial update
-      // - viewport from second partial update
+      // - viewport and containerDimensions from second partial update
       const context = newApp.getHostContext();
       expect(context?.theme).toBe("dark");
       expect(context?.locale).toBe("en-US");
-      expect(context?.viewport).toEqual({ width: 1024, height: 768 });
+      expect(context?.viewport).toEqual({
+        width: 1024,
+        height: 768,
+      });
+      expect(context?.containerDimensions).toEqual({
+        width: 1024,
+        maxHeight: 768,
+      });
 
       await newAppTransport.close();
       await newBridgeTransport.close();

src/app-bridge.ts

@@ -1008,7 +1008,8 @@ export class AppBridge extends Protocol<
    * ```typescript
    * bridge.setHostContext({
    *   theme: "dark",
-   *   viewport: { width: 800, height: 600 }
+   *   viewport: { width: 800, height: 600 },
+   *   containerDimensions: { maxHeight: 600, width: 800 }
    * });
    * ```
    *