[responsesAPI][bugfix] serialize harmony messages (#26185)

Signed-off-by: Andrew Xia <axia@meta.com>
Co-authored-by: Ye (Charlotte) Qi <yeq@meta.com>

vllm/entrypoints/openai/protocol.py

@@ -63,6 +63,7 @@ from pydantic import (
     Field,
     TypeAdapter,
     ValidationInfo,
+    field_serializer,
     field_validator,
     model_validator,
 )
@@ -2078,11 +2079,6 @@ class ResponsesResponse(OpenAIBaseModel):
     model: str
     object: Literal["response"] = "response"
     output: list[ResponseOutputItem]
-    # These are populated when enable_response_messages is set to True
-    # TODO: Currently an issue where content of harmony messages
-    # is not available when these are serialized. Metadata is available
-    input_messages: Optional[list[ChatCompletionMessageParam]] = None
-    output_messages: Optional[list[ChatCompletionMessageParam]] = None
     parallel_tool_calls: bool
     temperature: float
     tool_choice: ToolChoice
@@ -2102,6 +2098,49 @@ class ResponsesResponse(OpenAIBaseModel):
     usage: Optional[ResponseUsage] = None
     user: Optional[str] = None
 
+    # --8<-- [start:responses-extra-params]
+    # These are populated when enable_response_messages is set to True
+    # NOTE: custom serialization is needed
+    # see serialize_input_messages and serialize_output_messages
+    input_messages: Optional[list[ChatCompletionMessageParam]] = None
+    output_messages: Optional[list[ChatCompletionMessageParam]] = None
+    # --8<-- [end:responses-extra-params]
+
+    # NOTE: openAI harmony doesn't serialize TextContent properly,
+    # TODO: this fixes for TextContent, but need to verify for tools etc
+    # https://github.com/openai/harmony/issues/78
+    @field_serializer("output_messages", when_used="json")
+    def serialize_output_messages(self, msgs, _info):
+        if msgs:
+            serialized = []
+            for m in msgs:
+                if isinstance(m, dict):
+                    serialized.append(m)
+                elif hasattr(m, "__dict__"):
+                    serialized.append(m.to_dict())
+                else:
+                    # fallback to pyandic dump
+                    serialized.append(m.model_dump_json())
+            return serialized
+        return None
+
+    # NOTE: openAI harmony doesn't serialize TextContent properly, this fixes it
+    # https://github.com/openai/harmony/issues/78
+    @field_serializer("input_messages", when_used="json")
+    def serialize_input_messages(self, msgs, _info):
+        if msgs:
+            serialized = []
+            for m in msgs:
+                if isinstance(m, dict):
+                    serialized.append(m)
+                elif hasattr(m, "__dict__"):
+                    serialized.append(m.to_dict())
+                else:
+                    # fallback to pyandic dump
+                    serialized.append(m.model_dump_json())
+            return serialized
+        return None
+
     @classmethod
     def from_request(
         cls,

vllm/entrypoints/openai/serving_responses.py

@@ -1876,6 +1876,6 @@ class OpenAIServingResponses(OpenAIServing):
                 ResponseCompletedEvent(
                     type="response.completed",
                     sequence_number=-1,
-                    response=final_response.model_dump(),
+                    response=final_response,
                 )
             )