resources/realtime/subresources/sessions/methods/create/index.md 2026-07-10 23:02 UTC to 2026-07-12 06:58 UTC

Create session

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 Parameters

• 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.

  • number

  • "inf"

    • "inf"

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

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

  • "text"

  • "audio"

• output_audio_format: optional string

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

• prompt: optional ResponsePrompt

  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 or ResponseInputImage or ResponseInputFile]

    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.

    • string

    • ResponseInputText object { text, type, prompt_cache_breakpoint }

      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.

        • "input_text"

      • prompt_cache_breakpoint: optional object { mode }

        Marks the exact end of a reusable prompt prefix. The breakpoint inherits its TTL from the request's prompt_cache_options.ttl; the boundary is not rounded to a token block.

        • mode: "explicit"

          The breakpoint mode. Always explicit.

          • "explicit"

    • ResponseInputImage object { detail, type, file_id, 2 more }

      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.

        • "low"

        • "high"

        • "auto"

        • "original"

      • type: "input_image"

        The type of the input item. Always input_image.

        • "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.

      • prompt_cache_breakpoint: optional object { mode }

        Marks the exact end of a reusable prompt prefix. The breakpoint inherits its TTL from the request's prompt_cache_options.ttl; the boundary is not rounded to a token block.

        • mode: "explicit"

          The breakpoint mode. Always explicit.

          • "explicit"

    • ResponseInputFile object { type, detail, file_data, 4 more }

      A file input to the model.

      • type: "input_file"

        The type of the input item. Always input_file.

        • "input_file"

      • detail: optional "auto" or "low" or "high"

        The detail level of the file to be sent to the model. Use auto to let the system select the detail level; for GPT-5.6 and later models, auto uses high-quality rendering, which may increase input token usage. Use low for lower-cost rendering, or high to render the file at higher quality. Defaults to auto.

        • "auto"

        • "low"

        • "high"

      • 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.

      • prompt_cache_breakpoint: optional object { mode }

        Marks the exact end of a reusable prompt prefix. The breakpoint inherits its TTL from the request's prompt_cache_options.ttl; the boundary is not rounded to a token block.

        • mode: "explicit"

          The breakpoint mode. Always explicit.

          • "explicit"

  • 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.

• 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.

    • "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.

  • "auto"

    Default tracing mode for the session.

    • "auto"

  • 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.

  • "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.

    • "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.

    • type: "retention_ratio"

      Use retention ratio truncation.

      • "retention_ratio"

    • 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.

• 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.

  • string

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

    • "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.

Returns

• 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.

      • 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.

          • 24000

        • type: optional "audio/pcm"

          The audio format. Always audio/pcm.

          • "audio/pcm"

      • PCMUAudioFormat object { type }

        The G.711 μ-law format.

        • type: optional "audio/pcmu"

          The audio format. Always audio/pcmu.

          • "audio/pcmu"

      • PCMAAudioFormat object { type }

        The G.711 A-law format.

        • type: optional "audio/pcma"

          The audio format. Always audio/pcma.

          • "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.

        • "near_field"

        • "far_field"

    • transcription: optional object { language, model, prompt }

      Configuration for input audio transcription.

      • language: optional string

        The language of the input audio.

      • model: optional string or "whisper-1" or "gpt-4o-mini-transcribe" or "gpt-4o-mini-transcribe-2025-12-15" or 3 more

        The model used for transcription. Current options are whisper-1, gpt-4o-mini-transcribe, gpt-4o-mini-transcribe-2025-12-15, gpt-4o-transcribe, gpt-4o-transcribe-diarize, and gpt-realtime-whisper.

        • string

        • "whisper-1" or "gpt-4o-mini-transcribe" or "gpt-4o-mini-transcribe-2025-12-15" or 3 more

          The model used for transcription. Current options are whisper-1, gpt-4o-mini-transcribe, gpt-4o-mini-transcribe-2025-12-15, gpt-4o-transcribe, gpt-4o-transcribe-diarize, and gpt-realtime-whisper.

          • "whisper-1"

          • "gpt-4o-mini-transcribe"

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

          • "gpt-4o-transcribe"

          • "gpt-4o-transcribe-diarize"

          • "gpt-realtime-whisper"

      • prompt: optional string

        The prompt configured for input audio transcription, when present.

    • 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.

    • speed: optional number

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

      • string

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

        • "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.

  • "item.input_audio_transcription.logprobs"

• 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_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.

  • number

  • "inf"

    • "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"].

  • "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

  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.

    • "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.

  • "auto"

    Default tracing mode for the session.

    • "auto"

  • 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 }

  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.

Example

curl https://api.openai.com/v1/realtime/sessions \
    -H 'Content-Type: application/json' \
    -H "Authorization: Bearer $OPENAI_API_KEY" \
    -d '{
          "client_secret": {
            "expires_at": 0,
            "value": "value"
          }
        }'

Response

{
  "id": "id",
  "audio": {
    "input": {
      "format": {
        "rate": 24000,
        "type": "audio/pcm"
      },
      "noise_reduction": {
        "type": "near_field"
      },
      "transcription": {
        "language": "language",
        "model": "whisper-1",
        "prompt": "prompt"
      },
      "turn_detection": {
        "prefix_padding_ms": 0,
        "silence_duration_ms": 0,
        "threshold": 0,
        "type": "type"
      }
    },
    "output": {
      "format": {
        "rate": 24000,
        "type": "audio/pcm"
      },
      "speed": 0,
      "voice": "ash"
    }
  },
  "expires_at": 0,
  "include": [
    "item.input_audio_transcription.logprobs"
  ],
  "instructions": "instructions",
  "max_output_tokens": "inf",
  "model": "model",
  "object": "object",
  "output_modalities": [
    "text"
  ],
  "tool_choice": "tool_choice",
  "tools": [
    {
      "description": "description",
      "name": "name",
      "parameters": {},
      "type": "function"
    }
  ],
  "tracing": "auto",
  "turn_detection": {
    "prefix_padding_ms": 0,
    "silence_duration_ms": 0,
    "threshold": 0,
    "type": "type"
  }
}

Example

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."
  }'

Response

{
  "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
  }
}