diff --git a/litellm/litellm_core_utils/litellm_logging.py b/litellm/litellm_core_utils/litellm_logging.py
index d9d63681594..75b75f9421a 100644
--- a/litellm/litellm_core_utils/litellm_logging.py
+++ b/litellm/litellm_core_utils/litellm_logging.py
@@ -60,9 +60,6 @@ from litellm.integrations.deepeval.deepeval import DeepEvalLogger
 from litellm.integrations.mlflow import MlflowLogger
 from litellm.integrations.sqs import SQSLogger
 from litellm.litellm_core_utils.core_helpers import reconstruct_model_name
-from litellm.litellm_core_utils.llm_response_utils.response_metadata import (
-    get_response_hidden_params,
-)
 from litellm.litellm_core_utils.get_litellm_params import get_litellm_params
 from litellm.litellm_core_utils.llm_cost_calc.tool_call_cost_tracking import (
     StandardBuiltInToolCostTracking,
@@ -1705,7 +1702,17 @@ class Logging(LiteLLMLoggingBaseClass):
         """
         if logging_result is None:
             return
-        incoming = get_response_hidden_params(logging_result)
+        from litellm.litellm_core_utils.llm_response_utils.response_metadata import (
+            get_response_hidden_params,
+            hidden_params_to_plain_dict,
+        )
+
+        incoming_raw = get_response_hidden_params(logging_result)
+        incoming = hidden_params_to_plain_dict(incoming_raw)
+        if not incoming:
+            return
+        # Do not let explicit None values from model_dump wipe proxy-injected timing keys.
+        incoming = {k: v for k, v in incoming.items() if v is not None}
         if not incoming:
             return
         if self.model_call_details.get("litellm_params") is None:
@@ -1719,9 +1726,6 @@ class Logging(LiteLLMLoggingBaseClass):
         existing = md.get("hidden_params")
         if not isinstance(existing, dict):
             existing = {}
-        if not isinstance(incoming, dict):
-            md["hidden_params"] = existing or incoming
-            return
         md["hidden_params"] = {**existing, **incoming}
 
     def _process_hidden_params_and_response_cost(
@@ -1730,18 +1734,20 @@ class Logging(LiteLLMLoggingBaseClass):
         start_time,
         end_time,
     ):
-        hidden_params = get_response_hidden_params(logging_result)
-        if hidden_params:
-            if self.model_call_details.get("litellm_params") is not None:
-                self.model_call_details["litellm_params"].setdefault("metadata", {})
-                if self.model_call_details["litellm_params"]["metadata"] is None:
-                    self.model_call_details["litellm_params"]["metadata"] = {}
-                self.model_call_details["litellm_params"]["metadata"]["hidden_params"] = get_response_hidden_params(logging_result)  # type: ignore
+        from litellm.litellm_core_utils.llm_response_utils.response_metadata import (
+            get_response_hidden_params,
+            hidden_params_to_plain_dict,
+        )
+
+        hp_raw = get_response_hidden_params(logging_result)
+        if hp_raw:
+            self._merge_hidden_params_from_response_into_metadata(logging_result)
+        hp_dict = hidden_params_to_plain_dict(hp_raw) if hp_raw else {}
 
         if self.model_call_details.get("cache_hit") is True:
             self.model_call_details["response_cost"] = 0.0
-        elif "response_cost" in hidden_params:
-            self.model_call_details["response_cost"] = hidden_params["response_cost"]
+        elif "response_cost" in hp_dict:
+            self.model_call_details["response_cost"] = hp_dict["response_cost"]
         elif self.model_call_details.get("response_cost") is not None:
             # Preserve response_cost if already calculated (e.g., by pass-through
             # handlers like Gemini/Vertex which call completion_cost directly)
@@ -5316,6 +5322,10 @@ def _extract_response_obj_and_hidden_params(
         hidden_params = getattr(init_response_obj, "_hidden_params", None)
     elif isinstance(init_response_obj, dict):
         response_obj = init_response_obj
+        from litellm.litellm_core_utils.llm_response_utils.response_metadata import (
+            get_response_hidden_params,
+        )
+
         hp = get_response_hidden_params(init_response_obj)
         if hp:
             hidden_params = (
diff --git a/litellm/litellm_core_utils/llm_response_utils/response_metadata.py b/litellm/litellm_core_utils/llm_response_utils/response_metadata.py
index ffc981448f7..e72dd16bc96 100644
--- a/litellm/litellm_core_utils/llm_response_utils/response_metadata.py
+++ b/litellm/litellm_core_utils/llm_response_utils/response_metadata.py
@@ -33,6 +33,20 @@ def get_response_hidden_params(response: Any) -> Union[HiddenParams, dict]:
     return {}
 
