Extensions let you add custom functionality to your web chat widget. You can render interactive elements like file uploads, date pickers, or payment forms directly in the chat window, or trigger effects elsewhere on your site like updating a status icon or running custom scripts.

Extension types

There are two types of extensions: Response extensions render interactive widgets inside the chat window. Use these for file uploads, calendar pickers, custom forms, or any UI element users interact with during the conversation. Effect extensions don’t render anything in the chat, but trigger actions elsewhere on your site. Use these for deep-linking users, updating page elements, or running custom scripts based on conversation events.

How extensions work

Extensions are triggered by a Function step in a workflow. You define the extension in your site’s JavaScript, then register it in your chat.load() configuration. The action name must match across your extension code and your Voiceflow steps.

Response extension structure

const MyResponseExtension = {
  name: 'ExtensionName',
  type: 'response',
  match: ({ trace }) => trace.type === 'Custom_Action_Name' || trace.payload?.name === 'Custom_Action_Name',
  render: ({ trace, element }) => {
    // Create and append your custom UI to 'element'
    // Optionally return a cleanup function
  }
};

Effect extension structure

const MyEffectExtension = {
  name: 'ExtensionName',
  type: 'effect',
  match: ({ trace }) => trace.type === 'Custom_Action_Name' || trace.payload?.name === 'Custom_Action_Name',
  effect: ({ trace }) => {
    // Run your custom logic here
  }
};

Registering extensions

window.voiceflow.chat.load({
  verify: { projectID: 'YOUR_PROJECT_ID' },
  url: 'https://general-runtime.voiceflow.com',
  assistant: {
    extensions: [MyResponseExtension, MyEffectExtension]
  }
});

There’s no limit to the number of extensions you can register.

Example: building a form extension

Build a webchat extension that renders a pre-filled form, triggered by a Voiceflow function. The function sends user data (name, email) via a custom trace, the extension renders the form and collects missing fields, and submits the completed data back to the function which routes based on the response.

Architecture

Step 1: The Voiceflow Function

Create a function in Voiceflow that accepts name and email as input variables, emits a custom trace to trigger the extension, and listens for the form submission.

Function Configuration

Setting	Value
Name	`Send Form Trace`
Input Variables	`name` (string), `email` (string)
Output Variables	`form_name`, `form_email`, `form_phone`, `form_company`
Paths	`submitted`, `default`

Function Code

export default async function main(args) {
  const { name, email } = args.inputVars;

  return {
    trace: [
      {
        type: 'user_form',
        payload: {
          name: name || '',
          email: email || '',
        },
      },
    ],
    next: {
      listen: true,
      to: [
        {
          on: { 'event.type': 'form_submitted' },
          dest: 'submitted',
        },
      ],
      defaultTo: 'default',
    },
  };
}

What this does:

Reads name and email from the function’s input variables (collected earlier in the flow).
Emits a trace with type: 'user_form' — this is the custom trace type the webchat extension will match against.
Sets listen: true so the agent pauses and waits for the user to interact.
Routes to the submitted path when it receives an event with type: 'form_submitted'.

Step 2: The Webchat Extension

Register a response extension that matches the user_form trace, renders a form pre-filled with the payload data, and sends the completed form back via interact().

Extension Code

const FORM_FIELDS = [
  { name: 'name', label: 'Name', type: 'text', prefilled: true },
  { name: 'email', label: 'Email', type: 'email', prefilled: true },
  { name: 'phone', label: 'Phone', type: 'tel', placeholder: '(555) 123-4567' },
  { name: 'company', label: 'Company', type: 'text', placeholder: 'Acme Inc.' },
];

