1. What is a Machine Learning Engineer at Character.AI?

At Character.AI, a Machine Learning Engineer is not simply a model trainer; you are an architect of the next generation of interactive entertainment. This role sits at the intersection of cutting-edge research and massive-scale product engineering. Whether you are optimizing inference engines to serve millions of concurrent conversations or building recommendation systems that help users discover their next favorite character, your work directly powers the "brain" of the platform.

The impact of this position is immediate and tangible. With over 20 million monthly users and a platform that processes tens of thousands of queries per second (QPS), you are working on systems where efficiency and latency are critical. You will be responsible for taking Large Language Models (LLMs) from research concepts to production realities, ensuring that the magic of an infinite conversation feels instantaneous and seamless.

This role requires a unique blend of scientific curiosity and engineering rigor. You will work cross-functionally with research scientists, product managers, and data platform teams. Whether you are on the ML Systems team optimizing GPU kernels, the Applied ML team building discovery surfaces, or the Safety Engineering team ensuring integrity, you are building the infrastructure that defines the future of consumer AI.

2. Getting Ready for Your Interviews

Preparation for Character.AI is distinct from standard big-tech interviews. While algorithmic competence is required, the primary focus is on your ability to build practical, scalable AI systems. You should approach your preparation with a "builder's mindset," focusing on how you translate theoretical knowledge into working code.

Role-Related Knowledge (ML & Systems) – 2–3 sentences describing: This is the core of the evaluation. Interviewers assess your depth in modern ML frameworks (PyTorch), your understanding of LLM architectures (Transformers, Attention mechanisms), and your ability to optimize systems (CUDA, Triton, distributed training). You must demonstrate that you understand not just how to use a model, but how it works under the hood and how to serve it efficiently.

Problem-Solving Ability – 2–3 sentences describing: Character.AI values engineers who can navigate ambiguity. You will be tested on your ability to take a vague problem—such as "reduce inference latency by 20%" or "design a safety filter for generative content"—and break it down into a technical roadmap. Success here means showing a logical progression from high-level design to low-level implementation details.

Engineering Excellence & Coding – 2–3 sentences describing: Theoretical knowledge is insufficient; you must write clean, production-ready code. Expect to write working code for ML components (e.g., implementing an attention head or a custom loss function) and demonstrate familiarity with backend engineering principles like gRPC services, CI/CD, and cloud infrastructure.

Culture Fit / "Get Things Done" – 2–3 sentences describing: The company moves at a blistering pace. Interviewers look for proactive candidates who take ownership and have a history of shipping features end-to-end. They want to see that you are scrappy, collaborative, and capable of making an impact during your first week.

3. Interview Process Overview

The interview process at Character.AI is streamlined but rigorous, designed to identify high-agency engineers who can contribute immediately. It typically begins with a recruiter screen to align on your background and interests, followed by a technical screen. This technical screen is usually a live coding session focused on practical ML implementation or algorithmic problem-solving relevant to the role.

If you pass the screen, you will move to the onsite stage (often virtual), which consists of 3 to 5 rounds. These rounds are split between deep technical assessments—covering coding, ML system design, and ML theory—and behavioral interviews. For ML Systems roles, expect a heavy emphasis on GPU optimization and distributed systems. For Applied ML roles, the focus will shift toward modeling, recommendations, or safety infrastructure. The company values practical discussions over abstract puzzles; you might be asked to debug a training pipeline or architect a serving layer for a new model.

The timeline above represents the typical flow for an engineering candidate. Use this visual to plan your energy; the "Onsite" phase is an endurance test requiring deep focus across multiple domains. Note that the specific mix of System Design vs. Coding rounds may vary slightly depending on whether you are interviewing for a Systems, Applied, or Safety role.

4. Deep Dive into Evaluation Areas

Candidates are evaluated on their ability to bridge the gap between research and production. You must be prepared to discuss the full lifecycle of a machine learning model, from data ingestion to high-performance inference.

Machine Learning Theory & Architecture

This area tests your fundamental understanding of the models that power Character.AI. You cannot simply treat models as black boxes; you need to understand the mathematics and architecture defining them.

