Design a 1-to-1 Chat System

Design a chat system that supports only 1-to-1 messaging between users. Group chats, channels, and other multi-party features are explicitly out of scope.

This walkthrough follows the Interview Framework and focuses on what you'd actually present in a 45-60 minute interview.

Anthropic interviewers are known to dig deep into implementation details. Don't hand-wave any component—be prepared to explain the "how" behind every "what." If you mention WebSockets, be ready to discuss connection management. If you mention message ordering, be ready to discuss clock synchronization.

Disclaimer: This is a sample solution to help you get started. To better prepare for the interview, you should think through the question yourself and try to come up with your own solution. System design questions are open-ended and have multiple valid approaches.