const FORM_STYLES = `
  .vf-form {
    display: flex;
    flex-direction: column;
    gap: 12px;
    padding: 16px;
    font-family: sans-serif;
  }
  .vf-form label {
    display: flex;
    flex-direction: column;
    gap: 4px;
    font-size: 13px;
    font-weight: 500;
    color: #333;
  }
  .vf-form input {
    padding: 8px 12px;
    border: 1px solid #ddd;
    border-radius: 6px;
    font-size: 14px;
    outline: none;
    transition: border-color 0.15s;
  }
  .vf-form input:focus { border-color: #387dff; }
  .vf-form input.error { border-color: #e74c3c; }
  .vf-form button {
    padding: 10px 16px;
    background: #387dff;
    color: #fff;
    border: none;
    border-radius: 6px;
    font-size: 14px;
    font-weight: 600;
    cursor: pointer;
    margin-top: 4px;
  }
  .vf-form button:hover { background: #2f68db; }
  .vf-form button:disabled { background: #ccc; cursor: not-allowed; }
`;

function createFormField(field, prefillData) {
  const label = document.createElement('label');
  label.textContent = field.label;

  const input = document.createElement('input');
  input.type = field.type;
  input.name = field.name;
  input.required = true;

  if (field.prefilled && prefillData[field.name]) {
    input.value = prefillData[field.name];
  }
  if (field.placeholder) {
    input.placeholder = field.placeholder;
  }

  label.appendChild(input);
  return label;
}

const FormExtension = {
  name: 'UserForm',
  type: 'response',
  match: ({ trace }) => trace.type === 'user_form' || trace.payload?.name == 'user_form',
  render: ({ trace, element }) => {
    const prefillData = trace.payload || {};

    const style = document.createElement('style');
    style.textContent = FORM_STYLES;

    const form = document.createElement('form');
    const container = document.createElement('div');
    container.className = 'vf-form';

    FORM_FIELDS.forEach((field) => {
      container.appendChild(createFormField(field, prefillData));
    });

    const button = document.createElement('button');
    button.type = 'submit';
    button.textContent = 'Submit';
    container.appendChild(button);

    form.appendChild(style);
    form.appendChild(container);

    form.addEventListener('submit', (event) => {
      event.preventDefault();

      const data = {};
      let valid = true;

      FORM_FIELDS.forEach((field) => {
        const input = form.querySelector(`input[name="${field.name}"]`);
        const value = input.value.trim();
        input.classList.toggle('error', !value);
        if (!value) valid = false;
        data[field.name] = value;
      });

      if (!valid) return;

      button.disabled = true;
      form.querySelectorAll('input').forEach((input) => {
        input.disabled = true;
      });

      window.voiceflow.chat.interact({
        type: 'form_submitted',
        payload: data,
      });
    });

    element.appendChild(form);
  },
};

Register the Extension

Add it to your chat.load() call:

window.voiceflow.chat.load({
  verify: { projectID: 'YOUR_PROJECT_ID' },
  url: 'https://general-runtime.voiceflow.com',
  assistant: {
    extensions: [FormExtension],
  },
});

Step 3: Capture the Data in the Agent Flow

When the user submits the form, interact() sends an event with type: 'form_submitted' and a payload containing { name, email, phone, company }. The function’s listen picks this up and routes to the submitted path. To access the submitted data downstream, use a Code step (or a second function) connected to the submitted path:

// In a Code step after the "submitted" path
const name = last_event.payload.name;
const email = last_event.payload.email;
const phone = last_event.payload.phone;
const company = last_event.payload.company;

Alternatively, if using the function’s output variables, create a second function on the submitted path that reads last_event and maps values to output variables:

export default async function main(args) {
  const { last_event } = args.inputVars;

  return {
    outputVars: {
      form_name: last_event.payload.name,
      form_email: last_event.payload.email,
      form_phone: last_event.payload.phone,
      form_company: last_event.payload.company,
    },
    next: { path: 'success' },
  };
}

Step 4: Wire the Agent Flow

The overall canvas layout:

Before the function, collect name and email.
Wire those variables into the function’s input variables.
Connect the submitted output path to a Code step or a Function that reads last_event.payload.
Use the extracted variables in subsequent steps or back in your agent.

