diff --git a/packages/frontend/test/lib/nirax/fallbacks.test.ts b/packages/frontend/test/lib/nirax/fallbacks.test.ts
new file mode 100644
index 0000000000..fc18d78316
--- /dev/null
+++ b/packages/frontend/test/lib/nirax/fallbacks.test.ts
@@ -0,0 +1,90 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { assert, describe, test } from 'vitest';
+import { createRouter, loginFallbackComponent } from './fixture.js';
+
+describe('[NIRAX] フォールバック', () => {
+	test('pushの際、ページが見つからなかったらforcePushを発火する', () => {
+		const router = createRouter('/');
+		const forcePushes: string[] = [];
+
+		router.addListener('forcePush', ctx => {
+			forcePushes.push(ctx.fullPath);
+			assert.strictEqual(ctx.onInit, false);
+		});
+
+		router.init();
+
+		router.pushByPath('/missing');
+
+		assert.deepStrictEqual(forcePushes, ['/missing']);
+		assert.strictEqual(router.getCurrentFullPath(), '/');
+		assert.strictEqual(router.current.route.path, '/');
+	});
+
+	test('replaceの際、ページが見つからなかったらforceReplaceを発火する', () => {
+		const router = createRouter('/');
+		const forceReplacements: string[] = [];
+
+		router.addListener('forceReplace', ctx => {
+			forceReplacements.push(ctx.fullPath);
+			assert.strictEqual(ctx.onInit, false);
+		});
+
+		router.init();
+
+		router.replaceByPath('/also-missing');
+
+		assert.deepStrictEqual(forceReplacements, ['/also-missing']);
+		assert.strictEqual(router.getCurrentFullPath(), '/');
+		assert.strictEqual(router.current.route.path, '/');
+	});
+
+	test('初期ページが見つからない場合でも初回はforceReplaceを発火しない', () => {
+		const router = createRouter('/missing');
+		const forceReplacements: string[] = [];
+
+		router.addListener('forceReplace', ctx => {
+			forceReplacements.push(ctx.fullPath);
+			assert.strictEqual(ctx.onInit, true);
+		});
+
+		router.init();
+
+		assert.deepStrictEqual(forceReplacements, []); // 初回はforceReplaceを発火しない
+		assert.strictEqual(router.getCurrentFullPath(), '/missing');
+		assert.strictEqual(router.current.route.path, '/:(*)');
+	});
+
+	test('初期ページが見つからない場合でも、initで明示した場合はforceReplaceを発火する', () => {
+		const router = createRouter('/missing');
+		const forceReplacements: string[] = [];
+
+		router.addListener('forceReplace', ctx => {
+			forceReplacements.push(ctx.fullPath);
+			assert.strictEqual(ctx.onInit, true);
+		});
+
+		router.init(true); // forceReplaceを強制的に発火させる
+
+		assert.deepStrictEqual(forceReplacements, ['/missing']);
+		assert.strictEqual(router.getCurrentFullPath(), '/missing');
+		assert.strictEqual(router.current.route.path, '/:(*)');
+	});
+
+	test('loginRequiredなルートではコンポーネントを差し替えてshowLoginPopupを設定する', () => {
+		const router = createRouter('/', false);
+
+		router.init();
+
+		router.pushByPath('/private');
+
+		assert.strictEqual(router.current.route.path, '/private');
+		assert.ok('component' in router.current.route);
+		assert.strictEqual(router.current.route.component, loginFallbackComponent);
+		assert.strictEqual(router.current.props.get('showLoginPopup'), true);
+	});
+});
diff --git a/packages/frontend/test/lib/nirax/fixture.ts b/packages/frontend/test/lib/nirax/fixture.ts
new file mode 100644
index 0000000000..ef6cf1ed9b
--- /dev/null
+++ b/packages/frontend/test/lib/nirax/fixture.ts
@@ -0,0 +1,80 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import type { Component } from 'vue';
+import { Nirax } from '@/lib/nirax.js';
+import type { RouteDef } from '@/lib/nirax.js';
+
+export const homeComponent = { name: 'home-page' } as Component;
+export const postComponent = { name: 'post-page' } as Component;
