Add utility for using styles

src/app.ts

@@ -47,6 +47,7 @@ import { Transport } from "@modelcontextprotocol/sdk/shared/transport.js";
 
 export { PostMessageTransport } from "./message-transport";
 export * from "./types";
+export { applyHostStyles } from "./styles";
 
 /**
  * Metadata key for associating a resource URI with a tool call.

src/react/index.tsx

@@ -9,6 +9,7 @@
  * ## Main Exports
  *
  * - {@link useApp} - React hook to create and connect an MCP App
+ * - {@link useHostStyles} - React hook to apply host styles as CSS variables
  * - {@link useAutoResize} - React hook for manual auto-resize control (rarely needed)
  *
  * @module @modelcontextprotocol/ext-apps/react
@@ -32,3 +33,4 @@
  */
 export * from "./useApp";
 export * from "./useAutoResize";
+export * from "./useHostStyles";

src/react/useHostStyles.ts

@@ -0,0 +1,84 @@
+import { useEffect, useRef } from "react";
+import { App } from "../app";
+import { applyHostStyles } from "../styles";
+import { McpUiHostContext } from "../types";
+
+/**
+ * React hook that applies host styles as CSS custom properties.
+ *
+ * This hook listens to host context changes and automatically applies the
+ * `styles` CSS variables to `document.documentElement`. This allows your
+ * app to use the host's theming values via CSS variables like
+ * `var(--color-background-primary)`.
+ *
+ * The hook also applies styles from the initial host context when the app
+ * first connects.
+ *
+ * @param app - The connected App instance, or null during initialization
+ * @param initialContext - Initial host context from the connection (optional).
+ *   If provided, styles will be applied immediately on mount.
+ *
+ * @example Basic usage with useApp
+ * ```tsx
+ * import { useApp } from '@modelcontextprotocol/ext-apps/react';
+ * import { useHostStyles } from '@modelcontextprotocol/ext-apps/react';
+ *
+ * function MyApp() {
+ *   const { app, isConnected } = useApp({
+ *     appInfo: { name: "MyApp", version: "1.0.0" },
+ *     capabilities: {},
+ *   });
+ *
+ *   // Automatically apply host styles as CSS variables
+ *   useHostStyles(app);
+ *
+ *   return (
+ *     <div style={{ background: 'var(--color-background-primary)' }}>
+ *       Hello!
+ *     </div>
+ *   );
+ * }
+ * ```
+ *
+ * @example With initial context
+ * ```tsx
+ * const [hostContext, setHostContext] = useState<McpUiHostContext | null>(null);
+ *
+ * // ... get initial context from app.connect() result
+ *
+ * useHostStyles(app, hostContext);
+ * ```
+ *
+ * @see {@link applyHostStyles} for the underlying function
+ * @see {@link McpUiStyles} for available CSS variables
+ */
+export function useHostStyles(
+  app: App | null,
+  initialContext?: McpUiHostContext | null,
+): void {
+  const initialStylesApplied = useRef(false);
+
+  // Apply initial styles once on mount
+  useEffect(() => {
+    if (initialStylesApplied.current) {
+      return;
+    }
+    if (initialContext?.styles) {
+      applyHostStyles(initialContext.styles);
+      initialStylesApplied.current = true;
+    }
+  }, [initialContext]);
+
+  // Listen for host context changes and apply updated styles
+  useEffect(() => {
+    if (!app) {
+      return;
+    }
+
+    app.onhostcontextchanged = (params) => {
+      if (params.styles) {
+        applyHostStyles(params.styles);
+      }
+    };
+  }, [app]);
+}

src/styles.ts

@@ -0,0 +1,41 @@
+import { McpUiStyles } from "./types";
+
+/**
+ * Apply host styles as CSS custom properties on an element.
+ *
+ * This function takes the `styles` object from `McpUiHostContext` and sets
+ * each CSS variable on the specified root element (defaults to `document.documentElement`).
+ * This allows apps to use the host's theming values via CSS variables like
+ * `var(--color-background-primary)`.
+ *
+ * @param styles - The styles object from `McpUiHostContext.styles`
+ * @param root - The element to apply styles to (defaults to `document.documentElement`)
+ *
+ * @example Apply styles from host context
+ * ```typescript
+ * import { applyHostStyles } from '@modelcontextprotocol/ext-apps';
+ *
+ * app.onhostcontextchanged = (params) => {
+ *   if (params.styles) {
+ *     applyHostStyles(params.styles);
+ *   }
+ * };
+ * ```
+ *
+ * @example Apply to a specific element
+ * ```typescript
+ * const container = document.getElementById('app-root');
+ * applyHostStyles(hostContext.styles, container);
+ * ```
+ *
+ * @see {@link McpUiStyles} for the available CSS variables
+ * @see {@link McpUiHostContext} for the full host context structure
+ */
+export function applyHostStyles(
+  styles: McpUiStyles,
+  root: HTMLElement = document.documentElement,
+): void {
+  for (const [key, value] of Object.entries(styles)) {
+    root.style.setProperty(key, value);
+  }
+}