Be ready to go over:

• Transformer Architecture – Deep knowledge of self-attention, multi-head attention, positional encodings, and feed-forward networks.
• LLM Specifics – Concepts like RLHF (Reinforcement Learning from Human Feedback), KV caching, and tokenization strategies.
• Training Dynamics – Loss functions, optimizers (Adam, SGD), gradient descent nuances, and vanishing/exploding gradients.
• Advanced concepts – Mixture of Experts (MoE), LoRA (Low-Rank Adaptation), and quantization techniques.

Example questions or scenarios:

• "Derive and implement the Softmax function from scratch, ensuring numerical stability."
• "Explain how FlashAttention differs from standard attention and why it improves performance."
• "How would you modify a standard Transformer to handle infinite context lengths?"

ML Systems & Infrastructure

For the ML Systems track, this is the most critical evaluation area. For Applied roles, you still need a strong working knowledge of how models are deployed and served.

Be ready to go over:

• Inference Optimization – Techniques to maximize throughput and minimize latency (batching, pipelining, model parallelism).
• Distributed Computing – Data parallelism vs. model parallelism, sharding strategies, and communication overheads.
• Hardware utilization – Understanding GPU memory hierarchy, CUDA kernels, and Triton.
• Serving Infrastructure – Designing scalable inference services using gRPC, load balancing, and caching strategies.

Example questions or scenarios:

• "Design a system to serve an LLM with 20K QPS. How do you handle peak load?"
• "You are running out of GPU memory during training. What strategies do you use to debug and fix this?"
• "Write a custom kernel to optimize a specific matrix operation found in our models."

Coding & Algorithms (ML Context)

Coding interviews here are practical. You will rarely see dynamic programming puzzles that have no relevance to the job. Instead, you will likely code algorithms used in daily ML work.

Be ready to go over:

• Tensor Manipulation – Proficient use of PyTorch or NumPy to manipulate high-dimensional data.
• Algorithm Implementation – Writing the forward/backward pass of a layer or implementing a specific metric.
• Data Structures – Using heaps, trees, or graphs in the context of search or recommendation logic.

Example questions or scenarios:

• "Implement the attention mechanism in Python using only NumPy."
• "Given a stream of user queries, implement a sampling strategy to select data for fine-tuning."
• "Write a function to calculate the BLEU score or Perplexity for a given output."

5. Key Responsibilities

As a Machine Learning Engineer at Character.AI, your daily work is highly collaborative and fast-paced. You are expected to own projects from "idea to production." This means you won't just be handed a clean dataset and asked to train a model; you will likely build the data pipeline, design the model architecture, train it, and then write the serving code to deploy it to millions of users.

For Applied ML engineers, a typical week might involve working with Product Managers to define a new discovery feature, implementing a two-tower recommendation model, and tuning vector database queries to ensure relevant results. You will work closely with Data Science to analyze user engagement metrics and iterate on your models.

For ML Systems engineers, responsibilities focus on the "engine" of the company. You will profile GPU clusters to identify bottlenecks, write custom Triton kernels to speed up specific operations, and develop routing algorithms to optimize cache hit rates. You are the guardian of latency and efficiency, ensuring that the infrastructure can support the massive growth in user traffic.

6. Role Requirements & Qualifications

Character.AI hires for excellence and slope (rate of learning). While specific experience varies by level, the following core qualifications are essential for success.

• Technical Skills

  • Must-have: deep proficiency in Python and a major ML framework (PyTorch is preferred). Strong grasp of backend engineering (gRPC, REST, microservices).
  • Must-have: Experience with cloud infrastructure (GCP, AWS, or Azure) and containerization (Docker, Kubernetes).
  • Critical: For Systems roles, experience with CUDA, Triton, or distributed training frameworks (DeepSpeed, Megatron) is often required.

• Experience Level

  • Typically 5+ years of industry experience for Senior roles, though "All Industry Levels" are considered for Research Engineers with a PhD or exceptional track records.
  • Proven history of shipping AI/ML features to production, not just research prototypes.

• Soft Skills

  • A proactive "get things done" attitude is non-negotiable.
  • Ability to communicate complex technical concepts to cross-functional teams (Product, Safety, Ops).

• Nice-to-Have

  • Experience with vector databases (Pinecone, Milvus, etc.).
  • Publications in top-tier ML conferences (NeurIPS, ICML) for research-heavy roles.
  • Background in Trust & Safety or Content Integrity for Safety Engineering roles.

