diff --git a/litellm/proxy/common_request_processing.py b/litellm/proxy/common_request_processing.py
index 97f7d51970cd..f4d0cb52af6f 100644
--- a/litellm/proxy/common_request_processing.py
+++ b/litellm/proxy/common_request_processing.py
@@ -1427,6 +1427,30 @@ async def base_process_llm_request(
 
         response = responses[1]
 
+        # GH#30566: set overhead duration for non-chat-completions
+        # routes (/v1/messages, /v1/responses). Chat completions
+        # already have litellm_overhead_time_ms from the SDK.
+        # Only set the overhead field directly (don't call
+        # update_response_metadata which also touches cost).
+        _overhead_hidden_params = getattr(response, "_hidden_params", {}) or {}
+        if not _overhead_hidden_params.get("litellm_overhead_time_ms") and route_type not in (
+            "acompletion",
+            "completion",
+        ):
+            end_time = datetime.now()
+            _logging_obj = self.data.get("litellm_logging_obj")
+            if _logging_obj is not None and _logging_obj.start_time is not None:
+                overhead_ms = (end_time - _logging_obj.start_time).total_seconds() * 1000 - (
+                    _logging_obj.model_call_details.get("llm_api_duration_ms", 0)
+                )
+                if not isinstance(_overhead_hidden_params, dict):
+                    _overhead_hidden_params = {}
+                _overhead_hidden_params["litellm_overhead_time_ms"] = overhead_ms
+                if hasattr(response, "_hidden_params"):
+                    response._hidden_params = _overhead_hidden_params
+                elif isinstance(response, dict):
+                    response["_hidden_params"] = _overhead_hidden_params
+
         _exception_raised = False
         try:
             hidden_params = getattr(response, "_hidden_params", {}) or {}
diff --git a/tests/test_litellm/litellm_core_utils/llm_response_utils/test_response_metadata.py b/tests/test_litellm/litellm_core_utils/llm_response_utils/test_response_metadata.py
index 203c6d3da0d5..ca620dd7e7e4 100644
--- a/tests/test_litellm/litellm_core_utils/llm_response_utils/test_response_metadata.py
+++ b/tests/test_litellm/litellm_core_utils/llm_response_utils/test_response_metadata.py
@@ -8,6 +8,8 @@
 import datetime
 from unittest.mock import MagicMock
 
+import pytest
+
 import litellm.litellm_core_utils.llm_response_utils.response_metadata as response_metadata_mod
 import litellm.proxy.common_request_processing as common_request_processing_mod
 from litellm.litellm_core_utils.litellm_logging import Logging
@@ -91,6 +93,50 @@ def test_update_response_metadata_includes_callback_duration(self):
         # overhead should also be set
         assert hidden.get("litellm_overhead_time_ms") is not None
 
+    def test_overhead_computed_for_routes_without_pre_existing_value(self):
+        """GH#30566: overhead is set even when _hidden_params
+        does not already contain litellm_overhead_time_ms.
+        This simulates non-chat-completions routes that skip
+        the SDK-level update_response_metadata call."""
+        result = ModelResponse()
+        logging_obj = self._make_logging_obj(llm_api_duration_ms=900.0)
+        logging_obj._response_cost_calculator = MagicMock(return_value=0.001)
+        logging_obj.litellm_call_id = "test-gh30566"
+
+        start = datetime.datetime(2025, 1, 1, 0, 0, 0)
+        end = datetime.datetime(2025, 1, 1, 0, 0, 1)
+
+        update_response_metadata(
+            result=result,
+            logging_obj=logging_obj,
+            model="gpt-4",
+            kwargs={},
+            start_time=start,
+            end_time=end,
+        )
+
+        hidden = result._hidden_params
+        assert hidden.get("litellm_overhead_time_ms") == pytest.approx(100.0, rel=0.01)
+        assert hidden.get("_response_ms") == pytest.approx(1000.0, rel=0.01)
+
+    def test_overhead_guard_skips_when_already_present(self):
+        """GH#30566: the guard in base_process_llm_request prevents
+        calling update_response_metadata when litellm_overhead_time_ms
+        is already set by the SDK layer (e.g. /v1/chat/completions).
+        This test verifies the guard condition directly."""
+        # Chat-completions path: overhead already populated
+        result = ModelResponse()
+        result._hidden_params = {"litellm_overhead_time_ms": 50.0}
+        hidden_params = getattr(result, "_hidden_params", {}) or {}
+        should_skip = bool(hidden_params.get("litellm_overhead_time_ms"))
+        assert should_skip is True
+
+        # Non-chat path (/v1/messages, /v1/responses): no overhead yet
+        result2 = ModelResponse()
+        hidden_params2 = getattr(result2, "_hidden_params", {}) or {}
+        should_skip2 = bool(hidden_params2.get("litellm_overhead_time_ms"))
+        assert should_skip2 is False
+
 
 class TestCallbackDurationInCustomHeaders:
     """Test that callback_duration_ms flows into get_custom_headers."""