OpenAI Backend Engineer interview questions

23 Backend Engineers have shared notes from their OpenAI interviews, with the questions they were asked in each round. That is 3% of the 667 OpenAI interview notes on Screna AI.

23 Backend Engineer interview notes

OpenAI·Backend Engineer·Onsite - System Design / Architecture

Jun 2026
System design round at OpenAI for a backend engineer role, focused entirely on building a cloud-based IDE from scratch. The scope was massive and I felt like I was playing catch-up the whole time trying to cover everything they wanted.
  • Design a cloud-based IDE with features like multi-file editing, syntax highlighting, autocomplete, sandboxed code execution, terminal access, and real-time collaboration.
  • How would you handle scaling a large number of idle user sessions without burning through infrastructure costs?
  • Walk me through how you'd implement real-time collaborative editing across multiple users in the same file.
  • What are the security and isolation requirements for running arbitrary user code, and how would your architecture enforce them?
  • How would you keep keystroke latency low in the editor given that some operations like autocomplete depend on a remote language server?

“This is a monster of a question.”

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OpenAI·Backend Engineer·Onsite - System Design / Architecture

Jun 2026
System design round at OpenAI for a backend role, basically one big question about building YouTube from scratch. The scope was massive and I felt like I was constantly playing catch-up across components.
  • Design a large-scale video sharing and streaming platform similar to YouTube, covering upload, transcoding, storage, adaptive bitrate delivery, view counts, comments, recommendations, and search.

“This question swallowed the entire session.”

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OpenAI·Backend Engineer·Onsite - System Design / Architecture

Jun 2026
System design round at OpenAI for a backend engineer role, focused entirely on payment infrastructure. The question was dense and covered a lot of ground, probably more than I could fully address in one session.
  • Design a payment processing system that handles authorize, capture, and refund flows for card payments at high transaction volume. Requirements include multi-currency support, idempotency on retries, fraud screening, strong consistency on balances, exactly-once semantics for user-visible operations, an audit log, and PCI compliance. Walk through the ledger data model, how you'd integrate with payment service providers and card networks, async settlement, and monitoring.

“This is one of those questions where you can tell within the first two minutes you're not going to cover everything.”

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OpenAI·Backend Engineer·Onsite - System Design / Architecture

Jun 2026
System design round at OpenAI for a backend engineer role. The whole thing was focused on one big problem: designing a webhook delivery service end to end. Dense topic with a lot of moving parts, and I felt like I was constantly playing catch-up as the scope kept expanding.
  • Design a webhook delivery system where customers can register HTTP endpoints to receive event notifications, with at-least-once delivery, signed payloads, retries with exponential backoff, a dead-letter queue, and tenant isolation.
  • How would you design the worker pool and queue structure to support high throughput while maintaining ordering guarantees per subscription?
  • How would you handle consumer-side idempotency so that retried webhook deliveries don't cause duplicate processing on the receiving end?
  • Walk through your approach to securing webhook payloads, including signature verification and rate limiting per tenant.

“This was basically the whole interview.”

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OpenAI·Backend Engineer·Technical Phone Screen

Jun 2026
OpenAI backend interview with a deep ML systems debugging question. They handed me a broken Transformer implementation and wanted me to find every bug, fix it, and explain the reasoning. Pretty intense for a backend role but I guess that's what you sign up for there.
  • You're given a buggy Transformer implementation (could be multi-head attention, positional encoding, encoder/decoder block, etc.). Find all the bugs, fix them, and explain why each fix is correct. Expect follow-up questions about extending or training the model.

“The part that got me was the masking.”

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OpenAI·Backend Engineer·Onsite - System Design / Architecture

Jun 2026
System design round at OpenAI for a backend engineer role, centered entirely on building a Slack-style messaging platform. It went deep fast and covered a lot of ground in one question.
  • Design a team messaging platform similar to Slack, covering workspaces, channels, direct messages, real-time delivery, message history, search, threads, file uploads, and presence indicators. How do you handle millions of concurrent users with sub-200ms latency and high availability?

“This is essentially four or five hard problems stapled together and they want you to move through all of them.”

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OpenAI·Backend Engineer·Onsite - System Design / Architecture

Jun 2026
System design round at OpenAI for a backend role, centered entirely on building a geo-aware points-of-interest service from scratch. Dense problem with a lot of moving parts and not a lot of hand-holding.
  • Design a Points-of-Interest service (think Google Places or Yelp) that supports storing location data with categories and metadata, querying nearby POIs by radius or bounding box, filtering by category or keyword, and ranking results by distance and rating. The system needs to handle planet-scale traffic with low latency, hot-spot regions, and near-real-time edits.

“This is the kind of question where you think you know where to start and then realize the scope is enormous about five minutes in.”

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OpenAI·Backend Engineer·Technical Phone Screen

Jun 2026
Interviewed for a backend role at OpenAI and the whole thing basically centered on one deep project walkthrough. They wanted you to own the conversation, not just answer questions, which I was not fully prepared for.
  • Walk us through one of your most representative projects in depth, covering the scope, your role, technical decisions, how it touched different components, and the trade-offs you made along the way. Be ready to defend your choices.

“This is the kind of question that sounds easy until you're actually in it and realize they want you to lead, not just respond.”

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OpenAI·Backend Engineer·Technical Phone Screen

Jun 2026
OpenAI backend interview that threw a full interpreter design problem at me. Not your typical coding round, this one wanted tokenization, parsing strategy, expression evaluation, the works. Left feeling like I'd only scratched the surface of what they were actually looking for.
  • Build a toy language interpreter from scratch: support variable assignment, arithmetic, conditionals, and output statements. Walk through tokenization, your parsing approach, how you evaluate expressions, and how you handle edge cases like undefined variables and operator precedence.

“This wrecked me a little.”

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OpenAI·Backend Engineer·Technical Phone Screen

Jun 2026
Backend coding round at OpenAI that was basically a deep dive into IP address manipulation. One problem, but it had enough layers to keep you busy for the whole session.
  • Given a CIDR block like '192.168.0.0/24', write code to iterate over every IP address in the range. Then flip it: given a starting IP and a count, return the minimal set of CIDR blocks that exactly covers that range. Walk through your bit manipulation logic and handle edge cases at block boundaries.

“The first part felt manageable, converting the CIDR prefix length to a mask and stepping through the 32-bit integer space.”

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