Chrome Extension Development

Architecture Overview

Brief's Chrome extension provides AI chat directly in the browser using Side Panel architecture.

Key Components:

• Manifest V3: Modern Chrome extension format
• Side Panel: Chrome's native side panel API (not content scripts)
• OAuth Authentication: chrome.identity.launchWebAuthFlow with PKCE
• Shared Business Logic: Imports from

  @briefhq/chat-ui

  package
• Context Awareness: Tracks active tab URL for contextual assistance

Side Panel Architecture

Brief uses Chrome's Side Panel API, NOT content script injection:

// sidepanel.tsx - Main entry point
export default function SidePanel() {
  const [accessToken, setAccessToken] = useState<string | null>(null);
  const [contextUrl, setContextUrl] = useState<string | undefined>();

  // Check auth on mount
  useEffect(() => {
    const token = await getValidAccessToken();
    setAccessToken(token);
  }, []);

  // Track active tab URL
  useEffect(() => {
    chrome.tabs.query({ active: true, currentWindow: true }, (tabs) => {
      setContextUrl(tabs[0]?.url);
    });

    chrome.tabs.onActivated.addListener(handleTabChange);
    chrome.tabs.onUpdated.addListener(handleUrlChange);
  }, []);

  return <ChatInterface accessToken={accessToken} contextUrl={contextUrl} />;
}

Manifest Configuration:

{
  "manifest_version": 3,
  "permissions": ["sidePanel", "activeTab", "storage", "tabs", "identity"],
  "side_panel": {
    "default_path": "sidepanel.html"
  },
  "commands": {
    "toggle-side-panel": {
      "suggested_key": {
        "default": "Ctrl+Shift+B",
        "mac": "Command+Shift+B"
      }
    }
  }
}

Shared Code from @briefhq/chat-ui

The extension reuses business logic from the

@briefhq/chat-ui

Shared Hooks

useChatTransport

useConversationHistory

usePresets

useContextStatus

useMessageFeedback

useFileAttachments

useMentions

Shared UI Components

Conversation

ConversationContent

Message

MessageContent

Response

Tool

ToolHeader

ToolContent

ToolInput

ToolOutput

Reasoning

ReasoningTrigger

ReasoningContent

Loader

Import Pattern

// ✅ GOOD - Import shared hooks and components from @briefhq/chat-ui
import {
  useChatTransport,
  useConversationHistory,
  usePresets,
  Conversation,
  Message,
  Response,
  Tool,
  Loader,
} from "@briefhq/chat-ui";

// Extension-specific orchestration
export function ChatInterface({ accessToken, contextUrl }: Props) {
  const transport = useChatTransport({ defaultModel: "claude-sonnet-4-5" });
  const history = useConversationHistory({ api, onConversationLoaded });
  const presets = usePresets({ api });

  // Extension-specific logic here
  return (
    <div>
      <ChatHeader presets={presets} onSignOut={onSignOut} />
      <Conversation>
        {messages.map(msg => <Message key={msg.id} message={msg} />)}
      </Conversation>
      <ChatInputArea onSubmit={handleSubmit} />
    </div>
  );
}

// ❌ BAD - Don't recreate what exists in @briefhq/chat-ui
export function ChatInterface() {
  // Don't reimplement shared business logic
  const [messages, setMessages] = useState([]);
  const [isStreaming, setIsStreaming] = useState(false);

  // This should use useChatTransport instead
  const handleSubmit = async (input: string) => {
    // Streaming logic...
  };
}

When to Create Extension-Specific Components

Create components in

packages/chrome-extension/components/

1. Chrome API Integration: Component uses chrome.tabs, chrome.storage, chrome.identity
2. Extension-Specific UI: Component is unique to side panel context (e.g., sign-in flow, header with sign-out)
3. Extension-Specific Configuration: Wrapper needed for extension constraints

Example - Extension-specific header:

// components/chat/views/ChatHeader.tsx
// Extension-specific because it has sign-out, model selector, history toggle
export function ChatHeader({ presets, onSignOut }: Props) {
  return (
    <header className="flex items-center justify-between px-4 py-3 border-b">
      <PresetSelector presets={presets} />
      <ModelSelector models={AVAILABLE_MODELS} />
      <button onClick={onSignOut}>Sign out</button>
    </header>
  );
}

OAuth Authentication

Brief uses chrome.identity API with PKCE for OAuth 2.0:

