diff --git a/litellm/proxy/hooks/dynamic_rate_limiter_v3.py b/litellm/proxy/hooks/dynamic_rate_limiter_v3.py
index d827cf3067..2cbb527284 100644
--- a/litellm/proxy/hooks/dynamic_rate_limiter_v3.py
+++ b/litellm/proxy/hooks/dynamic_rate_limiter_v3.py
@@ -528,6 +528,37 @@ class _PROXY_DynamicRateLimitHandlerV3(CustomLogger):
                         },
                     )
 
+            # Fail-closed guard: overall_code says OVER_LIMIT but no status
+            # matched a descriptor key we know how to translate into a 429.
+            # Refuse the request rather than silently fall through and let an
+            # over-limit request proceed to the model. Without this, a future
+            # caller wiring an unfamiliar descriptor into enforced_descriptors
+            # would silently bypass the rate limit.
+            offending = next(
+                (s for s in atomic_response["statuses"] if s["code"] == "OVER_LIMIT"),
+                None,
+            )
+            verbose_proxy_logger.error(
+                f"Dynamic rate limiter: OVER_LIMIT response with unknown "
+                f"descriptor_key(s) — refusing request. response={atomic_response}"
+            )
+            raise HTTPException(
+                status_code=429,
+                detail={
+                    "error": "Rate limit exceeded",
+                    "descriptor_key": (
+                        offending["descriptor_key"] if offending else "unknown"
+                    ),
+                    "rate_limit_type": (
+                        str(offending["rate_limit_type"]) if offending else "unknown"
+                    ),
+                },
+                headers={
+                    "retry-after": str(self.v3_limiter.window_size),
+                    "x-litellm-priority": priority or "default",
+                },
+            )
+
         # If priority is NOT enforced (saturation below threshold) but
         # priority_descriptors exist, increment them for tracking only — no
         # check, no rollback. This matches the prior tracking semantics.
diff --git a/tests/test_litellm/proxy/hooks/test_rate_limiter_toctou.py b/tests/test_litellm/proxy/hooks/test_rate_limiter_toctou.py
index b8ced4e661..ceea5de799 100644
--- a/tests/test_litellm/proxy/hooks/test_rate_limiter_toctou.py
+++ b/tests/test_litellm/proxy/hooks/test_rate_limiter_toctou.py
@@ -412,3 +412,78 @@ async def test_batch_zero_token_consumes_rpm_only():
             batch_usage=zero_batch,
         )
     assert exc.value.status_code == 429
+
+
+@pytest.mark.asyncio
+async def test_dynamic_rate_limiter_v3_fails_closed_on_unknown_descriptor():
+    """
+    Fail-closed guard: when atomic_check_and_increment_by_n returns
+    overall_code=OVER_LIMIT but with a descriptor_key the dispatcher does
+    not recognize, the dynamic limiter must raise 429 rather than silently
+    fall through.
+
+    Reproduces by patching atomic_check_and_increment_by_n to return an
+    OVER_LIMIT response carrying an unknown descriptor_key.
+    """
+    from fastapi import HTTPException
+
+    os.environ["LITELLM_LICENSE"] = "test-license-key"
+    litellm.priority_reservation = {"high": 0.9, "low": 0.1}
+
+    dual_cache = DualCache()
+    handler = DynamicRateLimitHandler(internal_usage_cache=dual_cache)
+
+    model = "fail-closed-model"
+    llm_router = Router(
+        model_list=[
+            {
+                "model_name": model,
+                "litellm_params": {
+                    "model": "gpt-3.5-turbo",
+                    "api_key": "test-key",
+                    "api_base": "test-base",
+                    "tpm": 1000,
+                },
+            }
+        ]
+    )
+    handler.update_variables(llm_router=llm_router)
+
+    async def fake_atomic(*args, **kwargs):
+        return {
+            "overall_code": "OVER_LIMIT",
+            "statuses": [
+                {
+                    "code": "OVER_LIMIT",
+                    "current_limit": 100,
+                    "limit_remaining": 0,
+                    "rate_limit_type": "tokens",
+                    "descriptor_key": "future_unrecognized_descriptor",
+                }
+            ],
+        }
+
+    handler.v3_limiter.atomic_check_and_increment_by_n = fake_atomic
+
+    from litellm.types.router import ModelGroupInfo
+
+    user = UserAPIKeyAuth(api_key=hash_token("fail-closed-key"))
+    user.metadata = {"priority": "high"}
+
+    with pytest.raises(HTTPException) as exc:
+        await handler._check_rate_limits(
+            model=model,
+            model_group_info=ModelGroupInfo(
+                model_group=model,
+                providers=["openai"],
+                rpm=None,
+                tpm=1000,
+            ),
+            user_api_key_dict=user,
+            priority="high",
+            saturation=0.0,
+            data={},
+        )
+    assert (
+        exc.value.status_code == 429
+    ), f"Expected 429 fail-closed on unknown descriptor; got {exc.value.status_code}"