diff --git a/litellm/llms/anthropic/chat/guardrail_translation/handler.py b/litellm/llms/anthropic/chat/guardrail_translation/handler.py
index 9d50345d70d..eb16278c9bd 100644
--- a/litellm/llms/anthropic/chat/guardrail_translation/handler.py
+++ b/litellm/llms/anthropic/chat/guardrail_translation/handler.py
@@ -103,9 +103,24 @@ class ToolResultBlockTextTarget:
     block_idx: int
 
 
-InputWriteBackTarget = (
-    MessageContentTarget | ContentBlockTextTarget | ToolResultStringTarget | ToolResultBlockTextTarget
-)
+@dataclass(frozen=True, slots=True)
+class SystemStringTarget:
+    pass
+
+
+@dataclass(frozen=True, slots=True)
+class SystemBlockTextTarget:
+    block_idx: int
+
+
+@dataclass(frozen=True, slots=True)
+class ToolUseInputTarget:
+    msg_idx: int
+    content_idx: int
+
+
+MessageTextTarget = MessageContentTarget | ContentBlockTextTarget | ToolResultStringTarget | ToolResultBlockTextTarget
+InputWriteBackTarget = SystemStringTarget | SystemBlockTextTarget | MessageTextTarget
 
 
 def _as_str_mapping(value: Mapping[str, object]) -> Mapping[str, object]:
@@ -146,10 +161,17 @@ class ScannedText:
     target: InputWriteBackTarget
 
 
+@dataclass(frozen=True, slots=True)
+class ScannedToolCall:
+    tool_call: ChatCompletionToolCallChunk
+    target: ToolUseInputTarget
+
+
 @dataclass(frozen=True, slots=True)
 class ExtractedInput:
     scanned: tuple[ScannedText, ...]
     images: tuple[str, ...]
+    tool_calls: tuple[ScannedToolCall, ...] = ()
 
 
 EMPTY_EXTRACTED_INPUT: Final = ExtractedInput(scanned=(), images=())
@@ -161,6 +183,76 @@ class _ToolCallShape:
     arguments: str
 
 
+def _is_client_tool_use(block: Mapping[str, object]) -> bool:
+    return (
+        block.get("type") == "tool_use"
+        and isinstance(block.get("id"), str)
+        and isinstance(block.get("name"), str)
+        and isinstance(block.get("input"), Mapping)
+    )
+
+
+def _write_back_system_block(system: object, block_idx: int, response: str) -> None:
+    if not isinstance(system, list):
+        return
+    text_blocks: Final = tuple(block for block in system if isinstance(block, dict) and block.get("type") == "text")
+    if block_idx < len(text_blocks):
+        text_blocks[block_idx]["text"] = (
+            response  # mutable-ok: guardrails rewrite the caller's request payload in place
+        )
+
+
+def _write_back_message_text(message: _WritableMessage, target: MessageTextTarget, response: str) -> None:
+    content: Final = message.get("content", None)
+    if content is None:
+        return
+    match target:
+        case MessageContentTarget():
+            if isinstance(content, str):
+                message["content"] = response  # mutable-ok: guardrails rewrite the caller's request payload in place
+        case ContentBlockTextTarget(content_idx=content_idx):
+            if isinstance(content, list):
+                content[content_idx]["text"] = (
+                    response  # mutable-ok: guardrails rewrite the caller's request payload in place
+                )
+        case ToolResultStringTarget(content_idx=content_idx):
+            if isinstance(content, list):
+                content[content_idx]["content"] = (
+                    response  # mutable-ok: guardrails rewrite the caller's request payload in place
+                )
+        case ToolResultBlockTextTarget(content_idx=content_idx, block_idx=block_idx):
+            if isinstance(content, list):
+                content[content_idx]["content"][block_idx]["text"] = (
+                    response  # mutable-ok: guardrails rewrite the caller's request payload in place
+                )
+        case _:
+            assert_never(target)
+
+
+def _write_back_tool_use(message: _WritableMessage, target: ToolUseInputTarget, shape: _ToolCallShape) -> None:
+    content: Final = message.get("content", None)
+    block: Final = content[target.content_idx] if isinstance(content, list) else None
+    if not isinstance(block, dict):
+        return
+    try:
+        rewritten_input: Final = json.loads(shape.arguments)
+    except json.JSONDecodeError:
+        verbose_proxy_logger.warning(
+            "Anthropic Messages: guardrail returned non-JSON arguments for tool_use %s; keeping its input",
+            block.get("id"),
+        )
+        return
+    if not isinstance(rewritten_input, dict):
+        verbose_proxy_logger.warning(
+            "Anthropic Messages: guardrail returned non-object arguments for tool_use %s; keeping its input",
+            block.get("id"),
+        )
+        return
+    block["input"] = rewritten_input  # mutable-ok: guardrails rewrite the caller's request payload in place
+    if shape.name is not None and shape.name != block.get("name"):
+        block["name"] = shape.name  # mutable-ok: guardrails rewrite the caller's request payload in place
+
+
 @dataclass(frozen=True, slots=True)
 class _SSEFieldRewrite:
     """One field of one nested section of a buffered SSE event, rewritten."""
@@ -452,9 +544,8 @@ class AnthropicMessagesHandler(BaseTranslation):
         skip_tool: Final = effective_skip_tool_message_for_guardrail(guardrail_to_apply)
         scan_only_tool_results: Final = effective_scan_only_tool_results_for_guardrail(guardrail_to_apply)
 
