diff --git a/packages/backend/test-federation/test/api/ap/show.test.ts b/packages/backend/test-federation/test/api/ap/show.test.ts
new file mode 100644
index 0000000000..3e8c3b3c3e
--- /dev/null
+++ b/packages/backend/test-federation/test/api/ap/show.test.ts
@@ -0,0 +1,104 @@
+import { strictEqual, rejects } from 'node:assert';
+import { createAccount, fetchAdmin, resolveRemoteUser, sleep, type LoginUser } from '../../utils.js';
+
+const [aAdmin, bAdmin] = await Promise.all([
+	fetchAdmin('a.test'),
+	fetchAdmin('b.test'),
+]);
+
+describe('API ap/show', () => {
+	let alice: LoginUser, bob: LoginUser;
+
+	beforeEach(async () => {
+		[alice, bob] = await Promise.all([
+			createAccount('a.test'),
+			createAccount('b.test'),
+		]);
+	});
+
+	describe('User resolution', () => {
+		test('resolve a remote user by canonical user url (https://b.test/users/:id)', async () => {
+			const res = await alice.client.request('ap/show', { uri: `https://b.test/users/${bob.id}` });
+			strictEqual(res.type, 'User');
+			strictEqual(res.object.uri, `https://b.test/users/${bob.id}`);
+		});
+
+		test('resolve a remote user by user profile url (https://b.test/@bob)', async () => {
+			const res = await alice.client.request('ap/show', { uri: `https://b.test/@${bob.username}` });
+			strictEqual(res.type, 'User');
+			strictEqual(res.object.uri, `https://b.test/users/${bob.id}`);
+		});
+
+		test('resolve a local user by local uri', async () => {
+			const res = await alice.client.request('ap/show', { uri: `https://a.test/users/${alice.id}` });
+			strictEqual(res.type, 'User');
+			strictEqual(res.object.id, alice.id);
+		});
+
+		test('resolve a local user by local profile url', async () => {
+			const res = await alice.client.request('ap/show', { uri: `https://a.test/@${alice.username}` });
+			strictEqual(res.type, 'User');
+			strictEqual(res.object.id, alice.id);
+		});
+
+		test('resolve a fetched remote user by local profile url (https://a.test/@bob@b.test)', async () => {
+			await aAdmin.client.request('admin/update-meta', { allowExternalApRedirect: true });
+			await resolveRemoteUser('b.test', bob.id, alice);
+
+			const res = await alice.client.request('ap/show', { uri: `https://a.test/@${bob.username}@b.test` });
+			strictEqual(res.type, 'User');
+			strictEqual(res.object.uri, `https://b.test/users/${bob.id}`);
+		});
+
+		test('throws in resolving a fetched remote user by local profile url when allowExternalApRedirect: false', async () => {
+			await aAdmin.client.request('admin/update-meta', { allowExternalApRedirect: false });
+			await resolveRemoteUser('b.test', bob.id, alice);
+
+			await rejects(
+				async () => await alice.client.request('ap/show', { uri: `https://a.test/@${bob.username}@b.test` }),
+				(err: any) => {
+					strictEqual(err.code, 'NO_SUCH_OBJECT');
+					return true;
+				},
+			);
+		});
+
+		test('throws in resolving a non-fetched remote user by local profile url (https://a.test/@bob@b.test)', async () => {
+			// ユーザーがこのような問い合わせをすることは故意でない限りなさそうだし、ActivityPubServerServiceでもこのような仕様である
+
+			await aAdmin.client.request('admin/update-meta', { allowExternalApRedirect: true });
+
+			await rejects(
+				async () => await alice.client.request('ap/show', { uri: `https://a.test/@${bob.username}@b.test` }),
+				(err: any) => {
+					strictEqual(err.code, 'NO_SUCH_OBJECT');
+					return true;
+				},
+			);
+		});
+	});
+
+	describe('Note resolution', () => {
+		test('resolve a remote note by note uri', async () => {
+			const note = (await bob.client.request('notes/create', { text: 'hello from Bob' })).createdNote;
+			await sleep();
+
+			const res = await alice.client.request('ap/show', { uri: `https://b.test/notes/${note.id}` });
+			strictEqual(res.type, 'Note');
+			strictEqual(res.object.uri, `https://b.test/notes/${note.id}`);
+			// 投稿者が a.test 側で解決済みの Bob になること
+			strictEqual(res.object.user.username, bob.username);
+			strictEqual(res.object.user.host, 'b.test');
+		});
+
+		test('resolve a local note by note uri', async () => {
+			const note = (await alice.client.request('notes/create', { text: 'hello from Alice' })).createdNote;
+			await sleep();
+
+			const res = await alice.client.request('ap/show', { uri: `https://a.test/notes/${note.id}` });
+			strictEqual(res.type, 'Note');
+			strictEqual(res.object.id, note.id);
+			strictEqual(res.object.user.id, alice.id);
+		});
+	});
+});