@assemble-inc/chat-widget

An embeddable, general-purpose AI chat widget. Drop it into any React app — or any plain HTML page — to give users a floating chat interface backed by Anthropic Claude and MCP (Model Context Protocol). Fully configurable at embed time: system prompt, model, knowledge base, UI copy, and more.

Installation

npm install @assemble-inc/chat-widget
# or
yarn add @assemble-inc/chat-widget

Peer dependencies — make sure these are already in your project:

npm install react react-dom

Quick Start

1. Mount the widget

import { Widget } from "@assemble-inc/chat-widget";
import "@assemble-inc/chat-widget/style.css";

export default function App() {
  return (
    <>
      {/* your app */}
      <Widget
        config={{
          systemPrompt: "You are a helpful assistant for Acme Corp.",
          title: "Ask Us Anything",
          placeholder: "Type your question...",
        }}
      />
    </>
  );
}

The widget renders as a fixed floating button in the bottom-right corner (configurable). Clicking it opens the chat panel.

2. Add the server-side handler

The simplest setup is to mount the handler on your existing app server so that the widget's default endpoint: '/api/chat' resolves correctly:

// your existing Express/Next/etc. server
import { createChatHandler } from "@assemble-inc/chat-widget/server";

app.use(express.json());
app.post(
  "/api/chat",
  createChatHandler({
    apiKey: process.env.ANTHROPIC_API_KEY!,
    mcpServerUrl: process.env.MCP_SERVER_URL!,
    mcpBearerToken: process.env.MCP_BEARER_TOKEN,
  }),
);

If you prefer to run a separate API server (e.g. on port 3006), you must tell the widget where to find it — otherwise it will POST to your frontend server and get a 404:

// Pass the full URL when the API server is on a different port
<Widget config={{ endpoint: "http://localhost:3006/api/chat" }} />

Or proxy /api to the API server in your frontend dev config (Vite example):

// vite.config.ts
server: {
  proxy: {
    "/api": "http://localhost:3006",
  },
},

3. Environment variables

ANTHROPIC_API_KEY=sk-ant-...
MCP_SERVER_URL=https://your-mcp-server.example.com/mcp
MCP_BEARER_TOKEN=your-bearer-token   # optional
SYSTEM_PROMPT="You are..."           # optional server-side override

Script-tag Embed (no React required)

For non-React apps, use the self-contained IIFE bundle:

<link rel="stylesheet" href="https://cdn.example.com/asm-widget/index.css" />
<script src="https://cdn.example.com/asm-widget/embed.js"></script>
<script>
  AsmWidget.init({
    endpoint: "https://my-server.com/api/chat",
    systemPrompt: "You are a helpful assistant.",
    title: "Help",
  });
</script>

Build the embed bundle:

npm run build:embed   # outputs dist/embed.js

Configuration Reference

WidgetConfig

All configuration is passed as a single config prop. Every field is optional.

| Field | Type | Default | Description | | ----------------- | --------------------------------- | ---------------- | -------------------------------------------------------------------------------------------------------------- | | endpoint | string | '/api/chat' | Backend chat endpoint URL. | | systemPrompt | string | — | System prompt for this embed. Forwarded to the server; server-side value wins when both are set. | | context | string | — | Additional context appended after the system prompt (e.g. current page, user role). Forwarded to the server. | | mcpServerUrl | string | — | MCP server URL for this embed. Must appear in the server's mcpCredentials allowlist; falls back to default. | | model | string | — | Claude model ID (e.g. 'claude-opus-4-5'). Forwarded to the server; server-side value wins when both are set. | | temperature | number | — | Response creativity, 0–1. Forwarded to the server; server-side value wins when both are set. | | initialMessages | Array<{ role, content }> | — | Seed messages shown when the widget first opens. | | persist | boolean | false | Save the conversation to sessionStorage across page navigations. | | user | { id?, name?, role? } | — | Forwarded in every request body for server-side personalisation or logging. | | title | string | 'Ask ASMBL' | Header title. | | logo | string | Built-in logo | URL to a custom logo image. | | welcomeHeading | string | — | Heading shown in the empty state. | | placeholder | string | — | Textarea placeholder text. | | position | 'bottom-right' \| 'bottom-left' | 'bottom-right' | Widget anchor corner. |

