Python: Fix FoundryAgent stripping model from PromptAgent requests

python/packages/foundry/agent_framework_foundry/_agent.py

@@ -341,6 +341,7 @@ async def _prepare_options(
         if _uses_foundry_agent_session(conversation_id):
             run_options.pop("previous_response_id", None)
             run_options.pop("conversation", None)
+            run_options.pop("model", None)
             extra_body["agent_session_id"] = conversation_id
         if extra_body:
             run_options["extra_body"] = extra_body
@@ -350,7 +351,6 @@ async def _prepare_options(
         # Strip tools from request body - Foundry API rejects requests with both
         # agent endpoint and tools present. FunctionTools are invoked client-side
         # by the function invocation layer, not sent to the service.
-        run_options.pop("model", None)
         if not self.allow_preview:
             run_options.pop("tools", None)
             run_options.pop("tool_choice", None)

python/packages/foundry/tests/foundry/test_foundry_agent.py

@@ -200,7 +200,7 @@ def my_func() -> str:
 
 
 async def test_raw_foundry_agent_chat_client_prepare_options_strips_client_side_fields() -> None:
-    """Test that _prepare_options strips model and tool-loop fields from run_options."""
+    """Test that _prepare_options strips tool-loop fields but preserves model for non-session requests."""
 
     mock_project = MagicMock()
     mock_openai = MagicMock()
@@ -232,11 +232,74 @@ def my_func() -> str:
             options={"tools": [my_func]},
         )
 
-    assert "model" not in result
+    # model is preserved for non-session (PromptAgent) requests
+    assert result["model"] == "gpt-4.1"
     assert "tools" not in result
     assert "tool_choice" not in result
     assert "parallel_tool_calls" not in result
-    assert result == {}
+
+
+async def test_raw_foundry_agent_chat_client_prepare_options_strips_model_for_hosted_session() -> None:
+    """Test that model is stripped when using a hosted agent session (not a PromptAgent)."""
+
+    mock_project = MagicMock()
+    mock_openai = MagicMock()
+    mock_project.get_openai_client.return_value = mock_openai
+
+    client = RawFoundryAgentChatClient(
+        project_client=mock_project,
+        agent_name="test-agent",
+    )
+
+    with patch(
+        "agent_framework_openai._chat_client.RawOpenAIChatClient._prepare_options",
+        new_callable=AsyncMock,
+        return_value={
+            "model": "gpt-4.1",
+            "previous_response_id": "resp_abc",
+        },
+    ):
+        result = await client._prepare_options(
+            messages=[Message(role="user", contents="hi")],
+            options={"conversation_id": "agent-session-123"},
+        )
+
+    assert "model" not in result
+    assert "previous_response_id" not in result
+    assert result["extra_body"]["agent_session_id"] == "agent-session-123"
+
+
+async def test_raw_foundry_agent_chat_client_prepare_options_preserves_model_for_resp_continuation() -> None:
+    """Test that model is preserved when conversation_id is a resp_* continuation (HostedAgent v1 / v2-no-session)."""
+
+    mock_project = MagicMock()
+    mock_openai = MagicMock()
+    mock_project.get_openai_client.return_value = mock_openai
+
+    client = RawFoundryAgentChatClient(
+        project_client=mock_project,
+        agent_name="test-agent",
+    )
+
+    with patch(
+        "agent_framework_openai._chat_client.RawOpenAIChatClient._prepare_options",
+        new_callable=AsyncMock,
+        return_value={
+            "model": "gpt-4.1",
+            "previous_response_id": "resp_abc123",
+        },
+    ):
+        result = await client._prepare_options(
+            messages=[Message(role="user", contents="hi")],
+            options={"conversation_id": "resp_abc123"},
+        )
+
+    # model preserved — resp_* is standard Responses API continuity, not a hosted session
+    assert result["model"] == "gpt-4.1"
+    # previous_response_id preserved — not stripped outside hosted session path
+    assert result["previous_response_id"] == "resp_abc123"
+    # no agent_session_id injected
+    assert "extra_body" not in result or "agent_session_id" not in result.get("extra_body", {})
 
 
 async def test_raw_foundry_agent_chat_client_prepare_options_maps_agent_session_id_to_extra_body() -> None: