Create session

Deprecated

POST/realtime/sessions

Create an ephemeral API token for use in client-side applications with the Realtime API. Can be configured with the same session parameters as the session.update client event.

It responds with a session object, plus a client_secret key which contains a usable ephemeral API token that can be used to authenticate browser clients for the Realtime API.

Returns the created Realtime session object, plus an ephemeral key.

Body ParametersJSONExpand Collapse

client_secret: object { expires_at, value }

Ephemeral key returned by the API.

expires_at: number

Timestamp for when the token expires. Currently, all tokens expire after one minute.

value: string

Ephemeral key usable in client environments to authenticate connections to the Realtime API. Use this in client-side environments rather than a standard API token, which should only be used server-side.

input_audio_format: optional string

The format of input audio. Options are pcm16, g711_ulaw, or g711_alaw.

input_audio_transcription: optional object { model }

Configuration for input audio transcription, defaults to off and can be set to null to turn off once on. Input audio transcription is not native to the model, since the model consumes audio directly. Transcription runs asynchronously and should be treated as rough guidance rather than the representation understood by the model.

model: optional string

The model to use for transcription.

instructions: optional string

The default system instructions (i.e. system message) prepended to model calls. This field allows the client to guide the model on desired responses. The model can be instructed on response content and format, (e.g. "be extremely succinct", "act friendly", "here are examples of good responses") and on audio behavior (e.g. "talk quickly", "inject emotion into your voice", "laugh frequently"). The instructions are not guaranteed to be followed by the model, but they provide guidance to the model on the desired behavior. Note that the server sets default instructions which will be used if this field is not set and are visible in the session.created event at the start of the session.

max_response_output_tokens: optional number or "inf"

Maximum number of output tokens for a single assistant response, inclusive of tool calls. Provide an integer between 1 and 4096 to limit output tokens, or inf for the maximum available tokens for a given model. Defaults to inf.

One of the following:

number

"inf"

modalities: optional array of "text" or "audio"

The set of modalities the model can respond with. To disable audio, set this to ["text"].

One of the following:

"text"

"audio"

output_audio_format: optional string

The format of output audio. Options are pcm16, g711_ulaw, or g711_alaw.

prompt: optional ResponsePrompt { id, variables, version }

Reference to a prompt template and its variables. Learn more.

id: string

The unique identifier of the prompt template to use.

variables: optional map[string or ResponseInputText { text, type } or ResponseInputImage { detail, type, file_id, image_url } or ResponseInputFile { type, file_data, file_id, 2 more } ]

Optional map of values to substitute in for variables in your prompt. The substitution values can either be strings, or other Response input types like images or files.

One of the following:

string

ResponseInputText = object { text, type }

A text input to the model.

text: string

The text input to the model.

type: "input_text"

The type of the input item. Always input_text.

ResponseInputImage = object { detail, type, file_id, image_url }

An image input to the model. Learn about image inputs.

detail: "low" or "high" or "auto" or "original"

The detail level of the image to be sent to the model. One of high, low, auto, or original. Defaults to auto.

One of the following:

"low"

"high"

"auto"

"original"

type: "input_image"

The type of the input item. Always input_image.

file_id: optional string

The ID of the file to be sent to the model.

image_url: optional string

The URL of the image to be sent to the model. A fully qualified URL or base64 encoded image in a data URL.

ResponseInputFile = object { type, file_data, file_id, 2 more }

A file input to the model.

type: "input_file"

The type of the input item. Always input_file.

file_data: optional string

The content of the file to be sent to the model.

file_id: optional string

The ID of the file to be sent to the model.

file_url: optional string

The URL of the file to be sent to the model.

filename: optional string

The name of the file to be sent to the model.

version: optional string

Optional version of the prompt template.

speed: optional number

The speed of the model's spoken response. 1.0 is the default speed. 0.25 is the minimum speed. 1.5 is the maximum speed. This value can only be changed in between model turns, not while a response is in progress.

maximum1.5

minimum0.25

temperature: optional number

Sampling temperature for the model, limited to [0.6, 1.2]. Defaults to 0.8.

tool_choice: optional string

How the model chooses tools. Options are auto, none, required, or specify a function.

tools: optional array of object { description, name, parameters, type }

Tools (functions) available to the model.

description: optional string

The description of the function, including guidance on when and how to call it, and guidance about what to tell the user when calling (if anything).

name: optional string

The name of the function.

parameters: optional unknown

Parameters of the function in JSON Schema.

type: optional "function"

The type of the tool, i.e. function.

tracing: optional "auto" or object { group_id, metadata, workflow_name }

Configuration options for tracing. Set to null to disable tracing. Once tracing is enabled for a session, the configuration cannot be modified.

auto will create a trace for the session with default values for the workflow name, group id, and metadata.

One of the following:

"auto"

Default tracing mode for the session.

TracingConfiguration = object { group_id, metadata, workflow_name }

Granular configuration for tracing.

group_id: optional string

The group id to attach to this trace to enable filtering and grouping in the traces dashboard.

metadata: optional unknown

The arbitrary metadata to attach to this trace to enable filtering in the traces dashboard.

workflow_name: optional string

The name of the workflow to attach to this trace. This is used to name the trace in the traces dashboard.

truncation: optional RealtimeTruncation

When the number of tokens in a conversation exceeds the model's input token limit, the conversation be truncated, meaning messages (starting from the oldest) will not be included in the model's context. A 32k context model with 4,096 max output tokens can only include 28,224 tokens in the context before truncation occurs.

Clients can configure truncation behavior to truncate with a lower max token limit, which is an effective way to control token usage and cost.

Truncation will reduce the number of cached tokens on the next turn (busting the cache), since messages are dropped from the beginning of the context. However, clients can also configure truncation to retain messages up to a fraction of the maximum context size, which will reduce the need for future truncations and thus improve the cache rate.

Truncation can be disabled entirely, which means the server will never truncate but would instead return an error if the conversation exceeds the model's input token limit.

One of the following:

"auto" or "disabled"

The truncation strategy to use for the session. auto is the default truncation strategy. disabled will disable truncation and emit errors when the conversation exceeds the input token limit.

One of the following:

"auto"

"disabled"

RetentionRatioTruncation = object { retention_ratio, type, token_limits }

Retain a fraction of the conversation tokens when the conversation exceeds the input token limit. This allows you to amortize truncations across multiple turns, which can help improve cached token usage.

retention_ratio: number

Fraction of post-instruction conversation tokens to retain (0.0 - 1.0) when the conversation exceeds the input token limit. Setting this to 0.8 means that messages will be dropped until 80% of the maximum allowed tokens are used. This helps reduce the frequency of truncations and improve cache rates.

minimum0

maximum1

type: "retention_ratio"

Use retention ratio truncation.

token_limits: optional object { post_instructions }

Optional custom token limits for this truncation strategy. If not provided, the model's default token limits will be used.

post_instructions: optional number

Maximum tokens allowed in the conversation after instructions (which including tool definitions). For example, setting this to 5,000 would mean that truncation would occur when the conversation exceeds 5,000 tokens after instructions. This cannot be higher than the model's context window size minus the maximum output tokens.

minimum0

turn_detection: optional object { prefix_padding_ms, silence_duration_ms, threshold, type }

Configuration for turn detection. Can be set to null to turn off. Server VAD means that the model will detect the start and end of speech based on audio volume and respond at the end of user speech.

prefix_padding_ms: optional number

Amount of audio to include before the VAD detected speech (in milliseconds). Defaults to 300ms.

silence_duration_ms: optional number

Duration of silence to detect speech stop (in milliseconds). Defaults to 500ms. With shorter values the model will respond more quickly, but may jump in on short pauses from the user.

threshold: optional number

Activation threshold for VAD (0.0 to 1.0), this defaults to 0.5. A higher threshold will require louder audio to activate the model, and thus might perform better in noisy environments.

type: optional string

Type of turn detection, only server_vad is currently supported.

voice: optional string or "alloy" or "ash" or "ballad" or 7 more or object { id }

The voice the model uses to respond. Supported built-in voices are alloy, ash, ballad, coral, echo, sage, shimmer, verse, marin, and cedar. You may also provide a custom voice object with an id, for example { "id": "voice_1234" }. Voice cannot be changed during the session once the model has responded with audio at least once.

