Summarizer

The Summarizer API allows web developers to offer local, AI-powered text distillation directly within the browser using Gemini Nano in Chrome or Phi in Edge. This API supports various formats, including key points, headlines, and TL;DRs, while operating entirely on-device to ensure user privacy.

Getting Started

The Summarizer API is available starting in Chrome and Edge 138. It requires a one-time model download of Gemini Nano or Phi (respectively).

Hardware & Software Requirements

• OS: Windows 10/11, macOS 13+, Linux, or ChromeOS (Chromebook Plus).
• Storage: 22GB free space for the profile volume.
• RAM/CPU: 16GB+ RAM and 4+ CPU cores.
• VRAM: 4GB+ (if using GPU).

Model Download and Availability

Check if the model is ready, needs downloading, or is unavailable.

Mandatory Options Passing: You must pass the identical configuration options object to both Summarizer.availability(options) and Summarizer.create(options). Do NOT use the deprecated window.ai.summarizer API surface.

Mandatory Progress Monitoring: You MUST implement a monitor for model download progress by providing a monitor(m) callback to Summarizer.create() and adding a listener for the downloadprogress event.

User Gesture Requirement: When availability is 'downloadable' or 'downloading', triggering the actual download via Summarizer.create() requires a user gesture (such as a user click). You must place the creation call inside an event listener rather than calling it unconditionally on page load to prevent NotAllowedError.

const options = {  type: 'key-points',  format: 'plain-text',  length: 'medium'};
const availability = await Summarizer.availability(options);
if (availability === 'available') {  const summarizer = await Summarizer.create(options);  // Ready to use immediately} else if (availability === 'downloadable') {  // A user gesture is strictly required to start the download  document.getElementById('start-download-btn').addEventListener('click', async () => {    const summarizer = await Summarizer.create({      ...options,      monitor(m) {        m.addEventListener('downloadprogress', (e) => {          console.log(`Downloaded ${Math.round((e.loaded / e.total) * 100)}%`);        });      },    });  });}

API Functions & Configuration

When creating a summarizer via Summarizer.create(options), you can customize the output:

Example Configuration

const options = {  sharedContext: 'This is a scientific article',  type: 'key-points',  format: 'markdown',  length: 'medium',};
if (navigator.userActivation.isActive) {  const summarizer = await Summarizer.create(options);}

Language Support

You can specify expected languages to ensure the browser can handle the specific summary request.

const summarizer = await Summarizer.create({  type: 'key-points',  expectedInputLanguages: ['en', 'ja'],  outputLanguage: 'es',});

MANDATORY: if the output language is different from the overall page language, you MUST apply the lang attribute to the container element for the summary, and set its value to match the BCP 47 language code set using the outputLanguage property.

Summarization Methods

1. Batch Summarization

Processes the entire text at once and returns the result.

const longText = document.querySelector('article').innerText;const summary = await summarizer.summarize(longText, {  context: 'This article is intended for a tech-savvy audience.',});console.log(summary);

2. Stream Summarization

Returns results in real-time as the model generates them, providing a more responsive UI.

const stream = summarizer.summarizeStreaming(longText);for await (const chunk of stream) {  console.log(chunk);}

Security and Permissions

• Data Privacy: No data is sent to Google; processing happens on the local device.

• Cross-Origin: Access can be granted to iframes using the Permission Policy.

  html

  <iframe src="https://example.com/" allow="summarizer"></iframe>

• Web Workers: Currently not supported.

Fallback Strategy

Browser support for Summarizer: Limited availability. Supported by: Chrome 138 (Jun 2025) and Edge 138 (Jun 2025). Unsupported in: Firefox and Safari.

Before initializing or querying availability, check if the browser supports the Summarizer API:

if ('Summarizer' in self) {  // The Summarizer API is supported.} else {  // Execute fallback strategy}

If the Summarizer API is unsupported or availability checks return 'unavailable', you must gracefully fall back.

Recommended options:

1. Remote API Fallback: Direct the distillation request to a server endpoint or remote API (such as the Vertex AI Gemini API) so that users still get summaries.
2. Graceful Degradation: Visually disable the summarization controls in the UI, or hide the button while displaying a friendly message (e.g., "Local summarization is currently unsupported in this browser"). Do not allow interaction to trigger generic unhandled runtime exceptions.
3. Polyfill Fallback: You can use community-maintained polyfills like built-in-ai-task-apis-polyfills or prompt-api-polyfill to emulate the API surface using remote services with models in the cloud or on-device inference with local models.

Privacy and Cost Implications: These polyfills possibly proxy requests to remote servers (such as Gemini API over the cloud). This completely nullifies the on-device privacy guarantees of the native Built-in AI APIs and will incur server-side API usage costs.