diff --git a/examples/express-react-todo/src/tools/get.tsx b/examples/express-react-todo/src/tools/get.tsx
index ee40d99..b418f84 100644
--- a/examples/express-react-todo/src/tools/get.tsx
+++ b/examples/express-react-todo/src/tools/get.tsx
@@ -50,5 +50,8 @@ export default defineTool(
 				</ul>
 			</div>
 		),
+		preview: {
+			toolOutput: { todos },
+		},
 	},
 );
diff --git a/examples/hono-todo/src/tools/get.tsx b/examples/hono-todo/src/tools/get.tsx
index 4124b93..9a4cc22 100644
--- a/examples/hono-todo/src/tools/get.tsx
+++ b/examples/hono-todo/src/tools/get.tsx
@@ -54,5 +54,9 @@ export default defineTool(
 				</ul>
 			</div>
 		),
+		preview: {
+			toolOutput: { todos },
+			toolResponseMetadata: { timestamp: new Date().toISOString() },
+		},
 	},
 );
diff --git a/packages/chapplin/src/preview.ts b/packages/chapplin/src/preview.ts
new file mode 100644
index 0000000..0c306cd
--- /dev/null
+++ b/packages/chapplin/src/preview.ts
@@ -0,0 +1,97 @@
+import {
+	type DisplayMode,
+	SET_GLOBALS_EVENT_TYPE,
+	type OpenAiGlobals,
+} from "./openai.js";
+
+export type Preview = {
+	toolInput?: Record<string, unknown>;
+	toolOutput?: Record<string, unknown>;
+	toolResponseMetadata?: Record<string, unknown>;
+};
+
+export type PreviewDefaults = {
+	theme?: "light" | "dark";
+	locale?: string;
+	maxHeight?: number;
+	displayMode?: DisplayMode;
+};
+
+const defaultPreviewGlobals: PreviewDefaults = {
+	theme: "light",
+	locale: "en-US",
+	maxHeight: 600,
+	displayMode: "inline",
+};
+
+/**
+ * Check if running in development mode.
+ * Returns false in production to prevent preview stubs from being used.
+ */
+function isDevMode(): boolean {
+	try {
+		// Vite injects import.meta.env.DEV
+		// biome-ignore lint/suspicious/noExplicitAny: Vite-specific property
+		return (import.meta as any).env?.DEV === true;
+	} catch {
+		// Fallback: assume production if import.meta.env is not available
+		return false;
+	}
+}
+
+/**
+ * Initialize window.openai with preview data for local development.
+ * Only initializes in development mode when preview data is provided
+ * and window.openai is not already set by the host.
+ */
+export function initializePreview(
+	preview?: Preview,
+	defaults?: PreviewDefaults,
+): void {
+	// Only initialize preview in development mode
+	if (!preview || window.openai || !isDevMode()) return;
+
+	const config = { ...defaultPreviewGlobals, ...defaults };
+
+	// Create a mutable state object for widgetState
+	let widgetState: Record<string, unknown> | null = null;
+
+	window.openai = {
+		theme: config.theme ?? "light",
+		userAgent: {
+			device: { type: "desktop" },
+			capabilities: { hover: true, touch: false },
+		},
+		locale: config.locale ?? "en-US",
+		maxHeight: config.maxHeight ?? 600,
+		displayMode: config.displayMode ?? "inline",
+		safeArea: { insets: { top: 0, bottom: 0, left: 0, right: 0 } },
+		toolInput: preview.toolInput ?? {},
+		toolOutput: preview.toolOutput ?? null,
+		toolResponseMetadata: preview.toolResponseMetadata ?? null,
+		get widgetState() {
+			return widgetState;
+		},
+		// Stub API methods for preview
+		callTool: async () => {
+			throw new Error("callTool is not available in preview mode");
+		},
+		sendFollowUpMessage: async () => {
+			throw new Error("sendFollowUpMessage is not available in preview mode");
+		},
+		openExternal: () => {
+			throw new Error("openExternal is not available in preview mode");
+		},
+		requestDisplayMode: async () => ({ mode: config.displayMode ?? "inline" }),
+		setWidgetState: async (state) => {
+			widgetState = state as Record<string, unknown>;
+			// Dispatch event to notify subscribers (like useWidgetState hook)
+			window.dispatchEvent(
+				new CustomEvent<{ globals: Partial<OpenAiGlobals> }>(
+					SET_GLOBALS_EVENT_TYPE,
+					{ detail: { globals: { widgetState } } },
+				),
+			);
+		},
+	};
+}
diff --git a/packages/chapplin/src/tool-hono.ts b/packages/chapplin/src/tool-hono.ts
index 13d95ee..9bf5856 100644
--- a/packages/chapplin/src/tool-hono.ts
+++ b/packages/chapplin/src/tool-hono.ts
@@ -2,8 +2,12 @@ import type { Child, JSXNode } from "hono/jsx";
 import { jsx, render, useEffect, useState } from "hono/jsx/dom";
 import { createGlobalGetterHooks } from "./client.js";
 import type { OpenAiGlobals } from "./openai.js";
+import { type Preview, initializePreview } from "./preview.js";
 
