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Anthropic·Software Engineer·Technical Phone Screen·Senior

Senior
May 2026

Summary

Anthropic software engineer interview with a debugging-focused coding round. The problem was a broken tokenizer implementation and you had to find the bug before fixing it.

Questions Asked (1)

Q1

You're given a tokenizer implementation that produces incorrect outputs on certain inputs. Read through the code, test it against some inputs, find the first bug, explain the root cause (which line, which case, why it fails), and then propose a fix.

Root Cause AnalysisAlgorithms & Data StructuresTechnical 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 the tokenizer's expected behavior and edge cases, then systematically test with representative inputs to reproduce the bug. Trace the code execution to isolate the first failure, explain the root cause with a specific line and input, and propose a minimal fix with validation.

Pro tip: Demonstrate a methodical debugging process: state your hypothesis before testing, and after fixing, suggest adding a regression test to prevent recurrence. This shows engineering rigor and prevents future issues.

1. Clarify requirements and edge cases

Ask clarifying questions about the tokenizer's expected behavior, supported inputs, and edge cases (e.g., empty string, special characters, Unicode). This ensures you understand what 'correct' means.

2. Reproduce the bug with targeted tests

Design and run a few minimal test cases that cover normal and edge inputs. Identify an input that produces incorrect output and note the expected vs. actual result.

3. Trace execution to locate the first bug

Walk through the code line by line with the failing input, using print statements or a debugger. Pinpoint the exact line where the behavior diverges from expectations.

4. Explain root cause and propose a fix

Articulate why the bug occurs (e.g., off-by-one, incorrect condition, mishandled edge case) and suggest a concrete code change. Verify the fix with the failing test and additional cases.

5. Discuss prevention and trade-offs

Mention adding a regression test and consider if the fix introduces any performance or complexity trade-offs. Optionally, discuss alternative approaches.

Key Points to Mention

  • Systematic debugging: form hypotheses, test, and iterate rather than guessing.
  • Root cause analysis: identify the specific line and condition that causes the failure.
  • Edge case handling: consider empty inputs, delimiters, Unicode, and boundary conditions.
  • Regression testing: add a test case for the bug to prevent future regressions.
  • Code clarity: ensure the fix is minimal and doesn't introduce new issues.
  • Trade-offs: evaluate if the fix affects performance, readability, or maintainability.

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