Streaming output parameters for the Assistant API (being deprecated)

The Assistant API is being deprecated. Migrate to the Responses API, which offers built-in tools and multi-turn context management.

Message delta object

Event name: thread.message.delta

A message fragment generated by the Large Language Model (LLM) during a streaming run.

Field name	Type	Description
id	string	The message identifier.
object	string	The object type. It is the same as the event name. It is always `thread.message.delta`.
delta	object	The message fragment object generated by the LLM.
delta.role	string	The role that generated the message. The value can be `user` or `assistant`.
delta.file_ids	array	This field is not used.
delta.content	object	The message content object.
delta.content.type	string	The message content type. It is always `text`.
delta.content.text	object	The text content object in the message.
delta.content.text.annotations	array	This field is not used.
delta.content.text.value	string	The text content in the message.

Example of a message delta object

{
    "delta": {
        "content": {
            "text": {
                "annotations": [],
                "value": "[REDACTED]"
            },
            "type": "text"
        },
        "role": "[REDACTED]",
        "file_ids": []
    },
    "id": "[REDACTED]",
    "object": "thread.message.delta",
    "request_id": "[REDACTED]",
    "status_code": 200
}

Run step delta object

Event name: thread.run.step.delta

A message fragment returned by a tool call during a streaming run.

Field Name	Type	Description
id	string	The run step identifier.
object	string	The object type. It is the same as the event name. It is always `thread.run.step.delta`.
delta	object	The message fragment object returned by the tool call.
delta.step_details	object	The detailed step information object for the fragment.
delta.step_details.type	string	The step details type. The value can be `message_creation` or `tool_calls`.
delta.step_details.message_creation	object	The details of the message created in the step.
delta.step_details.message_creation.type	string	It is always `message_creation`.
delta.step_details.message_creation.message_id	string	The ID of the message created in this step.
delta.step_details.tool_calls	array	A list of details for the tool calls.
delta.step_details.tool_calls.type	string	The type of the tool call. The value can be `code_interpreter`, `quark_search`, `text_to_image`, or `calculator`.
delta.step_details.tool_calls.code_interpreter	object	The message object for the code interpreter.
delta.step_details.tool_calls.code_interpreter.arguments	string	The input parameters for the code interpreter. This includes the code type and the complete code. It is generated by the LLM.
delta.step_details.tool_calls.code_interpreter.output	string	The output parameters from the code interpreter. This includes the execution result. It is generated by the code interpreter.
delta.step_details.tool_calls.quark_search	object	The message object for Quark Search.
delta.step_details.tool_calls.quark_search.arguments	string	The input parameters for Quark Search. This includes the rewritten query statement. It is generated by the LLM.
delta.step_details.tool_calls.quark_search.output	string	The output parameters from Quark Search. This includes a success mark, error code, error message, and a list of search results (title, address, description, type, message source). It is generated by Quark Search.
delta.step_details.tool_calls.text_to_image	object	The message object for text-to-image.
delta.step_details.tool_calls.text_to_image.arguments	string	The input parameters for text-to-image. This includes the model name, style, image size, number of images, positive prompt, and negative prompt. It is generated by the LLM.
delta.step_details.tool_calls.text_to_image.output	string	The output parameters from text-to-image. This includes the request ID, task ID, task status, submission time, estimated completion time, end time, generated result, number of images, success mark, and failure mark. It is generated by the text-to-image tool.
delta.step_details.tool_calls.calculator	object	The message object for the calculator.
delta.step_details.tool_calls.calculator.arguments	string	The input parameters for the calculator. This includes the input formula and request header. It is generated by the LLM.
delta.step_details.tool_calls.calculator.output	string	The output parameters from the calculator. This includes the formula and the result. It is generated by the calculator.

The code interpreter, Quark Search, text-to-image, and calculator tools support streaming output.Other Assistant API tools do not support streaming output. To retrieve the results of these tool calls in the streaming output, use the standard tool calling method. For more information, see the relevant section in Tool calling overview.

Example of a run step delta object

{
    "delta": {
        "step_details": {
            "tool_calls": [
                {
                    "code_interpreter": {
                        "output": [],
                        "arguments": "[REDACTED]"
                    },
                    "type": "code_interpreter"
                }
            ],
            "type": "tool_calls"
        }
    },
    "id": "[REDACTED]",
    "object": "thread.run.step.delta",
    "request_id": "[REDACTED]",
    "status_code": 200
}

Assistant event stream

Events that occur when you run an Assistant in streaming mode. In the Assistant API, an event stream consists of an event and data:

event: The server sends an event when a new object is created, the state changes, or content is streamed. For example, an event is sent when a new run is created (thread.run.created) or when a run is completed (thread.run.completed).
data: Contains detailed data related to the event. For example, it can contain status, text, tool messages, or errors.

event: thread.created
data: {"id": "thread_123", "object": "thread", ...}

Event name	Data type	Description
thread.created	thread	Occurs when a new thread is created.
thread.run.created	run	Occurs when a new run is created.
thread.run.queued	run	Occurs when a run moves to the `queued` state.
thread.run.in_progress	run	Occurs when a run moves to the `in_progress` state.
thread.run.requires_action	run	Occurs when a run moves to the `requires_action` state.
thread.run.completed	run	Occurs when a run is completed.
thread.run.failed	run	Occurs when a run fails.
thread.run.cancelled	run	Occurs when a run is canceled.
thread.run.expired	run	Occurs when a run expires.
thread.run.step.created	run step	Occurs when a run step is created.
thread.run.step.in_progress	run step	Occurs when a run step moves to the `in_progress` state.
thread.run.step.delta	run step delta	Occurs when part of a run step's content is streamed.
thread.run.step.completed	run step	Occurs when a run step is completed.
thread.run.step.failed	run step	Occurs when a run step fails.
thread.run.step.cancelled	run step	Occurs when a run step is canceled.
thread.run.step.expired	run step	Occurs when a run step expires.
thread.message.created	message	Occurs when a message is created.
thread.message.in_progress	message	Occurs when a message enters the `in_progress` state.
thread.message.delta	message delta	Occurs when part of a message's content is streamed.
thread.message.completed	message	Occurs when a message is completed.
thread.message.incomplete	message	Occurs when a message ends before it is completed.
error	error	Triggered when an error occurs. This can be caused by an internal server error or a timeout.

FAQ

If you encounter a code execution error when you configure streaming output, see Error messages to troubleshoot the error.
To learn how to configure streaming output for the Assistant API, see Quick Start for Assistant API streaming output.

​Message delta object

​Run step delta object

​Assistant event stream

​FAQ

Message delta object

Run step delta object

Assistant event stream

FAQ