Implementation:Openai Openai python Response Reasoning Text Delta

Knowledge Sources	Openai_Openai_python OpenAI API Reference
Domains	API_Types, Responses_API
Last Updated	2026-02-15 00:00 GMT

Overview

Concrete type for representing a streaming delta event emitted when text is incrementally added to reasoning content, provided by the openai-python SDK.

Description

ResponseReasoningTextDeltaEvent is a Pydantic model representing the response.reasoning_text.delta streaming event. It is emitted each time a new text fragment (delta) is appended to the reasoning content (as opposed to the summary). The event includes content_index identifying which content part is being extended, item_id, output_index, and sequence_number.

Usage

Import this type when you need to process the raw reasoning text as it streams in from reasoning models such as o1 or o3.

Code Reference

Source Location

• Repository: openai-python
• File: src/openai/types/responses/response_reasoning_text_delta_event.py

Signature

class ResponseReasoningTextDeltaEvent(BaseModel):
    """Emitted when a delta is added to a reasoning text."""

    content_index: int
    delta: str
    item_id: str
    output_index: int
    sequence_number: int
    type: Literal["response.reasoning_text.delta"]

Import

from openai.types.responses import ResponseReasoningTextDeltaEvent

I/O Contract

Fields

Name	Type	Required	Description
content_index	int	Yes	The index of the reasoning content part this delta is associated with.
delta	str	Yes	The text delta that was added to the reasoning content.
item_id	str	Yes	The ID of the item this reasoning text delta is associated with.
output_index	int	Yes	The index of the output item this reasoning text delta is associated with.
sequence_number	int	Yes	The sequence number of this event.
type	Literal["response.reasoning_text.delta"]	Yes	The type of the event. Always response.reasoning_text.delta.

Usage Examples

import openai

client = openai.OpenAI()

stream = client.responses.create(
    model="o3-mini",
    input="Solve: What is 127 * 389?",
    stream=True,
)

reasoning_text = ""
for event in stream:
    if event.type == "response.reasoning_text.delta":
        reasoning_text += event.delta
        print(event.delta, end="", flush=True)

Related Pages

• Environment:Openai_Openai_python_Python_3_9_Plus