[Bugfix] Preserve tool call id/type/name in streaming finish chunk (#31438)

Signed-off-by: amittell <mittell@me.com>
Signed-off-by: Alex Mittell <mittell@me.com>

tests/entrypoints/openai/test_serving_chat.py

@@ -1506,3 +1506,142 @@ async def test_tool_choice_validation_without_parser():
     assert isinstance(response_named, ErrorResponse)
     assert "tool_choice" in response_named.error.message
     assert "--tool-call-parser" in response_named.error.message
+
+
+class TestCreateRemainingArgsDelta:
+    """Tests for _create_remaining_args_delta helper function.
+
+    This helper is used when streaming tool calls to preserve id/type/name
+    fields in the finish chunk, which would otherwise be lost.
+    """
+
+    def test_preserves_id_type_name(self):
+        """Test that id, type, and name are preserved from original delta."""
+        from vllm.entrypoints.openai.protocol import (
+            DeltaFunctionCall,
+            DeltaMessage,
+            DeltaToolCall,
+        )
+        from vllm.entrypoints.openai.serving_chat import OpenAIServingChat
+
+        original_delta = DeltaMessage(
+            tool_calls=[
+                DeltaToolCall(
+                    index=0,
+                    id="call_abc123",
+                    type="function",
+                    function=DeltaFunctionCall(
+                        name="get_weather",
+                        arguments='{"location": "Paris"}',
+                    ),
+                )
+            ]
+        )
+
+        result = OpenAIServingChat._create_remaining_args_delta(
+            original_delta, '", "unit": "celsius"}', 0
+        )
+
+        assert len(result.tool_calls) == 1
+        tc = result.tool_calls[0]
+        assert tc.index == 0
+        assert tc.id == "call_abc123"
+        assert tc.type == "function"
+        assert tc.function.name == "get_weather"
+        assert tc.function.arguments == '", "unit": "celsius"}'
+
+    def test_matches_by_index(self):
+        """Test that the correct tool call is matched by index."""
+        from vllm.entrypoints.openai.protocol import (
+            DeltaFunctionCall,
+            DeltaMessage,
+            DeltaToolCall,
+        )
+        from vllm.entrypoints.openai.serving_chat import OpenAIServingChat
+
+        original_delta = DeltaMessage(
+            tool_calls=[
+                DeltaToolCall(
+                    index=0,
+                    id="call_first",
+                    type="function",
+                    function=DeltaFunctionCall(name="func_a", arguments="{}"),
+                ),
+                DeltaToolCall(
+                    index=1,
+                    id="call_second",
+                    type="function",
+                    function=DeltaFunctionCall(name="func_b", arguments="{}"),
+                ),
+            ]
+        )
+
+        result = OpenAIServingChat._create_remaining_args_delta(
+            original_delta, '{"extra": true}', 1
+        )
+
+        assert len(result.tool_calls) == 1
+        tc = result.tool_calls[0]
+        assert tc.index == 1
+        assert tc.id == "call_second"
+        assert tc.function.name == "func_b"
+
+    def test_no_matching_tool_call(self):
+        """Test graceful handling when no matching tool call is found."""
+        from vllm.entrypoints.openai.protocol import (
+            DeltaFunctionCall,
+            DeltaMessage,
+            DeltaToolCall,
+        )
+        from vllm.entrypoints.openai.serving_chat import OpenAIServingChat
+
+        original_delta = DeltaMessage(
+            tool_calls=[
+                DeltaToolCall(
+                    index=0,
+                    id="call_zero",
+                    type="function",
+                    function=DeltaFunctionCall(name="func", arguments="{}"),
+                )
+            ]
+        )
+
+        result = OpenAIServingChat._create_remaining_args_delta(
+            original_delta, '{"arg": 1}', 5
+        )
+
+        assert len(result.tool_calls) == 1
+        tc = result.tool_calls[0]
+        assert tc.index == 5
+        assert tc.id is None
+        assert tc.type is None
+        assert tc.function.name is None
+        assert tc.function.arguments == '{"arg": 1}'
+
+    def test_function_is_none(self):
+        """Test handling when original tool call has no function."""
+        from vllm.entrypoints.openai.protocol import DeltaMessage, DeltaToolCall
+        from vllm.entrypoints.openai.serving_chat import OpenAIServingChat
+
+        original_delta = DeltaMessage(
+            tool_calls=[
+                DeltaToolCall(
+                    index=0,
+                    id="call_nofunc",
+                    type="function",
+                    function=None,
+                )
+            ]
+        )
+
+        result = OpenAIServingChat._create_remaining_args_delta(
+            original_delta, '{"data": "value"}', 0
+        )
+
+        assert len(result.tool_calls) == 1
+        tc = result.tool_calls[0]
+        assert tc.index == 0
+        assert tc.id == "call_nofunc"
+        assert tc.type == "function"
+        assert tc.function.name is None
+        assert tc.function.arguments == '{"data": "value"}'