diff --git a/litellm/llms/anthropic/pass_through/responses_adapters/handler.py b/litellm/llms/anthropic/pass_through/responses_adapters/handler.py
index abe0f2f419d..41411503a3d 100644
--- a/litellm/llms/anthropic/pass_through/responses_adapters/handler.py
+++ b/litellm/llms/anthropic/pass_through/responses_adapters/handler.py
@@ -16,7 +16,7 @@ from litellm.integrations.anthropic_cache_control_hook import (
 )
 from litellm.litellm_core_utils.core_helpers import as_str_mapping, iter_items, normalize_drop_params
 from litellm.litellm_core_utils.prompt_templates.common_utils import cache_control_as_prompt_cache_breakpoint
-from litellm.litellm_core_utils.tool_fields import drops_mcp_servers, tools_without_mcp_servers
+from litellm.litellm_core_utils.tool_fields import drop_policy, drops_mcp_servers, tools_without_mcp_servers
 from litellm.responses.streaming_iterator import BaseResponsesAPIStreamingIterator
 from litellm.types.integrations.custom_logger import CONVERTED_STREAM_KEYS
 from litellm.types.llms.anthropic import (
@@ -60,6 +60,33 @@ def _forwarded_kwargs(extra_kwargs: Mapping[str, object] | None) -> Mapping[str,
     return extra_kwargs or {}
 
 
+def _requests_background(forwarded_kwargs: Mapping[str, object]) -> bool:
+    extra_body: Final = as_str_mapping(forwarded_kwargs.get("extra_body")) or {}
+    return any(
+        value is not None and value is not False
+        for value in (forwarded_kwargs.get("background"), extra_body.get("background"))
+    )
+
+
+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 not _requests_background(forwarded_kwargs) or drop_policy(forwarded_kwargs).drops("background"):
+        return
+    provider: Final = forwarded_kwargs.get("custom_llm_provider")
+    raise litellm.UnsupportedParamsError(
+        model=model,
+        llm_provider=provider if isinstance(provider, str) else None,
+        message=(
+            f"/v1/messages does not support parameters: ['background'], for model={model}. "
+            "A background run answers with a response id the Anthropic Messages API has no way to retrieve. "
+            "To drop it, set `litellm.drop_params=True` or for proxy:\n\n`litellm_settings:\n drop_params: true`\n"
+        ),
+    )
+
+
 def _provider_returns_encrypted_reasoning(model: str, custom_llm_provider: object) -> bool:
     provider: Final = (
         custom_llm_provider if isinstance(custom_llm_provider, str) else litellm.get_llm_provider(model=model)[1]
@@ -147,6 +174,7 @@ def _build_responses_kwargs(
     Build the kwargs dict to pass directly to litellm.responses() / litellm.aresponses().
     """
     forwarded_kwargs: Final = _forwarded_kwargs(extra_kwargs)
+    _reject_background_mode(model, forwarded_kwargs)
     target_provider: Final = forwarded_kwargs.get("custom_llm_provider")
     request_tools: Final = tools_without_mcp_servers(
         forwarded_kwargs.get("mcp_servers"),
@@ -227,7 +255,7 @@ def _build_responses_kwargs(
         responses_kwargs["stream"] = True
 
     # Forward litellm-specific kwargs (api_key, api_base, logging obj, etc.)
-    excluded: Final = CONVERTED_STREAM_KEYS | {"anthropic_messages", "mcp_servers"}
+    excluded: Final = CONVERTED_STREAM_KEYS | {"anthropic_messages", "background", "mcp_servers"}
     for key, value in forwarded_kwargs.items():
         if key == "litellm_logging_obj" and value is not None:
             from litellm.litellm_core_utils.litellm_logging import (
@@ -241,6 +269,8 @@ 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
 
diff --git a/tests/unit/llms/anthropic/pass_through/responses_adapters/test_responses_adapters_handler.py b/tests/unit/llms/anthropic/pass_through/responses_adapters/test_responses_adapters_handler.py
index 526ead24d26..e64a59a3884 100644
--- a/tests/unit/llms/anthropic/pass_through/responses_adapters/test_responses_adapters_handler.py
+++ b/tests/unit/llms/anthropic/pass_through/responses_adapters/test_responses_adapters_handler.py
@@ -562,6 +562,69 @@ def test_client_cache_control_changes_nothing_for_a_deployment_that_does_not_rea
     assert marked == unmarked
 
