diff --git a/agent/transports/chat_completions.py b/agent/transports/chat_completions.py
index 7edb69e42c7..ed5d8b0ba43 100644
--- a/agent/transports/chat_completions.py
+++ b/agent/transports/chat_completions.py
@@ -122,7 +122,11 @@ class ChatCompletionsTransport(ProviderTransport):
         for msg in messages:
             if not isinstance(msg, dict):
                 continue
-            if "codex_reasoning_items" in msg or "codex_message_items" in msg:
+            if (
+                "codex_reasoning_items" in msg
+                or "codex_message_items" in msg
+                or "tool_name" in msg
+            ):
                 needs_sanitize = True
                 break
             tool_calls = msg.get("tool_calls")
@@ -145,6 +149,7 @@ class ChatCompletionsTransport(ProviderTransport):
                 continue
             msg.pop("codex_reasoning_items", None)
             msg.pop("codex_message_items", None)
+            msg.pop("tool_name", None)
             tool_calls = msg.get("tool_calls")
             if isinstance(tool_calls, list):
                 for tc in tool_calls:
diff --git a/tests/agent/transports/test_chat_completions.py b/tests/agent/transports/test_chat_completions.py
index 7ed0d4da634..2e7b9da2f8d 100644
--- a/tests/agent/transports/test_chat_completions.py
+++ b/tests/agent/transports/test_chat_completions.py
@@ -46,6 +46,26 @@ class TestChatCompletionsBasic:
         assert "codex_reasoning_items" in msgs[0]
         assert "codex_message_items" in msgs[0]
 
+    def test_convert_messages_strips_tool_name(self, transport):
+        """Internal `tool_name` (used for FTS indexing in the SQLite store) is
+        not part of the OpenAI Chat Completions schema. Strict providers like
+        Moonshot/Kimi reject it with HTTP 400 'Extra inputs are not permitted'.
+        """
+        msgs = [
+            {"role": "user", "content": "hi"},
+            {"role": "assistant", "content": None,
+             "tool_calls": [{"id": "call_1", "type": "function",
+                             "function": {"name": "execute_code", "arguments": "{}"}}]},
+            {"role": "tool", "tool_call_id": "call_1", "tool_name": "execute_code",
+             "content": "result"},
+        ]
+        result = transport.convert_messages(msgs)
+        assert "tool_name" not in result[2]
+        assert result[2]["content"] == "result"
+        assert result[2]["tool_call_id"] == "call_1"
+        # Original list untouched (deepcopy-on-demand)
+        assert msgs[2]["tool_name"] == "execute_code"
+
 
 class TestChatCompletionsBuildKwargs: