fix(anthropic): apply the background check to extra_body on the /v1/messages Responses bridge

litellm merges extra_body into the upstream Responses request, so background inside it started a background run past the top-level check. It now answers 400 without drop_params and is stripped from extra_body when dropped.

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

@@ -179,8 +179,19 @@ def _drops_background(forwarded_kwargs: Mapping[str, object]) -> bool:
     )
 
 
+def _requests_background(forwarded_kwargs: Mapping[str, object]) -> bool:
+    extra_body: Final = as_str_mapping(forwarded_kwargs.get("extra_body"))
+    return forwarded_kwargs.get("background") is not None or (
+        extra_body is not None and extra_body.get("background") is not None
+    )
+
+
+def _without_background(extra_body: Mapping[str, object]) -> Mapping[str, object]:
+    return {key: value for key, value in extra_body.items() if key != "background"}
+
+
 def _reject_background_mode(model: str, forwarded_kwargs: Mapping[str, object]) -> None:
-    if forwarded_kwargs.get("background") is None or _drops_background(forwarded_kwargs):
+    if not _requests_background(forwarded_kwargs) or _drops_background(forwarded_kwargs):
         return
     provider: Final = forwarded_kwargs.get("custom_llm_provider")
     raise litellm.UnsupportedParamsError(
@@ -396,6 +407,8 @@ def _build_responses_kwargs(
                 # Responses API result back to a ModelResponse for the row.
                 setattr(value, "call_type", CallTypes.anthropic_messages.value)
             responses_kwargs[key] = value
+        elif key == "extra_body" and (extra_body := as_str_mapping(value)) is not None:
+            responses_kwargs[key] = _without_background(extra_body)
         elif key not in excluded and key not in responses_kwargs and value is not None:
             responses_kwargs[key] = value
 

tests/unit/llms/anthropic/messages/responses_adapters/test_responses_adapters_handler.py

@@ -684,12 +684,19 @@ def test_tool_result_text_block_without_text_still_reaches_the_model_as_empty_te
     ]
 
 
-@pytest.mark.parametrize("background", [True, False])
-def test_background_answers_400_without_drop_params(background: bool) -> None:
+@pytest.mark.parametrize(
+    "background_request",
+    [
+        pytest.param({"background": True}, id="background-true"),
+        pytest.param({"background": False}, id="background-false"),
+        pytest.param({"extra_body": {"background": True}}, id="extra-body-background"),
+    ],
+)
+def test_background_answers_400_without_drop_params(background_request: Mapping[str, object]) -> None:
     with pytest.raises(litellm.UnsupportedParamsError) as rejected:
         _build_responses_kwargs(
             model="openai/gpt-5.6",
-            extra_kwargs={**OPENAI_API, "background": background},
+            extra_kwargs={**OPENAI_API, **background_request},
             max_tokens=1024,
             messages=MESSAGES,
         )
@@ -708,12 +715,18 @@ def test_background_answers_400_without_drop_params(background: bool) -> None:
 def test_dropped_background_never_reaches_the_responses_request(drop_setting: Mapping[str, object]) -> None:
     responses_kwargs: Final = _build_responses_kwargs(
         model="openai/gpt-5.6",
-        extra_kwargs={**OPENAI_API, **drop_setting, "background": True},
+        extra_kwargs={
+            **OPENAI_API,
+            **drop_setting,
+            "background": True,
+            "extra_body": {"background": True, "service_tier": "priority"},
+        },
         max_tokens=1024,
         messages=MESSAGES,
     )
 
     assert "background" not in responses_kwargs
+    assert responses_kwargs["extra_body"] == {"service_tier": "priority"}
     assert responses_kwargs["max_output_tokens"] == 1024