Python: fix: preserve reasoning with hosted MCP calls

python/packages/openai/agent_framework_openai/_chat_client.py

@@ -1491,10 +1491,20 @@ def _prepare_message_for_openai(
         # (replays_local_storage) still need stripping when the request also carries a continuation
         # marker, since the server-stored items would otherwise duplicate the inline ones. Without
         # storage, standalone reasoning items are invalid per the API ("reasoning was provided
-        # without its required following item"), so the reasoning branch always drops.
+        # without its required following item"). Hosted MCP calls are the exception: if we keep
+        # the mcp_call item without storage, keep its paired reasoning item too.
+        has_hosted_mcp_call = any(content.type == "mcp_server_tool_call" for content in message.contents)
         for content in message.contents:
             match content.type:
                 case "text_reasoning":
+                    if not request_uses_service_side_storage and has_hosted_mcp_call:
+                        reasoning = self._prepare_content_for_openai(
+                            message.role,
+                            content,
+                            replays_local_storage=replays_local_storage,
+                        )
+                        if reasoning:
+                            all_messages.append(reasoning)
                     continue
                 case "function_result":
                     if request_uses_service_side_storage:

python/packages/openai/tests/openai/test_openai_chat_client.py

@@ -5603,6 +5603,42 @@ def test_prepare_messages_for_openai_serializes_mcp_server_tool_call_as_mcp_call
     assert "output" not in item or item["output"] is None
 
 
+def test_prepare_messages_for_openai_keeps_reasoning_with_mcp_call_without_storage() -> None:
+    """Hosted MCP calls returned by reasoning models need their paired reasoning item."""
+    client = OpenAIChatClient(model="test-model", api_key="test-key")
+
+    messages = [
+        Message(
+            role="assistant",
+            contents=[
+                Content.from_text_reasoning(
+                    id="rs_abc123",
+                    text="I need to query the hosted MCP server.",
+                    additional_properties={"status": "completed"},
+                ),
+                Content.from_mcp_server_tool_call(
+                    call_id="mcp_abc123",
+                    tool_name="search",
+                    server_name="api_specs",
+                    arguments='{"q": "cats"}',
+                ),
+            ],
+        ),
+    ]
+
+    storage_on = client._prepare_messages_for_openai(messages, request_uses_service_side_storage=True)
+    assert "reasoning" not in [item.get("type") for item in storage_on]
+
+    storage_off = client._prepare_messages_for_openai(messages, request_uses_service_side_storage=False)
+    types = [item.get("type") for item in storage_off]
+    assert types == ["reasoning", "mcp_call"]
+
+    reasoning = storage_off[0]
+    assert reasoning["id"] == "rs_abc123"
+    assert reasoning["status"] == "completed"
+    assert reasoning["summary"] == [{"type": "summary_text", "text": "I need to query the hosted MCP server."}]
+
+
 def test_prepare_messages_for_openai_coalesces_mcp_call_and_result_into_single_item() -> None:
     """An mcp_server_tool_call followed by an mcp_server_tool_result with the
     same call_id (in same or separate Messages) must produce ONE mcp_call