-        # Exclude only the trusted top-level prompt. In-sequence system entries are untrusted
-        # and must stay aligned with texts_to_check for positional masking. When the top-level
-        # prompt is included, the pre-existing count mismatch disables positional masking.
+        # The top-level prompt is translated on its own below so it can be hoisted in front of
+        # any mid-turn system entries and scanned first, aligned with that structured position.
         translation_source: Final = {  # mutable-ok: API message payload
             key: value for key, value in data.items() if key != "system"
         }
@@ -490,7 +581,12 @@ class AnthropicMessagesHandler(BaseTranslation):
             ]
         )
 
-        # Step 1: Extract all text content and images
+        # Step 1: Extract all text content, images, and tool calls
+        top_level_system_scanned: Final = (
+            ()
+            if hoisted_system_message is None or scan_only_tool_results
+            else self._extract_top_level_system_text(hoisted_system_message)
+        )
         extracted: Final = tuple(
             self._extract_input_text_and_images(
                 message=message,
@@ -501,17 +597,27 @@ class AnthropicMessagesHandler(BaseTranslation):
             )
             for msg_idx, message in enumerate(messages)
         )
-        scanned: Final = tuple(item for one_message in extracted for item in one_message.scanned)
+        scanned: Final = (
+            *top_level_system_scanned,
+            *(item for one_message in extracted for item in one_message.scanned),
+        )
         texts_to_check: Final = [item.text for item in scanned]  # mutable-ok: GenericGuardrailAPIInputs takes list[str]
         images_to_check: Final = [
             image for one_message in extracted for image in one_message.images
         ]  # mutable-ok: GenericGuardrailAPIInputs takes list[str]
+        scanned_tool_calls: Final = tuple(item for one_message in extracted for item in one_message.tool_calls)
+        tool_calls_to_check: Final = [
+            item.tool_call for item in scanned_tool_calls
+        ]  # mutable-ok: GenericGuardrailAPIInputs takes list[ChatCompletionToolCallChunk]
+        pre_guardrail_tool_calls: Final = _tool_call_shapes(tool_calls_to_check)
 
