diff --git a/app/core/config.py b/app/core/config.py
index 8c79b69..126ea18 100644
--- a/app/core/config.py
+++ b/app/core/config.py
@@ -56,7 +56,15 @@ class Config:
     # omnibioai-billing are then not visible.
     V1_REDIS_URL = os.getenv("V1_REDIS_URL", "")
     V1_RATE_LIMIT_PER_MINUTE = int(os.getenv("V1_RATE_LIMIT_PER_MINUTE", "60"))
-    V1_IDEMPOTENCY_TTL = int(os.getenv("V1_IDEMPOTENCY_TTL", "86400"))
+    # M18 (design audit gap #11): this cached replay body is the real
+    # answer text/citations for /v1/literature/answers, not just
+    # metadata -- one of only two places that text survives anywhere in
+    # this system at all (the other is omnibioai-rag's own query
+    # cache). The design doc's 30-day question/answer deletion policy
+    # is enforced here as a hard ceiling on this already-short-lived
+    # cache's TTL, not a separate deletion job -- there is no durable
+    # store for one to run against.
+    V1_IDEMPOTENCY_TTL = min(int(os.getenv("V1_IDEMPOTENCY_TTL", "86400")), 30 * 24 * 60 * 60)
     # Design audit gap #7 ("concurrent-answer limits are absent"): the
     # most expensive /v1 call (it invokes an LLM, up to RAG's own
     # 300-second timeout) is capped on how many of a single key's or
diff --git a/tests/test_config.py b/tests/test_config.py
index f10aa45..c785867 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -41,6 +41,30 @@ def test_config_reads_iam_url_from_env(monkeypatch):
     importlib.reload(cfg_module)
 
 
+def test_v1_idempotency_ttl_is_capped_at_30_days_even_if_configured_higher(monkeypatch):
+    """M18 (design audit gap #11): the idempotency-replay cache holds
+    the real answer text/citations, so its TTL is the de facto
+    enforcement of the 30-day deletion policy -- a misconfigured env
+    var must not be able to silently violate that."""
+    monkeypatch.setenv("V1_IDEMPOTENCY_TTL", str(60 * 24 * 60 * 60))  # 60 days
+    import app.core.config as cfg_module
+
+    importlib.reload(cfg_module)
+    assert cfg_module.Config.V1_IDEMPOTENCY_TTL == 30 * 24 * 60 * 60
+    monkeypatch.delenv("V1_IDEMPOTENCY_TTL", raising=False)
+    importlib.reload(cfg_module)
+
+
+def test_v1_idempotency_ttl_below_the_30_day_cap_is_unaffected(monkeypatch):
+    monkeypatch.setenv("V1_IDEMPOTENCY_TTL", "60")
+    import app.core.config as cfg_module
+
+    importlib.reload(cfg_module)
+    assert cfg_module.Config.V1_IDEMPOTENCY_TTL == 60
+    monkeypatch.delenv("V1_IDEMPOTENCY_TTL", raising=False)
+    importlib.reload(cfg_module)
+
+
 def test_config_defaults_are_set():
     """IAM_URL/REDIS_URL/JWT_SECRET all have non-empty defaults, so the
     gateway can start with no environment variables configured."""