// lib/oauth.ts

// Start OAuth flow
export async function startOAuthFlow() {
  // Generate PKCE challenge
  const codeVerifier = generateCodeVerifier();
  const codeChallenge = await generateCodeChallenge(codeVerifier);

  // Launch web auth flow
  const redirectUrl = chrome.identity.getRedirectURL();
  const authUrl = `${BRIEF_URL}/oauth/authorize?` +
    `client_id=${CLIENT_ID}&` +
    `redirect_uri=${redirectUrl}&` +
    `response_type=code&` +
    `code_challenge=${codeChallenge}&` +
    `code_challenge_method=S256`;

  const responseUrl = await chrome.identity.launchWebAuthFlow({
    url: authUrl,
    interactive: true,
  });

  // Exchange code for tokens
  const code = extractCodeFromUrl(responseUrl);
  const tokens = await exchangeCodeForTokens(code, codeVerifier);

  // Store in chrome.storage.local
  await chrome.storage.local.set({
    access_token: tokens.access_token,
    refresh_token: tokens.refresh_token,
    expires_at: Date.now() + tokens.expires_in * 1000,
  });

  return tokens;
}

// Get valid token (auto-refresh if needed)
export async function getValidAccessToken(): Promise<string | null> {
  const { access_token, expires_at, refresh_token } =
    await chrome.storage.local.get(["access_token", "expires_at", "refresh_token"]);

  if (!access_token) return null;

  // Check if token needs refresh
  if (Date.now() >= expires_at - 60000) {
    return await refreshAccessToken(refresh_token);
  }

  return access_token;
}

API Integration

Extension calls Brief API with OAuth Bearer tokens:

// hooks/use-extension-api.ts
export function useExtensionApi(accessToken: string) {
  return {
    async chat(messages, options) {
      const response = await fetch(`${BRIEF_URL}/api/v1/chat`, {
        method: "POST",
        headers: {
          "Authorization": `Bearer ${accessToken}`,
          "Content-Type": "application/json",
        },
        body: JSON.stringify({
          messages,
          model: options.model,
          contextUrl: options.contextUrl, // Current tab URL
        }),
      });
      return response.body; // Streaming response
    },

    async fetchConversations() {
      const response = await fetch(`${BRIEF_URL}/api/v1/conversations`, {
        headers: { "Authorization": `Bearer ${accessToken}` },
      });
      return response.json();
    },

    async searchDocuments(query: string) {
      const response = await fetch(
        `${BRIEF_URL}/api/v1/documents/search?q=${query}`,
        {
          headers: { "Authorization": `Bearer ${accessToken}` },
        }
      );
      return response.json();
    },
  };
}

Component Organization

packages/chrome-extension/
├── sidepanel.tsx          # Main entry (auth + ChatInterface)
├── background.ts          # Service worker (keyboard shortcuts)
├── lib/
│   ├── oauth.ts           # OAuth 2.0 with PKCE
│   └── utils.ts           # Utility functions
├── components/chat/
│   ├── ChatInterface.tsx  # Main orchestrator (~400 LOC)
│   ├── hooks/
│   │   └── use-extension-api.ts  # Extension-specific API wrapper
│   ├── views/             # Extension-specific UI
│   │   ├── ChatHeader.tsx        # Header with sign-out
│   │   ├── ChatHistoryView.tsx   # History sidebar
│   │   ├── ChatEmptyState.tsx    # Empty state
│   │   └── ChatInputArea.tsx     # Input with @-mentions
│   ├── MentionExtension.tsx      # TipTap mention config
│   ├── MentionList.tsx           # Mention dropdown
│   ├── ContextRing.tsx           # Context indicator
│   └── EnhancedFileContent.tsx   # File preview
└── __tests__/             # Vitest unit tests

Rule: Extension components ONLY in

components/chat/

@briefhq/chat-ui

Development Workflow

Local Setup

cd packages/chrome-extension
pnpm install

# Create environment file
cp .env.dev.example .env.dev

# Start dev server (hot reload)
pnpm run dev

Load Extension in Chrome

1. Open

   chrome://extensions/

2. Enable "Developer mode"
3. Click "Load unpacked"
4. Select

   packages/chrome-extension/build/chrome-mv3-dev

Build for Different Environments

pnpm run build

pnpm run build:staging

pnpm run build:qa

pnpm run build:production

Package for Distribution

# QA build (.crx for internal testing)
pnpm run package:qa

# Production build (for Chrome Web Store)
pnpm run package:production

Testing

Brief extension uses Vitest for unit tests:

import { describe, it, expect, vi } from "vitest";
import { render, waitFor } from "@testing-library/react";
import { ChatInterface } from "./ChatInterface";

