diff --git a/litellm/llms/bedrock_mantle/responses/transformation.py b/litellm/llms/bedrock_mantle/responses/transformation.py
index 1451ac6abbf..2332cbc6b08 100644
--- a/litellm/llms/bedrock_mantle/responses/transformation.py
+++ b/litellm/llms/bedrock_mantle/responses/transformation.py
@@ -97,12 +97,8 @@ class BedrockMantleResponsesAPIConfig(OpenAIResponsesAPIConfig):
             or get_secret_str("BEDROCK_MANTLE_API_KEY")
             or get_secret_str("AWS_BEARER_TOKEN_BEDROCK")
         )
-        if not api_key:
-            raise ValueError(
-                "Bedrock Mantle API key is required. Set BEDROCK_MANTLE_API_KEY "
-                "(or AWS_BEARER_TOKEN_BEDROCK) or pass api_key."
-            )
-        headers["Authorization"] = f"Bearer {api_key}"
+        if api_key:
+            headers["Authorization"] = f"Bearer {api_key}"
         return headers
 
     def supports_native_file_search(self) -> bool:
diff --git a/tests/test_litellm/llms/bedrock_mantle/test_bedrock_mantle_responses_transformation.py b/tests/test_litellm/llms/bedrock_mantle/test_bedrock_mantle_responses_transformation.py
index aaf53a44d37..31cbe44daaf 100644
--- a/tests/test_litellm/llms/bedrock_mantle/test_bedrock_mantle_responses_transformation.py
+++ b/tests/test_litellm/llms/bedrock_mantle/test_bedrock_mantle_responses_transformation.py
@@ -114,16 +114,15 @@ class TestBedrockMantleResponsesAuth:
         )
         assert headers["Authorization"] == "Bearer bearer-key"
 
-    def test_missing_key_raises(self, monkeypatch):
+    def test_missing_bearer_does_not_raise_in_validate_environment(self, monkeypatch):
+        # SigV4 may still apply, so validate_environment must defer instead of raising.
         monkeypatch.delenv("BEDROCK_MANTLE_API_KEY", raising=False)
         monkeypatch.delenv("AWS_BEARER_TOKEN_BEDROCK", raising=False)
         cfg = BedrockMantleResponsesAPIConfig()
-        with pytest.raises(ValueError, match="Bedrock Mantle API key"):
-            cfg.validate_environment(
-                headers={},
-                model="openai.gpt-5.5",
-                litellm_params=GenericLiteLLMParams(),
-            )
+        headers = cfg.validate_environment(
+            headers={}, model="openai.gpt-5.5", litellm_params=GenericLiteLLMParams()
+        )
+        assert "Authorization" not in headers
 
     def test_custom_llm_provider(self):
         cfg = BedrockMantleResponsesAPIConfig()
@@ -530,6 +529,30 @@ class TestBedrockMantleResponsesSigV4:
         assert headers["Authorization"].startswith("AWS4-HMAC-SHA256")
         assert "Bearer stale-caller-token" not in headers["Authorization"]
 
+    def test_no_bearer_and_no_credentials_raises_both_paths(self, monkeypatch):
+        from unittest.mock import MagicMock
+        from botocore.exceptions import NoCredentialsError
+        from litellm.llms.bedrock.base_aws_llm import BaseAWSLLM
+
+        monkeypatch.delenv("BEDROCK_MANTLE_API_KEY", raising=False)
+        monkeypatch.delenv("AWS_BEARER_TOKEN_BEDROCK", raising=False)
+
+        signer = BaseAWSLLM()
+        signer.get_credentials = MagicMock(side_effect=NoCredentialsError())
+        cfg = BedrockMantleResponsesAPIConfig(aws_signer=signer)
+
+        with pytest.raises(ValueError) as exc:
+            cfg.sign_request(
+                headers={},
+                optional_params={"aws_region_name": "us-east-2"},
+                request_data={"input": "hi"},
+                api_base="https://bedrock-mantle.us-east-2.api.aws/openai/v1/responses",
+                api_key=None,
+            )
+        msg = str(exc.value)
+        assert "Bearer" in msg
+        assert "SigV4" in msg or "IAM" in msg
+
 
 class TestBedrockMantleResponsesPricing:
     def test_gpt_5_5_pricing_and_mode(self, local_cost_map):