-        # Step 2: Apply guardrail to all texts in batch
-        if texts_to_check:
+        # Step 2: Apply guardrail to all texts and tool calls in batch
+        if texts_to_check or tool_calls_to_check:
             inputs: Final = GenericGuardrailAPIInputs(texts=texts_to_check)
             if images_to_check:
                 inputs["images"] = images_to_check
+            if tool_calls_to_check:
+                inputs["tool_calls"] = tool_calls_to_check
             if tools_to_check:
                 inputs["tools"] = tools_to_check
             original_structured_messages: Final = structured_messages
@@ -572,10 +678,16 @@ class AnthropicMessagesHandler(BaseTranslation):
             else:
                 # Step 3: Map guardrail responses back to original message structure
                 await self._apply_guardrail_responses_to_input(
-                    messages=messages,
+                    data=data,
                     responses=guardrailed_texts,
                     scanned=scanned,
                 )
+                self._apply_guardrail_tool_calls_to_input(
+                    messages=messages,
+                    scanned_tool_calls=scanned_tool_calls,
+                    pre_guardrail_tool_calls=pre_guardrail_tool_calls,
+                    returned_tool_calls=guardrailed_inputs.get("tool_calls"),
+                )
 
         verbose_proxy_logger.debug("Anthropic Messages: Processed input messages: %s", messages)
 
@@ -598,6 +710,19 @@ class AnthropicMessagesHandler(BaseTranslation):
         hoisted: Final = probe.get("messages") or []  # mutable-ok: API message payload
         return hoisted[0] if hoisted else None
 
+    @staticmethod
+    def _extract_top_level_system_text(hoisted_system_message: AllMessageValues) -> tuple[ScannedText, ...]:
+        content: Final = hoisted_system_message.get("content")
+        if isinstance(content, str):
+            return (ScannedText(content, SystemStringTarget()),)
+        if not isinstance(content, list):
+            return ()
+        return tuple(
+            ScannedText(text_str, SystemBlockTextTarget(block_idx))
+            for block_idx, block in enumerate(content)
+            if isinstance(block, dict) and isinstance(text_str := block.get("text"), str) and text_str
+        )
+
     @staticmethod
     def _openai_system_message_to_anthropic(
         message: Mapping[str, object],
@@ -852,9 +977,25 @@ class AnthropicMessagesHandler(BaseTranslation):
             for content_idx, content_item in enumerate(content)
             if isinstance(content_item, dict)
         )
+        tool_use_blocks: Final = (
+            ()
+            if scan_only_tool_results
+            else tuple(
+                (content_idx, content_item)
+                for content_idx, content_item in enumerate(content)
+                if isinstance(content_item, dict) and _is_client_tool_use(content_item)
+            )
+        )
         return ExtractedInput(
             scanned=tuple(item for block in blocks for item in block.scanned),
             images=tuple(image for block in blocks for image in block.images),
+            tool_calls=tuple(
+                ScannedToolCall(
+                    tool_call=AnthropicConfig.convert_tool_use_to_openai_format(content_item, tool_call_idx),
+                    target=ToolUseInputTarget(msg_idx, content_idx),
+                )
+                for tool_call_idx, (content_idx, content_item) in enumerate(tool_use_blocks)
+            ),
         )
 
     @classmethod
@@ -940,43 +1081,48 @@ class AnthropicMessagesHandler(BaseTranslation):
 