// Mock Chrome APIs
vi.mock("chrome", () => ({
  tabs: {
    query: vi.fn(),
    onActivated: { addListener: vi.fn(), removeListener: vi.fn() },
    onUpdated: { addListener: vi.fn(), removeListener: vi.fn() },
  },
  storage: {
    local: {
      get: vi.fn(),
      set: vi.fn(),
    },
  },
}));

describe("ChatInterface", () => {
  it("renders chat UI when authenticated", () => {
    const { getByRole } = render(
      <ChatInterface accessToken="test-token" contextUrl="https://example.com" />
    );

    expect(getByRole("textbox")).toBeInTheDocument();
  });

  it("passes contextUrl to chat API", async () => {
    const { getByRole, getByText } = render(
      <ChatInterface
        accessToken="test-token"
        contextUrl="https://example.com/page"
      />
    );

    const input = getByRole("textbox");
    await userEvent.type(input, "Summarize this page");
    await userEvent.click(getByText("Send"));

    // Verify contextUrl passed to API
    expect(fetchMock).toHaveBeenCalledWith(
      expect.any(String),
      expect.objectContaining({
        body: expect.stringContaining("https://example.com/page"),
      })
    );
  });
});

Run Tests

pnpm test                # Run all tests
pnpm run test:watch      # Watch mode
pnpm run test:coverage   # Coverage report

Key Differences from Web App

/api/...

https://app.briefhq.ai/api/...

Common Patterns

Passing Context URL

// ✅ GOOD - Use current tab URL for context
export function ChatInterface({ contextUrl }: Props) {
  const handleSubmit = async (input: string) => {
    await api.chat(messages, {
      contextUrl, // Current tab URL
      model: selectedModel,
    });
  };
}

Model Selection

// ✅ GOOD - Store model selection in state
const [selectedModel, setSelectedModel] = useState<ModelId>("claude-sonnet-4-5");

// Pass to useChatTransport
const transport = useChatTransport({
  defaultModel: selectedModel,
});

File Attachments

// ✅ GOOD - Use useFileAttachments hook from @briefhq/chat-ui
const fileAttachments = useFileAttachments({
  maxFiles: 10,
  maxSize: 32 * 1024 * 1024, // 32MB
  allowedTypes: ["image/*", "application/pdf", "text/*"],
});

// In UI
<ChatInputArea
  files={fileAttachments.files}
  onFilesSelected={fileAttachments.addFiles}
  onFileRemove={fileAttachments.removeFile}
/>

Document @-Mentions

// ✅ GOOD - Use useMentions hook from @briefhq/chat-ui
const mentions = useMentions({
  api,
  onMentionSelected: (doc: MentionDocument) => {
    // Append to input
  },
});

// In TipTap editor
<Editor
  extensions={[
    StarterKit,
    Mention.configure({
      suggestion: mentions.suggestionOptions,
    }),
  ]}
/>

Troubleshooting

OAuth Issues

Problem: "Authentication failed" error

• Check: Extension ID in OAuth client redirect URIs
• Fix: Add

  https://{extension-id}.chromiumapp.org/

  to allowed redirect URIs

Problem: Token expired

• Check: Token refresh logic in

  lib/oauth.ts

• Fix: Implement automatic refresh 60 seconds before expiry

API Issues

Problem: CORS errors

• Check:

  host_permissions

  in manifest.json
• Fix: Add API domain to

  host_permissions

Build Issues

Problem: Hot reload not working

• Check: Plasmo dev server running
• Fix: Run

  pnpm run dev

  and reload extension

Documentation References

• Chrome Extension Architecture:

  /docs/CHROME_EXTENSION_ARCHITECTURE.md

• Extension README:

  /packages/chrome-extension/README.md

• Shared Chat UI:

  /packages/chat-ui/

Related Code

• OAuth flow:

  packages/chrome-extension/lib/oauth.ts

• Chat interface:

  packages/chrome-extension/components/chat/ChatInterface.tsx

• Side panel entry:

  packages/chrome-extension/sidepanel.tsx

• Extension API:

  packages/chrome-extension/components/chat/hooks/use-extension-api.ts