feat: store and expose hostContext in App class

Fixes #129 - The App class now stores the initial hostContext received
during initialization and exposes it via getHostContext().

Changes:
- Add _hostContext private property to App class
- Store result.hostContext in connect() method
- Add getHostContext() getter method (parallel to getHostCapabilities())
- Update onhostcontextchanged setter to merge partial updates into
  stored context
- Add default notification handler to update context even when
  user hasn't set onhostcontextchanged

This enables apps to:
- Access toolInfo immediately after connection (only in initial context)
- Render with correct theme/locale without waiting for first notification
- Synchronously access viewport, timezone, and other initial state

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ochafik

src/app-bridge.test.ts

@@ -91,6 +91,38 @@ describe("App <-> AppBridge integration", () => {
       const appCaps = bridge.getAppCapabilities();
       expect(appCaps).toEqual(appCapabilities);
     });
+
+    it("App receives initial hostContext after connect", async () => {
+      // Need fresh transports for new bridge
+      const [newAppTransport, newBridgeTransport] =
+        InMemoryTransport.createLinkedPair();
+
+      const testHostContext = {
+        theme: "dark" as const,
+        locale: "en-US",
+        viewport: { width: 800, height: 600 },
+      };
+      const newBridge = new AppBridge(
+        createMockClient() as Client,
+        testHostInfo,
+        testHostCapabilities,
+        { hostContext: testHostContext },
+      );
+      const newApp = new App(testAppInfo, {}, { autoResize: false });
+
+      await newBridge.connect(newBridgeTransport);
+      await newApp.connect(newAppTransport);
+
+      const hostContext = newApp.getHostContext();
+      expect(hostContext).toEqual(testHostContext);
+
+      await newAppTransport.close();
+      await newBridgeTransport.close();
+    });
+
+    it("getHostContext returns undefined before connect", () => {
+      expect(app.getHostContext()).toBeUndefined();
+    });
   });
 
   describe("Host -> App notifications", () => {
@@ -204,6 +236,129 @@ describe("App <-> AppBridge integration", () => {
       ]);
     });
 
+    it("getHostContext merges updates from onhostcontextchanged", async () => {
+      // Need fresh transports for new bridge
+      const [newAppTransport, newBridgeTransport] =
+        InMemoryTransport.createLinkedPair();
+
+      // Set up bridge with initial context
+      const initialContext = {
+        theme: "light" as const,
+        locale: "en-US",
+      };
+      const newBridge = new AppBridge(
+        createMockClient() as Client,
+        testHostInfo,
+        testHostCapabilities,
+        { hostContext: initialContext },
+      );
+      const newApp = new App(testAppInfo, {}, { autoResize: false });
+
+      await newBridge.connect(newBridgeTransport);
+
+      // Set up handler before connecting app
+      newApp.onhostcontextchanged = () => {
+        // User handler (can be empty, we're testing getHostContext behavior)
+      };
+
+      await newApp.connect(newAppTransport);
+
+      // Verify initial context
+      expect(newApp.getHostContext()).toEqual(initialContext);
+
+      // Update context
+      newBridge.setHostContext({ theme: "dark", locale: "en-US" });
+      await flush();
+
+      // getHostContext should reflect merged state
+      const updatedContext = newApp.getHostContext();
+      expect(updatedContext?.theme).toBe("dark");
+      expect(updatedContext?.locale).toBe("en-US");
+
+      await newAppTransport.close();
+      await newBridgeTransport.close();
+    });
+
+    it("getHostContext updates even without user setting onhostcontextchanged", async () => {
+      // Need fresh transports for new bridge
+      const [newAppTransport, newBridgeTransport] =
+        InMemoryTransport.createLinkedPair();
+
+      // Set up bridge with initial context
+      const initialContext = {
+        theme: "light" as const,
+        locale: "en-US",
+      };
+      const newBridge = new AppBridge(
+        createMockClient() as Client,
+        testHostInfo,
+        testHostCapabilities,
+        { hostContext: initialContext },
+      );
+      const newApp = new App(testAppInfo, {}, { autoResize: false });
+
+      await newBridge.connect(newBridgeTransport);
+      // Note: We do NOT set app.onhostcontextchanged here
+      await newApp.connect(newAppTransport);
+
+      // Verify initial context
+      expect(newApp.getHostContext()).toEqual(initialContext);
+
+      // Update context from bridge
+      newBridge.setHostContext({ theme: "dark", locale: "en-US" });
+      await flush();
+
+      // getHostContext should still update (default handler should work)
+      const updatedContext = newApp.getHostContext();
+      expect(updatedContext?.theme).toBe("dark");
+
+      await newAppTransport.close();
+      await newBridgeTransport.close();
+    });
+
+    it("getHostContext accumulates multiple partial updates", async () => {
+      // Need fresh transports for new bridge
+      const [newAppTransport, newBridgeTransport] =
+        InMemoryTransport.createLinkedPair();
+
+      const initialContext = {
+        theme: "light" as const,
+        locale: "en-US",
+        viewport: { width: 800, height: 600 },
+      };
+      const newBridge = new AppBridge(
+        createMockClient() as Client,
+        testHostInfo,
+        testHostCapabilities,
+        { hostContext: initialContext },
+      );
+      const newApp = new App(testAppInfo, {}, { autoResize: false });
+
+      await newBridge.connect(newBridgeTransport);
+      await newApp.connect(newAppTransport);
+
+      // Update only theme
+      newBridge.setHostContext({ ...initialContext, theme: "dark" });
+      await flush();
+
+      // Update viewport
+      newBridge.setHostContext({
+        theme: "dark",
+        locale: "en-US",
+        viewport: { width: 1024, height: 768 },
+      });
+      await flush();
+
+      // getHostContext should have all updates merged
+      const context = newApp.getHostContext();
+      expect(context?.theme).toBe("dark");
+      expect(context?.locale).toBe("en-US");
+      expect(context?.viewport).toEqual({ width: 1024, height: 768 });
+
+      await newAppTransport.close();
+      await newBridgeTransport.close();
+    });
+
     it("sendResourceTeardown triggers app.onteardown", async () => {
       let teardownCalled = false;
       app.onteardown = async () => {

