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AI Engineering

Building an LLM Eval Set That Catches Regressions Before Your Users Do

By

October 25, 2026

7 min read

Model retirements now force a migration every year or so, and every prompt change is a release. A small, honest eval set turns both into a day of work instead of a month of guessing. How to build one from real traffic, how to grade it, and what to automate.

LLM Evaluation
Evals
AI Engineering
Model Migration

Bedrock retired Claude Sonnet 4 in October 2026 and has more retirements scheduled through 2027. Every team on those models has to move, and the ones with an eval set will do it in a day: run the set on the new model, read the diffs, adjust two prompts, ship. The ones without will move and wait for complaints.

Where the cases come from

  • Real traffic first. Sample production inputs, stratified by type, and include the long, messy and ambiguous ones.

  • Every bug report becomes a case. The cases that once failed are the most valuable ones you have.

  • Edge cases you know about: empty input, other languages, adversarial phrasing, inputs that should be refused.

  • Start with 50 to 200 cases. A small set someone reads beats a large set nobody trusts.

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How to grade

Grader

Good for

Watch out for

Exact or programmatic checks

Structured output, extracted fields, classifications, valid JSON, tool choice

Too strict for free text

Assertions

Must mention X, must not mention Y, length limits, citations present

Easy to satisfy without being right

Model as judge

Tone, completeness, faithfulness to sources

Bias toward longer answers and toward its own model family; calibrate against human labels

Human review

The final call on a sample, and calibrating the judge

Slow; use it on diffs, not the whole set

What to automate

  1. Run the set on every prompt or model change, in CI, and fail the build on regressions in the programmatic checks.

  2. Report judge scores as a diff against the last release, case by case, so a reviewer reads the ten that changed rather than a single average.

  3. Track cost and latency per case alongside quality. A model that is 2% better and 3x the price is a business decision, not an engineering one.

  4. Keep the set versioned with the prompts it tests.

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