diff --git a/litellm/main.py b/litellm/main.py
index 2ace46a16fb8..96f1d2aded92 100644
--- a/litellm/main.py
+++ b/litellm/main.py
@@ -5343,6 +5343,15 @@ def completion(  # type: ignore
                 else:
                     optional_params["reasoning_effort"] = {"summary": rs_val}
 
+            # get_litellm_params() above is not passed **kwargs, so
+            # OPTIONAL_KWARGS_KEYS (e.g. AWS SigV4 aws_region_name /
+            # aws_access_key_id / aws_secret_access_key) never made it into
+            # litellm_params. Merge them in on the bridge path so the downstream
+            # responses() call can sign Bedrock Mantle requests with real creds.
+            for _param, _value in _supplemental_provider_params.items():
+                if litellm_params.get(_param) is None:
+                    litellm_params[_param] = _value
+
             return responses_api_bridge.completion(  # pyright: ignore[reportReturnType]  # bridge returns a coroutine on the acompletion path; awaited by the async caller
                 model=model,
                 messages=messages,
diff --git a/tests/test_litellm/completion_extras/test_responses_bridge_provider_propagation.py b/tests/test_litellm/completion_extras/test_responses_bridge_provider_propagation.py
index 8036c72679e8..30b9fbc589d8 100644
--- a/tests/test_litellm/completion_extras/test_responses_bridge_provider_propagation.py
+++ b/tests/test_litellm/completion_extras/test_responses_bridge_provider_propagation.py
@@ -151,3 +151,45 @@ async def _fake_aresponses(**kwargs):
         except Exception:
             pass
         assert _fake_aresponses.kwargs.get("aws_region_name") == "us-east-2"
+
+
+def test_completion_forwards_aws_creds_into_bridge_litellm_params():
+    """Regression test for https://github.com/BerriAI/litellm/issues/32336 -
+    litellm.completion() dropped aws_region_name / aws_access_key_id /
+    aws_secret_access_key before dispatching the chat->responses bridge for
+    bedrock_mantle gpt-5.* (Responses-only) models, so SigV4 signed with empty
+    creds and Bedrock Mantle rejected the request.
+
+    get_litellm_params() only extracts OPTIONAL_KWARGS_KEYS from its own
+    **kwargs, which completion() does not forward; the fix merges the
+    supplemental provider params back into litellm_params. This pins that the
+    AWS credentials survive into the litellm_params the bridge receives — the
+    handler-level tests above only cover forwarding once the params are already
+    present.
+    """
+    import litellm
+
+    captured = {}
+
+    def _fake_bridge_completion(**kwargs):
+        captured.update(kwargs)
+        return MagicMock(spec=[])
+
+    with patch(
+        "litellm.completion_extras.responses_api_bridge.completion",
+        _fake_bridge_completion,
+    ):
+        litellm.completion(
+            model="bedrock_mantle/openai.gpt-5.4",
+            messages=[{"role": "user", "content": "hi"}],
+            api_base="https://bedrock-mantle.us-east-2.api.aws/v1",
+            aws_region_name="us-east-2",
+            aws_access_key_id="AKIAEXAMPLE",
+            aws_secret_access_key="secretexample",
+        )
+
+    assert captured, "responses_api_bridge.completion was never called"
+    litellm_params = captured.get("litellm_params") or {}
+    assert litellm_params.get("aws_region_name") == "us-east-2"
+    assert litellm_params.get("aws_access_key_id") == "AKIAEXAMPLE"
+    assert litellm_params.get("aws_secret_access_key") == "secretexample"