     async def _apply_guardrail_responses_to_input(
         self,
-        messages: Sequence[_WritableMessage],
+        data: dict,
         responses: list[str],
         scanned: tuple[ScannedText, ...],
     ) -> None:
         """
-        Apply guardrail responses back to input messages.
+        Apply guardrail responses back to the top-level system prompt and the input messages.
         """
+        messages: Final[Sequence[_WritableMessage]] = data.get("messages") or ()
         for item, guardrail_response in zip(scanned, responses):
-            target = item.target
-            message = messages[target.msg_idx]
-            content = message.get("content", None)
-            if content is None:
-                continue
-
-            match target:
-                case MessageContentTarget():
-                    if isinstance(content, str):
-                        message["content"] = (
-                            guardrail_response  # mutable-ok: guardrails rewrite the caller's request payload in place
-                        )
-                case ContentBlockTextTarget(content_idx=content_idx):
-                    if isinstance(content, list):
-                        content[content_idx]["text"] = (
-                            guardrail_response  # mutable-ok: guardrails rewrite the caller's request payload in place
-                        )
-                case ToolResultStringTarget(content_idx=content_idx):
-                    if isinstance(content, list):
-                        content[content_idx]["content"] = (
-                            guardrail_response  # mutable-ok: guardrails rewrite the caller's request payload in place
-                        )
-                case ToolResultBlockTextTarget(content_idx=content_idx, block_idx=block_idx):
-                    if isinstance(content, list):
-                        content[content_idx]["content"][block_idx]["text"] = (
+            match item.target:
+                case SystemStringTarget():
+                    if isinstance(data.get("system"), str):
+                        data["system"] = (
                             guardrail_response  # mutable-ok: guardrails rewrite the caller's request payload in place
                         )
+                case SystemBlockTextTarget(block_idx=block_idx):
+                    _write_back_system_block(data.get("system"), block_idx, guardrail_response)
+                case (
+                    MessageContentTarget()
+                    | ContentBlockTextTarget()
+                    | ToolResultStringTarget()
+                    | ToolResultBlockTextTarget() as message_target
+                ):
+                    _write_back_message_text(messages[message_target.msg_idx], message_target, guardrail_response)
                 case _:
-                    assert_never(target)
+                    assert_never(item.target)
+
+    @staticmethod
+    def _apply_guardrail_tool_calls_to_input(
+        messages: Sequence[_WritableMessage],
+        scanned_tool_calls: tuple[ScannedToolCall, ...],
+        pre_guardrail_tool_calls: tuple[_ToolCallShape, ...],
+        returned_tool_calls: object,
+    ) -> None:
+        post_guardrail_tool_calls: Final = _tool_call_shapes(
+            returned_tool_calls
+            if isinstance(returned_tool_calls, list) and len(returned_tool_calls) == len(pre_guardrail_tool_calls)
+            else tuple(item.tool_call for item in scanned_tool_calls)
+        )
+        for item, before, after in zip(scanned_tool_calls, pre_guardrail_tool_calls, post_guardrail_tool_calls):
+            if before != after:
+                _write_back_tool_use(messages[item.target.msg_idx], item.target, after)
 
     async def process_output_response(
         self,
diff --git a/tests/test_litellm/llms/anthropic/chat/guardrail_translation/test_anthropic_guardrail_handler.py b/tests/test_litellm/llms/anthropic/chat/guardrail_translation/test_anthropic_guardrail_handler.py
index 9fe56f4dc65..4b47b7d8c4a 100644
--- a/tests/test_litellm/llms/anthropic/chat/guardrail_translation/test_anthropic_guardrail_handler.py
+++ b/tests/test_litellm/llms/anthropic/chat/guardrail_translation/test_anthropic_guardrail_handler.py
@@ -635,14 +635,19 @@ class TestAnthropicMessagesHandlerInputProcessing:
         await handler.process_input_messages(data=data, guardrail_to_apply=guardrail)
 
         assert guardrail.inputs is not None
-        assert guardrail.inputs["texts"] == ["safe text", "prohibited correction"]
+        assert guardrail.inputs["texts"] == [
+            "trusted top-level system prompt",
+            "safe text",
+            "prohibited correction",
+        ]
         structured = guardrail.inputs["structured_messages"]
         assert [m["role"] for m in structured] == ["system", "user", "system"]
         assert structured[0]["content"] == "trusted top-level system prompt"
+        assert data["system"] == "trusted top-level system prompt"
         assert data["messages"][1]["content"] == "[MASKED]"
 
