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Openai·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
Jul 2026Remote

Summary

OpenAI iOS Engineer interview that was more software architecture than anything mobile-specific. The whole session revolved around refactoring a chatbot codebase, which I wasn't expecting going in.

Questions Asked (4)

Q1

You're given a monolithic chatbot codebase. How would you break it apart into well-defined modules, and where do you draw the boundaries between them?

System DesignTechnical Trade-offs
Author's notes

I went with the obvious split: intent parsing, dialog management, response generation, persistence.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the chatbot's responsibilities and identifying natural seams based on domain-driven design and separation of concerns. Then propose a modular architecture with clear interfaces, explaining how you would incrementally refactor the monolith while maintaining functionality.

Pro tip: Emphasize that boundaries should be drawn around business capabilities, not technical layers, and that you'd validate boundaries by measuring coupling and cohesion before and after refactoring.

1. Understand the monolith

Map the current codebase: identify core responsibilities, data flows, dependencies, and pain points. Talk to stakeholders to understand business capabilities.

2. Identify candidate modules

Apply domain-driven design to find bounded contexts (e.g., conversation management, NLU, response generation, user profile, analytics). Group related functionality that changes together.

3. Define interfaces and contracts

For each module, specify clear APIs, data schemas, and communication patterns (sync/async). Ensure loose coupling and high cohesion.

4. Plan incremental extraction

Prioritize modules by business value and technical risk. Use strangler fig pattern to gradually extract modules, starting with least coupled. Maintain backward compatibility.

5. Validate and iterate

Measure coupling, cohesion, and performance. Gather feedback and adjust boundaries as needed. Automate testing and deployment for each module.

Key Points to Mention

  • Domain-driven design and bounded contexts
  • Separation of concerns (e.g., UI, business logic, data access)
  • Loose coupling and high cohesion
  • Clear interfaces and API contracts
  • Incremental refactoring (strangler fig pattern)
  • Trade-offs: performance vs. maintainability, short-term vs. long-term

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

Q2

How would you introduce dependency injection into this codebase so that each component can be tested in isolation?

System DesignTechnical Trade-offs
Author's notes

Talked through constructor injection and why I'd avoid a service locator.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the current architecture and pain points, then propose an incremental, risk-managed introduction of dependency injection (DI) using a lightweight container or manual injection. Emphasize how DI enables isolated testing, and discuss trade-offs like complexity and performance.

Pro tip: Advocate for starting with manual dependency injection in a few critical components to demonstrate value before adopting a full framework, as this minimizes disruption and builds team buy-in.

1. Assess Current State

Identify tightly coupled components, hard-coded dependencies, and testing pain points to prioritize where DI will have the most impact.

2. Choose DI Approach

Decide between manual DI, a lightweight container, or a full framework based on team size, project complexity, and long-term maintainability.

3. Introduce Incrementally

Start with a few high-value components, refactor them to accept dependencies via constructor or method injection, and write tests to validate isolation.

4. Establish Patterns and Guidelines

Document DI conventions, provide examples, and integrate DI into the development workflow to ensure consistency across the codebase.

5. Measure and Iterate

Track improvements in test coverage, test speed, and developer productivity, and adjust the DI strategy based on feedback and metrics.

Key Points to Mention

  • Types of dependency injection: constructor, setter, and interface injection
  • Benefits: improved testability, modularity, and separation of concerns
  • Trade-offs: increased complexity, potential performance overhead, and learning curve
  • Testing strategies: using mocks/stubs and in-memory databases for isolation
  • Incremental adoption to avoid disrupting the entire codebase
  • Tooling: DI containers (e.g., Spring, Guice, Dagger) and language-specific frameworks

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

Q3

There are hardcoded values scattered throughout the codebase. What's your approach to replacing them with configuration, and how do you decide what actually belongs in config versus code?

Technical Trade-offsAdaptability & Ambiguity
Author's notes

Short answer from me: anything environment-specific or likely to change without a code deploy goes in config.

