Anthropic·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Apr 2026

Summary

Anthropic SWE interview that centered on building an LRU cache, but with the twist of serialization support and constraints that kept shifting as you went. Not a bad experience, just more layered than I expected.

Questions Asked (1)

Implement an LRU cache that supports serialization, and be ready for the requirements to change as you go.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

Started with the standard doubly linked list plus hashmap setup, felt fine.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements: what operations must the cache support, what does serialization entail, and what are the expected performance characteristics. Then design a clean, modular implementation using a hash map and doubly linked list, and structure the code so that serialization and deserialization are separate concerns. Be prepared to adapt the design as new requirements are introduced, explaining your reasoning and trade-offs at each step.

Pro tip: Treat the interview as a collaborative design session: proactively ask clarifying questions and suggest extensions before the interviewer introduces them. This demonstrates foresight and the ability to anticipate changing requirements, which is highly valued at Anthropic.

1. Clarify Requirements and Constraints

Ask about the expected operations (get, put), capacity limits, serialization format (binary, JSON, custom), and performance goals. Confirm whether the cache needs to be thread-safe or persistent across sessions.

2. Design the Core Data Structure

Propose using a hash map for O(1) lookups and a doubly linked list to track access order, enabling O(1) eviction of the least recently used item. Explain how these two structures interact.

3. Implement Serialization and Deserialization

Decide on a serialization strategy that captures both the key-value pairs and the access order. Discuss trade-offs between simplicity (e.g., JSON) and efficiency (e.g., custom binary format), and ensure deserialization reconstructs the exact state.

4. Adapt to Changing Requirements

As the interviewer introduces new constraints (e.g., TTL, persistence, concurrency), explain how you would modify the design. Emphasize modularity and separation of concerns to make changes easier.

5. Analyze Trade-offs and Test

Discuss time/space complexity, potential edge cases (e.g., serializing an empty cache), and how you would test the implementation. Mention any limitations of your approach and possible improvements.

Key Points to Mention

O(1) time complexity for get and put operations using a hash map and doubly linked list.
Serialization format choices: JSON for readability vs. binary for efficiency, and how to handle versioning.
Thread safety considerations if the cache is to be used in a concurrent environment.
Eviction policy details: how to update the linked list on access and insertion.
Handling edge cases: empty cache, cache at capacity, deserialization errors.
Extensibility: designing with interfaces to allow swapping eviction policies or serialization methods.

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