From 6c9fd468c893cea1f1531b6962bdf61185a0a49f Mon Sep 17 00:00:00 2001
From: Yufeng He <40085740+he-yufeng@users.noreply.github.com>
Date: Fri, 15 May 2026 03:15:21 +0800
Subject: [PATCH] Python: fix Foundry handoff argument serialization

---
 .../_responses.py                             | 18 ++++++++---
 .../foundry_hosting/tests/test_responses.py   | 31 +++++++++++++++++++
 2 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/python/packages/foundry_hosting/agent_framework_foundry_hosting/_responses.py b/python/packages/foundry_hosting/agent_framework_foundry_hosting/_responses.py
index c34c65538c..8ad17803bb 100644
--- a/python/packages/foundry_hosting/agent_framework_foundry_hosting/_responses.py
+++ b/python/packages/foundry_hosting/agent_framework_foundry_hosting/_responses.py
@@ -9,8 +9,9 @@
 import os
 import tempfile
 import threading
-from collections.abc import AsyncIterable, AsyncIterator, Generator, Mapping, Sequence
+from collections.abc import AsyncIterable, AsyncIterator, Generator, Sequence
 from contextlib import suppress
+from dataclasses import asdict, is_dataclass
 from pathlib import Path
 from typing import Protocol, cast
 
@@ -1394,11 +1395,20 @@ def _convert_message_content(content: MessageContent) -> Content:
 # region Output Item Conversion
 
 
-def _arguments_to_str(arguments: str | Mapping[str, Any] | None) -> str:
+def _argument_json_default(value: Any) -> Any:
+    if is_dataclass(value) and not isinstance(value, type):
+        return asdict(value)
+    to_dict = getattr(value, "to_dict", None)
+    if callable(to_dict):
+        return to_dict()
+    raise TypeError(f"Object of type {type(value).__name__} is not JSON serializable")
+
+
+def _arguments_to_str(arguments: Any | None) -> str:
     """Convert arguments to a JSON string.
 
     Args:
-        arguments: The arguments to convert, can be a string, mapping, or None.
+        arguments: The arguments to convert, can be a string, JSON-like object, or None.
 
     Returns:
         The arguments as a JSON string.
@@ -1407,7 +1417,7 @@ def _arguments_to_str(arguments: str | Mapping[str, Any] | None) -> str:
         return ""
     if isinstance(arguments, str):
         return arguments
-    return json.dumps(arguments)
+    return json.dumps(arguments, default=_argument_json_default)
 
 
 async def _to_outputs(
diff --git a/python/packages/foundry_hosting/tests/test_responses.py b/python/packages/foundry_hosting/tests/test_responses.py
index e4d545d6d7..8b1c5aa8d0 100644
--- a/python/packages/foundry_hosting/tests/test_responses.py
+++ b/python/packages/foundry_hosting/tests/test_responses.py
@@ -12,6 +12,7 @@
 
 import json
 from collections.abc import AsyncIterator, Callable
+from dataclasses import dataclass
 from unittest.mock import AsyncMock, MagicMock
 
 import httpx
@@ -401,6 +402,36 @@ async def test_function_call_streaming(self) -> None:
         assert len(args_done) == 1
         assert args_done[0]["data"]["arguments"] == '{"q": "hello"}'
 
+    async def test_function_call_streaming_serializes_dataclass_arguments(self) -> None:
+        @dataclass
+        class HandoffLikeRequest:
+            agent_response: AgentResponse
+
+        request = HandoffLikeRequest(
+            agent_response=AgentResponse(
+                messages=[Message(role="assistant", contents=[Content.from_text("Need more details")])]
+            )
+        )
+        agent = _make_agent(
+            stream_updates=[
+                AgentResponseUpdate(
+                    contents=[Content.from_function_call("call_1", "handoff_to_refund", arguments=request)],
+                    role="assistant",
+                ),
+            ]
+        )
+        server = _make_server(agent)
+        resp = await _post(server, stream=True)
+
+        assert resp.status_code == 200
+        events = _parse_sse_events(resp.text)
+        args_done = [e for e in events if e["event"] == "response.function_call_arguments.done"]
+        assert len(args_done) == 1
+
+        payload = json.loads(args_done[0]["data"]["arguments"])
+        assert payload["agent_response"]["type"] == "agent_response"
+        assert payload["agent_response"]["messages"][0]["contents"][0]["text"] == "Need more details"
+
     async def test_alternating_text_and_function_call(self) -> None:
         agent = _make_agent(
             stream_updates=[