test file : cover unsupported follow platform error

apps/backend/src/__tests__/follow.test.ts

@@ -0,0 +1,57 @@
+import Fastify from 'fastify';
+import { describe, expect, it, vi } from 'vitest';
+
+import { followRoutes } from '../routes/follow.js';
+
+vi.mock('../utils/encryption.js', () => ({
+  decrypt: vi.fn(() => 'fake-access-token'),
+}));
+
+describe('POST /api/follow/:platform/:targetUsername', () => {
+  it('returns 400 when API follow is not supported for the platform', async () => {
+    const app = Fastify({ logger: false });
+
+    const findUnique = vi.fn().mockResolvedValue({
+      id: 'token-1',
+      userId: 'user-1',
+      platform: 'unknown',
+      accessToken: 'encrypted-token',
+    });
+
+    app.decorate('prisma', {
+      oAuthToken: {
+        findUnique,
+      },
+      followLog: {
+        create: vi.fn(),
+      },
+    }as any);
+
+    app.decorate('authenticate', async (request: any) => {
+      request.user = { id: 'user-1' };
+    });
+
+    await app.register(followRoutes, { prefix: '/api/follow' });
+    await app.ready();
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/api/follow/unknown/targetUser',
+    });
+
+    const body = response.json();
+
+    expect(response.statusCode).toBe(400);
+    expect(body.error).toContain('API follow not supported');
+    expect(findUnique).toHaveBeenCalledWith({
+      where: {
+        userId_platform: {
+          userId: 'user-1',
+          platform: 'unknown',
+        },
+      },
+    });
+
+    await app.close();
+  });
+});