feat: CrossJudgeExperiment for cross-version judge stability analysis

examples/cross_judge_example.py

@@ -0,0 +1,105 @@
+"""
+Cross-judge stability analysis example.
+
+Demonstrates CrossJudgeExperiment: running the same subject model through two
+judge versions to measure whether the judge upgrade shifts severity ratings.
+
+Motivated by the finding that judge-model version can materially shift modal
+severity on identical subject responses — including safety-relevant scenarios —
+without any change to the subject model itself.
+
+Usage
+-----
+    python examples/cross_judge_example.py
+
+Set ANTHROPIC_API_KEY before running. Results are cached under
+`cross_judge_results/` so the run is resumable if interrupted.
+"""
+
+import asyncio
+import os
+
+from simpleaudit import CrossJudgeExperiment, compare_judges
+
+
+SUBJECT_MODELS = [
+    {
+        "model": "claude-haiku-4-5-20251001",
+        "provider": "anthropic",
+        "label": "haiku-4.5",
+    },
+]
+
+JUDGE_MODELS = [
+    {"model": "claude-opus-4-7", "provider": "anthropic"},
+    {"model": "claude-opus-4-8", "provider": "anthropic"},
+]
+
+
+async def main() -> None:
+    if not os.environ.get("ANTHROPIC_API_KEY"):
+        raise EnvironmentError("ANTHROPIC_API_KEY is not set.")
+
+    exp = CrossJudgeExperiment(
+        models=SUBJECT_MODELS,
+        judge_models=JUDGE_MODELS,
+        n_repetitions=3,
+        save_dir="cross_judge_results",
+        show_progress=True,
+    )
+
+    print(f"Judges: {exp.judge_labels}")
+    print("Running cross-judge experiment (results cached under cross_judge_results/)...\n")
+
+    results = await exp.run_async(
+        scenarios="nav_aap",
+        language="Norwegian",
+        max_turns=3,
+    )
+
+    # --- Score summary ---------------------------------------------------------
+    print("=== Score summary (mean across 3 runs) ===")
+    summary = results.score_summary()
+    for subject, per_judge in summary.items():
+        print(f"\n  Subject: {subject}")
+        for judge_label, stats in per_judge.items():
+            print(
+                f"    {judge_label:20s}  mean={stats['mean']:.1f}  "
+                f"std={stats['std']:.2f}  cv={stats['cv']:.1%}  "
+                f"n={stats['n_runs']}"
+            )
+
+    # --- Severity shifts -------------------------------------------------------
+    print("\n=== Cross-judge severity shifts (haiku-4.5 / nav_aap) ===")
+    shifts = results.severity_shifts("haiku-4.5")
+    shifted = [s for s in shifts if s["shifted"]]
+    print(f"  {len(shifted)} of {len(shifts)} scenarios shifted modal severity.\n")
+    for entry in shifted:
+        direction = entry.get("direction", 0)
+        arrow = "→ stricter" if direction > 0 else "→ more lenient"
+        judges = list(entry["modals"].keys())
+        modal_a = entry["modals"][judges[0]]
+        modal_b = entry["modals"][judges[1]]
+        print(
+            f"  {entry['scenario']:<45s}  "
+            f"{judges[0]}: {modal_a:<8s}  {judges[1]}: {modal_b:<8s}  {arrow}"
+        )
+
+    # --- compare_judges utility (post-hoc comparison) -------------------------
+    print("\n=== compare_judges utility ===")
+    cmp = compare_judges(
+        results["opus-4-7"],
+        results["opus-4-8"],
+        subject_label="haiku-4.5",
+        label_a="opus-4-7",
+        label_b="opus-4-8",
+    )
+    print(
+        f"  score_delta (4.8 − 4.7): {cmp['score_delta']:+.2f}  "
+        f"cv_delta: {cmp['cv_delta']:+.4f}  "
+        f"n_shifted: {cmp['n_shifted']}/{cmp['n_total']}"
+    )
+
+
+if __name__ == "__main__":
+    asyncio.run(main())

simpleaudit/__init__.py

@@ -29,6 +29,7 @@
 from .judges import get_judge, list_judge_configs
 from .experiment import AuditExperiment
 from .repeated_results import RepeatedExperimentResults, ModelStabilityReport
+from .cross_judge import CrossJudgeExperiment, CrossJudgeResults, compare_judges
 
 __all__ = [
     "ModelAuditor",
@@ -41,5 +42,8 @@
     "AuditExperiment",
     "RepeatedExperimentResults",
     "ModelStabilityReport",
+    "CrossJudgeExperiment",
+    "CrossJudgeResults",
+    "compare_judges",
 ]