diff --git a/packages/backend/src/server/api/endpoints/ap/show.ts b/packages/backend/src/server/api/endpoints/ap/show.ts
index a7e6aa5b3f..f62baf79e1 100644
--- a/packages/backend/src/server/api/endpoints/ap/show.ts
+++ b/packages/backend/src/server/api/endpoints/ap/show.ts
@@ -66,7 +66,7 @@ export const meta = {
 		somethingHappenedInFetchingUri: {
 			message: 'Something happened while fetching the URI.',
 			code: 'SOMETHING_HAPPENED_IN_FETCHING_URI',
-			id: '14d45054-9df7-4f85-9e60-343b22f16b05,
+			id: '14d45054-9df7-4f85-9e60-343b22f16b05',
 		},
 	},
 
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..7c35026269
--- /dev/null
+++ b/packages/backend/test-federation/test/api/ap/show.test.ts
@@ -0,0 +1,66 @@
+import { strictEqual, rejects } from 'node:assert';
+import * as Misskey from 'misskey-js';
+import { createAccount, resolveRemoteUser, sleep, type LoginUser } from '../../utils.js';
+
+describe('API ap/show', () => {
+  let alice: LoginUser, bob: LoginUser;
+  let bobInA: Misskey.entities.UserDetailedNotMe;
+
+  beforeAll(async () => {
+    [alice, bob] = await Promise.all([
+      createAccount('a.test'),
+      createAccount('b.test'),
+    ]);
+
+    // 事前に bob を a.test に解決してキャッシュ・同一性を取る
+    bobInA = await resolveRemoteUser('b.test', bob.id, alice);
+  });
+
+  describe('User resolution', () => {
+    test('resolve by acct (bob@b.test)', async () => {
+      const res = await alice.client.request('ap/show', { uri: `${bob.username}@b.test` });
+      strictEqual(res.type, 'User');
+      strictEqual(res.object.id, bobInA.id);
+      strictEqual(res.object.uri, `https://b.test/users/${bob.id}`);
+    });
+
+    test('resolve by canonical user URL (https://b.test/users/:id)', async () => {
+      const res = await alice.client.request('ap/show', { uri: `https://b.test/users/${bob.id}` });
+      strictEqual(res.type, 'User');
+      strictEqual(res.object.id, bobInA.id);
+      strictEqual(res.object.uri, `https://b.test/users/${bob.id}`);
+    });
+
+    test('resolve by cross-origin non-canonical URL (https://a.test/@bob@b.test)', async () => {
+      const res = await alice.client.request('ap/show', { uri: `https://a.test/@${bob.username}@b.test` });
+      strictEqual(res.type, 'User');
+      // 非正規URLから正規IDに追従して同一ユーザーになること
+      strictEqual(res.object.id, bobInA.id);
+      strictEqual(res.object.uri, `https://b.test/users/${bob.id}`);
+    });
+
+    test('onlyUriFetch=true with acct string should fail with URI_INVALID', async () => {
+      await rejects(
+        async () => await alice.client.request('ap/show', { uri: `${bob.username}@b.test`, onlyUriFetch: true }),
+        (err: any) => {
+          strictEqual(err.code, 'URI_INVALID');
+          return true;
+        },
+      );
+    });
+  });
+
+  describe('Note resolution', () => {
+    test('resolve by note URL (https://b.test/notes/:id)', async () => {
+      const note = (await bob.client.request('notes/create', { text: 'hello from Bob' })).createdNote;
+      // 伝搬待ち
+      await sleep();
+
+      const res = await alice.client.request('ap/show', { uri: `https://b.test/notes/${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.userId, bobInA.id);
+    });
+  });
+});