fix(bedrock): keep native structured output in Mantle requests

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

litellm/llms/bedrock/chat/invoke_transformations/anthropic_claude_transformation.py

@@ -360,6 +360,7 @@ class AmazonAnthropicClaudeConfig(AmazonInvokeConfig, AnthropicConfig):
             model=model,
             request_body=anthropic_request,
             model_info=model_info,
+            custom_llm_provider=self.custom_llm_provider or "bedrock",
         )
         strip_unsupported_bedrock_invoke_output_config_keys(
             model=model,

litellm/llms/bedrock/common_utils.py

@@ -561,16 +561,16 @@ def apply_bedrock_invoke_structured_output(
     model: str,
     request_body: dict[str, object],  # mutable-ok: edited in place like siblings
     model_info: Mapping[str, object] | None = None,
+    custom_llm_provider: str = "bedrock",
 ) -> None:
     """
-    Route Anthropic structured-output params to what the Bedrock model supports.
+    Route Anthropic structured-output params to the provider's native format.
 
     Consumes the legacy top-level ``output_format`` and the newer
     ``output_config.format``, keeping the pre-existing precedence of the legacy
-    field when a request carries both. Models flagged
-    ``supports_native_structured_output`` in the model map get the schema
-    forwarded as ``output_config.format``, which Bedrock relays to the model for
-    enforced structured output. For every other model the schema is inlined into
+    field when a request carries both. Native Mantle models keep ``output_format``
+    in the Anthropic Messages body. Other native Bedrock models receive
+    ``output_config.format``. For every other model the schema is inlined into
     the last user message as best-effort text, with a warning because nothing
     enforces it.
     """
@@ -584,9 +584,13 @@ def apply_bedrock_invoke_structured_output(
     native_format: Final = bedrock_native_output_format(
         model,
         schema_format,
+        custom_llm_provider=custom_llm_provider,
         model_info=model_info,
     )
     if native_format is not None:
+        if custom_llm_provider == "bedrock_mantle":
+            request_body["output_format"] = native_format  # rebind-ok: mutate the caller's request body
+            return
         existing_output_config: Final = request_body.get("output_config")
         if isinstance(existing_output_config, dict):
             existing_output_config["format"] = native_format

tests/unit/llms/bedrock/chat/mantle/test_bedrock_chat_mantle_transformation.py

@@ -286,10 +286,10 @@ def test_mantle_claude_that_cannot_force_a_tool_keeps_json_schema_on_the_json_to
 
 @pytest.mark.usefixtures("local_model_cost_map")
 def test_mantle_uses_native_output_only_for_schemas_within_the_property_limit() -> None:
-    from litellm.llms.bedrock.chat.mantle.transformation import AmazonMantleConfig
+    from litellm.llms.bedrock_mantle.chat.claude_transformation import BedrockMantleClaudeChatConfig
 
     model: Final = "anthropic.claude-haiku-4-5"
-    config: Final = AmazonMantleConfig()
+    config: Final = BedrockMantleClaudeChatConfig()
 
     def transform(size: int) -> dict[str, object]:
         response_format: Final = {
@@ -310,11 +310,18 @@ def test_mantle_uses_native_output_only_for_schemas_within_the_property_limit()
             model=model,
             drop_params=False,
         )
-        return optional_params
+        return config.transform_request(
+            model=model,
+            messages=[{"role": "user", "content": "Return a JSON object."}],
+            optional_params=optional_params,
+            litellm_params={},
+            headers={},
+        )
 
     requests: Final = tuple(transform(size) for size in (2, 120))
     assert "output_format" in requests[0]
     assert "tools" not in requests[0]
+    assert "field_000" not in json.dumps(requests[0]["messages"])
     assert "output_format" not in requests[1]
     assert requests[1]["tool_choice"] == {"type": "tool", "name": "json_tool_call"}
     assert requests[1]["tools"][0]["name"] == "json_tool_call"