Do I need a separate vector database like Pinecone or Weaviate?

For MERN apps already on MongoDB Atlas M10+ clusters, no. MongoDB Atlas Vector Search is native to your existing Atlas cluster since GA in June 2024. You avoid a second database to procure, back up, secure, and pay for, plus the change-data-capture sync layer between them that used to fail silently. Pinecone and Weaviate still make sense when the vector workload dominates the application, when you need sub-50ms global multi-region latency, or when you need advanced features Atlas does not yet cover (real-time hybrid ranking with custom scoring functions, for example). For most 2026 MERN apps that need semantic search, Atlas Vector Search is the right first choice.