[Bugfix][Frontend] Normalize constrained Harmony recipients

tests/parser/test_harmony.py

@@ -728,6 +728,27 @@ def test_single_channel(self, harmony_parser):
             (s.channel, s.recipient, s.delta) for s in result.segments if s.delta
         ] == [("final", None, "Hello")]
 
+    def test_constrained_output_segment_recipient_normalized(self, harmony_parser):
+        result = harmony_parser.process_chunk(
+            encode_output(
+                '<|channel|>final <|constrain|>json<|message|>{"result":true}<|end|>'
+            )
+        )
+
+        content_segments = [segment for segment in result.segments if segment.delta]
+        assert all(segment.channel == "final" for segment in content_segments)
+        assert all(segment.recipient is None for segment in content_segments)
+        assert (
+            "".join(segment.delta for segment in content_segments) == '{"result":true}'
+        )
+        completed_messages = [
+            segment.completed_message
+            for segment in result.segments
+            if segment.completed_message is not None
+        ]
+        assert len(completed_messages) == 1
+        assert completed_messages[0].recipient is None
+
     def test_cross_channel(self, harmony_parser):
         result = harmony_parser.process_chunk(
             encode_output(

vllm/parser/harmony.py

@@ -100,6 +100,7 @@ def _poll_completed_message(self) -> Message | None:
         if len(messages) <= self._num_processed_messages:
             return None
         msg = messages[self._num_processed_messages]
+        msg.recipient = self._normalize_recipient(msg.recipient)
         self._num_processed_messages += 1
         return msg
 
@@ -192,7 +193,9 @@ def parse_delta(
         *,
         finished: bool,
     ) -> DeltaMessage | None:
-        prev_recipient = self._harmony_parser.current_recipient
+        prev_recipient = self._normalize_recipient(
+            self._harmony_parser.current_recipient
+        )
         result = self.process_chunk(delta_token_ids)
         if finished:
             flushed_segment = self.flush()
@@ -274,7 +277,9 @@ def process_chunk(self, token_ids: Sequence[int]) -> ChunkResult:
         for token_id in token_ids:
             self._harmony_parser.process(token_id)
             channel = self._harmony_parser.current_channel
-            recipient = self._harmony_parser.current_recipient
+            recipient = self._normalize_recipient(
+                self._harmony_parser.current_recipient
+            )
             delta = self._harmony_parser.last_content_delta or ""
             completed_message = self._poll_completed_message()
 
@@ -298,3 +303,14 @@ def process_chunk(self, token_ids: Sequence[int]) -> ChunkResult:
             segments=segments,
             reasoning_token_count=reasoning_token_count,
         )
+
+    @staticmethod
+    def _normalize_recipient(recipient: str | None) -> str | None:
+        """Remove constrained formats misparsed into recipients by older Harmony."""
+        if recipient is None:
+            return None
+
+        constrain_index = recipient.find("<|constrain|>")
+        if constrain_index == -1:
+            return recipient
+        return recipient[:constrain_index].rstrip() or None