One of the following:

string

"alloy" or "ash" or "ballad" or 7 more

One of the following:

"alloy"

"ash"

"ballad"

"coral"

"echo"

"sage"

"shimmer"

"verse"

"marin"

"cedar"

ID = object { id }

Custom voice reference.

id: string

The custom voice ID, e.g. voice_1234.

ReturnsExpand Collapse

id: optional string

Unique identifier for the session that looks like sess_1234567890abcdef.

audio: optional object { input, output }

Configuration for input and output audio for the session.

input: optional object { format, noise_reduction, transcription, turn_detection }

format: optional RealtimeAudioFormats

The PCM audio format. Only a 24kHz sample rate is supported.

One of the following:

PCMAudioFormat = object { rate, type }

The PCM audio format. Only a 24kHz sample rate is supported.

rate: optional 24000

The sample rate of the audio. Always 24000.

type: optional "audio/pcm"

The audio format. Always audio/pcm.

PCMUAudioFormat = object { type }

The G.711 μ-law format.

type: optional "audio/pcmu"

The audio format. Always audio/pcmu.

PCMAAudioFormat = object { type }

The G.711 A-law format.

type: optional "audio/pcma"

The audio format. Always audio/pcma.

noise_reduction: optional object { type }

Configuration for input audio noise reduction.

type: optional NoiseReductionType

Type of noise reduction. near_field is for close-talking microphones such as headphones, far_field is for far-field microphones such as laptop or conference room microphones.

One of the following:

"near_field"

"far_field"

transcription: optional AudioTranscription { language, model, prompt }

Configuration for input audio transcription.

language: optional string

The language of the input audio. Supplying the input language in ISO-639-1 (e.g. en) format will improve accuracy and latency.

model: optional string or "whisper-1" or "gpt-4o-mini-transcribe" or "gpt-4o-mini-transcribe-2025-12-15" or 2 more

The model to use for transcription. Current options are whisper-1, gpt-4o-mini-transcribe, gpt-4o-mini-transcribe-2025-12-15, gpt-4o-transcribe, and gpt-4o-transcribe-diarize. Use gpt-4o-transcribe-diarize when you need diarization with speaker labels.

One of the following:

string

"whisper-1" or "gpt-4o-mini-transcribe" or "gpt-4o-mini-transcribe-2025-12-15" or 2 more

One of the following:

"whisper-1"

"gpt-4o-mini-transcribe"

"gpt-4o-mini-transcribe-2025-12-15"

"gpt-4o-transcribe"

"gpt-4o-transcribe-diarize"

prompt: optional string

An optional text to guide the model's style or continue a previous audio segment. For whisper-1, the prompt is a list of keywords. For gpt-4o-transcribe models (excluding gpt-4o-transcribe-diarize), the prompt is a free text string, for example "expect words related to technology".

turn_detection: optional object { prefix_padding_ms, silence_duration_ms, threshold, type }

Configuration for turn detection.

prefix_padding_ms: optional number

silence_duration_ms: optional number

threshold: optional number

type: optional string

Type of turn detection, only server_vad is currently supported.

output: optional object { format, speed, voice }

format: optional RealtimeAudioFormats

The PCM audio format. Only a 24kHz sample rate is supported.

One of the following:

PCMAudioFormat = object { rate, type }

The PCM audio format. Only a 24kHz sample rate is supported.

rate: optional 24000

The sample rate of the audio. Always 24000.

type: optional "audio/pcm"

The audio format. Always audio/pcm.

PCMUAudioFormat = object { type }

The G.711 μ-law format.

type: optional "audio/pcmu"

The audio format. Always audio/pcmu.

PCMAAudioFormat = object { type }

The G.711 A-law format.

type: optional "audio/pcma"

The audio format. Always audio/pcma.

speed: optional number

voice: optional string or "alloy" or "ash" or "ballad" or 7 more

One of the following:

string

"alloy" or "ash" or "ballad" or 7 more

One of the following:

"alloy"

"ash"

"ballad"

"coral"

"echo"

"sage"

"shimmer"

"verse"

"marin"

"cedar"

expires_at: optional number

Expiration timestamp for the session, in seconds since epoch.

include: optional array of "item.input_audio_transcription.logprobs"

Additional fields to include in server outputs.

item.input_audio_transcription.logprobs: Include logprobs for input audio transcription.

instructions: optional string

Note that the server sets default instructions which will be used if this field is not set and are visible in the session.created event at the start of the session.

max_output_tokens: optional number or "inf"

One of the following:

number

"inf"

model: optional string

The Realtime model used for this session.

object: optional string

The object type. Always realtime.session.

output_modalities: optional array of "text" or "audio"

The set of modalities the model can respond with. To disable audio, set this to ["text"].

