feat!: Extensible addon system for third-party integrations

[jraylan]

Summary

This PR transforms Seamless Agent into an extensible extension, allowing third-party extensions (addons) to extend its functionality without modifying the core extension's primary purpose - the ask_user tool.

Motivation

Users have different needs for AI agent tools. Rather than bloating the core extension with every possible feature, this architecture allows:

• Modularity: Users install only the tools they need
• Ecosystem Growth: Third-party developers can create specialized addons
• Maintainability: Core extension stays focused on its primary goal

Changes

Architecture

Component	Description
src/api/	Public API contract (ISeamlessAgentAPI) for addon integration
src/addons/	Addon registry and lifecycle management
src/core/	Shared core types and utilities
src/agent/	Agent context and utilities

Public API (ISeamlessAgentAPI)

Addons can integrate with Seamless Agent through:

• api.registerAddon(addon) - Register addon with capabilities
• api.ui - UI integration (tabs, history, settings)
• api.tools - LLM tools integration
• api.events - Event subscription system
• api.storage - Namespaced data persistence

Addon Capabilities

Capability	Description
Custom Tabs	Register new tabs in Seamless Agent webview
History Providers	Add custom item types to session history
Settings Sections	Add configuration UI for addon settings
LLM Tools	Register tools executable by AI agents
Events	Subscribe to extension events
Storage	Persist addon-specific data

Types-Only NPM Package

A separate npm package (seamless-agent-addon-api) is generated containing only TypeScript definitions for addon authors:

• Build:
  npm run build:addon-typedefs
• Output: dist-addon-api/
• Published via release-please workflow

New Files

src/
 addon-typedefs/index.ts    # Types-only entrypoint
 addons/
    index.ts               # Addon system exports
    registry.ts            # AddonRegistry implementation
    types.ts               # Internal type aliases
    README.md              # Contributor documentation
 api/
    index.ts               # API exports
    SeamlessAgentAPI.ts    # API implementation
    events.ts              # Event emitter system
    types.ts               # Public interface definitions
    README.md              # Addon author documentation
 core/
    index.ts               # Core utilities
    types.ts               # Shared types
 agent/
     index.ts               # Agent context utilities

build-addon-typedefs.js        # Types package generator
tsconfig.addon-typedefs.json   # TypeScript config for types generation

CI/CD Updates

• Updated .github/workflows/release-please.yml to build and publish `seamless-agent-addon-api´ to npm on release

Usage Example (Addon Author)

import * as vscode from 'vscode';
import type { ISeamlessAgentAPI, IAddon } from 'seamless-agent-addon-api';

export async function activate(context: vscode.ExtensionContext) {
    const seamlessExt = vscode.extensions.getExtension('jraylan.seamless-agent');
    if (!seamlessExt) return;

    const api: ISeamlessAgentAPI = await seamlessExt.activate();

    const addon: IAddon = {
        id: 'my-extension.my-addon',
        name: 'My Addon',
        version: '1.0.0',
        ai: {
            tools: [
                {
                    name: 'my_tool',
                    description: 'My custom tool',
                    inputSchema: { type: 'object', properties: {} },
                    execute: async (input, context) => {
                        return { success: true };
                    }
                }
            ]
        }
    };

    const registration = api.registerAddon(addon);
    context.subscriptions.push(registration);
}

Breaking Changes

• Extension architecture refactored; internal imports may have changed
• Extension entry points consolidated into modular structure

Testing

• Extension activates correctly
• Core tools (ask_user, plan_review, walkthrough_review) work as expected
• Addon registration and unregistration works
• Types package builds correctly (npm run build:addon-typedefs)
• Webview renders addon tabs correctly

[Copilot]

Pull request overview

This PR introduces a comprehensive addon system that transforms Seamless Agent into an extensible platform, allowing third-party extensions to integrate with its core functionality without modifying the main extension.

Key Changes

• New extensible architecture with a public API contract for addon integration
• Modular addon registry for lifecycle management of third-party addons
• Types-only NPM package (seamless-agent-addon-api) for addon authors
• Settings tab in the webview for managing addon configurations

Reviewed changes

Copilot reviewed 37 out of 41 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
src/api/*	Public API contract defining interfaces for addon integration including UI, tools, events, and storage
src/addons/*	Addon registry and lifecycle management system
src/core/*	Refactored core extension initialization and types
src/extension.ts	Simplified to return public API for addon access
src/webview/*	Added settings tab and UI integration for addon management
build-addon-typedefs.js	Script to generate types-only package for NPM distribution
tsconfig.addon-typedefs.json	TypeScript configuration for generating type definitions
package.json	Added types field, extension capabilities, and build script for addon types
media/*	HTML and CSS for new settings tab UI
Localization files	Translations for settings tab strings

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[Copilot]

Pull request overview

Copilot reviewed 40 out of 46 changed files in this pull request and generated 5 comments.

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[Copilot]

@jraylan I've opened a new pull request, #55, to work on those changes. Once the pull request is ready, I'll request review from you.