diff --git a/packages/testing/README.md b/packages/testing/README.md
new file mode 100644
index 0000000..7a3229e
--- /dev/null
+++ b/packages/testing/README.md
@@ -0,0 +1,35 @@
+# @vue-tui/testing
+
+Test harness for [`@vue-tui/runtime`](../runtime).
+
+## Install
+
+```bash
+pnpm add -D @vue-tui/testing vue
+```
+
+## Quickstart
+
+```ts
+import { defineComponent, h } from "vue";
+import { test, expect } from "vitest";
+import { render, flush } from "@vue-tui/testing";
+import { Text } from "@vue-tui/runtime";
+
+test("renders hello", async () => {
+  const App = defineComponent({ render: () => h(Text, null, "hello") });
+  const r = render(App);
+  await flush();
+  expect(r.lastFrame()).toContain("hello");
+  r.unmount();
+});
+```
+
+## API
+
+- **`render(app, { columns?, rows? })`** — mount in a fake-TTY environment, collect frames.
+- **`flush()`** — await Vue's post-flush queue + Node's immediate queue.
+- **`result.lastFrame()` / `result.frames`** — frame snapshots.
+- **`result.stdin.write(data)`** — inject input that reaches `useInput` handlers.
+- **`result.app`** — the underlying [`TuiApp`](../runtime/README.md#tuiapp-interface) (use `.waitUntilExit()` if needed).
+- **`result.unmount()` / `result.waitUntilExit()`** — convenience pass-throughs.
diff --git a/packages/testing/package.json b/packages/testing/package.json
new file mode 100644
index 0000000..8300723
--- /dev/null
+++ b/packages/testing/package.json
@@ -0,0 +1,37 @@
+{
+  "name": "@vue-tui/testing",
+  "version": "0.0.0",
+  "description": "Test harness for @vue-tui/runtime.",
+  "license": "MIT",
+  "files": [
+    "dist"
+  ],
+  "type": "module",
+  "exports": {
+    ".": "./dist/index.mjs",
+    "./package.json": "./package.json"
+  },
+  "publishConfig": {
+    "access": "public"
+  },
+  "scripts": {
+    "build": "vp pack",
+    "dev": "vp pack --watch",
+    "test": "vp test",
+    "check": "vp check",
+    "prepublishOnly": "vp run build"
+  },
+  "dependencies": {
+    "@vue-tui/runtime": "workspace:*"
+  },
+  "devDependencies": {
+    "@types/node": "^25.6.2",
+    "@vitejs/plugin-vue-jsx": "catalog:",
+    "typescript": "^6.0.3",
+    "vite-plus": "^0.1.20",
+    "vue": "^3.4.0"
+  },
+  "peerDependencies": {
+    "vue": "^3.4.0"
+  }
+}
diff --git a/packages/testing/src/cleanup.ts b/packages/testing/src/cleanup.ts
new file mode 100644
index 0000000..740b06b
--- /dev/null
+++ b/packages/testing/src/cleanup.ts
@@ -0,0 +1,19 @@
+/// <reference types="vite-plus/test/globals" />
+import type { TuiApp } from "@vue-tui/runtime";
+
+const activeApps: TuiApp[] = [];
+
+export function trackApp(app: TuiApp): void {
+  activeApps.push(app);
+}
+
+export function cleanup(): void {
+  for (const app of activeApps) {
+    app.unmount();
+  }
+  activeApps.length = 0;
+}
+
+if (typeof afterEach === "function") {
+  afterEach(() => cleanup());
+}
diff --git a/packages/testing/src/index.ts b/packages/testing/src/index.ts
new file mode 100644
index 0000000..7fafa3e
--- /dev/null
+++ b/packages/testing/src/index.ts
@@ -0,0 +1,3 @@
+export { render, type RenderOptions, type RenderResult, type Terminal } from "./render.ts";
+export { type RawModeState } from "./streams.ts";
+export { cleanup } from "./cleanup.ts";
diff --git a/packages/testing/src/render.ts b/packages/testing/src/render.ts
new file mode 100644
index 0000000..818fbd7
--- /dev/null
+++ b/packages/testing/src/render.ts
@@ -0,0 +1,97 @@
+import { PassThrough } from "node:stream";
+import { nextTick, type Component } from "vue";
+import { createApp, type TuiApp } from "@vue-tui/runtime";
+import { makeFakeStdin, makeFakeWritable, type RawModeState } from "./streams.ts";
