diff --git a/litellm/llms/bedrock_mantle/responses/transformation.py b/litellm/llms/bedrock_mantle/responses/transformation.py
index 326233fd24b..769226c8b4d 100644
--- a/litellm/llms/bedrock_mantle/responses/transformation.py
+++ b/litellm/llms/bedrock_mantle/responses/transformation.py
@@ -5,7 +5,7 @@ Routes /v1/responses to the provider's native Responses endpoint instead of
 the chat-completions translation fallback.
 """
 
-from typing import Optional
+from typing import Optional, cast
 
 from litellm.llms.bedrock_mantle.chat.transformation import BedrockMantleChatConfig
 from litellm.llms.openai.responses.transformation import OpenAIResponsesAPIConfig
@@ -45,17 +45,16 @@ class BedrockMantleResponsesAPIConfig(OpenAIResponsesAPIConfig):
             api_base=api_base or litellm_params.get("api_base"),
             api_key=litellm_params.get("api_key"),
         )
-        if not resolved_api_base:
-            raise ValueError(
-                "api_base is required for bedrock_mantle Responses API. "
-                "Set BEDROCK_MANTLE_API_BASE or BEDROCK_MANTLE_REGION."
-            )
-        api_base = resolved_api_base.rstrip("/")
+        api_base = cast(str, resolved_api_base).rstrip("/")
         if api_base.endswith("/responses"):
             return api_base
+        if api_base.endswith("/openai/v1"):
+            return f"{api_base}/responses"
+        if api_base.endswith("/openai"):
+            return f"{api_base}/v1/responses"
         if api_base.endswith("/v1"):
             return f"{api_base}/responses"
-        return f"{api_base}/v1/responses"
+        return f"{api_base}/openai/v1/responses"
 
     def supports_native_websocket(self) -> bool:
         return False
diff --git a/tests/test_litellm/llms/bedrock_mantle/test_bedrock_mantle_transformation.py b/tests/test_litellm/llms/bedrock_mantle/test_bedrock_mantle_transformation.py
index 729dd2d6514..68fb4a27c2d 100644
--- a/tests/test_litellm/llms/bedrock_mantle/test_bedrock_mantle_transformation.py
+++ b/tests/test_litellm/llms/bedrock_mantle/test_bedrock_mantle_transformation.py
@@ -107,6 +107,14 @@ class TestBedrockMantleResponsesConfig:
         url = cfg.get_complete_url(api_base=None, litellm_params={})
         assert url == "https://bedrock-mantle.us-east-1.api.aws/openai/v1/responses"
 
+    def test_responses_complete_url_bare_api_base(self, monkeypatch):
+        monkeypatch.setenv(
+            "BEDROCK_MANTLE_API_BASE", "https://bedrock-mantle.us-east-1.api.aws"
+        )
+        cfg = BedrockMantleResponsesAPIConfig()
+        url = cfg.get_complete_url(api_base=None, litellm_params={})
+        assert url == "https://bedrock-mantle.us-east-1.api.aws/openai/v1/responses"
+
     def test_provider_config_manager_registers_responses(self):
         cfg = ProviderConfigManager.get_provider_responses_api_config(
             provider="bedrock_mantle",