fix(anthropic_messages): only Mantle consumes get_llm_provider's api_base

The /v1/messages handler passed the api_base get_llm_provider resolved to every
provider's native messages config, which shadowed DEEPSEEK_ANTHROPIC_API_BASE and
TENCENT_ANTHROPIC_API_BASE with the chat default and changed the azure_ai
precedence. Messages configs now opt in through uses_get_llm_provider_api_base(),
true only for Bedrock Mantle, whose region-prefixed model must resolve to a
region host before the prefix is stripped. Also registers
BedrockMantleAnthropicMessagesConfig in the lazy import registry.

litellm/__init__.py

@@ -1684,6 +1684,9 @@ if TYPE_CHECKING:
     from .llms.bedrock.messages.mantle_transformation import (
         AmazonMantleMessagesConfig as AmazonMantleMessagesConfig,
     )
+    from .llms.bedrock_mantle.messages.transformation import (
+        BedrockMantleAnthropicMessagesConfig as BedrockMantleAnthropicMessagesConfig,
+    )
     from .llms.together_ai.chat import TogetherAIConfig as TogetherAIConfig
     from .llms.together_ai.chat.transformation import (
         TogetherAIChatConfig as TogetherAIChatConfig,

litellm/_lazy_imports_registry.py

@@ -176,6 +176,7 @@ LLM_CONFIG_NAMES: Final = (
     "BedrockClaudePlatformMessagesConfig",
     "AmazonAnthropicClaudeMessagesConfig",
     "AmazonMantleMessagesConfig",
+    "BedrockMantleAnthropicMessagesConfig",
     "TogetherAIConfig",
     "TogetherAIChatConfig",
     "NLPCloudConfig",
@@ -746,6 +747,10 @@ _LLM_CONFIGS_IMPORT_MAP: Final = {
         ".llms.bedrock.messages.mantle_transformation",
         "AmazonMantleMessagesConfig",
     ),
+    "BedrockMantleAnthropicMessagesConfig": (
+        ".llms.bedrock_mantle.messages.transformation",
+        "BedrockMantleAnthropicMessagesConfig",
+    ),
     "TogetherAIConfig": (".llms.together_ai.chat", "TogetherAIConfig"),
     "TogetherAIChatConfig": (
         ".llms.together_ai.chat.transformation",

litellm/llms/anthropic/experimental_pass_through/messages/handler.py

@@ -501,7 +501,6 @@ def anthropic_messages_handler(
         api_base=litellm_params.api_base,
         api_key=litellm_params.api_key,
     )
-    resolved_api_base: Final = dynamic_api_base if dynamic_api_base is not None else api_base
 
     # Store agentic loop params in logging object for agentic hooks
     # This provides original request context needed for follow-up calls
@@ -652,6 +651,11 @@ def anthropic_messages_handler(
                 "display": "summarized",
             }
 
+    resolved_api_base: Final = (
+        dynamic_api_base
+        if dynamic_api_base is not None and anthropic_messages_provider_config.uses_get_llm_provider_api_base()
+        else api_base
+    )
     return base_llm_http_handler.anthropic_messages_handler(
         model=model,
         messages=strip_provider_specific_fields_from_anthropic_messages(messages),

litellm/llms/base_llm/anthropic_messages/transformation.py

@@ -128,6 +128,9 @@ class BaseAnthropicMessagesConfig(ABC):
         """
         return True
 
+    def uses_get_llm_provider_api_base(self) -> bool:
+        return False
+
     def get_async_streaming_response_iterator(
         self,
         model: str,

litellm/llms/bedrock_mantle/messages/transformation.py

@@ -61,6 +61,9 @@ class BedrockMantleAnthropicMessagesConfig(BedrockMantleAuthMixin, AmazonMantleM
     def custom_llm_provider(self) -> str | None:
         return "bedrock_mantle"
 
+    def uses_get_llm_provider_api_base(self) -> bool:
+        return True
+
     def get_complete_url(
         self,
         api_base: str | None,

tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_experimental_pass_through_messages_handler.py

@@ -1438,3 +1438,45 @@ async def test_anthropic_messages_leaves_non_provider_failures_unmapped():
         )
 
     assert "Traceback" not in str(excinfo.value)
+
+
+def _recording_client(seen_urls: list[str]) -> AsyncHTTPHandler:
+    def record_and_answer(request: httpx.Request) -> httpx.Response:
+        seen_urls.append(str(request.url))
+        return httpx.Response(
+            200,
+            json={
+                "id": "msg_test",
+                "type": "message",
+                "role": "assistant",
+                "model": "deepseek-chat",
+                "content": [{"type": "text", "text": "pong"}],
+                "stop_reason": "end_turn",
+                "stop_sequence": None,
+                "usage": {"input_tokens": 3, "output_tokens": 1},
+            },
+        )
+
+    upstream = AsyncHTTPHandler()
+    upstream.client = httpx.AsyncClient(transport=httpx.MockTransport(record_and_answer))
+    return upstream
+
+
+@pytest.mark.asyncio
+async def test_provider_messages_api_base_env_is_not_shadowed_by_the_chat_default(monkeypatch):
+    from litellm.llms.anthropic.experimental_pass_through.messages import handler
+
+    monkeypatch.delenv("DEEPSEEK_API_BASE", raising=False)
+    monkeypatch.setenv("DEEPSEEK_ANTHROPIC_API_BASE", "https://deepseek.internal.example/anthropic")
+    seen_urls: list[str] = []
+
+    await handler.anthropic_messages(
+        max_tokens=16,
+        messages=[{"role": "user", "content": "ping"}],
+        model="deepseek/deepseek-chat",
+        api_key="sk-test",
+        client=_recording_client(seen_urls),
+    )
+
+    assert seen_urls == ["https://deepseek.internal.example/anthropic/v1/messages"]
+