@@ -133,6 +133,8 @@ A browser tab shows one site at a time, so the tools always belong to the active
 
 `document.modelContext` is provided by Playground's mu-plugin on `wp_head` and `admin_head`, which covers the front end and wp-admin.
 
+Chrome 150 deprecated `navigator.modelContext` in favour of `document.modelContext` but still serves it, so Playground does the same: the deprecated global returns the very same registry and warns once on first access. A plugin that has not migrated keeps working here exactly as it does in Chrome. Write new code against `document.modelContext` — the alias goes when Chrome removes it.
+
 **The login screen is deliberately not covered.** `wp-login.php` fires neither hook, so that document has no registry and proxies no tools; a site's tools come back when the user leaves it. `login_head` still fires there, so a plugin hooking it must feature-detect rather than assume the registry exists — worth doing anywhere, since most browsers do not implement WebMCP:
 
 ```php
 
@@ -91,6 +91,7 @@ export function installWebMCPPolyfill(): boolean {
 			configurable: true,
 			value: modelContext,
 		});
+		installDeprecatedNavigatorAlias(modelContext);
 	} catch (error) {
 		// A non-configurable `modelContext` cannot be replaced. Callers wire
 		// up the MCP bridge right after this, so failing loudly here would
@@ -101,6 +102,32 @@ export function installWebMCPPolyfill(): boolean {
 	return true;
 }
 
+/**
+ * Serves the deprecated `navigator.modelContext` alongside the canonical
+ * `document.modelContext`.
+ *
+ * Chrome 150 deprecated the `navigator` getter but still serves it, so code
+ * written against it keeps working there and would break only where this
+ * polyfill stands in. Warn once on first access, as the platform does, and
+ * drop this when Chrome removes it.
+ */
+function installDeprecatedNavigatorAlias(modelContext: ModelContext) {
+	let warned = false;
+	Object.defineProperty(navigator, 'modelContext', {
+		configurable: true,
+		get() {
+			if (!warned) {
+				warned = true;
+				logger.warn(
+					'navigator.modelContext is deprecated and will be removed. ' +
+						'Use document.modelContext instead.'
+				);
+			}
+			return modelContext;
+		},
+	});
+}
+
 /**
  * The `client` argument a WebMCP agent passes to `execute()`. Playground's own
  * tools ignore it, and a caller reaching a tool through the polyfill has no
 
@@ -65,10 +65,12 @@ add_action('wp_head', function () {
 	});
 
 	// Lets the test add and withdraw a tool while the page stays put.
+	// Registers through the deprecated \`navigator\` alias on purpose: Chrome
+	// still serves it, so an unmigrated plugin must reach the same registry.
 	var extra = null;
 	window.__addExtraTool = function () {
 		extra = new AbortController();
-		document.modelContext.registerTool({
+		navigator.modelContext.registerTool({
 			name: 'site_extra',
 			description: 'Registered after the document loaded.',
 			inputSchema: { type: 'object', properties: {} },
 
@@ -498,6 +498,25 @@ function addTool(tool, options) {
 				value: modelContext
 			});
 
+			// Chrome 150 deprecated `navigator.modelContext` in favour of
+			// `document.modelContext` but still serves it, so a plugin that
+			// has not migrated works there and would break only here. Mirror
+			// the platform, warning once, and drop this when Chrome does.
+			var warnedAboutNavigator = false;
+			Object.defineProperty(navigator, 'modelContext', {
+				configurable: true,
+				get: function () {
+					if (!warnedAboutNavigator) {
+						warnedAboutNavigator = true;
+						console.warn(
+							'navigator.modelContext is deprecated and will be ' +
+							'removed. Use document.modelContext instead.'
+						);
+					}
+					return modelContext;
+				}
+			});
+
 			// A minimal WebMCP client. The agent lives in the top-level page,
 			// so this document cannot prompt the user itself.
 			var toolClient = {