diff --git a/app/policy/audit.py b/app/policy/audit.py
new file mode 100644
index 0000000..8b15ca5
--- /dev/null
+++ b/app/policy/audit.py
@@ -0,0 +1,56 @@
+"""Audit — 4B 가 자체 답변한 경우 금지 패턴 검출.
+
+escalate_to_26b=False 인 이벤트에만 호출. 위반 검출 시 policy_violation=true 로
+analyze_events 에 기록되고 야간 sweep 에서 under_escalation 후보로 포획된다.
+
+detection_patterns 는 Python re.search() 로 평가 (Postgres regex 아님).
+"""
+
+from __future__ import annotations
+
+import re
+from functools import lru_cache
+from typing import Iterable
+
+from app.policy.loader import load_policy
+from app.policy.schema import DomainPolicy, ForbiddenRule
+
+
+@lru_cache(maxsize=256)
+def _compiled_patterns(pattern_tuple: tuple[str, ...]) -> tuple[re.Pattern[str], ...]:
+    return tuple(re.compile(p) for p in pattern_tuple)
+
+
+def _rules_for_subject(
+    policy: DomainPolicy, subject_domain: str
+) -> Iterable[ForbiddenRule]:
+    for rule in policy.forbidden_for_4b:
+        if subject_domain in rule.applies_when_subject_in:
+            yield rule
+
+
+def check_4b_output_violations(
+    output_text: str,
+    subject_domain: str,
+    *,
+    policy: DomainPolicy | None = None,
+) -> list[str]:
+    """Return list of violated forbidden-rule IDs (빈 리스트면 위반 없음).
+
+    Parameters
+    ----------
+    output_text: 4B 가 생성한 자체 답변 텍스트.
+    subject_domain: routing 에서 결정된 도메인 이름. fallback 도메인은 `generic`.
+    policy: 주입용 (테스트). None 이면 load_policy().
+    """
+    if not output_text:
+        return []
+    if policy is None:
+        policy = load_policy()
+
+    violations: list[str] = []
+    for rule in _rules_for_subject(policy, subject_domain):
+        patterns = _compiled_patterns(tuple(rule.detection_patterns))
+        if any(p.search(output_text) for p in patterns):
+            violations.append(rule.id)
+    return violations