smoothStream()

smoothStream is a utility function that creates a TransformStream for the streamText transform option to smooth out text and reasoning streaming by buffering and releasing complete chunks with configurable delays. This creates a more natural reading experience when streaming text and reasoning responses.

import { smoothStream, streamText } from 'ai';

const result = streamText({
  model,
  prompt,
  experimental_transform: smoothStream({
    delayInMs: 20, // optional: defaults to 10ms
    chunking: 'line', // optional: defaults to 'word'
  }),
});

Import

<Snippet text={import { smoothStream } from "ai"} prompt={false} />

API Signature

Parameters

<PropertiesTable content={[ { name: 'delayInMs', type: 'number | null', isOptional: true, description: 'The delay in milliseconds between outputting each chunk. Defaults to 10ms. Set to null to disable delays.', }, { name: 'chunking', type: '"word" | "line" | RegExp | Intl.Segmenter | (buffer: string) => string | undefined | null', isOptional: true, description: 'Controls how text and reasoning content is chunked for streaming. Use "word" to stream word by word (default), "line" to stream line by line, an Intl.Segmenter for locale-aware word segmentation (recommended for CJK languages), or provide a custom callback or RegExp pattern for custom chunking.', }, ]} />

Word chunking caveats with non-latin languages

The word based chunking does not work well with the following languages that do not delimit words with spaces:

• Chinese
• Japanese
• Korean
• Vietnamese
• Thai

Using Intl.Segmenter (recommended)

For these languages, we recommend using Intl.Segmenter for proper locale-aware word segmentation. This is the preferred approach as it provides accurate word boundaries for CJK and other languages.

import { smoothStream, streamText } from 'ai';
__PROVIDER_IMPORT__;

const segmenter = new Intl.Segmenter('ja', { granularity: 'word' });

const result = streamText({
  model: __MODEL__,
  prompt: 'Your prompt here',
  experimental_transform: smoothStream({
    chunking: segmenter,
  }),
});

import { smoothStream, streamText } from 'ai';
__PROVIDER_IMPORT__;

const segmenter = new Intl.Segmenter('zh', { granularity: 'word' });

const result = streamText({
  model: __MODEL__,
  prompt: 'Your prompt here',
  experimental_transform: smoothStream({
    chunking: segmenter,
  }),
});

Regex based chunking

To use regex based chunking, pass a RegExp to the chunking option.

// To split on underscores:
smoothStream({
  chunking: /_+/,
});

// Also can do it like this, same behavior
smoothStream({
  chunking: /[^_]*_/,
});

Custom callback chunking

To use a custom callback for chunking, pass a function to the chunking option.

smoothStream({
  chunking: text => {
    const findString = 'some string';
    const index = text.indexOf(findString);

    if (index === -1) {
      return null;
    }

    return text.slice(0, index) + findString;
  },
});

Returns

Returns a TransformStream that:

• Buffers incoming text and reasoning chunks
• Releases content when the chunking pattern is encountered
• Adds configurable delays between chunks for smooth output
• Passes through non-text/reasoning chunks (like tool calls, step-finish events) immediately