[Bugfix] Set stop_sequence explicitly in streaming message_delta event (#55325)

Signed-off-by: Colin McNamara <colin@2cups.com>

tests/entrypoints/anthropic/test_anthropic_messages_conversion.py

@@ -1610,6 +1610,32 @@ class TestStopSequenceReason:
         assert msg_deltas[0]["delta"]["stop_reason"] == "stop_sequence"
         assert msg_deltas[0]["delta"]["stop_sequence"] == "</tool>"
 
+    @pytest.mark.asyncio
+    async def test_streaming_no_stop_string_emits_explicit_null_stop_sequence(self):
+        """exclude_unset=True drops stop_sequence unless it is set explicitly."""
+
+        async def sse_input():
+            yield _make_stream_chunk(delta=DeltaMessage(role="assistant"))
+            yield _make_stream_chunk(delta=DeltaMessage(content="hi"))
+            yield _make_stream_chunk(finish_reason="stop")
+            yield _make_stream_chunk(
+                choices=[],
+                usage=UsageInfo(prompt_tokens=5, total_tokens=8, completion_tokens=3),
+            )
+            yield "data: [DONE]"
+
+        converter = _make_stream_converter()
+        output = []
+        async for event in converter.message_stream_converter(sse_input()):
+            output.append(event)
+
+        events = _parse_sse_events(output)
+        msg_deltas = [data for ev_type, data in events if ev_type == "message_delta"]
+        assert len(msg_deltas) == 1
+        assert msg_deltas[0]["delta"]["stop_reason"] == "end_turn"
+        assert "stop_sequence" in msg_deltas[0]["delta"]
+        assert msg_deltas[0]["delta"]["stop_sequence"] is None
+
 
 # ======================================================================
 # Client-caused errors are 4xx, not 500 (Issue #52088)

vllm/entrypoints/anthropic/serving.py

@@ -842,7 +842,9 @@ class AnthropicServingMessages(OpenAIServingChat):
                                 stop_delta = AnthropicDelta(
                                     stop_reason=self.stop_reason_map.get(
                                         finish_reason or "stop"
-                                    )
+                                    ),
+                                    # Set explicitly so exclude_unset=True keeps it.
+                                    stop_sequence=None,
                                 )
                             chunk = AnthropicStreamEvent(
                                 type="message_delta",