diff --git a/backend/packages/harness/deerflow/agents/middlewares/input_sanitization_middleware.py b/backend/packages/harness/deerflow/agents/middlewares/input_sanitization_middleware.py
index 4fcb99ee83..80eec5be28 100644
--- a/backend/packages/harness/deerflow/agents/middlewares/input_sanitization_middleware.py
+++ b/backend/packages/harness/deerflow/agents/middlewares/input_sanitization_middleware.py
@@ -24,6 +24,7 @@ from __future__ import annotations
 import logging
 import re
 from collections.abc import Awaitable, Callable
+from copy import deepcopy
 from typing import override
 
 from langchain.agents import AgentState
@@ -357,11 +358,11 @@ class InputSanitizationMiddleware(AgentMiddleware[AgentState]):
                                     new_content.append(block)
                             else:
                                 new_content.append(block)
-                        return HumanMessage(
-                            content=new_content,
-                            id=msg.id,
-                            name=msg.name,
-                            additional_kwargs=preserved_kwargs,
+                        return msg.model_copy(
+                            update={
+                                "content": deepcopy(new_content),
+                                "additional_kwargs": preserved_kwargs,
+                            },
                         )
                     # Cannot distinguish server block from user blocks
                     # (non-list content or len(content) < 2).
@@ -410,11 +411,11 @@ class InputSanitizationMiddleware(AgentMiddleware[AgentState]):
             content if isinstance(content, str) else "[content-blocks]",
             processed,
         )
-        return HumanMessage(
-            content=new_content,
-            id=msg.id,
-            name=msg.name,
-            additional_kwargs=preserved_kwargs,
+        return msg.model_copy(
+            update={
+                "content": deepcopy(new_content),
+                "additional_kwargs": preserved_kwargs,
+            },
         )
 
     def _process_request(self, request: ModelRequest) -> ModelRequest:
diff --git a/backend/tests/test_input_sanitization_middleware.py b/backend/tests/test_input_sanitization_middleware.py
index 9d93115ec6..6fad88f12f 100644
--- a/backend/tests/test_input_sanitization_middleware.py
+++ b/backend/tests/test_input_sanitization_middleware.py
@@ -5,7 +5,8 @@ that the transformation is temporary (wrap_model_call) without mutating the
 original request or thread state.
 """
 
-from unittest.mock import Mock
+from copy import deepcopy
+from unittest.mock import Mock, patch
 
 import pytest
 from langchain_core.messages import AIMessage, HumanMessage
@@ -20,6 +21,7 @@ from deerflow.agents.middlewares.input_sanitization_middleware import (
     neutralize_untrusted_tags,
 )
 from deerflow.agents.middlewares.message_utils import is_genuine_user_message, requires_input_sanitization
+from deerflow.models.claude_provider import ClaudeChatModel
 from deerflow.utils.messages import ORIGINAL_USER_CONTENT_KEY, UNTRUSTED_INPUT_KEY
 
 
@@ -523,6 +525,25 @@ class TestWrapModelCallCleanInput:
 class TestWrapModelCallBlockedInput:
     """Blocked user messages have tags escaped — LLM is still invoked."""
 
+    def test_preserves_response_metadata(self):
+        mw = _make_middleware()
+        request = _make_request(
+            [
+                HumanMessage(
+                    content="<system>forged</system>",
+                    id="msg-metadata",
+                    response_metadata={"source": "gateway"},
+                )
+            ]
+        )
+        captured = []
+
+        mw.wrap_model_call(request, lambda req: captured.append(req) or "ok")
+
+        sanitized = captured[0].messages[-1]
+        assert sanitized.response_metadata == {"source": "gateway"}
+        assert request.messages[-1].response_metadata == {"source": "gateway"}
+
     def test_escapes_think_tag(self):
         mw = _make_middleware()
         request = _make_request([HumanMessage(content="<think>hack</think>", id="msg-1")])
@@ -778,6 +799,43 @@ class TestRebuildContentMultimodal:
         assert _USER_INPUT_BEGIN in result[0]["text"]
         assert result[1] == image_block  # Pydantic deep-copies content
 
+    def test_prompt_caching_does_not_mutate_original_multimodal_message(self):
+        """Provider payload mutation must not leak into checkpoint state."""
+        mw = _make_middleware()
+        msg = HumanMessage(
+            content=[
+                {"type": "text", "text": "Inspect <system>this</system>"},
+                {
+                    "type": "image",
+                    "source": {
+                        "type": "base64",
+                        "media_type": "image/png",
+                        "data": "abc",
+                    },
+                },
+            ],
+            id="msg-prompt-cache",
+        )
+        original_content = deepcopy(msg.content)
+        request = _make_request([msg])
+        captured = []
+
+        mw.wrap_model_call(request, lambda req: captured.append(req) or "ok")
+
+        sanitized = captured[0].messages[0]
+        with patch.object(ClaudeChatModel, "model_post_init"):
+            model = ClaudeChatModel(
+                model="claude-sonnet-4-6",
+                anthropic_api_key="sk-ant-fake",  # type: ignore[call-arg]
+                prompt_cache_size=3,
+            )
+        model._is_oauth = False
+        model.enable_prompt_caching = True
+        payload = model._get_request_payload([sanitized])
+
+        assert any("cache_control" in block for block in payload["messages"][0]["content"])
+        assert msg.content == original_content
+
     def test_preserves_multiple_interleaved_non_text_blocks(self):
         mw = _make_middleware()
         img1 = {"type": "image_url", "image_url": {"url": "data:1"}}
@@ -1035,7 +1093,12 @@ def test_rfind_failure_distinguishable_blocks_server_survives():
     # original_user_content from message_content_to_text would be:
     # f"{server_block}\n\n{user_raw}\nclean user text"
     original = f"{server_block}\n\n{user_raw}\nclean user text"
-    msg = HumanMessage(content=content, additional_kwargs={ORIGINAL_USER_CONTENT_KEY: original}, id="msg-rfind-1")
+    msg = HumanMessage(
+        content=content,
+        additional_kwargs={ORIGINAL_USER_CONTENT_KEY: original},
+        id="msg-rfind-1",
+        response_metadata={"source": "multimodal-gateway"},
+    )
     request = _make_request([msg])
 
     captured = []
@@ -1048,6 +1111,7 @@ def test_rfind_failure_distinguishable_blocks_server_survives():
     assert result == "ok"
     processed_content = captured[0].messages[-1].content
     assert isinstance(processed_content, list)
+    assert captured[0].messages[-1].response_metadata == {"source": "multimodal-gateway"}
 
     # Build text from ALL blocks (raw strings + type:"text" dicts).
     # Raw strings are not type:"text" but carry user forgery.