WidgetProps

| Prop | Type | Default | Description | | -------------- | -------------------------------------------------- | ------- | ------------------------------------------------------------------------------------ | | config | WidgetConfig | — | All widget configuration (see above). | | open | boolean | — | Controls open/close state from outside (controlled mode). | | defaultOpen | boolean | false | Initial open state when uncontrolled. | | onOpenChange | (open: boolean) => void | — | Called whenever the widget opens or closes. | | onMessage | (msg: { role: string; content: string }) => void | — | Called after each new assistant message is received. | | onError | (error: Error) => void | — | Called when a stream error occurs. | | children | ReactNode | — | Custom content shown in the empty state. Replaces the built-in empty state entirely. |

Server Reference

createChatHandler(config)

Returns an Express-compatible (req, res) => Promise<void> request handler that streams Claude responses via your MCP server.

| Option | Type | Required | Description | | ---------------- | ------------------------ | -------- | --------------------------------------------------------------------------------------------------------------------------- | | apiKey | string | Yes | Anthropic API key. | | mcpServerUrl | string | Yes | Default MCP server URL. Used when no per-embed URL is provided or the requested URL is not in the allowlist. | | mcpBearerToken | string | No | Bearer token for the default MCP server. | | model | string | No | Claude model ID. Takes precedence over any model sent by the widget. Defaults to claude-sonnet-4-5. | | systemPrompt | string | No | Operator system prompt. Takes precedence over any system prompt sent by the widget. | | temperature | number | No | Model temperature (0–1). Takes precedence over any temperature sent by the widget. | | mcpCredentials | Record<string, string> | No | Allowlist of additional MCP server URLs → bearer tokens. Enables per-embed MCP routing. Credentials never leave the server. |

Precedence rules

For systemPrompt, model, temperature, and mcpServerUrl, the server-side value always takes precedence over what the widget sends. The resolution order is:

ChatHandlerConfig value  →  widget config value  →  built-in default

Multi-context deployments

A single server can serve multiple embeds pointing at different MCP servers:

createChatHandler({
  apiKey: process.env.ANTHROPIC_API_KEY!,
  mcpServerUrl: process.env.MCP_SERVER_URL!, // default fallback
  mcpBearerToken: process.env.MCP_BEARER_TOKEN,
  mcpCredentials: {
    "https://mcp.acme.com/hr": process.env.MCP_HR_TOKEN,
    "https://mcp.acme.com/it": process.env.MCP_IT_TOKEN,
  },
});

Each embed specifies which server it wants via config.mcpServerUrl. Bearer tokens are looked up server-side — they are never exposed to the browser.

Examples

Minimal embed

<Widget config={{ systemPrompt: "You are a helpful assistant." }} />

Fully configured embed

<Widget
  config={{
    endpoint: "https://api.acme.com/chat",
    systemPrompt: "You are an HR assistant for Acme Corp.",
    mcpServerUrl: "https://mcp.acme.com/hr",
    model: "claude-opus-4-5",
    temperature: 0.3,
    title: "Ask HR",
    welcomeHeading: "How can we help?",
    placeholder: "Ask an HR question...",
    position: "bottom-left",
    persist: true,
    user: { id: currentUser.id, name: currentUser.name },
  }}
  onMessage={({ content }) => analytics.track("chat_message", { content })}
  onOpenChange={(open) => setHelpOpen(open)}
/>

Custom empty state (children)

<Widget config={{ title: "Support", placeholder: "Describe your issue..." }}>
  <div>
    <h3>How can we help?</h3>
    <button onClick={() => sendMessage("Reset my password")}>
      Reset my password
    </button>
    <button onClick={() => sendMessage("Billing question")}>
      Billing question
    </button>
  </div>
</Widget>

Local Development

# Install dependencies
npm install

# Copy and fill in environment variables
cp .env.example .env

# Start the Vite dev server (port 3003) + Express API server (port 3006)
npm run dev

The Vite dev server proxies /api/* to http://localhost:3006.

npm run lint          # type-check without emitting
npm run build         # build the React library (dist/)
npm run build:embed   # build the standalone IIFE script (dist/embed.js)

Publishing

npm version patch   # bug fixes (0.1.x)
npm version minor   # new features (0.x.0)
npm version major   # breaking changes (x.0.0)

npm publish