LanguageModel: promptStreaming() method

Syntax

promptStreaming(input)
promptStreaming(input, options)

Parameters

input

The content to prompt the model with. This is either:

• A string — Shorthand for a single textual message.
• An array of objects, each representing a single message in a conversation with a language model. Objects may have the following properties:

  role

  A string indicating the point of view the message is phrased from. Must be one of:

  system

  A system-level instruction that guides the model's overall behavior. This must be the first instruction passed to the model.

  user

  A message from the user, which the API should respond to.

  assistant

  An input that provides context for the AI assistant, such as its persona or the format of its responses. Such messages mainly serve to provide context/history, and further shape how the model responds.

  content

  A string representing a textual prompt, or an array of objects. Each object includes the following properties:

  type

  An enumerated value representing the type of content. This can be one of:

  audio

  Audio content.

  image

  Image content.

  text

  Textual content.

  tool-call

  A tool invocation issued by the model.

  tool-response

  The result of a tool invocation.

  value

  The content of the message. If the type is text, this is always a string. If the type is audio or image, the value can be one of several different object types; see What data types are accepted?.

  prefix Optional

  A boolean, defaulting to false. When true, the message is treated as a prefix for the model's next generated response rather than a complete turn.

options Optional

Options for creating a prompt. Properties include:

responseConstraint

An object following the structure defined by JSON Schema defining the precise format the model's output should be delivered in. When provided and omitResponseConstraintInput is false, any implementation-defined constraint-description message is included in the measurement.

omitResponseConstraintInput

A boolean; when true, the automatic constraint-description message is excluded from the measurement.

signal

An AbortSignal to cancel the operation.

Return value

A ReadableStream of String chunks. Each chunk represents a piece of the model's response as it is generated. The stream closes when generation completes.

Exceptions

Errors are surfaced as stream errors rather than as rejected promises. Consumers should handle errors using a stream's standard error-handling mechanisms.

AbortError DOMException

Thrown if the operation was cancelled via the signal option.

NotAllowedError DOMException

Thrown if usage of the method is blocked by a language-model Permissions-Policy.

NotSupportedError DOMException

Thrown if:

• A message's role is assistant and its type is anything other than text.
• A message's type is text and its value is not a string.
• The input or output text is in a language the user agent doesn't support for prompting.
• A message's type is image or audio but the type was not listed in expectedInputs, or the value is not an accepted data type.

OperationError DOMException

Thrown if the prompt fails for any other reason not listed in the other exception types.

QuotaExceededError DOMException

Thrown if the prompt would cause the session's context usage to exceed the model's LanguageModel.contextWindow.

SyntaxError DOMException

Thrown if:

• No messages are included in the messages array.
• A message's prefix property is set to true and:
  • The message's role is not assistant.
  • The message is not the last item in the messages array.

TypeError

Thrown if:

• omitResponseConstraintInput is true but responseConstraint is not provided.
• A message's role is system but it was not the first message passed to the context.

Description

The promptStreaming() method adds the provided input to the context window and generates a response. The entire response is receives incrementally as a ReadableStream.

To receive the response as one complete string, use LanguageModel.prompt() instead. To add content to the context window without generating a response, use LanguageModel.append().

Each call to promptStreaming() adds to the session's context. To branch from a given state without affecting the original session, call LanguageModel.clone().

Examples

Streaming a response to the page

This example writes out chunks from a promptStreaming() call's ReadableStream as they arrive.

const session = await LanguageModel.create();
const output = document.querySelector("#output");

const stream = session.promptStreaming("Write a short poem about the ocean.");

for await (const chunk of stream) {
  output.textContent += chunk;
}

See also Using the Prompt API > Complete streaming example.

Streaming with an abort signal

This example shows how to use an AbortController with promptStreaming().

const controller = new AbortController();
document
  .querySelector("#stop")
  .addEventListener("click", () => controller.abort());

const stream = session.promptStreaming("Tell me a long story.", {
  signal: controller.signal,
});

try {
  for await (const chunk of stream) {
    output.textContent += chunk;
  }
} catch (err) {
  if (err.name === "AbortError") {
    console.log("Streaming was stopped by the user.");
  }
}

Collecting streamed chunks into a single string

In this example, chunks from a ReadableStream are collected before the whole stream is written out.

const session = await LanguageModel.create();
const stream = session.promptStreaming("Explain quantum entanglement.");
const chunks = [];

for await (const chunk of stream) {
  chunks.push(chunk);
}

const fullResponse = chunks.join("");
console.log(fullResponse);

Specifications

Browser compatibility

See also

• LanguageModel.prompt()
• ReadableStream
• Prompt API
• Using the Prompt API
• Adding context with initial and ongoing prompt inputs