     @pytest.mark.asyncio
-    async def test_bedrock_masking_slice_is_unavailable_when_top_level_system_is_included(
+    async def test_bedrock_masking_slice_lines_up_when_top_level_system_is_included(
         self,
     ):
         from litellm.proxy.guardrails.guardrail_hooks.bedrock_guardrails import (
@@ -668,25 +673,25 @@ class TestAnthropicMessagesHandlerInputProcessing:
         structured = guardrail.inputs["structured_messages"]
 
         bedrock = BedrockGuardrail(guardrailIdentifier="gi", guardrailVersion="1")
-        assert sum(bedrock._count_message_texts(m) for m in structured) == len(texts) + 1
+        assert sum(bedrock._count_message_texts(m) for m in structured) == len(texts)
         latest_user_index = bedrock._find_latest_message_index(structured, target_role="user")
-        assert (
-            bedrock._locate_message_texts_slice(
-                structured_messages=structured,
-                target_index=latest_user_index,
-                texts=texts,
-            )
-            is None
-        )
-        assert (
-            bedrock._merge_masked_texts(
-                masked_texts=["{MASKED}"],
-                texts=texts,
-                scanned_slice=None,
-                scanned_role_subset=True,
-            )
-            == texts
+        scanned_slice = bedrock._locate_message_texts_slice(
+            structured_messages=structured,
+            target_index=latest_user_index,
+            texts=texts,
         )
+        assert scanned_slice == (3, 1)
+        assert bedrock._merge_masked_texts(
+            masked_texts=["{MASKED}"],
+            texts=texts,
+            scanned_slice=scanned_slice,
+            scanned_role_subset=True,
+        ) == [
+            "trusted top-level system prompt",
+            "safe text",
+            "prohibited correction",
+            "{MASKED}",
+        ]
 
     @pytest.mark.asyncio
     @pytest.mark.parametrize("skip_system_message_in_guardrail", [True, None])
@@ -1611,7 +1616,8 @@ class TestAnthropicMessagesIncrementalScan:
             )
             assert mock_api.call_count == 1
             assert [m["content"] for m in mock_api.call_args.kwargs["messages"]] == [
-                "What is the capital of France?"
+                "You are a helpful geography assistant.",
+                "What is the capital of France?",
             ]
             mock_api.reset_mock()
             await handler.process_input_messages(
@@ -2150,6 +2156,208 @@ class TestAnthropicMessagesScanOnlyToolResults:
         assert guardrail.captured_inputs.get("images") == ["TOOL_IMG"]
 