 
+def hidden_params_to_plain_dict(hp: Any) -> dict:
+    """
+    Normalize ``get_response_hidden_params`` output (``dict`` or ``HiddenParams``) to a
+    plain ``dict`` for merging into ``metadata['hidden_params']``.
+    """
+    if not hp:
+        return {}
+    if isinstance(hp, dict):
+        return dict(hp)
+    if hasattr(hp, "model_dump"):
+        return hp.model_dump(exclude_none=True)
+    return {}
+
+
 def strip_litellm_internal_keys_from_dict_response(response: Any) -> None:
     """Remove internal keys from dict API responses before JSON serialization."""
     if isinstance(response, dict):
diff --git a/litellm/proxy/common_request_processing.py b/litellm/proxy/common_request_processing.py
index dea0cb3980d..6b749886379 100644
--- a/litellm/proxy/common_request_processing.py
+++ b/litellm/proxy/common_request_processing.py
@@ -36,10 +36,6 @@ from litellm.litellm_core_utils.litellm_logging import Logging as LiteLLMLogging
 from litellm.litellm_core_utils.llm_response_utils.get_headers import (
     get_response_headers,
 )
-from litellm.litellm_core_utils.llm_response_utils.response_metadata import (
-    merge_hidden_params_with_logging_timings,
-    strip_litellm_internal_keys_from_dict_response,
-)
 from litellm.litellm_core_utils.safe_json_dumps import safe_dumps
 from litellm.proxy._types import ProxyException, UserAPIKeyAuth
 from litellm.proxy.auth.auth_utils import check_response_size_is_safe
@@ -1042,6 +1038,10 @@ class ProxyBaseLLMRequestProcessing:
 
         _exception_raised = False
         try:
+            from litellm.litellm_core_utils.llm_response_utils.response_metadata import (
+                merge_hidden_params_with_logging_timings,
+            )
+
             # Async generators (e.g. Anthropic /v1/messages SSE) have no _hidden_params;
             # merge timing from logging_obj.model_call_details (llm_api_duration_ms, etc.).
             hidden_params = merge_hidden_params_with_logging_timings(
@@ -1287,6 +1287,11 @@ class ProxyBaseLLMRequestProcessing:
                 log_context=f"litellm_call_id={logging_obj.litellm_call_id}",
             )
 
+        from litellm.litellm_core_utils.llm_response_utils.response_metadata import (
+            merge_hidden_params_with_logging_timings,
+            strip_litellm_internal_keys_from_dict_response,
+        )
+
         # Re-merge after post_call_success_hook so dict / CSW hidden_params + logging timings align.
         hidden_params = merge_hidden_params_with_logging_timings(
             response,
diff --git a/tests/test_litellm/litellm_core_utils/test_litellm_logging.py b/tests/test_litellm/litellm_core_utils/test_litellm_logging.py
index 792bbdad59a..cf5976e3849 100644
--- a/tests/test_litellm/litellm_core_utils/test_litellm_logging.py
+++ b/tests/test_litellm/litellm_core_utils/test_litellm_logging.py
@@ -2333,6 +2333,43 @@ def test_merge_hidden_params_from_response_preserves_proxy_injected_timing():
     assert hp["model_id"] == "mid-2"
 
 
+def test_merge_hidden_params_from_response_preserves_proxy_injected_timing_with_hidden_params_model():
+    """Assembled streaming response may use HiddenParams object; merge must not drop fields."""
+    from litellm.types.llms.base import HiddenParams
+
+    from litellm.litellm_core_utils.litellm_logging import Logging as LiteLLMLoggingObj
+
+    logging_obj = LiteLLMLoggingObj(
+        model="claude-3-5-sonnet",
+        messages=[{"role": "user", "content": "hi"}],
+        stream=True,
+        call_type="anthropic_messages",
+        start_time=time.time(),
+        litellm_call_id="merge-hp-hp-model",
+        function_id="merge-hp-hp-model-fn",
+    )
+    logging_obj.model_call_details = {
+        "litellm_params": {
+            "metadata": {
+                "hidden_params": {
+                    "litellm_overhead_time_ms": 273.329,
+                    "_response_ms": 1590.0,
+                }
+            }
+        },
+    }
+
+    class _Assembled:
+        _hidden_params = HiddenParams(response_cost=0.001, model_id="mid-2")
+
+    logging_obj._merge_hidden_params_from_response_into_metadata(_Assembled())
+    hp = logging_obj.model_call_details["litellm_params"]["metadata"]["hidden_params"]
+    assert hp["litellm_overhead_time_ms"] == 273.329
+    assert hp["_response_ms"] == 1590.0
+    assert hp["response_cost"] == 0.001
+    assert hp["model_id"] == "mid-2"
+
+
 def test_get_standard_logging_payload_includes_metadata_hidden_params_overhead():
     """Spend logs read overhead from standard_logging_object.hidden_params; merge metadata.hidden_params."""
     from datetime import datetime