From a little tool to a shared extension

Build with what you know.

HTML for structure. CSS for style. JavaScript for behavior. GitHub for your source, packages and releases.

Current compatibility: This site uses Agent Developer’s schemaVersion 1 package metadata. Agent Developer presently reads explicitly registered built-in metadata and sanitized README content. Community installation and isolated execution are future work. There is no public host JavaScript API to call yet.

1Start with web files

Create a local project containing the files below. Open index.html in your own browser to preview the starter. This website and Agent Developer never render or run package HTML, scripts, styles or icons.

my-extension/
  agentic-extension.json
  index.html
  styles.css
  main.js
  icon.svg
  README.md
  CHANGELOG.md
  LICENSE

The complete, minimal starter source is shown here. Copy each file into your own repository; no build framework is required.

index.html

<!doctype html>
<html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1"><title>Hello World</title><link rel="stylesheet" href="styles.css"><script src="main.js" defer></script></head>
<body><main><h1>Hello, builder.</h1><p>A little tool starts here.</p><button id="greet" type="button">Say hello</button><p id="message" role="status"></p></main></body></html>

styles.css

body { font: 1rem/1.6 system-ui, sans-serif; margin: 3rem; color: #17213b; background: #f5f8ff; }
main { max-width: 36rem; margin: auto; }
button { font: inherit; padding: .6rem 1rem; border: 0; border-radius: .4rem; color: white; background: #0757f9; cursor: pointer; }
button:focus-visible { outline: 3px solid #17213b; outline-offset: 3px; }

main.js

document.querySelector('#greet').addEventListener('click', () => {
    document.querySelector('#message').textContent = 'Hello from your extension!';
});

README.md

# Hello World

A small, offline HTML/CSS/JavaScript starter.

## Local preview

Open index.html in your own browser to try the greeting button. The source uses ordinary DOM APIs; it has no Agent Developer host API, filesystem access or remote dependencies.

## Compatibility

This package follows the current Agent Developer schemaVersion 1 metadata shape. Agent Developer currently registers application-owned built-ins only and does not run uploaded HTML/CSS/JS/SVG. Runtime execution, command dispatch and isolated community-package installation are future host work. Manifest contributions describe intended UI, not a currently callable runtime API.

## Publishing

Replace github-12345 in the manifest with your authenticated publisher namespace. Commit all source files into a public personal GitHub repository. Enable GitHub release immutability. ZIP these files at the archive root as extension.zip; attach it to a draft GitHub release before publishing tag v1.0.0. Import owner/repository into The Extension Market, review your catalog draft, then publish it.

## Maintenance

Use your repository issue tracker. Record changes in CHANGELOG.md, increment the manifest version and publish a matching immutable GitHub release. The website does not execute source or independently verify skills or security.

CHANGELOG.md

# Changelog

## 1.0.0

Initial offline greeting example and package metadata.

icon.svg

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64"><rect width="64" height="64" rx="14" fill="#0757f9"/><path d="M24 20 12 32l12 12m16-24 12 12-12 12" fill="none" stroke="white" stroke-width="5"/></svg>

LICENSE

MIT License

Copyright (c) 2026 Garth

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

2Describe your extension

Sign in to see your immutable publisher namespace, such as github-12345. Replace it in both publisher and id below. Names use lowercase letters, numbers and hyphens. Versions are stable major.minor.patch numbers. The release tag must match the version, optionally prefixed by v.