 
+class ToolCallArgumentsMaskingGuardrail(InputsRecordingGuardrail):
+    """Masks the canary inside tool-call arguments, in place or through a fresh list of plain dicts."""
+
+    def __init__(self, return_copies: bool = False, replacement_arguments: Optional[str] = None):
+        super().__init__()
+        self.return_copies = return_copies
+        self.replacement_arguments = replacement_arguments
+        self.seen_tool_calls: list[dict] = []
+
+    async def apply_guardrail(
+        self,
+        inputs: GenericGuardrailAPIInputs,
+        request_data: dict,
+        input_type: Literal["request", "response"],
+        logging_obj: Optional[Any] = None,
+    ) -> GenericGuardrailAPIInputs:
+        outputs = await super().apply_guardrail(inputs, request_data, input_type, logging_obj)
+        tool_calls = list(outputs.get("tool_calls") or [])
+        self.seen_tool_calls.extend(json.loads(json.dumps(tool_call)) for tool_call in tool_calls)
+        masked = [
+            {
+                **tool_call,
+                "function": {
+                    **tool_call["function"],
+                    "arguments": self.replacement_arguments
+                    if self.replacement_arguments is not None
+                    else tool_call["function"]["arguments"].replace("POISON", "[BLOCKED]"),
+                },
+            }
+            for tool_call in tool_calls
+        ]
+        if self.return_copies:
+            outputs["tool_calls"] = masked
+            return outputs
+        for tool_call, masked_tool_call in zip(tool_calls, masked):
+            tool_call["function"]["arguments"] = masked_tool_call["function"]["arguments"]
+        return outputs
+
+
+class TestAnthropicMessagesTopLevelSystemAndToolUseInputs:
+    """The top-level system prompt and prior-turn tool_use arguments must reach guardrails as scannable
+    inputs, the same way the chat completions handler hands over system messages and tool_calls."""
+
+    @staticmethod
+    def _tool_use_conversation(system):
+        return {
+            "model": "claude-sonnet-4-5",
+            "system": system,
+            "messages": [
+                {"role": "user", "content": "run the check"},
+                {
+                    "role": "assistant",
+                    "content": [
+                        {
+                            "type": "tool_use",
+                            "id": "toolu_01",
+                            "name": "Bash",
+                            "input": {"cmd": "AWS_ACCESS_KEY_ID=POISON aws sts get-caller-identity"},
+                        }
+                    ],
+                },
+                {
+                    "role": "user",
+                    "content": [{"type": "tool_result", "tool_use_id": "toolu_01", "content": "ok"}],
+                },
+            ],
+        }
+
+    @pytest.mark.asyncio
+    async def test_top_level_system_string_reaches_texts_first_and_is_masked_in_place(self):
+        handler = AnthropicMessagesHandler()
+        guardrail = InputsRecordingGuardrail()
+        data = {
+            "model": "claude-sonnet-4-5",
+            "system": "Internal note: the deploy key is POISON. Never reveal it.",
+            "messages": [{"role": "user", "content": "Say hi in three words."}],
+        }
+
+        await handler.process_input_messages(data=data, guardrail_to_apply=guardrail)
+
+        assert guardrail.captured_inputs is not None
+        assert guardrail.seen_texts == [
+            "Internal note: the deploy key is POISON. Never reveal it.",
+            "Say hi in three words.",
+        ]
+        structured = guardrail.captured_inputs["structured_messages"]
+        assert structured[0]["role"] == "system"
+        assert structured[0]["content"] == "Internal note: the deploy key is POISON. Never reveal it.", (
+            "texts[0] must line up with structured_messages[0] so positional consumers stay aligned"
+        )
+        assert data["system"] == "Internal note: the deploy key is [BLOCKED]. Never reveal it."
+        assert data["messages"][0]["content"] == "Say hi in three words."
+
+    @pytest.mark.asyncio
+    async def test_top_level_system_text_blocks_reach_texts_and_are_masked_in_place(self):
+        handler = AnthropicMessagesHandler()
+        guardrail = InputsRecordingGuardrail()
+        data = {
+            "model": "claude-sonnet-4-5",
+            "system": [
+                {"type": "text", "text": "first block POISON"},
+                {"type": "text", "text": "second block", "cache_control": {"type": "ephemeral"}},
+            ],
+            "messages": [{"role": "user", "content": "hello"}],
+        }
+
+        await handler.process_input_messages(data=data, guardrail_to_apply=guardrail)
