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Openai·Software Engineer·Onsite - System Design / Architecture·Senior

SeniorPrefer not to say
Jun 2026

Summary

OpenAI full-stack interview that blended frontend and system design into one question about building an AI playground. The scope was bigger than I expected and the conversation went pretty deep on sharing semantics and cost control, which I hadn't really prepped for.

Questions Asked (6)

Q1

Design a document-style AI playground where users can insert prompts at any position in the document and see responses rendered inline directly below each prompt. Walk through the data model and component structure.

System DesignData ModelingTechnical Trade-offs
Author's notes

This tripped me up more than I expected.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements (e.g., real-time collaboration, streaming responses, document persistence) and then present a data model that treats the document as a sequence of blocks, each block being either text or a prompt-response pair. Then describe the component hierarchy and state management, emphasizing how to handle asynchronous updates and rendering inline responses without disrupting the document flow.

Pro tip: Discuss how you would handle streaming responses and partial updates to avoid re-rendering the entire document, perhaps using a normalized state shape and memoization. Also, mention trade-offs between storing responses as separate entities vs. embedding them in the document structure.

1. Clarify Requirements and Scope

Ask questions to understand key constraints: Is the document collaborative? Should responses stream? What's the expected scale? This shows you think about the problem holistically before diving into design.

2. Define the Data Model

Propose a document model as an ordered list of blocks, where each block has a type (text or prompt) and content. For prompt blocks, include fields for the prompt text, response, status (e.g., loading, complete), and any metadata. Consider using a normalized store (e.g., byId) for efficient updates.

3. Design the Component Structure

Outline a component hierarchy: a Document component that renders a list of Block components. Each Block can be a TextBlock or PromptBlock. PromptBlock renders the prompt and, conditionally, a Response component below it. Use a state management solution (e.g., Redux, Zustand) to handle updates.

4. Handle Asynchronous Updates and Streaming

Explain how to manage API calls for prompt responses, including streaming updates. Use optimistic UI or loading states, and update only the affected block to avoid full re-renders. Discuss error handling and retries.

5. Discuss Trade-offs and Extensibility

Compare different approaches: e.g., storing responses inline vs. separately, using a flat vs. nested data model, and implications for collaboration, undo/redo, and performance. Mention how the design supports future features like editing prompts or re-running.

Key Points to Mention

  • Normalized data model to avoid duplication and simplify updates
  • Component separation: Document, Block, PromptBlock, Response
  • State management for asynchronous operations and streaming
  • Performance optimizations: memoization, virtualization for large documents
  • Handling concurrent edits and real-time collaboration (if applicable)
  • Trade-offs between different data models (e.g., flat vs. nested, inline vs. separate responses)

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

Q2

How would you handle streaming LLM responses in this inline editor, including partial rendering and allowing the user to cancel mid-stream?

System DesignTechnical Trade-offsAPI & Integrations
Author's notes

Went okay.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and constraints of the inline editor, such as expected latency, token throughput, and cancellation semantics. Then propose a streaming architecture using server-sent events (SSE) or WebSockets, with a client-side buffer that incrementally renders tokens and supports cancellation via AbortController. Finally, discuss trade-offs around partial rendering, error handling, and user experience.

Pro tip: Emphasize the importance of graceful degradation: if streaming fails, fall back to a non-streaming response or show a retry option. Also, mention that cancellation should not only stop the stream but also clean up resources on both client and server to avoid leaks.

1. Clarify requirements and constraints

Ask about expected response size, latency tolerance, and whether the editor supports multiple concurrent streams. Confirm if cancellation should be immediate or allow partial results to persist.

2. Choose a streaming transport

Select between SSE, WebSockets, or fetch with ReadableStream based on bidirectional needs and browser support. Justify your choice with trade-offs like overhead, compatibility, and ease of cancellation.

3. Design client-side rendering and buffering

Implement a buffer that accumulates tokens and triggers UI updates at a throttled rate to avoid excessive re-renders. Use a virtual DOM or direct DOM manipulation for efficient partial rendering.

4. Implement cancellation and cleanup

Use AbortController to cancel the fetch/stream, and ensure the server stops generation and releases resources. On the client, clear buffers and update UI to reflect cancellation.

5. Handle errors and edge cases

Define behavior for network failures, timeouts, and partial responses. Consider fallback to non-streaming mode and provide user feedback (e.g., retry button).