+import { trackApp } from "./cleanup.ts";
+
+export interface RenderOptions {
+  columns?: number;
+  rows?: number;
+  props?: Record<string, unknown>;
+  exitOnCtrlC?: boolean;
+}
+
+export interface Terminal {
+  readonly columns: number;
+  readonly rows: number;
+  resize(columns: number, rows: number): Promise<void>;
+  rawMode: RawModeState;
+}
+
+export interface RenderResult {
+  lastFrame(this: void): string | undefined;
+  frames: string[];
+  stdin: {
+    write(data: string): Promise<void>;
+  };
+  terminal: Terminal;
+  unmount(this: void): void;
+  waitUntilExit(this: void): Promise<void>;
+}
+
+function trimFrame(raw: string): string {
+  return raw
+    .split("\n")
+    .map((line) => line.trimEnd())
+    .join("\n")
+    .trimEnd();
+}
+
+export async function render(
+  component: Component,
+  options: RenderOptions = {},
+): Promise<RenderResult> {
+  const stdout = makeFakeWritable({
+    columns: options.columns ?? 100,
+    rows: options.rows ?? 100,
+  });
+  const stderr = makeFakeWritable({
+    columns: options.columns ?? 100,
+    rows: options.rows ?? 100,
+  });
+  const { stream: stdin, rawMode } = makeFakeStdin();
+
+  const frames: string[] = [];
+  stdout.on("data", (chunk) => {
+    frames.push(trimFrame(chunk.toString()));
+  });
+
+  const app: TuiApp = createApp(component, options.props ?? undefined);
+  app.mount({ stdout, stdin, stderr, debug: true, exitOnCtrlC: options.exitOnCtrlC ?? false });
+
+  trackApp(app);
+
+  await nextTick();
+
+  const terminal: Terminal = {
+    get columns() {
+      return stdout.columns;
+    },
+    get rows() {
+      return stdout.rows;
+    },
+    async resize(columns: number, rows: number) {
+      stdout.columns = columns;
+      stdout.rows = rows;
+      (stderr as NodeJS.WriteStream).columns = columns;
+      (stderr as NodeJS.WriteStream).rows = rows;
+      (stdout as unknown as PassThrough).emit("resize");
+      await nextTick();
+    },
+    rawMode,
+  };
+
+  return {
+    lastFrame: () => frames.at(-1),
+    frames,
+    stdin: {
+      async write(data: string): Promise<void> {
+        stdin.emit("data", data);
+        await nextTick();
+      },
+    },
+    terminal,
+    unmount: app.unmount.bind(app),
+    waitUntilExit: app.waitUntilExit.bind(app),
+  };
+}
diff --git a/packages/testing/src/streams.ts b/packages/testing/src/streams.ts
new file mode 100644
index 0000000..3f8c64b
--- /dev/null
+++ b/packages/testing/src/streams.ts
@@ -0,0 +1,38 @@
+import { PassThrough } from "node:stream";
+
+export interface FakeWritableOptions {
+  columns?: number;
+  rows?: number;
+}
+
+export function makeFakeWritable(options: FakeWritableOptions = {}): NodeJS.WriteStream {
+  const s = new PassThrough() as unknown as NodeJS.WriteStream;
+  Object.assign(s, {
+    columns: options.columns ?? 100,
+    rows: options.rows ?? 100,
+    isTTY: true,
+  });
+  return s;
+}
+
+export interface RawModeState {
+  readonly current: boolean;
+  readonly history: readonly boolean[];
+}
+
+export function makeFakeStdin(): { stream: NodeJS.ReadStream; rawMode: RawModeState } {
+  const rawMode = { current: false, history: [] as boolean[] };
+  const s = new PassThrough() as unknown as NodeJS.ReadStream;
+  Object.assign(s, {
+    isTTY: true,
+    setRawMode(this: NodeJS.ReadStream, mode: boolean) {
+      rawMode.current = mode;
+      (rawMode.history as boolean[]).push(mode);
+      return this;
+    },
+    setEncoding(this: NodeJS.ReadStream) {
+      return this;
+    },
+  });
+  return { stream: s, rawMode };
+}
diff --git a/packages/testing/tests/render.test.tsx b/packages/testing/tests/render.test.tsx