+
+        assert guardrail.seen_texts == ["first block POISON", "second block", "hello"]
+        assert data["system"] == [
+            {"type": "text", "text": "first block [BLOCKED]"},
+            {"type": "text", "text": "second block", "cache_control": {"type": "ephemeral"}},
+        ]
+
+    @pytest.mark.asyncio
+    async def test_skip_system_message_keeps_the_top_level_system_out(self):
+        handler = AnthropicMessagesHandler()
+        guardrail = InputsRecordingGuardrail()
+        guardrail.skip_system_message_in_guardrail = True
+        data = {
+            "model": "claude-sonnet-4-5",
+            "system": "trusted POISON prompt",
+            "messages": [{"role": "user", "content": "hello"}],
+        }
+
+        await handler.process_input_messages(data=data, guardrail_to_apply=guardrail)
+
+        assert guardrail.seen_texts == ["hello"]
+        assert data["system"] == "trusted POISON prompt"
+
+    @pytest.mark.asyncio
+    async def test_prior_turn_tool_use_input_reaches_tool_calls_in_openai_shape(self):
+        handler = AnthropicMessagesHandler()
+        guardrail = InputsRecordingGuardrail()
+        data = self._tool_use_conversation(system="You are a careful agent harness.")
+
+        await handler.process_input_messages(data=data, guardrail_to_apply=guardrail)
+
+        assert guardrail.captured_inputs is not None
+        tool_calls = guardrail.captured_inputs.get("tool_calls")
+        assert tool_calls is not None and len(tool_calls) == 1
+        assert tool_calls[0]["id"] == "toolu_01"
+        assert tool_calls[0]["type"] == "function"
+        assert tool_calls[0]["function"]["name"] == "Bash"
+        assert json.loads(tool_calls[0]["function"]["arguments"]) == {
+            "cmd": "AWS_ACCESS_KEY_ID=POISON aws sts get-caller-identity"
+        }
+        assert data["messages"][1]["content"][0]["input"] == {
+            "cmd": "AWS_ACCESS_KEY_ID=POISON aws sts get-caller-identity"
+        }, "a guardrail that leaves tool_calls alone must leave the tool_use input alone"
+
+    @pytest.mark.asyncio
+    @pytest.mark.parametrize("return_copies", [False, True])
+    async def test_masked_tool_call_arguments_write_back_into_the_tool_use_input(self, return_copies: bool):
+        handler = AnthropicMessagesHandler()
+        guardrail = ToolCallArgumentsMaskingGuardrail(return_copies=return_copies)
+        data = self._tool_use_conversation(system="You are a careful agent harness.")
+
+        await handler.process_input_messages(data=data, guardrail_to_apply=guardrail)
+
+        assert [tool_call["function"]["name"] for tool_call in guardrail.seen_tool_calls] == ["Bash"]
+        tool_use = data["messages"][1]["content"][0]
+        assert tool_use == {
+            "type": "tool_use",
+            "id": "toolu_01",
+            "name": "Bash",
+            "input": {"cmd": "AWS_ACCESS_KEY_ID=[BLOCKED] aws sts get-caller-identity"},
+        }
+        assert data["messages"][2]["content"][0]["tool_use_id"] == "toolu_01"
+
+    @pytest.mark.asyncio
+    async def test_non_json_rewritten_arguments_keep_the_tool_use_input(self):
+        handler = AnthropicMessagesHandler()
+        guardrail = ToolCallArgumentsMaskingGuardrail(replacement_arguments="[REDACTED]")
+        data = self._tool_use_conversation(system="You are a careful agent harness.")
+
+        await handler.process_input_messages(data=data, guardrail_to_apply=guardrail)
+
+        assert data["messages"][1]["content"][0]["input"] == {
+            "cmd": "AWS_ACCESS_KEY_ID=POISON aws sts get-caller-identity"
+        }
+
+    @pytest.mark.asyncio
+    async def test_scan_only_tool_results_keeps_system_and_tool_use_out(self):
+        handler = AnthropicMessagesHandler()
+        guardrail = InputsRecordingGuardrail()
+        guardrail.scan_only_tool_results = True
+        data = self._tool_use_conversation(system="trusted POISON prompt")
+        data["messages"][2]["content"][0]["content"] = "fetched POISON page"
+
+        await handler.process_input_messages(data=data, guardrail_to_apply=guardrail)
+
+        assert guardrail.seen_texts == ["fetched POISON page"]
+        assert guardrail.captured_inputs is not None
+        assert guardrail.captured_inputs.get("tool_calls") is None
+        assert data["system"] == "trusted POISON prompt"
+        assert data["messages"][1]["content"][0]["input"] == {
+            "cmd": "AWS_ACCESS_KEY_ID=POISON aws sts get-caller-identity"
+        }
+        assert data["messages"][2]["content"][0]["content"] == "fetched [BLOCKED] page"
+
+
 class TestStructuredWriteBackKeepsToolResults:
     """A guardrail rewrite must never leave a tool_use without its tool_result (Claude Code ToolSearch, LIT-6103)."""