diff --git a/litellm/proxy/guardrails/guardrail_endpoints.py b/litellm/proxy/guardrails/guardrail_endpoints.py
index 70e7aac12b5..1d1174d72c3 100644
--- a/litellm/proxy/guardrails/guardrail_endpoints.py
+++ b/litellm/proxy/guardrails/guardrail_endpoints.py
@@ -2331,6 +2331,26 @@ async def test_custom_code_guardrail(
         )
 
 
+_GUARDRAIL_METADATA_ADAPTER: Final = TypeAdapter(Mapping[str, object])
+
+
+def _metadata_fields(value: object) -> Mapping[str, object]:
+    try:
+        return _GUARDRAIL_METADATA_ADAPTER.validate_python(value)
+    except ValidationError:
+        return {}
+
+
+def _guardrail_request_metadata(caller: object, proxy: object) -> Mapping[str, object]:
+    caller_fields: Final = tuple(
+        (key, value) for key, value in _metadata_fields(caller).items() if not key.startswith("user_api_key_")
+    )
+    key_identity: Final = tuple(
+        (key, value) for key, value in _metadata_fields(proxy).items() if key.startswith("user_api_key_")
+    )
+    return dict((*caller_fields, *key_identity))
+
+
 def _execution_timeout_response(timeout: float) -> TestCustomCodeGuardrailResponse:
     return TestCustomCodeGuardrailResponse(
         success=False,
@@ -2493,9 +2513,16 @@ async def apply_guardrail(
         if litellm_logging_obj is not None:
             _patch_logging_obj_for_guardrail(litellm_logging_obj, request)
 
+        metadata: Final = _guardrail_request_metadata(
+            request.metadata, _GUARDRAIL_METADATA_ADAPTER.validate_python(data).get("metadata")
+        )
         request_data: Final[dict] = {
-            **({"messages": request.messages} if request.messages is not None else {}),
-            **({"metadata": request.metadata} if request.metadata is not None else {}),
+            key: value
+            for key, value in (
+                ("messages", request.messages),
+                ("metadata", metadata if request.metadata is not None or metadata else None),
+            )
+            if value is not None
         }
         _input_type: Final = _resolve_guardrail_input_type(active_guardrail, request.input_type)
         guardrailed_inputs: Final = await active_guardrail.apply_guardrail(
diff --git a/tests/unit/proxy/guardrails/test_guardrail_endpoints.py b/tests/unit/proxy/guardrails/test_guardrail_endpoints.py
index 6807a4fb569..a22c30cacc0 100644
--- a/tests/unit/proxy/guardrails/test_guardrail_endpoints.py
+++ b/tests/unit/proxy/guardrails/test_guardrail_endpoints.py
@@ -2167,7 +2167,7 @@ async def test_apply_guardrail_invokes_logging_pipeline(mocker):
     }
 
 
-def _patch_apply_guardrail_env(mocker, guardrail_result):
+def _patch_apply_guardrail_env(mocker, guardrail_result, processed_data=None):
     mock_guardrail = mocker.Mock()
     mock_guardrail.apply_guardrail = AsyncMock(return_value=guardrail_result)
 
@@ -2180,7 +2180,7 @@ def _patch_apply_guardrail_env(mocker, guardrail_result):
     mock_logging_obj.model_call_details = {}
     mock_processor = mocker.Mock()
     mock_processor.common_processing_pre_call_logic = AsyncMock(
-        return_value=({"guardrail_name": "test-guardrail"}, mock_logging_obj)
+        return_value=(processed_data or {"guardrail_name": "test-guardrail"}, mock_logging_obj)
     )
     mocker.patch(
         "litellm.proxy.common_request_processing.ProxyBaseLLMRequestProcessing",
@@ -2251,6 +2251,68 @@ async def test_apply_guardrail_forwards_metadata_and_messages_together(mocker):
     )
 
 
+@pytest.mark.asyncio
+async def test_apply_guardrail_replaces_caller_identity_with_the_authenticated_key(mocker):
+    """Identity fields come from the proxy's own sanitized metadata, never from
+    the caller, so a request cannot impersonate another key or probe its policy."""
+    mock_guardrail = _patch_apply_guardrail_env(
+        mocker,
+        {"texts": ["ok"]},
+        processed_data={
+            "guardrail_name": "test-guardrail",
+            "metadata": {
+                "route": "/apply_guardrail",
+                "user_api_key_alias": "billing-app",
+                "user_api_key_user_id": "u-1",
+            },
+        },
+    )
+
+    request = ApplyGuardrailRequest(
+        guardrail_name="test-guardrail",
+        text="hello",
+        metadata={
+            "forbidden_topics": ["tax"],
+            "user_api_key_alias": "someone-else",
+            "user_api_key_user_email": "victim@example.com",
+        },
+    )
+    await apply_guardrail(
+        fastapi_request=mocker.Mock(),
+        request=request,
+        user_api_key_dict=UserAPIKeyAuth(key_alias="billing-app", user_id="u-1"),
+    )
+
+    forwarded = mock_guardrail.apply_guardrail.await_args.kwargs["request_data"]["metadata"]
+    assert forwarded == {
+        "forbidden_topics": ["tax"],
+        "user_api_key_alias": "billing-app",
+        "user_api_key_user_id": "u-1",
+    }
+
+
+@pytest.mark.asyncio
+async def test_apply_guardrail_attaches_key_identity_when_caller_sends_no_metadata(mocker):
+    """A caller that sends no metadata still gets the authenticated identity
+    forwarded, the same shape every LLM route gives a guardrail."""
+    mock_guardrail = _patch_apply_guardrail_env(
+        mocker,
+        {"texts": ["ok"]},
+        processed_data={"guardrail_name": "test-guardrail", "metadata": {"user_api_key_alias": "billing-app"}},
+    )
+
+    request = ApplyGuardrailRequest(guardrail_name="test-guardrail", text="hello")
+    await apply_guardrail(
+        fastapi_request=mocker.Mock(),
+        request=request,
+        user_api_key_dict=UserAPIKeyAuth(key_alias="billing-app"),
+    )
+
+    assert mock_guardrail.apply_guardrail.await_args.kwargs["request_data"] == {
+        "metadata": {"user_api_key_alias": "billing-app"}
+    }
+
+
 @pytest.mark.asyncio
 async def test_apply_guardrail_omits_metadata_when_not_sent(mocker):
     """Without metadata, request_data stays empty (backward-compatible)."""