Key Points to Mention

  • Use of AbortController for cancellation and its integration with fetch/SSE.
  • Throttling UI updates to balance responsiveness and performance.
  • Server-side handling of cancellation to stop token generation and free resources.
  • Trade-offs between SSE, WebSockets, and fetch streams (e.g., SSE is unidirectional but simpler; WebSockets allow bidirectional but heavier).
  • Error handling and fallback strategies for robustness.
  • User experience considerations: showing a loading indicator, allowing partial results to remain, and providing clear cancellation feedback.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

Q3

A user wants to save a prompt for later reuse. Where do you persist it, and how does the data model account for per-user vs. per-workspace ownership?

Data ModelingSystem Design
Author's notes

Pretty natural to talk through.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements: is the prompt saved for personal reuse or shared within a workspace? Then propose a persistence layer (e.g., a database table) with a flexible ownership model that supports both per-user and per-workspace scoping, and discuss how to enforce access control and query efficiently.

Pro tip: Mention that you'd model ownership with a polymorphic owner (owner_type + owner_id) or separate nullable foreign keys, and add a unique constraint to prevent duplicate prompts per owner. Also highlight the importance of indexing owner columns for fast lookups.

1. Clarify requirements and scope

Ask whether prompts can be personal, workspace-shared, or both, and whether users can belong to multiple workspaces. This determines the ownership model.

2. Choose persistence store

Select a durable database (e.g., PostgreSQL) for structured metadata and possibly a blob store for large prompt text. Justify based on query patterns and scale.

3. Design the data model

Propose a prompts table with columns like id, content, created_at, and ownership fields. Use either a polymorphic owner (owner_type, owner_id) or separate user_id and workspace_id with a check constraint.

4. Enforce access control and constraints

Add foreign keys, unique constraints (e.g., unique per owner and name), and application-level authorization to ensure users can only access prompts they own or that belong to their workspaces.

5. Optimize for queries and scale

Index owner columns and consider partitioning or sharding if needed. Discuss how to handle listing prompts for a user across personal and workspace scopes.

Key Points to Mention

  • Polymorphic ownership (owner_type + owner_id) vs. separate nullable foreign keys (user_id, workspace_id) with a check constraint.
  • Unique constraint to prevent duplicate prompt names per owner.
  • Indexing on owner columns for efficient retrieval.
  • Access control: ensure users can only read/write prompts they have permission for.
  • Consideration for soft deletes and audit trails.
  • Scalability: partitioning or sharding by owner if data grows large.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

Q4

Walk through the sharing model: how do you scope access (link-based, specific users, public), handle permissions, and support revocation?

System DesignTechnical Trade-offs
Author's notes

I jumped straight to short links and ACL tables but then they asked about revocation and I realized I hadn't thought through what happens to cached or already-opened links.

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AI HintsAI Generated

Suggested Approach

Start by outlining the core entities (resources, users, groups, links) and the access control model (ACL/RBAC). Then walk through the sharing flow: how a user selects a scope (public, link, specific users), how permissions are enforced at read/write time, and how revocation propagates. Emphasize trade-offs between simplicity, security, and scalability.

Pro tip: Mention that link-based sharing should use unguessable tokens with optional expiration and that revocation must be immediate—consider a centralized permission service or token blacklist to avoid stale caches. Also note that public sharing should be explicit and audited.

1. Define the access control model

Choose between ACLs, RBAC, or a hybrid. Explain how permissions are stored (e.g., per-resource ACL) and inherited (e.g., folder-level).

2. Describe sharing scopes

Cover public (anyone), link-based (anyone with link), and specific users/groups. For link-based, discuss token generation, expiration, and optional password.

3. Explain permission enforcement

Detail how checks happen at request time: authenticate user, resolve effective permissions (considering groups, inheritance), and authorize action. Mention caching and invalidation.

4. Handle revocation

Describe how to revoke access: remove ACL entries, invalidate link tokens, and ensure propagation (e.g., via pub/sub or short TTL caches). Discuss immediate vs eventual consistency.

5. Discuss trade-offs and edge cases

Address scalability, performance, security (e.g., token leakage), and usability (e.g., accidental public sharing). Mention audit logs and monitoring.

Key Points to Mention

  • Use of ACLs or RBAC for fine-grained permissions
  • Link-based sharing with unguessable tokens and expiration
  • Permission inheritance and group-based access
  • Caching strategies and invalidation for performance
  • Immediate revocation via centralized service or token blacklist
  • Audit logging and monitoring for security

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

Q5

When a user shares a prompt, what data actually gets included? How do you handle privacy concerns around responses that might contain sensitive context or PII?

