DSPy Evaluation Harness (3.2.x)

The metric is usually more important than the program. For dspy.GEPA especially, the quality of textual feedback in your metric determines whether optimization converges.

Two rules

1. Return a dspy.Prediction(score=..., feedback=...), not a dict. dspy.Evaluate's parallel executor aggregates scores via sum, which breaks on dict outputs (TypeError: unsupported operand type(s) for +: 'int' and 'dict'). dspy.Prediction supports float/add and is what GEPA's adapter natively unwraps. A bare float still works for pure dspy.Evaluate scoring, but GEPA needs the score+feedback pair.
2. Separate valset. Never optimize and evaluate on the same examples. Optimizers overfit fast.

Canonical rich-feedback metric

import dspy

def rich_metric(gold: dspy.Example, pred: dspy.Prediction, trace=None,
                pred_name: str | None = None, pred_trace=None):
    # 1. Compute sub-scores — multi-axis beats scalar
    correctness = 1.0 if _normalize(pred.answer) == _normalize(gold.answer) else 0.0
    cited = _has_citation(pred.answer, gold.sources) if hasattr(gold, "sources") else 1.0
    concise = 1.0 if len(pred.answer.split()) <= 50 else 0.5
    score = 0.6 * correctness + 0.25 * cited + 0.15 * concise

    # 2. Write feedback that teaches the optimizer
    parts = []
    if correctness < 1.0:
        parts.append(
            f"Answer mismatch. Predicted: {pred.answer!r}. Expected: {gold.answer!r}. "
            f"Likely cause: reasoning skipped the units/quantity in the question."
        )
    if cited < 1.0:
        parts.append("Did not ground the claim in the provided sources. Quote a source fragment.")
    if concise < 1.0:
        parts.append("Answer exceeded 50 words — tighten to one sentence.")
    if not parts:
        parts.append("Correct, grounded, and concise.")
    feedback = " ".join(parts)

    return dspy.Prediction(score=score, feedback=feedback)

Canonical harness

evaluator = dspy.Evaluate(
    devset=valset,
    metric=rich_metric,
    num_threads=8,
    display_progress=True,
    display_table=10,          # pretty-print first 10 rows
    provide_traceback=True,    # surface exceptions, don't swallow them
    max_errors=5,
    failure_score=0.0,
    save_as_json="eval_runs/baseline.json",
)
result = evaluator(program)
print("Overall:", result.score)
for example_result in result.results[:3]:
    print(example_result)

dspy.Evaluate returns an EvaluationResult with .score (aggregate float) and .results (list of (example, pred, score) tuples).

Dataset hygiene

• Size: 20–50 examples is enough for GEPA's reflective loop; 100–500 for MIPROv2-style bootstrapping.
• Split: hand-curate two disjoint sets — trainset (for optimization) and valset (for metric-on-optimized-program). A test set you never look at during development is gold.
• Representativeness beats size. Include edge cases, ambiguity, adversarial inputs.
• Build dspy.Example(...).withinputs("question", "context") — the withinputs call marks which fields are inputs vs. gold outputs.

trainset = [
    dspy.Example(question="…", answer="…").with_inputs("question"),
    ...
]

Multi-axis metrics (recommended)

Combine correctness, faithfulness, format adherence, latency, and cost. Each axis should be a 0–1 float with a written definition. Weight them explicitly; don't hide weights inside magic numbers — make them constants so optimizers can be told to trade off.

CI-ready eval suite

# tests/test_dspy_eval.py
import dspy, pytest
from my_program import program, valset, rich_metric

@pytest.fixture(scope="module")
def evaluator():
    return dspy.Evaluate(devset=valset, metric=rich_metric, num_threads=8,
                         display_progress=False, provide_traceback=True)

def test_program_meets_threshold(evaluator):
    result = evaluator(program)
    assert result.score >= 0.75, f"Regression: {result.score:.3f}"

Run offline in CI with a cached LM (dspy.LM(..., cache=True)) + pre-populated DSPY_CACHEDIR.

Tracing & observability

• trackusage=True on dspy.configure accumulates token counts on predictions (pred.getlm_usage()).
• MLflow: import mlflow; mlflow.dspy.autolog() → traces every prediction.
• W&B: pass use_wandb=True to dspy.GEPA to log Pareto fronts.
• Always log eval results to a versioned JSON file (saveasjson=...) so you can diff runs.

Anti-patterns

• Scalar-only metrics (float but no feedback) when using GEPA — wasted signal.
• return {"score": s, "feedback": f} (dict) — crashes dspy.Evaluate's parallel aggregator. Use dspy.Prediction(score=s, feedback=f).
• Exact-match metrics on open-ended generation tasks — use semantic or LM-as-judge scoring.
• Evaluating on the trainset — optimistic by 10–30 points.
• Silently swallowing exceptions (provide_traceback=False) — you'll blame the LM for a KeyError.
• Changing the metric mid-experiment without re-baselining — prior numbers become incomparable.

Next

• Feed this metric into dspy-gepa-optimizer.
• Full harness pattern → [reference.md](reference.md).
• Runnable example → [examplemetric.py](examplemetric.py).