fix(bedrock-mantle): honor api_base for VPC endpoint routing on bedrock/mantle/... (#31141)

The bedrock/mantle chat and messages paths hardcoded the public Mantle host and ignored api_base, so private VPC/VPCE/GovCloud endpoints could not be used. Route URL construction through a shared helper that prefers api_base and aws_bedrock_runtime_endpoint before falling back to the regional public host.

Co-authored-by: Shivam Rawat <shivamrawat@Shivams-MacBook-Pro.local>
Co-authored-by: Cursor <cursoragent@cursor.com>

litellm/llms/bedrock/chat/mantle/transformation.py

@@ -12,6 +12,7 @@ from typing import TYPE_CHECKING, Any, List, Optional
 from litellm.llms.bedrock.chat.invoke_transformations.anthropic_claude3_transformation import (
     AmazonAnthropicClaudeConfig,
 )
+from litellm.llms.bedrock.common_utils import build_mantle_messages_url
 from litellm.types.llms.openai import AllMessageValues
 
 if TYPE_CHECKING:
@@ -21,10 +22,6 @@ if TYPE_CHECKING:
 else:
     LiteLLMLoggingObj = Any
 
-MANTLE_ENDPOINT_TEMPLATE = (
-    "https://bedrock-mantle.{region}.api.aws/anthropic/v1/messages"
-)
-
 
 class AmazonMantleConfig(AmazonAnthropicClaudeConfig):
     """
@@ -46,7 +43,13 @@ class AmazonMantleConfig(AmazonAnthropicClaudeConfig):
         stream: Optional[bool] = None,
     ) -> str:
         region = self._get_aws_region_name(optional_params=optional_params, model=model)
-        return MANTLE_ENDPOINT_TEMPLATE.format(region=region)
+        return build_mantle_messages_url(
+            api_base=api_base,
+            aws_bedrock_runtime_endpoint=optional_params.get(
+                "aws_bedrock_runtime_endpoint"
+            ),
+            region=region,
+        )
 
     def validate_environment(
         self,

litellm/llms/bedrock/common_utils.py

@@ -622,6 +622,31 @@ def strip_bedrock_throughput_suffix(model: str) -> str:
     return model
 
 
+MANTLE_MESSAGES_PATH = "/anthropic/v1/messages"
+
+
+def build_mantle_messages_url(
+    api_base: Optional[str],
+    aws_bedrock_runtime_endpoint: Optional[str],
+    region: str,
+) -> str:
+    """Build the bedrock-mantle Anthropic /messages URL.
+
+    Honors an explicit endpoint override (``api_base``, then
+    ``aws_bedrock_runtime_endpoint``) so private VPC / VPCE / GovCloud Mantle
+    endpoints are reachable; otherwise falls back to the public regional host.
+    The mantle messages path is appended unless the override already carries it,
+    so callers can pass either the host or the full messages URL.
+    """
+    override = api_base or aws_bedrock_runtime_endpoint
+    if override:
+        base = override.rstrip("/")
+        if base.endswith(MANTLE_MESSAGES_PATH):
+            return base
+        return f"{base}{MANTLE_MESSAGES_PATH}"
+    return f"https://bedrock-mantle.{region}.api.aws{MANTLE_MESSAGES_PATH}"
+
+
 def get_bedrock_base_model(model: str) -> str:
     """
     Get the base model from the given model name.

litellm/llms/bedrock/messages/mantle_transformation.py

@@ -8,6 +8,7 @@ stripping that are specific to the bedrock-mantle endpoint.
 
 from typing import TYPE_CHECKING, Any, Dict, List, Optional, Tuple
 
+from litellm.llms.bedrock.common_utils import build_mantle_messages_url
 from litellm.llms.bedrock.messages.invoke_transformations.anthropic_claude3_transformation import (
     AmazonAnthropicClaudeMessagesConfig,
 )
@@ -20,10 +21,6 @@ if TYPE_CHECKING:
 else:
     LiteLLMLoggingObj = Any
 
-MANTLE_ENDPOINT_TEMPLATE = (
-    "https://bedrock-mantle.{region}.api.aws/anthropic/v1/messages"
-)
-
 
 class AmazonMantleMessagesConfig(AmazonAnthropicClaudeMessagesConfig):
     """
@@ -43,7 +40,13 @@ class AmazonMantleMessagesConfig(AmazonAnthropicClaudeMessagesConfig):
         stream: Optional[bool] = None,
     ) -> str:
         region = self._get_aws_region_name(optional_params=optional_params, model=model)
-        return MANTLE_ENDPOINT_TEMPLATE.format(region=region)
+        return build_mantle_messages_url(
+            api_base=api_base,
+            aws_bedrock_runtime_endpoint=optional_params.get(
+                "aws_bedrock_runtime_endpoint"
+            ),
+            region=region,
+        )
 
     def validate_anthropic_messages_environment(
         self,

tests/test_litellm/llms/bedrock/test_mantle.py

@@ -125,6 +125,74 @@ def test_mantle_messages_url_construction():
     assert url == "https://bedrock-mantle.us-east-1.api.aws/anthropic/v1/messages"
 
