feat(responses): improve streaming for function calls (#3124)

Emit streaming events for function calls

## Test Plan

Improved the test case

llama_stack/providers/inline/agents/meta_reference/openai_responses.py

@@ -33,6 +33,10 @@ from llama_stack.apis.agents.openai_responses import (
     OpenAIResponseObjectStream,
     OpenAIResponseObjectStreamResponseCompleted,
     OpenAIResponseObjectStreamResponseCreated,
+    OpenAIResponseObjectStreamResponseFunctionCallArgumentsDelta,
+    OpenAIResponseObjectStreamResponseFunctionCallArgumentsDone,
+    OpenAIResponseObjectStreamResponseOutputItemAdded,
+    OpenAIResponseObjectStreamResponseOutputItemDone,
     OpenAIResponseObjectStreamResponseOutputTextDelta,
     OpenAIResponseOutput,
     OpenAIResponseOutputMessageContent,
@@ -73,7 +77,9 @@ from llama_stack.apis.tools import ToolGroups, ToolInvocationResult, ToolRuntime
 from llama_stack.apis.vector_io import VectorIO
 from llama_stack.log import get_logger
 from llama_stack.models.llama.datatypes import ToolDefinition, ToolParamDefinition
-from llama_stack.providers.utils.inference.openai_compat import convert_tooldef_to_openai_tool
+from llama_stack.providers.utils.inference.openai_compat import (
+    convert_tooldef_to_openai_tool,
+)
 from llama_stack.providers.utils.responses.responses_store import ResponsesStore
 
 logger = get_logger(name=__name__, category="openai_responses")
@@ -82,7 +88,7 @@ OPENAI_RESPONSES_PREFIX = "openai_responses:"
 
 
 async def _convert_response_content_to_chat_content(
-    content: str | list[OpenAIResponseInputMessageContent] | list[OpenAIResponseOutputMessageContent],
+    content: (str | list[OpenAIResponseInputMessageContent] | list[OpenAIResponseOutputMessageContent]),
 ) -> str | list[OpenAIChatCompletionContentPartParam]:
     """
     Convert the content parts from an OpenAI Response API request into OpenAI Chat Completion content parts.
@@ -150,7 +156,9 @@ async def _convert_response_input_to_chat_messages(
     return messages
 
 
-async def _convert_chat_choice_to_response_message(choice: OpenAIChoice) -> OpenAIResponseMessage:
+async def _convert_chat_choice_to_response_message(
+    choice: OpenAIChoice,
+) -> OpenAIResponseMessage:
     """
     Convert an OpenAI Chat Completion choice into an OpenAI Response output message.
     """
@@ -172,7 +180,9 @@ async def _convert_chat_choice_to_response_message(choice: OpenAIChoice) -> Open
     )
 
 
-async def _convert_response_text_to_chat_response_format(text: OpenAIResponseText) -> OpenAIResponseFormatParam:
+async def _convert_response_text_to_chat_response_format(
+    text: OpenAIResponseText,
+) -> OpenAIResponseFormatParam:
     """
     Convert an OpenAI Response text parameter into an OpenAI Chat Completion response format.
     """
@@ -228,7 +238,9 @@ class OpenAIResponsesImpl:
         self.vector_io_api = vector_io_api
 
     async def _prepend_previous_response(
-        self, input: str | list[OpenAIResponseInput], previous_response_id: str | None = None
+        self,
+        input: str | list[OpenAIResponseInput],
+        previous_response_id: str | None = None,
     ):
         if previous_response_id:
             previous_response_with_input = await self.responses_store.get_response_object(previous_response_id)
@@ -446,6 +458,8 @@ class OpenAIResponsesImpl:
 
             # Create a placeholder message item for delta events
             message_item_id = f"msg_{uuid.uuid4()}"
+            # Track tool call items for streaming events
+            tool_call_item_ids: dict[int, str] = {}
 
             async for chunk in completion_result:
                 chat_response_id = chunk.id
@@ -472,18 +486,62 @@ class OpenAIResponsesImpl:
                     if chunk_choice.delta.tool_calls:
                         for tool_call in chunk_choice.delta.tool_calls:
                             response_tool_call = chat_response_tool_calls.get(tool_call.index, None)
-                            if response_tool_call:
-                                # Don't attempt to concatenate arguments if we don't have any new argumentsAdd commentMore actions
-                                if tool_call.function.arguments:
-                                    # Guard against an initial None argument before we concatenate
-                                    response_tool_call.function.arguments = (
-                                        response_tool_call.function.arguments or ""
-                                    ) + tool_call.function.arguments
-                            else:
+                            # Create new tool call entry if this is the first chunk for this index
+                            is_new_tool_call = response_tool_call is None
+                            if is_new_tool_call:
                                 tool_call_dict: dict[str, Any] = tool_call.model_dump()
                                 tool_call_dict.pop("type", None)
                                 response_tool_call = OpenAIChatCompletionToolCall(**tool_call_dict)
-                            chat_response_tool_calls[tool_call.index] = response_tool_call
+                                chat_response_tool_calls[tool_call.index] = response_tool_call
+
+                                # Create item ID for this tool call for streaming events
+                                tool_call_item_id = f"fc_{uuid.uuid4()}"
+                                tool_call_item_ids[tool_call.index] = tool_call_item_id
+
+                                # Emit output_item.added event for the new function call
+                                sequence_number += 1
+                                function_call_item = OpenAIResponseOutputMessageFunctionToolCall(
+                                    arguments="",  # Will be filled incrementally via delta events
+                                    call_id=tool_call.id or "",
+                                    name=tool_call.function.name if tool_call.function else "",
+                                    id=tool_call_item_id,
+                                    status="in_progress",
+                                )
+                                yield OpenAIResponseObjectStreamResponseOutputItemAdded(
+                                    response_id=response_id,
+                                    item=function_call_item,
+                                    output_index=len(output_messages),
+                                    sequence_number=sequence_number,
+                                )
+
+                            # Stream function call arguments as they arrive
+                            if tool_call.function and tool_call.function.arguments:
+                                tool_call_item_id = tool_call_item_ids[tool_call.index]
+                                sequence_number += 1
+                                yield OpenAIResponseObjectStreamResponseFunctionCallArgumentsDelta(
+                                    delta=tool_call.function.arguments,
+                                    item_id=tool_call_item_id,
+                                    output_index=len(output_messages),
+                                    sequence_number=sequence_number,
+                                )
+
+                                # Accumulate arguments for final response (only for subsequent chunks)
+                                if not is_new_tool_call:
+                                    response_tool_call.function.arguments = (
+                                        response_tool_call.function.arguments or ""
+                                    ) + tool_call.function.arguments
+
+            # Emit function_call_arguments.done events for completed tool calls
+            for tool_call_index in sorted(chat_response_tool_calls.keys()):
+                tool_call_item_id = tool_call_item_ids[tool_call_index]
+                final_arguments = chat_response_tool_calls[tool_call_index].function.arguments or ""
+                sequence_number += 1
+                yield OpenAIResponseObjectStreamResponseFunctionCallArgumentsDone(
+                    arguments=final_arguments,
+                    item_id=tool_call_item_id,
+                    output_index=len(output_messages),
+                    sequence_number=sequence_number,
+                )
 