src/app.ts

@@ -22,6 +22,7 @@ import {
   LATEST_PROTOCOL_VERSION,
   McpUiAppCapabilities,
   McpUiHostCapabilities,
+  McpUiHostContext,
   McpUiHostContextChangedNotification,
   McpUiHostContextChangedNotificationSchema,
   McpUiInitializedNotification,
@@ -191,6 +192,7 @@ type RequestHandlerExtra = Parameters<
 export class App extends Protocol<Request, Notification, Result> {
   private _hostCapabilities?: McpUiHostCapabilities;
   private _hostInfo?: Implementation;
+  private _hostContext?: McpUiHostContext;
 
   /**
    * Create a new MCP App instance.
@@ -219,6 +221,10 @@ export class App extends Protocol<Request, Notification, Result> {
       console.log("Received ping:", request.params);
       return {};
     });
+
+    // Set up default handler to update _hostContext when notifications arrive.
+    // Users can override this by setting onhostcontextchanged.
+    this.onhostcontextchanged = () => {};
   }
 
   /**
@@ -276,6 +282,42 @@ export class App extends Protocol<Request, Notification, Result> {
     return this._hostInfo;
   }
 
+  /**
+   * Get the host context discovered during initialization.
+   *
+   * Returns the host context that was provided in the initialization response,
+   * including tool info, theme, viewport, locale, and other environment details.
+   * This context is automatically updated when the host sends
+   * `ui/notifications/host-context-changed` notifications.
+   *
+   * Returns `undefined` if called before connection is established.
+   *
+   * @returns Host context, or `undefined` if not yet connected
+   *
+   * @example Access host context after connection
+   * ```typescript
+   * await app.connect(transport);
+   * const context = app.getHostContext();
+   * if (context === undefined) {
+   *   console.error("Not connected");
+   *   return;
+   * }
+   * if (context.theme === "dark") {
+   *   document.body.classList.add("dark-theme");
+   * }
+   * if (context.toolInfo) {
+   *   console.log("Tool:", context.toolInfo.tool.name);
+   * }
+   * ```
+   *
+   * @see {@link connect} for the initialization handshake
+   * @see {@link onhostcontextchanged} for context change notifications
+   * @see {@link McpUiHostContext} for the context structure
+   */
+  getHostContext(): McpUiHostContext | undefined {
+    return this._hostContext;
+  }
+
   /**
    * Convenience handler for receiving complete tool input from the host.
    *
@@ -463,7 +505,11 @@ export class App extends Protocol<Request, Notification, Result> {
   ) {
     this.setNotificationHandler(
       McpUiHostContextChangedNotificationSchema,
-      (n) => callback(n.params),
+      (n) => {
+        // Merge the partial update into the stored context
+        this._hostContext = { ...this._hostContext, ...n.params };
+        callback(n.params);
+      },
     );
   }
 
@@ -961,6 +1007,7 @@ export class App extends Protocol<Request, Notification, Result> {
 
       this._hostCapabilities = result.hostCapabilities;
       this._hostInfo = result.hostInfo;
+      this._hostContext = result.hostContext;
 
       await this.notification(<McpUiInitializedNotification>{
         method: "ui/notifications/initialized",