7. Common Interview Questions

The questions below are representative of what you might face. They are not a script, but they illustrate the depth and practicality Character.AI expects. You should prepare to answer these with code, system diagrams, and mathematical derivations where appropriate.

Technical & Implementation

This category tests your raw coding ability and familiarity with ML primitives.

• Implement a multi-head attention layer from scratch in PyTorch.
• Write a function to perform Non-Maximum Suppression (NMS) or a similar filtering algorithm.
• How would you implement a custom loss function for a ranking problem?
• Given a large dataset of text, write a script to efficiently tokenize and batch it for training.
• Implement a simplified version of the Key-Value (KV) cache mechanism used in LLM inference.

System Design & Architecture

These questions test your ability to scale.

• Design a recommendation system for millions of character interactions. How do you handle latency constraints?
• How would you architect a distributed training job across hundreds of GPUs?
• Design a real-time safety filter that detects harmful content in generated text without adding significant latency.
• How do you design a system to handle "hot" characters (characters that suddenly become very popular)?
• Explain how you would route requests in a prefix-aware serving system to maximize cache hits.

Behavioral & Culture

• Tell me about a time you had to optimize a system that was already in production. What was the impact?
• Describe a situation where you had to make a technical trade-off between model accuracy and inference speed.
• How do you prioritize tasks when you have requests from Product, Safety, and Research simultaneously?
• Tell me about a feature you shipped from end-to-end. What was the hardest part?

8. Frequently Asked Questions

Q: How much research experience is required for the Engineering roles? For the Research Engineer track, a PhD or equivalent research experience is highly valued. However, for Applied ML and Software Engineer roles, strong industry engineering experience and the ability to implement papers are often more important than a publication record.

Q: What is the primary coding language used in interviews? Python is the standard for ML interviews. You should be extremely comfortable with Python's standard library and data manipulation libraries (NumPy, Pandas). For backend-heavy roles, Go or C++ might be relevant, but Python remains the primary language for ML discussions.

Q: How intense is the interview process? The process is rigorous. Character.AI is a high-performance environment, and the interviews reflect that. Expect deep dives into technical details. However, interviewers are generally collaborative and want to see you succeed, provided you can demonstrate the necessary skills.

Q: Is the work location flexible? Most roles are based in Redwood City, CA. The company values in-person collaboration, especially given the rapid pace of innovation and the complex nature of the hardware/software integration.

Q: What differentiates a Senior candidate from a Junior one? Senior candidates are expected to drive system design and strategy. They should be able to spot potential bottlenecks before they happen and mentor other engineers. Junior candidates are evaluated more on their raw coding speed, ability to learn, and grasp of fundamentals.

9. Other General Tips

Code like it's production: When writing code in the interview, do not take shortcuts. Use meaningful variable names, handle edge cases, and consider modularity. Interviewers often view your interview code as a proxy for the code you will push to the repository on day one.

Know the "Why": Don't just memorize how to use a library. Understand why a Transformer uses LayerNorm instead of BatchNorm, or why a specific optimizer is chosen for LLMs. Deep understanding separates good engineers from great ones.

Showcase your "Builder" side: If you have side projects, open-source contributions, or have built your own LLM wrappers, mention them. Character.AI loves builders who tinker with AI technology in their spare time.

Be ready for ambiguity: You might be asked open-ended questions like "How do we make our characters feel more human?" Approach this systematically: define "human-like" metrics, propose a technical approach, and discuss how you would measure success.

10. Summary & Next Steps

Becoming a Machine Learning Engineer at Character.AI puts you at the forefront of the consumer AI revolution. This is a role for engineers who are not content with just training models but want to build the massive systems that bring those models to life for millions of users. The work is challenging, high-impact, and deeply technical.

To succeed, focus your preparation on the intersection of theory (Transformers, LLMs) and practice (System Design, GPU optimization, Python coding). Review your fundamentals, practice implementing ML components from scratch, and be ready to discuss how you build scalable, resilient systems. The bar is high, but the opportunity to shape the future of interactive AI is unmatched.

The compensation data above reflects the high value Character.AI places on top-tier engineering talent. Note that total compensation often includes significant equity components, which can be substantial given the company's growth trajectory and "unicorn" status. Ensure you view the salary range in the context of the total rewards package.

Good luck with your preparation. Dig deep into the technology, stay curious, and approach the interview with the confidence of a builder ready to ship. For more insights and community-sourced questions, continue exploring Dataford.