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Library rep  ·  Wednesday, May 13

Hamel Husain & Shreya Shankar on AI evals: ground them in your product's actual failures, not fashionable metrics.

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Based on Lenny’s
Newsletter · Hamel Husain & Shreya Shankar · Sep 9, 2025

Building eval systems that improve your AI product

Start from the original episode or newsletter, then use the ideas below in the reps.

Key ideas to remember

  1. 01 Don't start with 'hallucination' or 'toxicity' — those metrics rarely match your users' actual problems. Start with 100 traces and a domain expert reading them.
  2. 02 If you are the PM on an AI product, you are probably the principal domain expert. Stop waiting for someone else to define quality.
  3. 03 Code-based evaluators for objective rules; LLM-as-a-judge only for subjective qualities. Pick the failure mode first, the tool second.
  4. 04 An eval suite that doesn't catch regressions before they ship isn't a flywheel — it's a vanity dashboard.
3-minute summary

Hamel Husain and Shreya Shankar argue that most AI eval dashboards are theater. Teams reach for ready-made metrics like 'hallucination' or 'toxicity' because they sound rigorous, but those scores rarely correlate with the actual problems users face. The result: pretty dashboards that nobody trusts and that don't drive shipping decisions.

Their alternative is a three-phase process. Phase 1 is error analysis: ground your evals in reality by appointing a single principal domain expert (often the PM) as the 'benevolent dictator' on quality. Arm them with ~100 representative user interactions, sampled randomly at first, and have them do open coding — free-form pass/fail critiques detailed enough that a brand-new employee could understand them. The output is a prioritized list of your product's most common failure modes.

Phase 2 is building the eval suite. Each prioritized failure mode gets the right tool: code-based evaluators for objective, rule-based checks ('does the output contain a user ID?'), and LLM-as-a-judge for subjective qualities like tone or reasoning quality. Don't pick the tool first; pick the failure mode first, then the tool.

Phase 3 is operationalizing — making the suite a flywheel that catches regressions before they ship and feeds back into product improvement. The throughline: evals are a process, not a dashboard. They're the PM's domain, not a thing you outsource to ML engineers.

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