new file mode 100644
index 0000000..53f006d
--- /dev/null
+++ b/packages/testing/tests/render.test.tsx
@@ -0,0 +1,39 @@
+import { nextTick, ref } from "vue";
+import { expect, test } from "vite-plus/test";
+import { render } from "../src/index.ts";
+import { Text } from "@vue-tui/runtime";
+
+test("lastFrame captures rendered output", async () => {
+  const { lastFrame } = await render(() => <Text>hello</Text>);
+  expect(lastFrame()).toContain("hello");
+});
+
+test("frames accumulate on reactive updates", async () => {
+  const message = ref("first");
+  const { lastFrame, frames } = await render(() => <Text>{message.value}</Text>);
+  const initialCount = frames.length;
+  expect(lastFrame()).toContain("first");
+
+  message.value = "second";
+  await nextTick();
+  expect(lastFrame()).toContain("second");
+  expect(frames.length).toBeGreaterThan(initialCount);
+});
+
+test("render with custom columns", async () => {
+  const { lastFrame } = await render(() => <Text>hello</Text>, { columns: 20 });
+  expect(lastFrame()).toContain("hello");
+});
+
+test("lastFrame trims trailing whitespace", async () => {
+  const { lastFrame } = await render(() => <Text>hi</Text>);
+  const frame = lastFrame()!;
+  for (const line of frame.split("\n")) {
+    expect(line).toBe(line.trimEnd());
+  }
+});
+
+test("auto cleanup — no manual unmount needed", async () => {
+  const { lastFrame } = await render(() => <Text>auto</Text>);
+  expect(lastFrame()).toContain("auto");
+});
diff --git a/packages/testing/tests/streams.test.ts b/packages/testing/tests/streams.test.ts
new file mode 100644
index 0000000..759f1d8
--- /dev/null
+++ b/packages/testing/tests/streams.test.ts
@@ -0,0 +1,23 @@
+import { expect, test } from "vite-plus/test";
+import { makeFakeStdin, makeFakeWritable } from "../src/streams.ts";
+
+test("fake stdout reports columns/rows and isTTY", () => {
+  const s = makeFakeWritable({ columns: 50, rows: 10 });
+  expect(s.columns).toBe(50);
+  expect(s.rows).toBe(10);
+  expect(s.isTTY).toBe(true);
+});
+
+test("fake stdin supports setRawMode and emits data", () => {
+  const { stream: s, rawMode } = makeFakeStdin();
+  expect(s.isTTY).toBe(true);
+  let observed = "";
+  s.on("data", (c) => {
+    observed += c.toString();
+  });
+  s.setRawMode(true);
+  expect(rawMode.current).toBe(true);
+  expect(rawMode.history).toEqual([true]);
+  s.emit("data", "hello");
+  expect(observed).toBe("hello");
+});
diff --git a/packages/testing/tsconfig.json b/packages/testing/tsconfig.json
new file mode 100644
index 0000000..5baeccb
--- /dev/null
+++ b/packages/testing/tsconfig.json
@@ -0,0 +1,22 @@
+{
+  "compilerOptions": {
+    "target": "esnext",
+    "lib": ["es2023"],
+    "moduleDetection": "force",
+    "module": "nodenext",
+    "moduleResolution": "nodenext",
+    "resolveJsonModule": true,
+    "types": ["node"],
+    "strict": true,
+    "noUnusedLocals": true,
+    "declaration": true,
+    "noEmit": true,
+    "allowImportingTsExtensions": true,
+    "esModuleInterop": true,
+    "isolatedModules": true,
+    "verbatimModuleSyntax": true,
+    "skipLibCheck": true,
+    "jsx": "preserve",
+    "jsxImportSource": "vue"
+  }
+}
diff --git a/packages/testing/vite.config.ts b/packages/testing/vite.config.ts
new file mode 100644
index 0000000..bbd9ff3
--- /dev/null
+++ b/packages/testing/vite.config.ts
@@ -0,0 +1,14 @@
+import { defineConfig } from "vite-plus";
+import vueJsx from "@vitejs/plugin-vue-jsx";
+
+export default defineConfig({
+  plugins: [vueJsx()],
+  pack: {
+    dts: { tsgo: true },
+    exports: true,
+  },
+  lint: {
+    options: { typeAware: true, typeCheck: true },
+  },
+  fmt: {},
+});