diff --git a/tests/entrypoints/openai/responses/test_harmony_utils.py b/tests/entrypoints/openai/responses/test_harmony_utils.py
index f1434ce2bd58..b9540c5c1313 100644
--- a/tests/entrypoints/openai/responses/test_harmony_utils.py
+++ b/tests/entrypoints/openai/responses/test_harmony_utils.py
@@ -2,6 +2,8 @@
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 """Unit tests for vllm.entrypoints.openai.responses.harmony."""
 
+from types import SimpleNamespace
+
 from openai.types.responses import (
     ResponseFunctionToolCall,
     ResponseOutputMessage,
@@ -10,11 +12,20 @@
 from openai.types.responses.response_output_item import McpCall
 from openai_harmony import Author, Message, Role, TextContent
 
+from vllm.entrypoints.openai.parser.harmony_utils import (
+    get_encoding,
+    get_streamable_parser_for_assistant,
+)
 from vllm.entrypoints.openai.responses.harmony import (
     harmony_to_response_output,
     parser_state_to_response_output,
     response_previous_input_to_harmony,
 )
+from vllm.entrypoints.openai.responses.streaming_events import (
+    StreamingState,
+    emit_content_delta_events,
+    emit_previous_item_done_events,
+)
 
 
 class TestResponsePreviousInputToHarmony:
@@ -631,3 +642,49 @@ def test_parser_state_to_response_output_analysis_channel() -> None:
     assert len(builtin_items) == 1
     assert not isinstance(builtin_items[0], McpCall)
     assert builtin_items[0].type == "reasoning"
+
+
+class TestConstrainedOutputWithoutRecipient:
+    @staticmethod
+    def _parse(text: str):
+        parser = get_streamable_parser_for_assistant()
+        for token in get_encoding().encode(text, allowed_special="all"):
+            parser.process(token)
+        return parser
+
+    def test_completed_output(self) -> None:
+        parser = self._parse(
+            '<|channel|>final <|constrain|>json<|message|>{"result":true}<|return|>',
+        )
+
+        message = parser.messages[0]
+        output_items = harmony_to_response_output(message)
+        assert len(output_items) == 1
+        assert isinstance(output_items[0], ResponseOutputMessage)
+        assert output_items[0].content[0].text == '{"result":true}'
+
+        state = StreamingState(
+            current_content_index=0,
+            current_item_id="msg_test",
+        )
+        events = emit_previous_item_done_events(message, state)
+        assert "response.output_text.done" in [event.type for event in events]
+
+    def test_in_progress_output(self) -> None:
+        parser = self._parse(
+            '<|channel|>final <|constrain|>json<|message|>{"result":true}',
+        )
+
+        output_items = parser_state_to_response_output(parser)
+        assert len(output_items) == 1
+        assert isinstance(output_items[0], ResponseOutputMessage)
+        assert output_items[0].content[0].text == '{"result":true}'
+
+        context = SimpleNamespace(
+            last_content_delta=parser.current_content,
+            parser=parser,
+            function_tool_names=None,
+        )
+        events = emit_content_delta_events(context, StreamingState())
+        event_types = [event.type for event in events]
+        assert "response.output_text.delta" in event_types
diff --git a/vllm/entrypoints/openai/responses/harmony.py b/vllm/entrypoints/openai/responses/harmony.py
index 8dee0d993d5f..19fdfcf99893 100644
--- a/vllm/entrypoints/openai/responses/harmony.py
+++ b/vllm/entrypoints/openai/responses/harmony.py
@@ -424,6 +424,13 @@ def _parse_message_no_recipient(
 # ---------------------------------------------------------------------------
 
 
+def get_response_output_recipient(recipient: str | None) -> str | None:
+    """Exclude constrained formats misparsed as recipients by older Harmony."""
+    if recipient and recipient.startswith("<|constrain|>"):
+        return None
+    return recipient
+
+
 def harmony_to_response_output(
     message: Message,
     function_tool_names: frozenset[str] | None = None,
@@ -439,7 +446,7 @@ def harmony_to_response_output(
         return []
 
     output_items: list[ResponseOutputItem] = []
-    recipient = message.recipient
+    recipient = get_response_output_recipient(message.recipient)
 
     if recipient is not None:
         # Browser tool calls (browser.search, browser.open, browser.find)
@@ -478,7 +485,7 @@ def parser_state_to_response_output(
         return []
     if parser.current_role != Role.ASSISTANT:
         return []
-    current_recipient = parser.current_recipient
+    current_recipient = get_response_output_recipient(parser.current_recipient)
     if current_recipient is not None and current_recipient.startswith("browser."):
         return []
 
diff --git a/vllm/entrypoints/openai/responses/streaming_events.py b/vllm/entrypoints/openai/responses/streaming_events.py
index 7447347fba66..472c0f79a9b3 100644
--- a/vllm/entrypoints/openai/responses/streaming_events.py
+++ b/vllm/entrypoints/openai/responses/streaming_events.py
@@ -67,6 +67,7 @@
     is_function_recipient,
 )
 from vllm.entrypoints.openai.responses.context import StreamingHarmonyContext
+from vllm.entrypoints.openai.responses.harmony import get_response_output_recipient
 from vllm.entrypoints.openai.responses.protocol import (
     ResponseReasoningPartAddedEvent,
     ResponseReasoningPartDoneEvent,
@@ -571,7 +572,7 @@ def emit_content_delta_events(
         return []
 
     channel = ctx.parser.current_channel
-    recipient = ctx.parser.current_recipient
+    recipient = get_response_output_recipient(ctx.parser.current_recipient)
 
     if channel in ("final", "commentary") and recipient is None:
         # Preambles (commentary with no recipient) and final messages
@@ -605,7 +606,7 @@ def emit_previous_item_done_events(
     Harmony parser's message object and delegates to shared leaf helpers.
     """
     text = previous_item.content[0].text
-    if previous_item.recipient is not None:
+    if get_response_output_recipient(previous_item.recipient) is not None:
         # Deal with tool call
         if is_function_recipient(previous_item.recipient, function_tool_names):
             function_name = extract_function_from_recipient(previous_item.recipient)