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AI HintsAI Generated

Suggested Approach

Start by emphasizing a systematic, incremental approach: audit the codebase to identify hardcoded values, categorize them by volatility and environment-dependence, then extract only those that truly benefit from externalization. Balance the trade-off between flexibility and complexity, and highlight the importance of testing and documentation.

Pro tip: Mention that not all hardcoded values should be moved to config—constants that are truly invariant and part of the domain logic (e.g., mathematical constants, protocol limits) are better left in code for clarity and performance. This shows you understand the nuance and avoid over-engineering.

1. Audit and categorize

Search for hardcoded values and classify them by type: environment-specific (URLs, credentials), operational (timeouts, retries), business rules (tax rates), and true constants (pi, HTTP status codes).

2. Define criteria for config

Decide what belongs in config based on whether the value changes across environments, deployments, or over time without code changes, and whether it needs to be tuned by non-developers.

3. Prioritize and plan

Prioritize extraction based on impact and risk: start with high-value, low-risk items (e.g., API keys) and defer low-value or high-risk changes. Create a phased plan to avoid disrupting the codebase.

4. Implement with safeguards

Introduce a configuration management system (e.g., environment variables, config files, feature flags) with validation, defaults, and documentation. Ensure backward compatibility and use feature toggles for safe rollout.

5. Test and iterate

Add tests to verify config loading and behavior, monitor for issues, and gather feedback. Continuously refine the criteria as the system evolves.

Key Points to Mention

  • Distinguish between environment-specific values, operational tunables, and true constants that should remain in code.
  • Consider the trade-offs: increased flexibility vs. complexity, performance overhead, and potential for misconfiguration.
  • Use a configuration management strategy that includes validation, defaults, and clear documentation.
  • Adopt an incremental approach to avoid breaking changes and allow for rollback.
  • Ensure security for sensitive values (e.g., secrets management, encryption).
  • Align with team conventions and existing patterns to maintain consistency.

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

Q4

Walk me through how you'd add tests to a codebase that wasn't designed with testability in mind.

System DesignTechnical Trade-offs
Author's notes

Started with characterization tests to lock down existing behavior before touching anything, then refactor toward seams you can actually inject.

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AI HintsAI Generated

Suggested Approach

Start by emphasizing an incremental, risk-driven approach: identify high-value, low-coupling areas to test first, then use seams and dependency injection to isolate units. Show how you balance short-term wins with long-term testability improvements, and tie decisions to trade-offs like coverage vs. velocity.

Pro tip: Frame testing as a tool for enabling safe refactoring, not just bug catching—this shows you understand that testability and design are intertwined. Also, mention that you'd track metrics like defect escape rate to justify the investment to stakeholders.

1. Assess and Prioritize

Map the codebase to find critical paths, high-risk areas, and natural seams (e.g., pure functions, boundaries). Prioritize based on business impact and change frequency.

2. Introduce Seams and Test Doubles

Use techniques like dependency injection, wrapping legacy code, or extracting interfaces to create testable units without rewriting everything. Start with characterization tests to capture current behavior.

3. Write Incremental Tests

Begin with unit tests for isolated logic, then add integration tests for critical flows. Use test doubles (mocks, stubs) to break dependencies and keep tests fast and reliable.

4. Refactor for Testability

As tests provide safety, refactor code to improve design (e.g., separate concerns, reduce coupling). This makes future testing easier and improves overall code quality.

5. Automate and Monitor

Integrate tests into CI/CD, track coverage and defect rates, and iterate. Use metrics to demonstrate value and guide further testing efforts.

Key Points to Mention

  • Characterization tests to document existing behavior before changes
  • Dependency injection and seams to decouple code for testing
  • Prioritization based on risk and business value (e.g., critical paths)
  • Incremental approach: start small, deliver value early, and expand
  • Trade-offs between test coverage, development speed, and refactoring effort
  • Metrics like defect escape rate and coverage to measure progress and justify investment

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