System DesignAdaptability & AmbiguityTechnical Trade-offs
Author's notes

Honestly the question I was least prepared for.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the scope—distinguish between data included in the prompt itself versus data used for model training or logging—then outline a privacy-by-design pipeline that classifies, redacts, and governs sensitive data. Emphasize trade-offs between utility and privacy, and propose concrete mechanisms like PII detection, differential privacy, and user controls.

Pro tip: Show you understand that privacy isn't just about redaction—it's about data minimization, purpose limitation, and giving users transparency and control. Mention that you'd design for auditability and compliance from day one, not as an afterthought.

1. Clarify data flow and scope

Define what 'prompt data' includes: user input, system instructions, conversation history, metadata (timestamps, user IDs), and any retrieved context. Distinguish between data processed in real-time versus data stored or used for training.

2. Identify sensitive data and risks

Discuss types of sensitive data (PII, PHI, credentials, proprietary info) and risks like memorization, leakage, or unauthorized access. Consider both user-provided data and model-generated responses that might contain sensitive context.

3. Design privacy-preserving mechanisms

Propose layered defenses: PII detection and redaction, data minimization, encryption, access controls, and techniques like differential privacy or federated learning. Explain how these apply at ingestion, processing, and storage stages.

4. Implement governance and user controls

Outline policies for data retention, user consent, opt-outs, and transparency (e.g., data usage dashboards). Include audit trails and compliance with regulations like GDPR or CCPA.

5. Evaluate trade-offs and iterate

Acknowledge trade-offs between privacy, utility, and performance. Suggest metrics (e.g., false positive/negative rates for PII detection) and a feedback loop to improve over time.

Key Points to Mention

  • Data minimization: only collect and retain what's necessary for the stated purpose.
  • PII detection and redaction techniques (e.g., NER, regex, ML-based classifiers) with false positive/negative considerations.
  • Differential privacy and its role in training and analytics.
  • User transparency and control: clear privacy policies, opt-in/opt-out mechanisms, and data access requests.
  • Compliance with regulations (GDPR, CCPA) and internal policies like data retention limits.
  • Trade-offs between privacy and model utility, and how to measure and balance them.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

Q6

How do you design rate limiting and cost control for LLM API calls in this system?

System DesignAPI & Integrations
Author's notes

Token bucket per user, hard caps per workspace, async usage tracking.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the system's requirements: expected traffic, user tiers, and cost constraints. Then propose a multi-layered approach combining rate limiting (e.g., token bucket per user/API key) and cost controls (e.g., budget quotas, token counting, caching). Emphasize trade-offs between user experience, fairness, and cost efficiency, and mention monitoring and dynamic adjustments.

Pro tip: Show awareness of OpenAI's specific challenges: handling streaming responses, token-based rate limits, and the need for graceful degradation. Mention that rate limiting should be applied at multiple levels (user, organization, global) and that cost control includes optimizing prompts and using cheaper models for certain tasks.

1. Clarify Requirements and Constraints

Ask about expected request volume, user tiers, cost budget, and latency requirements. Understand if rate limiting is per user, per API key, or global, and whether cost control is about preventing abuse or managing expenses.

2. Design Rate Limiting Strategy

Propose algorithms like token bucket or sliding window for rate limiting. Consider multiple dimensions: requests per minute, tokens per minute, and concurrent requests. Implement at API gateway and application layers.

3. Implement Cost Control Mechanisms

Use token counting to estimate costs before processing. Set budget quotas per user/organization. Implement caching for repeated queries and suggest cheaper models for non-critical tasks. Monitor and alert on cost anomalies.

4. Handle Edge Cases and Failures

Define behavior when limits are exceeded: return 429 with retry-after, queue requests, or degrade gracefully. Ensure distributed rate limiting using Redis or similar. Handle streaming responses by counting tokens as they arrive.

5. Monitor, Analyze, and Iterate

Set up logging and metrics for rate limit hits, cost per user, and system load. Use this data to adjust limits dynamically and optimize costs. Consider A/B testing different limits to balance user experience and cost.

Key Points to Mention

  • Token bucket algorithm for rate limiting with burst capacity
  • Distributed rate limiting using Redis or centralized service
  • Token counting and cost estimation before processing
  • Budget quotas and alerts per user/organization
  • Caching and prompt optimization to reduce costs
  • Graceful degradation and clear error responses (e.g., 429 with Retry-After)

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.