agentic-extension.json · complete schemaVersion 1 example

{
    "schemaVersion": 1,
    "id": "github-12345.hello-world",
    "publisher": "github-12345",
    "name": "hello-world",
    "displayName": "Hello World",
    "description": "A small starter for learning the extension package format.",
    "version": "1.0.0",
    "compatibility": {
        "agenticTeamUi": ">=0.1.0"
    },
    "runtime": "web",
    "entry": {
        "html": "index.html",
        "styles": "styles.css",
        "script": "main.js"
    },
    "activationEvents": [
        "onCommand:hello-world.greet"
    ],
    "contributes": {
        "commands": [
            {
                "id": "hello-world.greet",
                "title": "Hello World: Greet"
            }
        ],
        "panels": [
            {
                "id": "hello-world.panel",
                "title": "Hello World"
            }
        ],
        "statusItems": []
    },
    "permissions": [
        "ui.commands",
        "ui.panels"
    ],
    "categories": [
        "Education",
        "Starter"
    ],
    "keywords": [
        "hello",
        "starter"
    ],
    "icon": "icon.svg",
    "license": "LICENSE",
    "readme": "README.md",
    "changelog": "CHANGELOG.md",
    "operatingSystems": [
        "windows",
        "macos",
        "linux"
    ],
    "dependencies": []
}

compatibility.agenticTeamUi accepts an exact numeric version or a single >=, ^ or ~ range. This is your compatibility declaration, not an executed compatibility test. Only ui.commands, ui.panels and ui.status declarations are supported; dependencies must be empty. Contributions record intended UI and do not invoke host code.

3Release on GitHub

Commit your source to a public personal repository owned by the GitHub account you use here. Organization ownership will need a future GitHub App integration.
In the repository’s Settings → General → Releases, enable release immutability.
Create extension.zip with the manifest and source files directly at the ZIP root. Exclude Git history, credentials, dependencies, build caches and this ZIP itself.
Create a draft GitHub release tagged v1.0.0. Attach extension.zip, then publish the release. Immutability locks the tag and asset contents once published.
Import owner/repository in your creator dashboard. Choose a tag or leave it empty for GitHub’s latest stable release.

Validation temporarily downloads the asset, checks its SHA-256 and bounded package structure, reads documentation, then deletes the temporary archive. Only metadata and documentation remain here. Downloads go directly to GitHub.

Package limits

2 MiB ZIP · 8 MiB expanded · 1 MiB per file · 128 entries · 64 KiB manifest · 256 KiB each README and changelog. UTF-8 text only: HTML, CSS, JS, SVG, JSON, Markdown, TXT and LICENSE. No traversal, links, hidden/reserved paths, duplicate paths, encryption, executable server files or excessive compression. These checks do not establish that code is safe.

4Publish & maintain

Review the imported catalog draft in your dashboard. Publish it explicitly to make it discoverable here. The GitHub release is already public; “draft” describes only its catalog visibility.

For an update, change source and changelog, increment the manifest version, and publish a new matching immutable GitHub release. “Check GitHub for updates” imports the latest. Optional automatic publication lets future accepted latest releases appear after scheduled checks; otherwise they remain drafts. The initial catalog supports ten repositories per creator and fifty recorded versions per repository.

Withdrawing hides a catalog version and disables its catalog download route. The GitHub release remains under your control; copies already downloaded cannot be recalled. Withdrawn versions are not republished automatically. Publish a new version to replace them. Keep support and maintenance status current.

Creator profiles are opt-in. Minimal GitHub attribution remains attached to published packages even if your full creator profile is nonpublic. Optional Developer Market contribution consent is a separate preference; its authenticated cross-service connection is not activated yet.

Catalog interface for Agent Developer

GET /api/v1/extensions returns published catalog metadata with pagination and an optional ?q= search. GET /api/v1/extensions/{id}/versions/{version} returns one published version. Drafts and withdrawn versions return 404.

The response includes the original manifest, numeric publisher/repository/release/asset IDs, GitHub URL, SHA-256, byte count, check time, maintenance and availability. Clients must verify the downloaded digest and size and evaluate manifest compatibility themselves. A non-available or stale result has no download URL. Discovery does not grant installation or execution trust. Agent Developer must keep its privileged page isolated from community code.

Scheduled checks are conservative and subject to GitHub’s shared rate limits. Missing, private, renamed, deleted or changed upstream releases become unavailable when observed. Transient failures and rate limiting are shown separately. Last-check timestamps are observations, not a real-time availability guarantee; checks older than 24 hours are stale.