Define how components communicate with each other and with the server.
Component interfaces: What props does each component accept? What events does it emit?
Server APIs: What endpoints do you need? What's the request/response shape?
Example API contract:
GET /api/feed?cursor=abc&limit=20
Response: { posts: [...], nextCursor: "xyz" }
Be specific about pagination strategy (cursor vs offset), error responses, and authentication requirements.