diff --git a/sandboxes/mcp_local/app/mocks/openai_tool.py b/sandboxes/mcp_local/app/mocks/openai_tool.py
index b286a2d..d4d5b26 100644
--- a/sandboxes/mcp_local/app/mocks/openai_tool.py
+++ b/sandboxes/mcp_local/app/mocks/openai_tool.py
@@ -68,6 +68,11 @@ async def chat_completions(
                     if not response_message.tool_calls:
                         return response
 
+                    # Preserve the assistant's tool-call request before adding results.
+                    request.messages.append(
+                        response_message.model_dump(exclude_none=True)
+                    )
+
                     for tool_call in response_message.tool_calls:
                         tool_name = tool_call.function.name
                         tool_args_raw = tool_call.function.arguments
diff --git a/sandboxes/mcp_local/tests/test_openai_tool.py b/sandboxes/mcp_local/tests/test_openai_tool.py
new file mode 100644
index 0000000..44d3ef7
--- /dev/null
+++ b/sandboxes/mcp_local/tests/test_openai_tool.py
@@ -0,0 +1,169 @@
+"""Regression tests for MCP tool-call message ordering."""
+
+import copy
+from types import SimpleNamespace
+from unittest import IsolatedAsyncioTestCase
+from unittest.mock import AsyncMock, patch
+
+from app.mocks import openai_tool
+from app.mocks.openai import ChatCompletionRequest
+
+
+class AsyncContext:
+    """Minimal async context manager used by the MCP client fakes."""
+
+    def __init__(self, value):
+        self.value = value
+
+    async def __aenter__(self):
+        return self.value
+
+    async def __aexit__(self, exc_type, exc, traceback):
+        return False
+
+
+class FakeAssistantMessage:
+    """Assistant response containing one or more tool calls."""
+
+    def __init__(self, tool_call_count=1):
+        self.tool_calls = [
+            SimpleNamespace(
+                id=f"call-{index}",
+                function=SimpleNamespace(
+                    name=f"lookup-{index}",
+                    arguments='{"query": "test"}',
+                ),
+            )
+            for index in range(1, tool_call_count + 1)
+        ]
+
+    def model_dump(self, *, exclude_none):
+        assert exclude_none is True
+        return {
+            "role": "assistant",
+            "tool_calls": [
+                {
+                    "id": tool_call.id,
+                    "type": "function",
+                    "function": {
+                        "name": tool_call.function.name,
+                        "arguments": tool_call.function.arguments,
+                    },
+                }
+                for tool_call in self.tool_calls
+            ],
+        }
+
+
+class FakeCompletions:
+    """Record messages passed to consecutive completion calls."""
+
+    def __init__(self, tool_call_count=1):
+        self.calls = []
+        self.responses = iter(
+            [
+                SimpleNamespace(
+                    choices=[
+                        SimpleNamespace(message=FakeAssistantMessage(tool_call_count))
+                    ]
+                ),
+                SimpleNamespace(
+                    choices=[SimpleNamespace(message=SimpleNamespace(tool_calls=[]))]
+                ),
+            ]
+        )
+
+    def create(self, **kwargs):
+        self.calls.append(copy.deepcopy(kwargs["messages"]))
+        return next(self.responses)
+
+
+class ToolMessageOrderingTests(IsolatedAsyncioTestCase):
+    async def run_completion(self, tool_result, tool_call_count=1):
+        completions = FakeCompletions(tool_call_count)
+        if isinstance(tool_result, BaseException):
+            call_tool = AsyncMock(side_effect=tool_result)
+        elif tool_call_count > 1:
+            call_tool = AsyncMock(
+                side_effect=[tool_result for _ in range(tool_call_count)]
+            )
+        else:
+            call_tool = AsyncMock(return_value=tool_result)
+        session = SimpleNamespace(
+            initialize=AsyncMock(),
+            call_tool=call_tool,
+        )
+        request = ChatCompletionRequest(
+            model="test",
+            messages=[{"role": "user", "content": "hello"}],
+        )
+
+        with (
+            patch.object(
+                openai_tool,
+                "client",
+                SimpleNamespace(chat=SimpleNamespace(completions=completions)),
+            ),
+            patch.object(
+                openai_tool,
+                "sse_client",
+                lambda **kwargs: AsyncContext((object(), object())),
+            ),
+            patch.object(
+                openai_tool,
+                "ClientSession",
+                lambda read, write: AsyncContext(session),
+            ),
+            patch.object(
+                openai_tool,
+                "fetch_mcp_tools",
+                AsyncMock(return_value=[{"type": "function"}]),
+            ),
+        ):
+            await openai_tool.chat_completions(request, token="test")
+
+        return completions.calls
+
+    def assert_valid_tool_message_order(self, calls):
+        self.assertEqual(
+            [message["role"] for message in calls[1]],
+            ["user", "assistant", "tool"],
+        )
+        assistant_message, tool_message = calls[1][1:]
+        self.assertEqual(
+            assistant_message["tool_calls"][0]["id"],
+            tool_message["tool_call_id"],
+        )
+
+    async def test_tool_result_follows_assistant_tool_call(self):
+        calls = await self.run_completion(
+            SimpleNamespace(content=[SimpleNamespace(text="result")])
+        )
+
+        self.assert_valid_tool_message_order(calls)
+        self.assertEqual(calls[1][2]["content"], "result")
+
+    async def test_tool_error_follows_assistant_tool_call(self):
+        calls = await self.run_completion(RuntimeError("tool failed"))
+
+        self.assert_valid_tool_message_order(calls)
+        self.assertEqual(
+            calls[1][2]["content"],
+            "Error executing tool: tool failed",
+        )
+
+    async def test_multiple_tool_results_follow_one_assistant_message(self):
+        calls = await self.run_completion(
+            SimpleNamespace(content=[SimpleNamespace(text="result")]),
+            tool_call_count=2,
+        )
+
+        self.assertEqual(
+            [message["role"] for message in calls[1]],
+            ["user", "assistant", "tool", "tool"],
+        )
+        self.assertEqual(
+            [message["tool_call_id"] for message in calls[1][2:]],
+            ["call-1", "call-2"],
+        )
+        self.assertEqual(len(calls[1][1]["tool_calls"]), 2)