diff --git a/metrics/behavioral_drift/README.md b/metrics/behavioral_drift/README.md
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+# Behavioral Drift
+
+**A fine-tuning quality metric that catches what loss curves miss.**
+
+## The gap
+
+Everyone checks loss after fine-tuning. But loss can drop while every output collapses to `"199999999999..."`. Perplexity won't flag this. BLEU won't flag this. Only reading actual outputs catches it — and nobody reads every output.
+
+**Standard metrics measure token-level quality. Behavioral Drift measures output integrity.**
+
+## How it complements existing tools
+
+| Tool | Measures | Blind spot |
+|------|----------|------------|
+| Perplexity | Token prediction | Output coherence |
+| BLEU/ROUGE | n-gram overlap | Mode collapse, degeneration |
+| Loss curve | Convergence | Behavioral degradation |
+| **drift_score** | Output diversity + integrity | — |
+
+Not a replacement. An addition. Use alongside perplexity — not instead of it.
+
+## Three signals
+
+- **self-BLEU**: pairwise similarity among outputs (high = mode collapse)
+- **digit density**: fraction of numeric chars (high = garbage output)
+- **repetition ratio**: unique/total token ratio (low = looping)
+
+=> **drift_score** (0-1): 1.0 = healthy, 0.0 = collapsed.
+
+## Usage
+
+```python
+import evaluate
+drift = evaluate.load("./behavioral_drift.py")
+r = drift.compute(
+    predictions=ft_outputs,
+    references=base_outputs,
+)
+print(r["drift_score"])  # 0.95 = healthy, 0.05 = collapse
+```
+
+## Origin
+
+Extracted from 6 failed LoRA experiments (Qwen2.5-0.5B/1.5B). Loss curves looked fine. Outputs were broken. The checks that finally caught the collapse were formalized into this reusable metric — so others don't debug the same way.
+
+## License
+
+MIT
diff --git a/metrics/behavioral_drift/app.py b/metrics/behavioral_drift/app.py
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+"""Demo: Behavioral Drift metric — interactive test."""
+import gradio as gr
+import evaluate
+
+drift = evaluate.load("./behavioral_drift.py")
+
+def check(predictions_text, references_text):
+    preds = [p.strip() for p in predictions_text.split("|||") if p.strip()]
+    refs = [r.strip() for r in references_text.split("|||") if r.strip()]
+    if not preds or not refs:
+        return "Error: need at least one prediction and reference (separate multiple with |||)"
+    if len(preds) != len(refs):
+        return f"Error: predictions ({len(preds)}) and references ({len(refs)}) must have same count"
+    r = drift.compute(predictions=preds, references=refs)
+    return f"""drift_score: {r["drift_score"]}
+self_bleu: {r["self_bleu"]}
+digit_density: {r["digit_density"]} (baseline: {r["digit_density_baseline"]})
+repetition_ratio: {r["repetition_ratio"]} (baseline: {r["repetition_ratio_baseline"]})
+diagnosis: {r["diagnosis"]}"""
+
+demo = gr.Interface(
+    fn=check,
+    inputs=[
+        gr.Textbox(label="FT outputs (separate with |||)", value="正常的中文回答|||199999999999999"),
+        gr.Textbox(label="Base outputs (separate with |||)", value="base输出A|||base输出B"),
+    ],
+    outputs=gr.Textbox(label="Result"),
+    title="Behavioral Drift — Fine-Tuning Quality Metric",
+    description="Detects output collapse invisible to loss curves. 0=collapse, 1=healthy.",
+)
+demo.launch()
diff --git a/metrics/behavioral_drift/behavioral_drift.py b/metrics/behavioral_drift/behavioral_drift.py
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+"""Behavioral Drift metric for HuggingFace Evaluate.
+
+Detects fine-tuning output collapse that complements perplexity.
+Concatenated-reference self-BLEU | digit density | repetition ratio
+=> drift_score (0=collapse, 1=healthy)
+
+Motivated by observations from LoRA fine-tuning where training loss
+improved while outputs collapsed to repetitive patterns.
+"""
+import evaluate
+import datasets
+from collections import Counter
+
+_CITATION = """\
+@misc{behavioral_drift,
+  author = {Yuhao Lin},
+  title = {Behavioral Drift: A Fine-Tuning Quality Metric},
+  year = {2026},
+}
+"""
+
+_DESCRIPTION = """\
+Behavioral Drift catches fine-tuning output collapse earlier than perplexity alone.
+
+Three signals composited into a 0-1 drift_score:
+1. self-BLEU: concatenated-reference BLEU-1 across outputs (>0.8 = mode collapse)
+2. digit_density: fraction of numeric chars, vs reference baseline
+3. repetition_ratio: unique/total token ratio (<0.4 = degenerate loops)
+
+Real-world case: Qwen2.5-1.5B-Instruct fine-tuned on 80 custom samples.
+Loss: 9.2 -> 8.8 (improvement). But all outputs collapsed to "1999999...".
+Drift_score caught this while perplexity did not.
+
+Unified thresholds across formula and diagnosis:
+self-BLEU>0.8, digit_density_delta>0.2, repetition_ratio<0.4
+"""
+
+_KWARGS_DESCRIPTION = """\
+Args:
+    predictions: list of fine-tuned model outputs on test prompts.
+    references: list of base model outputs on same prompts (baseline).
+Returns:
+    drift_score (0-1), self_bleu, digit_density, digit_density_baseline,
+    repetition_ratio, repetition_ratio_baseline, diagnosis
+"""
+
+# Unified thresholds — same in drift_score formula and diagnosis
+SB_T = 0.8        # self-BLEU threshold
+DD_DELTA_T = 0.2  # digit_density increase vs baseline
+RR_T = 0.4        # repetition_ratio threshold
+
+
+def _bleu1(candidate, reference):
+    """BLEU-1: unigram precision. Each ref token matched at most once (standard)."""
+    cand = candidate.split()
+    ref_counts = Counter(reference.split())
+    if not cand: return 0.0
+    hits = 0
+    for t in cand:
+        if ref_counts.get(t, 0) > 0:
+            hits += 1
+            ref_counts[t] -= 1
+    return hits / len(cand)
+
+
+def _self_bleu(outputs):
+    """Concatenated-reference self-BLEU-1 approximation.
+    Each output is candidate, remaining N-1 joined as single reference.
+    Conservative: inflates scores ~0.05-0.15 vs multi-reference BLEU.
+    """
+    if len(outputs) < 2: return 0.0
+    scores = []
+    for i in range(len(outputs)):
+        ref = " ".join(outputs[:i] + outputs[i+1:])
+        scores.append(_bleu1(outputs[i], ref))
+    return sum(scores) / len(scores)
+
+
+def _digit_density(text):
+    if not text: return 0.0
+    return sum(1 for c in text if c.isnumeric()) / len(text)
+
+
+def _repetition_ratio(text):
+    tokens = text.split()
+    if len(tokens) < 5: return 1.0
+    return len(set(tokens)) / len(tokens)
+
+
+class BehavioralDrift(evaluate.Metric):
+    def _info(self):
+        return evaluate.MetricInfo(
+            description=_DESCRIPTION,
+            citation=_CITATION,
+            inputs_description=_KWARGS_DESCRIPTION,
+            features=datasets.Features({
+                "predictions": datasets.Value("string"),
+                "references": datasets.Value("string"),
+            }),
+            reference_urls=[],
+        )
+
+    def _compute(self, predictions, references):
+        if not predictions:
+            return {"drift_score": 0.0, "self_bleu": 0.0,
+                    "digit_density": 0.0, "digit_density_baseline": 0.0,
+                    "repetition_ratio": 0.0, "repetition_ratio_baseline": 0.0,
+                    "diagnosis": "empty_input"}
+
+        dd_pred = [_digit_density(p) for p in predictions]
+        rr_pred = [_repetition_ratio(p) for p in predictions]
+        avg_dd = sum(dd_pred) / len(dd_pred)
+        avg_rr = sum(rr_pred) / len(rr_pred)
+        sb = _self_bleu(predictions)
+
+        # References used as baseline — compared against
+        dd_base = sum(_digit_density(r) for r in references) / len(references) if references else 0.0
+        rr_base = sum(_repetition_ratio(r) for r in references) / len(references) if references else 0.0
+
+        # Unified thresholds
+        sb_score = max(0.0, 1.0 - sb / SB_T)
+        dd_delta = max(0.0, avg_dd - dd_base)  # ft minus base
+        dd_score = max(0.0, 1.0 - dd_delta / DD_DELTA_T)
+        rr_score = min(1.0, avg_rr / RR_T)
+        drift_score = round(sb_score * dd_score * rr_score, 4)
+
+        issues = []
+        if sb > SB_T: issues.append(f"mode_collapse(self_bleu={sb:.2f})")
+        if dd_delta > DD_DELTA_T: issues.append(f"digit_degradation(+{dd_delta:.2f})")
+        if avg_rr < RR_T: issues.append(f"token_repetition(ratio={avg_rr:.2f})")
+        diagnosis = "healthy" if not issues else ";".join(issues)
+
+        return {"drift_score": drift_score, "self_bleu": round(sb, 4),
+                "digit_density": round(avg_dd, 4), "digit_density_baseline": round(dd_base, 4),
+                "repetition_ratio": round(avg_rr, 4), "repetition_ratio_baseline": round(rr_base, 4),
+                "diagnosis": diagnosis}
diff --git a/metrics/behavioral_drift/requirements.txt b/metrics/behavioral_drift/requirements.txt
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+# No external dependencies ¡ª uses only Python stdlib + evaluate