 
+_VPC_ENDPOINT = "https://vpce-0a1b2c3d.bedrock-mantle.us-gov-west-1.vpce.amazonaws.com"
+
+
+def test_mantle_chat_url_honors_api_base_host():
+    config = AmazonMantleConfig()
+    url = config.get_complete_url(
+        api_base=_VPC_ENDPOINT,
+        api_key=None,
+        model="mantle/anthropic.claude-mythos-preview",
+        optional_params={"aws_region_name": "us-gov-west-1"},
+        litellm_params={},
+    )
+    assert url == f"{_VPC_ENDPOINT}/anthropic/v1/messages"
+
+
+def test_mantle_chat_url_honors_api_base_full_path_without_duplication():
+    config = AmazonMantleConfig()
+    full = f"{_VPC_ENDPOINT}/anthropic/v1/messages"
+    url = config.get_complete_url(
+        api_base=full,
+        api_key=None,
+        model="mantle/anthropic.claude-mythos-preview",
+        optional_params={"aws_region_name": "us-gov-west-1"},
+        litellm_params={},
+    )
+    assert url == full
+
+
+def test_mantle_messages_url_honors_api_base_host():
+    config = AmazonMantleMessagesConfig()
+    url = config.get_complete_url(
+        api_base=_VPC_ENDPOINT,
+        api_key=None,
+        model="mantle/anthropic.claude-mythos-preview",
+        optional_params={"aws_region_name": "us-gov-west-1"},
+        litellm_params={},
+    )
+    assert url == f"{_VPC_ENDPOINT}/anthropic/v1/messages"
+    assert "api.aws" not in url
+
+
+def test_mantle_messages_url_honors_api_base_with_trailing_slash():
+    config = AmazonMantleMessagesConfig()
+    url = config.get_complete_url(
+        api_base=f"{_VPC_ENDPOINT}/",
+        api_key=None,
+        model="mantle/anthropic.claude-mythos-preview",
+        optional_params={"aws_region_name": "us-gov-west-1"},
+        litellm_params={},
+    )
+    assert url == f"{_VPC_ENDPOINT}/anthropic/v1/messages"
+
+
+def test_mantle_messages_url_honors_aws_bedrock_runtime_endpoint():
+    config = AmazonMantleMessagesConfig()
+    url = config.get_complete_url(
+        api_base=None,
+        api_key=None,
+        model="mantle/anthropic.claude-mythos-preview",
+        optional_params={
+            "aws_region_name": "us-gov-west-1",
+            "aws_bedrock_runtime_endpoint": _VPC_ENDPOINT,
+        },
+        litellm_params={},
+    )
+    assert url == f"{_VPC_ENDPOINT}/anthropic/v1/messages"
+
+
 def test_mantle_transform_request_strips_prefix_and_adds_model():
     config = AmazonMantleConfig()
     request = config.transform_request(
@@ -247,3 +315,35 @@ async def test_mantle_anthropic_messages_sends_workspace_header_and_clean_body()
     assert requests[0]["path"] == "/anthropic/v1/messages"
     assert requests[0]["headers"]["anthropic-workspace"] == "proj_abc123def456"
     assert "aws_bedrock_project_id" not in requests[0]["body"]
+
+
+@pytest.mark.asyncio
+async def test_mantle_anthropic_messages_routes_to_vpc_api_base():
+    import litellm
+
+    urls = []
+
+    async def mock_post(self, url, data=None, headers=None, **kwargs):
+        urls.append(str(url))
+        return _anthropic_response(str(url))
+
+    try:
+        with patch(
+            "litellm.llms.custom_httpx.http_handler.AsyncHTTPHandler.post",
+            new=mock_post,
+        ):
+            await litellm.anthropic_messages(
+                model="bedrock/mantle/anthropic.claude-mythos-preview",
+                messages=[{"role": "user", "content": "hello"}],
+                max_tokens=10,
+                api_base=_VPC_ENDPOINT,
+                aws_access_key_id="fake-key",
+                aws_secret_access_key="fake-secret",
+                aws_region_name="us-gov-west-1",
+            )
+    finally:
+        await litellm.close_litellm_async_clients()
+
+    assert len(urls) == 1
+    assert urls[0] == f"{_VPC_ENDPOINT}/anthropic/v1/messages"
+    assert "api.aws" not in urls[0]