             # Convert collected chunks to complete response
             if chat_response_tool_calls:
@@ -532,18 +590,56 @@ class OpenAIResponsesImpl:
                 tool_call_log, tool_response_message = await self._execute_tool_call(tool_call, ctx)
                 if tool_call_log:
                     output_messages.append(tool_call_log)
+
+                    # Emit output_item.done event for completed non-function tool call
+                    # Find the item_id for this tool call
+                    matching_item_id = None
+                    for index, item_id in tool_call_item_ids.items():
+                        response_tool_call = chat_response_tool_calls.get(index)
+                        if response_tool_call and response_tool_call.id == tool_call.id:
+                            matching_item_id = item_id
+                            break
+
+                    if matching_item_id:
+                        sequence_number += 1
+                        yield OpenAIResponseObjectStreamResponseOutputItemDone(
+                            response_id=response_id,
+                            item=tool_call_log,
+                            output_index=len(output_messages) - 1,
+                            sequence_number=sequence_number,
+                        )
+
                 if tool_response_message:
                     next_turn_messages.append(tool_response_message)
 
             for tool_call in function_tool_calls:
-                output_messages.append(
-                    OpenAIResponseOutputMessageFunctionToolCall(
-                        arguments=tool_call.function.arguments or "",
-                        call_id=tool_call.id,
-                        name=tool_call.function.name or "",
-                        id=f"fc_{uuid.uuid4()}",
-                        status="completed",
-                    )
+                # Find the item_id for this tool call from our tracking dictionary
+                matching_item_id = None
+                for index, item_id in tool_call_item_ids.items():
+                    response_tool_call = chat_response_tool_calls.get(index)
+                    if response_tool_call and response_tool_call.id == tool_call.id:
+                        matching_item_id = item_id
+                        break
+
+                # Use existing item_id or create new one if not found
+                final_item_id = matching_item_id or f"fc_{uuid.uuid4()}"
+
+                function_call_item = OpenAIResponseOutputMessageFunctionToolCall(
+                    arguments=tool_call.function.arguments or "",
+                    call_id=tool_call.id,
+                    name=tool_call.function.name or "",
+                    id=final_item_id,
+                    status="completed",
+                )
+                output_messages.append(function_call_item)
+
+                # Emit output_item.done event for completed function call
+                sequence_number += 1
+                yield OpenAIResponseObjectStreamResponseOutputItemDone(
+                    response_id=response_id,
+                    item=function_call_item,
+                    output_index=len(output_messages) - 1,
+                    sequence_number=sequence_number,
                 )
 
             if not function_tool_calls and not non_function_tool_calls:
@@ -779,7 +875,8 @@ class OpenAIResponsesImpl:
                 )
             elif function.name == "knowledge_search":
                 response_file_search_tool = next(
-                    (t for t in ctx.response_tools if isinstance(t, OpenAIResponseInputToolFileSearch)), None
+                    (t for t in ctx.response_tools if isinstance(t, OpenAIResponseInputToolFileSearch)),
+                    None,
                 )
                 if response_file_search_tool:
                     # Use vector_stores.search API instead of knowledge_search tool
@@ -798,7 +895,9 @@ class OpenAIResponsesImpl:
             error_exc = e
 
         if function.name in ctx.mcp_tool_to_server:
-            from llama_stack.apis.agents.openai_responses import OpenAIResponseOutputMessageMCPCall
+            from llama_stack.apis.agents.openai_responses import (
+                OpenAIResponseOutputMessageMCPCall,
+            )
 
             message = OpenAIResponseOutputMessageMCPCall(
                 id=tool_call_id,
@@ -850,7 +949,10 @@ class OpenAIResponsesImpl:
             if isinstance(result.content, str):
                 content = result.content
             elif isinstance(result.content, list):
-                from llama_stack.apis.common.content_types import ImageContentItem, TextContentItem
+                from llama_stack.apis.common.content_types import (
+                    ImageContentItem,
+                    TextContentItem,
+                )
 
                 content = []
                 for item in result.content: