Character AI

Mid-sized

Character AI is an artificial intelligence company that develops conversational AI chatbots allowing users to create and interact with customizable virtual characters. It is known for its consumer-facing platform that enables lifelike, personalized dialogue powered by large language models.

5 interview notes · updated Jul 2026

Character AI·Software Engineer·Onsite - System Design / Architecture

Jun 2026

System design round at Character AI for a software engineer role. The whole session was one big question about building an online chess platform, and it went pretty deep into real-time communication, timer logic, and scaling. Felt like a solid 60-minute grind.

Design an online chess game service that supports matchmaking, real-time gameplay with move validation, per-player timers, and resign/draw mechanics.
How would you handle ELO-based matchmaking and pairing players in a queue?
How do you approach real-time bidirectional communication between the client and game server, and what are the trade-offs between WebSockets and polling?
How would you shard or scale the game session service as the number of concurrent games grows?
Walk through how the server should handle clock updates: should the timer be pushed to clients continuously or pulled by clients on demand?

“This was the whole interview basically.”

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Character AI·Software Engineer·Technical Phone Screen

Jun 2026

Phone screen for a Software Engineer role at Character AI. It was mostly a coding round with LeetCode 76 (Minimum Window Substring), and the interviewer kept pushing for optimizations after each working solution. Wrote clean code, thought it went fine, then got rejected the next day with the vaguest possible feedback.

Implement the Minimum Window Substring algorithm (find the smallest substring of a source string that contains all characters of a target string), then iteratively optimize your solution for space and time efficiency.

“I got this working pretty quickly with two hash maps and a helper that checked if the window was valid.”

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Character AI·Software Engineer·Technical Phone Screen

May 2026

Character AI coding round, pretty much one meaty question about building a HashMap from scratch. They wanted the full picture: implementation, tradeoffs, complexity. Felt like a solid mid-level systems-flavored coding problem.

Design and implement a HashMap from scratch without using any built-in hash table libraries. Your implementation should support put, get, and remove operations. Walk through your design choices including collision handling, hash function selection, load factor management, and dynamic resizing, and analyze the time and space complexity.

“I went with separate chaining first since it felt safer to explain, bucket array with a linked list at each slot.”

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Character AI·Software Engineer·Onsite - System Design / Architecture

May 2026

A technical deep-dive round at Character AI where the whole thing basically revolves around one big project walkthrough and then a bunch of follow-up questions that spiral out in every direction. Be ready to defend every decision you made.

Walk me through your most technically challenging project, including the architecture, data flow, and the specific design decisions you made along the way.
What alternatives did you consider for your design, and why did you go with the approach you chose over those alternatives?
How did you navigate the tradeoffs in your project, for example between latency and cost, or consistency and availability, or build versus buy decisions?
How did you measure whether the project was successful?
Looking back, what would you do differently on that project?
Tell me about a time you disagreed with a teammate or stakeholder on a technical or product decision. How did you handle it?
How do you prioritize when you're juggling multiple things and not everything can get done?
How do you operate when requirements are unclear or the problem is still being defined?

“This is the core of the whole interview so you better have a project you can actually go deep on.”

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Character AI·Software Engineer·Onsite - System Design / Architecture

May 2026

System design round at Character AI for a software engineer role. The whole thing was one meaty question about building a spreadsheet engine, and they really wanted to dig into the internals rather than just get a working answer.

Design a spreadsheet data structure where cells can hold either literal values or formulas referencing other cells. Implement set_cell and get_cell, where get_cell recursively resolves formula dependencies. Cover the dependency graph representation, evaluation order, update propagation, cycle detection, and edge cases like self-references and missing cells. Also discuss memoization and lazy vs eager evaluation with time complexity for each operation.

“This one kept expanding.”

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