diff --git a/.vitepress/theme/Layout.vue b/.vitepress/theme/Layout.vue
new file mode 100644
index 0000000..7c903b4
--- /dev/null
+++ b/.vitepress/theme/Layout.vue
@@ -0,0 +1,14 @@
+<script setup>
+import DefaultTheme from 'vitepress/theme'
+import NotFound from './NotFound.vue'
+
+const { Layout } = DefaultTheme
+</script>
+
+<template>
+  <Layout>
+    <template #not-found>
+      <NotFound />
+    </template>
+  </Layout>
+</template>
diff --git a/.vitepress/theme/NotFound.vue b/.vitepress/theme/NotFound.vue
new file mode 100644
index 0000000..a5b779b
--- /dev/null
+++ b/.vitepress/theme/NotFound.vue
@@ -0,0 +1,114 @@
+<script setup>
+import { useRouter } from 'vitepress'
+
+const { go } = useRouter()
+</script>
+
+<template>
+  <div class="not-found">
+    <p class="code">404</p>
+    <h1 class="title">Page Not Found</h1>
+    <p class="description">The page you're looking for doesn't exist or has been moved.</p>
+
+    <div class="links">
+      <h2>Where to go</h2>
+      <ul>
+        <li><a href="/quickstart/what-is-cycles">What is Cycles?</a> — start with the basics</li>
+        <li><a href="/quickstart/end-to-end-tutorial">End-to-End Tutorial</a> — build a budget-guarded app in 10 minutes</li>
+        <li><a href="/api/">API Reference</a> — interactive endpoint documentation</li>
+        <li><a href="/how-to/adding-cycles-to-an-existing-application">How-To Guides</a> — integration patterns and recipes</li>
+      </ul>
+    </div>
+
+    <div class="action">
+      <a class="link" href="/" aria-label="Go to home">
+        Take me home
+      </a>
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.not-found {
+  padding: 64px 24px 96px;
+  text-align: center;
+}
+
+.code {
+  font-size: 64px;
+  font-weight: 700;
+  line-height: 1;
+  color: var(--vp-c-brand-1);
+}
+
+.title {
+  padding-top: 12px;
+  font-size: 20px;
+  font-weight: 700;
+  letter-spacing: 0.02em;
+  line-height: 28px;
+}
+
+.description {
+  padding-top: 12px;
+  font-size: 14px;
+  color: var(--vp-c-text-2);
+  line-height: 24px;
+}
+
+.links {
+  padding-top: 24px;
+  text-align: left;
+  max-width: 480px;
+  margin: 0 auto;
+}
+
+.links h2 {
+  font-size: 16px;
+  font-weight: 600;
+  margin-bottom: 12px;
+}
+
+.links ul {
+  list-style: none;
+  padding: 0;
+}
+
+.links li {
+  padding: 6px 0;
+  font-size: 14px;
+  line-height: 22px;
+  color: var(--vp-c-text-2);
+}
+
+.links a {
+  color: var(--vp-c-brand-1);
+  font-weight: 500;
+  text-decoration: none;
+}
+
+.links a:hover {
+  text-decoration: underline;
+}
+
+.action {
+  padding-top: 28px;
+}
+
+.link {
+  display: inline-block;
+  border: 1px solid var(--vp-c-brand-1);
+  border-radius: 16px;
+  padding: 3px 16px;
+  font-size: 14px;
+  font-weight: 500;
+  color: var(--vp-c-brand-1);
+  text-decoration: none;
+  transition: border-color 0.25s, color 0.25s;
+}
+
+.link:hover {
+  color: var(--vp-c-brand-2);
+  border-color: var(--vp-c-brand-2);
+}
+</style>
diff --git a/.vitepress/theme/index.ts b/.vitepress/theme/index.ts
index c91a013..61f54d9 100644
--- a/.vitepress/theme/index.ts
+++ b/.vitepress/theme/index.ts
@@ -6,9 +6,11 @@ import { nextTick, onMounted, watch } from 'vue'
 import 'vitepress-openapi/dist/style.css'
 import './custom.css'
 import spec from '../../public/openapi.json'
+import Layout from './Layout.vue'
 
 export default {
   extends: DefaultTheme,
+  Layout,
   async enhanceApp({ app }) {
     useOpenapi({ spec })
     theme.enhanceApp({ app })