-type Widget = { app: (props: OpenAiGlobals) => Child };
+type Widget = {
+	app: (props: OpenAiGlobals) => Child;
+	preview?: Preview;
+};
 type Component = (props: unknown) => JSXNode;
 
 const hooks = createGlobalGetterHooks({ useState, useEffect });
@@ -15,6 +19,9 @@ export function defineTool(
 	widget?: Widget,
 ): void {
 	if (!widget) return;
+
+	initializePreview(widget.preview);
+
 	const container = document.getElementById("app");
 	if (container) render(jsx(App as Component, { app: widget.app }), container);
 }
diff --git a/packages/chapplin/src/tool-preact.ts b/packages/chapplin/src/tool-preact.ts
index 516324a..3e01bcf 100644
--- a/packages/chapplin/src/tool-preact.ts
+++ b/packages/chapplin/src/tool-preact.ts
@@ -4,8 +4,12 @@ import { useEffect, useState } from "preact/hooks";
 import { jsx } from "preact/jsx-runtime";
 import { createGlobalGetterHooks } from "./client.js";
 import type { OpenAiGlobals } from "./openai.js";
+import { type Preview, initializePreview } from "./preview.js";
 
-type Widget = { app: ComponentType<OpenAiGlobals> };
+type Widget = {
+	app: ComponentType<OpenAiGlobals>;
+	preview?: Preview;
+};
 
 const hooks = createGlobalGetterHooks({ useState, useEffect });
 
@@ -16,6 +20,9 @@ export function defineTool(
 	widget?: Widget,
 ): void {
 	if (!widget) return;
+
+	initializePreview(widget.preview);
+
 	const container = document.getElementById("app");
 	if (container) render(jsx(App, { app: widget.app }), container);
 }
diff --git a/packages/chapplin/src/tool-react.ts b/packages/chapplin/src/tool-react.ts
index 8453680..b9cacf7 100644
--- a/packages/chapplin/src/tool-react.ts
+++ b/packages/chapplin/src/tool-react.ts
@@ -4,8 +4,12 @@ import { jsx } from "react/jsx-runtime";
 import { createRoot } from "react-dom/client";
 import { createGlobalGetterHooks } from "./client.js";
 import type { OpenAiGlobals } from "./openai.js";
+import { type Preview, initializePreview } from "./preview.js";
 
-type Widget = { app: ComponentType<OpenAiGlobals> };
+type Widget = {
+	app: ComponentType<OpenAiGlobals>;
+	preview?: Preview;
+};
 
 const hooks = createGlobalGetterHooks({ useState, useEffect });
 