One of the following:

"text"

"audio"

tool_choice: optional string

How the model chooses tools. Options are auto, none, required, or specify a function.

tools: optional array of RealtimeFunctionTool { description, name, parameters, type }

Tools (functions) available to the model.

description: optional string

The description of the function, including guidance on when and how to call it, and guidance about what to tell the user when calling (if anything).

name: optional string

The name of the function.

parameters: optional unknown

Parameters of the function in JSON Schema.

type: optional "function"

The type of the tool, i.e. function.

tracing: optional "auto" or object { group_id, metadata, workflow_name }

Configuration options for tracing. Set to null to disable tracing. Once tracing is enabled for a session, the configuration cannot be modified.

auto will create a trace for the session with default values for the workflow name, group id, and metadata.

One of the following:

"auto"

Default tracing mode for the session.

TracingConfiguration = object { group_id, metadata, workflow_name }

Granular configuration for tracing.

group_id: optional string

The group id to attach to this trace to enable filtering and grouping in the traces dashboard.

metadata: optional unknown

The arbitrary metadata to attach to this trace to enable filtering in the traces dashboard.

workflow_name: optional string

The name of the workflow to attach to this trace. This is used to name the trace in the traces dashboard.

turn_detection: optional object { prefix_padding_ms, silence_duration_ms, threshold, type }

prefix_padding_ms: optional number

Amount of audio to include before the VAD detected speech (in milliseconds). Defaults to 300ms.

silence_duration_ms: optional number

Duration of silence to detect speech stop (in milliseconds). Defaults to 500ms. With shorter values the model will respond more quickly, but may jump in on short pauses from the user.

threshold: optional number

Activation threshold for VAD (0.0 to 1.0), this defaults to 0.5. A higher threshold will require louder audio to activate the model, and thus might perform better in noisy environments.

type: optional string

Type of turn detection, only server_vad is currently supported.

Create session

curl -X POST https://api.openai.com/v1/realtime/sessions \
  -H "Authorization: Bearer $OPENAI_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-realtime",
    "modalities": ["audio", "text"],
    "instructions": "You are a friendly assistant."
  }'

{
  "id": "sess_001",
  "object": "realtime.session",
  "model": "gpt-realtime-2025-08-25",
  "modalities": ["audio", "text"],
  "instructions": "You are a friendly assistant.",
  "voice": "alloy",
  "input_audio_format": "pcm16",
  "output_audio_format": "pcm16",
  "input_audio_transcription": {
      "model": "whisper-1"
  },
  "turn_detection": null,
  "tools": [],
  "tool_choice": "none",
  "temperature": 0.7,
  "max_response_output_tokens": 200,
  "speed": 1.1,
  "tracing": "auto",
  "client_secret": {
    "value": "ek_abc123", 
    "expires_at": 1234567890
  }
}

Returns Examples

{
  "id": "sess_001",
  "object": "realtime.session",
  "model": "gpt-realtime-2025-08-25",
  "modalities": ["audio", "text"],
  "instructions": "You are a friendly assistant.",
  "voice": "alloy",
  "input_audio_format": "pcm16",
  "output_audio_format": "pcm16",
  "input_audio_transcription": {
      "model": "whisper-1"
  },
  "turn_detection": null,
  "tools": [],
  "tool_choice": "none",
  "temperature": 0.7,
  "max_response_output_tokens": 200,
  "speed": 1.1,
  "tracing": "auto",
  "client_secret": {
    "value": "ek_abc123", 
    "expires_at": 1234567890
  }
}

Suggested

Create session

Body ParametersJSONExpand Collapse

ReturnsExpand Collapse

Create session

Returns Examples