AI

smoothStream()

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

import { smoothStream } from "ai"

API Signature

Parameters

delayInMs?:

number | null
The delay in milliseconds between outputting each chunk. Defaults to 10ms. Set to `null` to disable delays.

chunking?:

"word" | "line" | RegExp | (buffer: string) => string | undefined | null
Controls how the text is chunked for streaming. Use "word" to stream word by word (default), "line" to stream line by line, 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:

For these languages we recommend using a custom regex, like the following:

  • Chinese - /[\u4E00-\u9FFF]|\S+\s+/
  • Japanese - /[\u3040-\u309F\u30A0-\u30FF]|\S+\s+/
Japanese example
import { smoothStream, streamText } from 'ai';


const result = streamText({
  model: 'openai/gpt-4.1',
  prompt: 'Your prompt here',
  experimental_transform: smoothStream({
    chunking: /[\u3040-\u309F\u30A0-\u30FF]|\S+\s+/,
  }),
});
Chinese example
import { smoothStream, streamText } from 'ai';


const result = streamText({
  model: 'openai/gpt-4.1',
  prompt: 'Your prompt here',
  experimental_transform: smoothStream({
    chunking: /[\u4E00-\u9FFF]|\S+\s+/,
  }),
});

For these languages you could pass your own language aware chunking function:

  • Vietnamese
  • Thai
  • Javanese (Aksara Jawa)

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 chunks
  • Releases text when the chunking pattern is encountered
  • Adds configurable delays between chunks for smooth output
  • Passes through non-text chunks (like step-finish events) immediately