Model Benchmarking

The Law

A MODEL IS NOT CHOSEN UNTIL IT HAS BEEN TESTED ON YOUR TASK DATA.
Leaderboard scores are averaged over tasks you're not building.
"It's the best model" is a claim about benchmarks someone else ran.
Task data + defined metric + three tested tiers IS a model selection.

When to Use

Trigger when:

• Choosing a model for any new production feature
• Considering a switch to a newer, cheaper, or faster model
• Validating that a smaller model can replace a larger one
• Comparing providers (OpenAI, Anthropic, Mistral, local)

When NOT to Use

• Initial feasibility exploration (before task definition is stable) — benchmark when you know what you're measuring
• Model compatibility checks (does this model support tool use, JSON mode, etc.) — that's a capability query, not a benchmark
• Leaderboard research to narrow the candidate list — that's input to Stage 3, not a substitute for it

The Process

Four stages. Do not collapse them.

Stage 1 — Define the Bar

Before running any model, write down:

If you cannot define the metric and threshold first, the selection criteria are not clear enough to proceed.

Stage 2 — Build a Representative Test Set

• Minimum 50 examples; 200+ for features used at volume
• Label ground truth before running any model. Seeing outputs first contaminates labels.
• Include: easy cases, hard cases, edge cases, adversarial inputs
• Tag each example by difficulty: easy / medium / hard
• Store in benchmarks/<feature>/test-set.jsonl

Stage 3 — Benchmark at Least Three Tiers

Always test at least one tier below your expected choice:

For each model:

1. Run all examples through the same prompt (version-controlled via prompt-versioning)
2. Run 3× and average — model outputs are stochastic
3. Record: accuracy, latency p50/p95, cost per 1k input+output tokens

Stage 4 — Cost-Quality Decision

Build this table:

Decision rule: if a cheaper model is within 5% of frontier on your metric AND within the latency constraint → use the cheaper model. The quality difference at 100k calls/month is worth the cost differential analysis.

Document decision in benchmarks/<feature>/<date>.md.

Rationalization Red Flags

These thoughts mean a model was chosen without benchmarking — stop:

• "GPT-4 is the best model" — best on MMLU, not necessarily on your invoice extraction pipeline
• "I tested a few examples and it was good" — you self-selected; the test set tests the distribution
• "The new model is clearly better" — "clearly better" at which task? at which cost?
• "We don't have time to benchmark" — you have time to migrate away from the wrong model after launch?
• "The efficient tier can't do this task" — you know this from how many labelled examples?
• "We should use the best model to be safe" — using a 20× more expensive model when Haiku passes the threshold is not safe, it's wasteful

Completion Statement Format

When model-benchmarking is satisfied, state it like this:

Model selected: <model-id>
Task: <one-line description>
Metric: <metric name>, threshold: ≥ X%
Test set: benchmarks/<feature>/test-set.jsonl — N examples

Results:
  <Frontier model>: A%, p95=Xs, $Y/1k
  <Mid model>:      B%, p95=Xs, $Y/1k ← selected
  <Efficient model>: C%, p95=Xs, $Y/1k (below threshold)

Decision: <mid model> meets threshold (B% ≥ X%) at Z× lower cost than frontier
Stored: benchmarks/<feature>/<date>.md ✓

BLOCK: This statement cannot be completed if fewer than 3 tiers were tested. State which tiers are missing and why — do not produce a recommendation.

Why This Matters

A model that underperforms on your task produces silent quality degradation at scale. A model that over-performs costs more than necessary. The benchmark is how you know which you have before 100k calls tell you.