diff --git a/litellm/proxy/hooks/tag_rate_limiter.py b/litellm/proxy/hooks/tag_rate_limiter.py
index 875fc8338a7..b55c472f89d 100644
--- a/litellm/proxy/hooks/tag_rate_limiter.py
+++ b/litellm/proxy/hooks/tag_rate_limiter.py
@@ -616,7 +616,16 @@ class _PROXY_TagRateLimiter(  # pyright: ignore[reportUnusedClass]  # only refer
         internal_usage_cache: DualCache,
         time_provider: Callable[[], datetime] | None = None,
     ) -> None:
-        self.internal_usage_cache = InternalUsageCache(dual_cache=internal_usage_cache)
+        # A dedicated in-memory layer, not the proxy-wide `internal_usage_cache`
+        # passed in: that instance is shared with the key/team parallel-request
+        # limiter's own authentication-bound counters, and its default
+        # InMemoryCache evicts at 200 items. Without this isolation, a caller
+        # flooding this hook's own caller-controlled tag buckets past that
+        # ceiling could evict an unrelated, authentication-bound counter and
+        # exceed a limit nothing here configured. The real Redis connection
+        # (if any) is still shared, so cross-instance correctness is unaffected.
+        isolated_dual_cache: Final = DualCache(redis_cache=internal_usage_cache.redis_cache)
+        self.internal_usage_cache = InternalUsageCache(dual_cache=isolated_dual_cache)
         self._v3 = _PROXY_MaxParallelRequestsHandler_v3(self.internal_usage_cache, time_provider=time_provider)
         self._time_provider = time_provider or datetime.now
         self._index = _TagRateLimitIndex(time_provider=self._time_provider)
diff --git a/tests/test_litellm/proxy/hooks/test_tag_rate_limiter.py b/tests/test_litellm/proxy/hooks/test_tag_rate_limiter.py
index e1fcf76ab5e..c0c943c4b5f 100644
--- a/tests/test_litellm/proxy/hooks/test_tag_rate_limiter.py
+++ b/tests/test_litellm/proxy/hooks/test_tag_rate_limiter.py
@@ -2131,3 +2131,53 @@ async def test_concurrency_scope_by_key_hash_gives_independent_reservations_per_
             messages=None,
             request_kwargs={"metadata": {"tags": ["end_user_id:u1"], "user_api_key": "keyB"}},
         )
+
+
+# ---------------------------------------------------------------------------
+# in-memory cache isolation -- caller-controlled tag buckets must never evict
+# the shared cache's other, authentication-bound counters
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_flooding_tag_buckets_does_not_evict_the_shared_cache_authentication_bound_counter(
+    time_controller,
+):
+    """
+    The proxy-wide internal_usage_cache passed into this limiter is also
+    used by the key/team parallel-request limiter for its own,
+    authentication-bound counters, and its default InMemoryCache evicts at
+    200 items. Without a dedicated in-memory layer for this hook's own
+    caller-controlled tag buckets, an attacker sending 200+ distinct tag
+    values could evict an unrelated authentication-bound counter and let
+    some other caller exceed a limit nothing here configured.
+    """
+    shared_cache = DualCache()
+    await shared_cache.async_set_cache(key="authentication_bound_counter", value="do-not-evict")
+
+    limiter = _PROXY_TagRateLimiter(internal_usage_cache=shared_cache, time_provider=time_controller.now)
+    router = litellm.Router(
+        model_list=[
+            _deployment(
+                "grp",
+                "dep-1",
+                {
+                    "request_limits": {
+                        "limits": [{"name": "per_minute", "tag_id": "end_user_id", "limit": 1000, "period_seconds": 60}]
+                    }
+                },
+            )
+        ]
+    )
+    limiter.update_variables(llm_router=router)
+    healthy = router.model_list
+
+    for i in range(250):
+        await limiter.async_filter_deployments(
+            model="grp",
+            healthy_deployments=healthy,
+            messages=None,
+            request_kwargs={"metadata": {"tags": [f"end_user_id:flood-{i}"]}},
+        )
+
+    assert await shared_cache.async_get_cache(key="authentication_bound_counter") == "do-not-evict"