diff --git a/agent/auxiliary_client.py b/agent/auxiliary_client.py
index f8927a3a36..04487ccb07 100644
--- a/agent/auxiliary_client.py
+++ b/agent/auxiliary_client.py
@@ -2430,6 +2430,9 @@ def _relay_sync_completion(
     client: Any, kwargs: dict[str, Any], *, provider: str | None = None,
     api_mode: str | None = None, create: Callable[[dict[str, Any]], Any] | None = None,
 ) -> Any:
+    from agent.auxiliary_wire import prepare_chat_messages
+
+    kwargs = prepare_chat_messages(client, kwargs)
     callback = create or (lambda request: client.chat.completions.create(**request))
     route = _relay_auxiliary_metadata(provider=provider, api_mode=api_mode)
     # Isolate only the provider callback so the owning thread can unwind its lease/DB
@@ -2449,6 +2452,9 @@ async def _relay_async_completion(
     client: Any, kwargs: dict[str, Any], *, provider: str | None = None,
     api_mode: str | None = None, create: Callable[[dict[str, Any]], Any] | None = None,
 ) -> Any:
+    from agent.auxiliary_wire import prepare_chat_messages
+
+    kwargs = prepare_chat_messages(client, kwargs)
     callback = create or (lambda request: client.chat.completions.create(**request))
     route = _relay_auxiliary_metadata(provider=provider, api_mode=api_mode)
     if route is None:
@@ -2464,6 +2470,9 @@ async def _relay_async_completion(
 def _relay_sync_stream(
     client: Any, kwargs: dict[str, Any], *, provider: str | None = None, api_mode: str | None = None
 ) -> Any:
+    from agent.auxiliary_wire import prepare_chat_messages
+
+    kwargs = prepare_chat_messages(client, kwargs)
     route = _relay_auxiliary_metadata(provider=provider, api_mode=api_mode)
     if route is None:
         return client.chat.completions.create(**kwargs)
diff --git a/agent/auxiliary_wire.py b/agent/auxiliary_wire.py
new file mode 100644
index 0000000000..f698c4a17a
--- /dev/null
+++ b/agent/auxiliary_wire.py
@@ -0,0 +1,20 @@
+"""Message hygiene at the resolved auxiliary client boundary."""
+
+from openai import AsyncOpenAI, OpenAI
+
+from agent.transports.chat_completions import ChatCompletionsTransport
+
+
+def prepare_chat_messages(client, kwargs: dict) -> dict:
+    """Sanitize actual Chat Completions SDK requests, not native adapter replay.
+
+    Auxiliary and MoA callers can retain a prepared request before the virtual
+    transport sanitizes its copy. The resolved SDK client identifies the wire;
+    native Messages/Responses adapters must retain their reasoning sidecars.
+    """
+    if not isinstance(client, (OpenAI, AsyncOpenAI)) or "messages" not in kwargs:
+        return kwargs
+    messages = ChatCompletionsTransport().convert_messages(
+        kwargs["messages"], model=kwargs.get("model")
+    )
+    return {**kwargs, "messages": messages}
diff --git a/tests/agent/test_auxiliary_wire_messages.py b/tests/agent/test_auxiliary_wire_messages.py
new file mode 100644
index 0000000000..e682d48395
--- /dev/null
+++ b/tests/agent/test_auxiliary_wire_messages.py
@@ -0,0 +1,49 @@
+"""The final auxiliary destination owns message sanitation, not virtual MoA."""
+import asyncio
+import copy
+from types import SimpleNamespace
+
+import pytest
+from openai import AsyncOpenAI, OpenAI
+
+from agent.auxiliary_client import _relay_async_completion, _relay_sync_completion
+
+
+@pytest.mark.parametrize("async_mode", [False, True])
+def test_auxiliary_chat_wire_sanitizes_without_mutating_history(async_mode):
+    history = [{"role": "assistant", "content": "answer", "_db_persisted": True,
+                "timestamp": 12, "tool_calls": [], "reasoning": "private"}]
+    original = copy.deepcopy(history)
+    kwargs = {"model": "fixture-model", "messages": history}
+    captured = []
+    if async_mode:
+        async def run():
+            async with AsyncOpenAI(api_key="fixture", base_url="http://127.0.0.1:1/v1") as client:
+                async def send(request):
+                    captured.append(request)
+                await _relay_async_completion(client, kwargs, create=send)
+        asyncio.run(run())
+    else:
+        with OpenAI(api_key="fixture", base_url="http://127.0.0.1:1/v1") as client:
+            _relay_sync_completion(client, kwargs, create=captured.append)
+    assert captured[0]["messages"] == [{"role": "assistant", "content": "answer"}]
+    assert kwargs["messages"] == original
+
+
+@pytest.mark.parametrize("async_mode", [False, True])
+def test_auxiliary_native_adapters_keep_replay_and_tool_fields(async_mode):
+    history = [{"role": "assistant", "content": "", "_db_persisted": True,
+                "codex_reasoning_items": [{"type": "reasoning", "id": "rs_fixture"}],
+                "thinking_blocks": [{"type": "thinking", "thinking": "native", "signature": "sig"}],
+                "tool_calls": [{"id": "call_fixture", "type": "function",
+                                "function": {"name": "fixture", "arguments": "{}"}}]}]
+    kwargs = {"model": "native", "messages": history}
+    captured = []
+    if async_mode:
+        async def send(request):
+            captured.append(request)
+        asyncio.run(_relay_async_completion(SimpleNamespace(), kwargs, create=send))
+    else:
+        _relay_sync_completion(SimpleNamespace(), kwargs, create=captured.append)
+    assert captured[0] is kwargs
+    assert captured[0]["messages"] is history