awesome-copilot/plugins/phoenix/skills/phoenix-evals/references/fundamentals-anti-patterns.md

Anti-Patterns

Common mistakes and fixes.

Anti-Pattern	Problem	Fix
Generic metrics	Pre-built scores don't match your failures	Build from error analysis
Vibe-based	No quantification	Measure with experiments
Ignoring humans	Uncalibrated LLM judges	Validate >80% TPR/TNR
Premature automation	Evaluators for imagined problems	Let observed failures drive
Saturation blindness	100% pass = no signal	Keep capability evals at 50-80%
Similarity metrics	BERTScore/ROUGE for generation	Use for retrieval only
Model switching	Hoping a model works better	Error analysis first
Single-run scoring	LLM judges and non-deterministic tasks add per-run noise that can drown the signal from a prompt change on a small dataset	Set repetitions on runExperiment (or grow the dataset) when the task or judge is an LLM call

Quantify Changes

from phoenix.client import Client

client = Client()
baseline = client.experiments.run_experiment(dataset=dataset, task=old_prompt, evaluators=evaluators)
improved = client.experiments.run_experiment(dataset=dataset, task=new_prompt, evaluators=evaluators)
print(f"Improvement: {improved.pass_rate - baseline.pass_rate:+.1%}")

Don't Use Similarity for Generation

# BAD
score = bertscore(output, reference)

# GOOD
correct_facts = check_facts_against_source(output, context)

Error Analysis Before Model Change

# BAD
for model in models:
    results = test(model)

# GOOD
failures = analyze_errors(results)
# Then decide if model change is warranted