 
+@pytest.mark.parametrize(
+    "background_request",
+    [
+        pytest.param({"background": True}, id="background"),
+        pytest.param({"extra_body": {"background": True}}, id="extra-body-background"),
+        pytest.param({"extra_body": {"background": "true"}}, id="extra-body-background-string"),
+        pytest.param({"background": 0}, id="background-zero"),
+    ],
+)
+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_request},
+            max_tokens=1024,
+            messages=MESSAGES,
+        )
+
+    assert rejected.value.status_code == 400
+    assert "['background']" in rejected.value.message
+
+
+@pytest.mark.parametrize(
+    ("drop_setting", "global_drop_params"),
+    [
+        pytest.param({"drop_params": True}, False, id="drop_params"),
+        pytest.param({"additional_drop_params": ["background"]}, False, id="additional_drop_params"),
+        pytest.param({}, True, id="litellm.drop_params"),
+    ],
+)
+def test_dropped_background_never_reaches_the_responses_request(
+    drop_setting: Mapping[str, object], global_drop_params: bool, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    monkeypatch.setattr(litellm, "drop_params", global_drop_params)
+    responses_kwargs: Final = _build_responses_kwargs(
+        model="openai/gpt-5.6",
+        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
+
+
+def test_background_false_still_succeeds_without_reaching_the_responses_request() -> None:
+    responses_kwargs: Final = _build_responses_kwargs(
+        model="openai/gpt-5.6",
+        extra_kwargs={**OPENAI_API, "background": False, "extra_body": {"background": False}},
+        max_tokens=1024,
+        messages=MESSAGES,
+    )
+
+    assert "background" not in responses_kwargs
+    assert responses_kwargs["extra_body"] == {}
+
+
 REASONING_MODEL: Final = "gpt-bridge-reasoning-cost"
 REASONING_RATES: Final = {
     "input_cost_per_token": 2e-06,
diff --git a/tests/unit/llms/bedrock/messages/invoke_transformations/test_anthropic_claude3_transformation.py b/tests/unit/llms/bedrock/messages/invoke_transformations/test_anthropic_claude3_transformation.py
index 56272436857..fe2b6f69e7d 100644
--- a/tests/unit/llms/bedrock/messages/invoke_transformations/test_anthropic_claude3_transformation.py
+++ b/tests/unit/llms/bedrock/messages/invoke_transformations/test_anthropic_claude3_transformation.py
@@ -4171,17 +4171,6 @@ _THINKING_HISTORY: Final = (
 )
 
 
-@dataclass(slots=True)
-class ScriptedBedrockInvoke:
-    replies: tuple[tuple[int, Mapping[str, object]], ...]
-    bodies: tuple[dict[str, object], ...] = ()
-
-    def __call__(self, request: httpx.Request) -> httpx.Response:
-        self.bodies = (*self.bodies, REASONING_BODY_ADAPTER.validate_json(request.content))
-        status_code, body = self.replies[len(self.bodies) - 1]
-        return httpx.Response(status_code, json=body)
-
-
 async def _send_bedrock_messages(
     model: str,
     model_info: Mapping[str, object] | None,
diff --git a/tests/unit/test_utils.py b/tests/unit/test_utils.py
index 58ed951540e..a9e3aa7da5d 100644
--- a/tests/unit/test_utils.py
+++ b/tests/unit/test_utils.py
@@ -1288,7 +1288,6 @@ def test_aaamodel_prices_and_context_window_json_is_valid():
                 "supports_native_streaming": {"type": "boolean"},
                 "supports_image_size": {"type": "boolean"},
                 "supports_native_structured_output": {"type": "boolean"},
-                "supports_bedrock_native_structured_output": {"type": "boolean"},
                 "use_openai_responses_path": {"type": "boolean"},
                 "off_peak_pricing": {
                     "type": "object",