+export const fileComponent = { name: 'file-page' } as Component;
+export const optionalComponent = { name: 'optional-page' } as Component;
+export const userComponent = { name: 'user-page' } as Component;
+export const followersComponent = { name: 'followers-page' } as Component;
+export const privateComponent = { name: 'private-page' } as Component;
+export const notFoundRouteComponent = { name: 'not-found-route' } as Component;
+export const loginFallbackComponent = { name: 'login-fallback' } as Component;
+
+export const routes = [
+	{
+		path: '/',
+		component: homeComponent,
+	},
+	{
+		path: '/posts/:postId',
+		component: postComponent,
+		query: {
+			from: 'source',
+		},
+		hash: 'section',
+	},
+	{
+		path: '/files/:path(*)',
+		component: fileComponent,
+	},
+	{
+		path: '/optional/:slug?',
+		component: optionalComponent,
+	},
+	{
+		path: '/old',
+		redirect: '/posts/redirected',
+	},
+	{
+		path: '/legacy/:postId',
+		redirect: props => `/posts/${props.get('postId')}`,
+	},
+	{
+		path: '/loop-a',
+		redirect: '/loop-b',
+	},
+	{
+		path: '/loop-b',
+		redirect: '/loop-a',
+	},
+	{
+		path: '/user/:id',
+		component: userComponent,
+		children: [
+			{
+				path: '/followers',
+				component: followersComponent,
+			},
+		],
+	},
+	{
+		path: '/private',
+		component: privateComponent,
+		loginRequired: true,
+	},
+	{
+		path: '/:(*)',
+		component: notFoundRouteComponent,
+	},
+] as const satisfies RouteDef[];
+
+export function createRouter(currentFullPath = '/', isLoggedIn = true) {
+	return new Nirax(routes, currentFullPath, isLoggedIn, loginFallbackComponent);
+}
diff --git a/packages/frontend/test/lib/nirax/navigation.test.ts b/packages/frontend/test/lib/nirax/navigation.test.ts
new file mode 100644
index 0000000000..b0fcedcc70
--- /dev/null
+++ b/packages/frontend/test/lib/nirax/navigation.test.ts
@@ -0,0 +1,105 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { assert, describe, test } from 'vitest';
+import { createRouter } from './fixture.js';
+
+describe('[NIRAX] ナビゲーションイベント', () => {
+	test('init時にリダイレクトを解決してreplaceを発火する', () => {
+		const router = createRouter('/old?from=legacy#intro');
+		const changes: string[] = [];
+		const replacements: string[] = [];
+
+		router.addListener('change', ctx => {
+			changes.push(ctx.fullPath);
+		});
+		router.addListener('replace', ctx => {
+			replacements.push(ctx.fullPath);
+		});
+
+		router.init();
+
+		assert.strictEqual(router.getCurrentFullPath(), '/posts/redirected?from=legacy#intro');
+		assert.strictEqual(router.current.redirected, true);
+		assert.deepStrictEqual(changes, []); // 初回はchangeを発火しない
+		assert.deepStrictEqual(replacements, ['/posts/redirected?from=legacy#intro']);
+	});
+
+	test('push時に動的リダイレクトを解決してpushとchangeを発火する', () => {
+		const router = createRouter('/');
+		const pushed: string[] = [];
+		const changed: string[] = [];
+
+		router.addListener('push', ctx => {
+			pushed.push(ctx.fullPath);
+			assert.strictEqual(ctx.route?.path, '/posts/:postId');
+			assert.strictEqual(ctx.props?.get('postId'), 'abc123');
+		});
+		router.addListener('change', ctx => {
+			changed.push(ctx.fullPath);
+		});
+
+		router.init();
+
+		router.pushByPath('/legacy/abc123');
+
+		assert.strictEqual(router.getCurrentFullPath(), '/posts/abc123');
+		assert.deepStrictEqual(pushed, ['/posts/abc123']);
+		assert.deepStrictEqual(changed, ['/posts/abc123']);
+	});
+
+	test('無限リダイレクトはエラーになる', () => {
+		const router = createRouter('/');
+
+		router.init();
+
+		assert.throws(() => {
+			router.pushByPath('/loop-a');