@@ -16,6 +20,9 @@ export function defineTool(
 	widget?: Widget,
 ): void {
 	if (!widget) return;
+
+	initializePreview(widget.preview);
+
 	const container = document.getElementById("app");
 	if (container) createRoot(container).render(jsx(App, { app: widget.app }));
 }
diff --git a/packages/chapplin/src/tool-solid.ts b/packages/chapplin/src/tool-solid.ts
index 5b34a4b..b5948b9 100644
--- a/packages/chapplin/src/tool-solid.ts
+++ b/packages/chapplin/src/tool-solid.ts
@@ -2,8 +2,12 @@ import { type Component, createEffect, createSignal } from "solid-js";
 import { createComponent, render } from "solid-js/web";
 import { createGlobalGetterHook, createGlobalsSubscribe } from "./client.js";
 import type { OpenAiGlobals } from "./openai.js";
+import { type Preview, initializePreview } from "./preview.js";
 
-type Widget = { app: Component<OpenAiGlobals> };
+type Widget = {
+	app: Component<OpenAiGlobals>;
+	preview?: Preview;
+};
 
 export function defineTool(
 	_name: unknown,
@@ -12,6 +16,9 @@ export function defineTool(
 	widget?: Widget,
 ): void {
 	if (!widget) return;
+
+	initializePreview(widget.preview);
+
 	const container = document.getElementById("app");
 	if (container)
 		render(
diff --git a/packages/chapplin/src/tool.ts b/packages/chapplin/src/tool.ts
index 19c4f8f..4454446 100644
--- a/packages/chapplin/src/tool.ts
+++ b/packages/chapplin/src/tool.ts
@@ -50,6 +50,17 @@ export function defineTool<
 				OutputMeta extends undefined ? UnknownObject : OutputMeta
 			>,
 		) => JSXElement;
+		/**
+		 * Preview data for local development.
+		 * Used to display mock data before MCP response is available.
+		 */
+		preview?: {
+			toolInput?: Shape<InputArgs>;
+			toolOutput?: Shape<OutputArgs>;
+			toolResponseMetadata?: OutputMeta extends undefined
+				? UnknownObject
+				: OutputMeta;
+		};
 	},
 ): Tool<Shape<InputArgs>, Shape<OutputArgs>, OutputMeta> {
 	type TypedTool = Tool<Shape<InputArgs>, Shape<OutputArgs>, OutputMeta>;
diff --git a/packages/create-chapplin/templates/hono/src/tools/get.tsx b/packages/create-chapplin/templates/hono/src/tools/get.tsx
index ee40d99..b418f84 100644
--- a/packages/create-chapplin/templates/hono/src/tools/get.tsx
+++ b/packages/create-chapplin/templates/hono/src/tools/get.tsx
@@ -50,5 +50,8 @@ export default defineTool(
 				</ul>
 			</div>
 		),
+		preview: {
+			toolOutput: { todos },
+		},
 	},
 );
diff --git a/packages/create-chapplin/templates/preact/src/tools/get.tsx b/packages/create-chapplin/templates/preact/src/tools/get.tsx
index ee40d99..b418f84 100644
--- a/packages/create-chapplin/templates/preact/src/tools/get.tsx
+++ b/packages/create-chapplin/templates/preact/src/tools/get.tsx
@@ -50,5 +50,8 @@ export default defineTool(
 				</ul>
 			</div>
 		),
+		preview: {
+			toolOutput: { todos },
+		},
 	},
 );
diff --git a/packages/create-chapplin/templates/react/src/tools/get.tsx b/packages/create-chapplin/templates/react/src/tools/get.tsx
index ee40d99..b418f84 100644
--- a/packages/create-chapplin/templates/react/src/tools/get.tsx
+++ b/packages/create-chapplin/templates/react/src/tools/get.tsx
@@ -50,5 +50,8 @@ export default defineTool(
 				</ul>
 			</div>
 		),
+		preview: {
+			toolOutput: { todos },
+		},
 	},
 );
diff --git a/packages/create-chapplin/templates/solid/src/tools/get.tsx b/packages/create-chapplin/templates/solid/src/tools/get.tsx
index 8ff0292..4672c36 100644
--- a/packages/create-chapplin/templates/solid/src/tools/get.tsx
+++ b/packages/create-chapplin/templates/solid/src/tools/get.tsx
@@ -50,5 +50,8 @@ export default defineTool(
 				</ul>
 			</div>
 		),
+		preview: {
+			toolOutput: { todos },
+		},
 	},
 );