diff --git a/litellm/litellm_core_utils/response_header_helpers.py b/litellm/litellm_core_utils/response_header_helpers.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/litellm/utils.py b/litellm/utils.py
index 559e91203..8b257cc42 100644
--- a/litellm/utils.py
+++ b/litellm/utils.py
@@ -1002,6 +1002,10 @@ def client(original_function):
                 result._hidden_params["response_cost"] = (
                     logging_obj._response_cost_calculator(result=result)
                 )
+
+                result._hidden_params["additional_headers"] = process_response_headers(
+                    result._hidden_params.get("additional_headers") or {}
+                )  # GUARANTEE OPENAI HEADERS IN RESPONSE
             result._response_ms = (
                 end_time - start_time
             ).total_seconds() * 1000  # return response latency in ms like openai
@@ -1410,6 +1414,9 @@ def client(original_function):
                 result._hidden_params["response_cost"] = (
                     logging_obj._response_cost_calculator(result=result)
                 )
+                result._hidden_params["additional_headers"] = process_response_headers(
+                    result._hidden_params.get("additional_headers") or {}
+                )  # GUARANTEE OPENAI HEADERS IN RESPONSE
             if (
                 isinstance(result, ModelResponse)
                 or isinstance(result, EmbeddingResponse)
@@ -6394,24 +6401,9 @@ class CustomStreamWrapper:
             "model_id": (_model_info.get("id", None)),
         }  # returned as x-litellm-model-id response header in proxy
 
-        if _response_headers is not None:
-            openai_headers = {}
-            processed_headers = {}
-            additional_headers = {}
-            for k, v in _response_headers.items():
-                if k in OPENAI_RESPONSE_HEADERS:  # return openai-compatible headers
-                    openai_headers[k] = v
-                if k.startswith(
-                    "llm_provider-"
-                ):  # return raw provider headers (incl. openai-compatible ones)
-                    processed_headers[k] = v
-                else:
-                    additional_headers["{}-{}".format("llm_provider", k)] = v
-            self._hidden_params["additional_headers"] = {
-                **openai_headers,
-                **processed_headers,
-                **additional_headers,
-            }
+        self._hidden_params["additional_headers"] = process_response_headers(
+            _response_headers or {}
+        )  # GUARANTEE OPENAI HEADERS IN RESPONSE
 
         self._response_headers = _response_headers
         self.response_id = None
@@ -9263,3 +9255,29 @@ def has_tool_call_blocks(messages: List[AllMessageValues]) -> bool:
         if message.get("tool_calls") is not None:
             return True
     return False
+
+
+def process_response_headers(response_headers: Union[httpx.Headers, dict]) -> dict:
+    openai_headers = {}
+    processed_headers = {}
+    additional_headers = {}
+    for k, v in response_headers.items():
+        if k in OPENAI_RESPONSE_HEADERS:  # return openai-compatible headers
+            openai_headers[k] = v
+        if k.startswith(
+            "llm_provider-"
+        ):  # return raw provider headers (incl. openai-compatible ones)
+            processed_headers[k] = v
+        else:
+            additional_headers["{}-{}".format("llm_provider", k)] = v
+    ## GUARANTEE OPENAI HEADERS IN RESPONSE
+    for item in OPENAI_RESPONSE_HEADERS:
+        if item not in openai_headers:
+            openai_headers[item] = None
+
+    additional_headers = {
+        **openai_headers,
+        **processed_headers,
+        **additional_headers,
+    }
+    return additional_headers
diff --git a/tests/local_testing/test_completion.py b/tests/local_testing/test_completion.py
index f589bc2d4..20783f6a8 100644
--- a/tests/local_testing/test_completion.py
+++ b/tests/local_testing/test_completion.py
@@ -4547,7 +4547,12 @@ async def test_completion_ai21_chat():
 
 @pytest.mark.parametrize(
     "model",
-    ["gpt-4o", "azure/chatgpt-v-2", "claude-3-sonnet-20240229"],  #
+    [
+        "gpt-4o",
+        "azure/chatgpt-v-2",
+        "claude-3-sonnet-20240229",
+        "fireworks_ai/mixtral-8x7b-instruct",
+    ],
 )
 @pytest.mark.parametrize(
     "stream",