+		}, /redirect loop detected/);
+		assert.strictEqual(router.getCurrentFullPath(), '/');
+	});
+
+	test('同じパスへの遷移ではsameを発火する', () => {
+		const router = createRouter('/posts/123');
+		let sameCount = 0;
+		let pushCount = 0;
+
+		router.addListener('same', () => {
+			sameCount++;
+		});
+		router.addListener('push', () => {
+			pushCount++;
+		});
+
+		router.init();
+
+		router.pushByPath('/posts/123');
+
+		assert.strictEqual(sameCount, 1);
+		assert.strictEqual(pushCount, 0); // sameのときはpushを発火しない
+	});
+
+	test('navHookでナビゲーションをキャンセルできる', () => {
+		const router = createRouter('/posts/123');
+		const navHookCalls: string[] = [];
+		let pushCount = 0;
+
+		router.addListener('push', () => {
+			pushCount++;
+		});
+		router.navHook = fullPath => {
+			navHookCalls.push(fullPath);
+			return true;
+		};
+
+		router.init();
+
+		router.pushByPath('/posts/456');
+
+		assert.deepStrictEqual(navHookCalls, ['/posts/456']);
+		assert.strictEqual(pushCount, 0);
+		assert.strictEqual(router.getCurrentFullPath(), '/posts/123');
+	});
+});
diff --git a/packages/frontend/test/lib/nirax/resolve.test.ts b/packages/frontend/test/lib/nirax/resolve.test.ts
new file mode 100644
index 0000000000..42cc0bae04
--- /dev/null
+++ b/packages/frontend/test/lib/nirax/resolve.test.ts
@@ -0,0 +1,73 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { assert, describe, test } from 'vitest';
+import { createRouter } from './fixture.js';
+
+describe('[NIRAX] resolve', () => {
+	test('staticなルートを解決できる', () => {
+		const router = createRouter();
+		const resolved = router.resolve('/');
+
+		assert.ok(resolved);
+		assert.strictEqual(resolved.route.path, '/');
+		assert.strictEqual(resolved.props.size, 0);
+	});
+
+	test('パスパラメータ付きルートを解決できる', () => {
+		const router = createRouter();
+		const resolved = router.resolve('/posts/abc%2Fdef');
+
+		assert.ok(resolved);
+		assert.strictEqual(resolved.route.path, '/posts/:postId');
+		assert.strictEqual(resolved.props.get('postId'), 'abc/def');
+	});
+
+	test('queryとhashのエイリアスを解決できる', () => {
+		const router = createRouter();
+		const resolved = router.resolve('/posts/abc?from=timeline#thread');
+
+		assert.ok(resolved);
+		assert.strictEqual(resolved.props.get('source'), 'timeline');
+		assert.strictEqual(resolved.props.get('section'), 'thread');
+	});
+
+	test('wildcardルートのパラメータを解決できる', () => {
+		const router = createRouter();
+		const resolved = router.resolve('/files/images/icons/logo%20mark.svg');
+
+		assert.ok(resolved);
+		assert.strictEqual(resolved.route.path, '/files/:path(*)');
+		assert.strictEqual(resolved.props.get('path'), 'images/icons/logo mark.svg');
+	});
+
+	test('optionalなパスパラメータが省略されたルートを解決できる', () => {
+		const router = createRouter();
+		const resolved = router.resolve('/optional');
+
+		assert.ok(resolved);
+		assert.strictEqual(resolved.route.path, '/optional/:slug?');
+		assert.strictEqual(resolved.props.has('slug'), false);
+	});
+
+	test('optionalなパスパラメータが存在するルートを解決できる', () => {
+		const router = createRouter();
+		const resolved = router.resolve('/optional/topic');
+
+		assert.ok(resolved);
+		assert.strictEqual(resolved.props.get('slug'), 'topic');
+	});
+
+	test('ネストされたルートを解決できる', () => {
+		const router = createRouter();
+		const resolved = router.resolve('/user/alice/followers');
+
+		assert.ok(resolved);
+		assert.strictEqual(resolved.route.path, '/user/:id');
+		assert.strictEqual(resolved.props.get('id'), 'alice');
+		assert.ok(resolved.child);
+		assert.strictEqual(resolved.child.route.path, '/followers');
+	});
+});