Complete Working Example

Two files: form.js with the extension, and index.html that loads it.

`form.js`

const FORM_FIELDS = [
  { name: "name", label: "Name", type: "text", prefilled: true },
  { name: "email", label: "Email", type: "email", prefilled: true },
  { name: "phone", label: "Phone", type: "tel", placeholder: "(555) 123-4567" },
  { name: "company", label: "Company", type: "text", placeholder: "Acme Inc." },
];

const FORM_CSS = [
  ".vf-form { display:flex; flex-direction:column; gap:12px; padding:16px; font-family:sans-serif; }",
  ".vf-form label { display:flex; flex-direction:column; gap:4px; font-size:13px; font-weight:500; color:#333; }",
  ".vf-form input { padding:8px 12px; border:1px solid #ddd; border-radius:6px; font-size:14px; outline:none; transition:border-color 0.15s; }",
  ".vf-form input:focus { border-color:#387dff; }",
  ".vf-form input.error { border-color:#e74c3c; }",
  ".vf-form button { padding:10px 16px; background:#387dff; color:#fff; border:none; border-radius:6px; font-size:14px; font-weight:600; cursor:pointer; margin-top:4px; }",
  ".vf-form button:hover { background:#2f68db; }",
  ".vf-form button:disabled { background:#ccc; cursor:not-allowed; }",
].join("\n");

function createFormField(field, prefillData) {
  const label = document.createElement("label");
  label.textContent = field.label;

  const input = document.createElement("input");
  input.type = field.type;
  input.name = field.name;
  input.required = true;
  if (field.prefilled && prefillData[field.name]) {
    input.value = prefillData[field.name];
  }
  if (field.placeholder) {
    input.placeholder = field.placeholder;
  }

  label.appendChild(input);
  return label;
}

const FormExtension = {
  name: "UserForm",
  type: "response",
  match: function (ctx) {
    return ctx.trace.type === "user_form";
  },
  render: function (ctx) {
    const prefillData = ctx.trace.payload || {};

    const style = document.createElement("style");
    style.textContent = FORM_CSS;

    const form = document.createElement("form");
    const container = document.createElement("div");
    container.className = "vf-form";

    FORM_FIELDS.forEach(function (field) {
      container.appendChild(createFormField(field, prefillData));
    });

    const button = document.createElement("button");
    button.type = "submit";
    button.textContent = "Submit";
    container.appendChild(button);

    form.appendChild(style);
    form.appendChild(container);

    form.addEventListener("submit", function (event) {
      event.preventDefault();
      const data = {};
      let valid = true;

      FORM_FIELDS.forEach(function (field) {
        const input = form.querySelector('input[name="' + field.name + '"]');
        const value = input.value.trim();
        input.classList.toggle("error", !value);
        if (!value) valid = false;
        data[field.name] = value;
      });

      if (!valid) return;

      button.disabled = true;
      form.querySelectorAll("input").forEach(function (input) {
        input.disabled = true;
      });

      window.voiceflow.chat.interact({
        type: "form_submitted",
        payload: data,
      });
    });

    ctx.element.appendChild(form);
  },
};

`index.html`

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Form Extension Demo</title>
</head>
<body>
  <h1>Form Extension Demo</h1>

  <script src="form.js"></script>
  <script type="text/javascript">
    (function (d, t) {
      var v = d.createElement(t), s = d.getElementsByTagName(t)[0];
      v.onload = function () {
        window.voiceflow.chat.load({
          verify: { projectID: 'YOUR_PROJECT_ID' },
          url: 'https://general-runtime.voiceflow.com',
          assistant: {
            extensions: [FormExtension],
          },
        });
      };
      v.src = 'https://cdn.voiceflow.com/widget-next/bundle.mjs';
      v.type = 'text/javascript';
      s.parentNode.insertBefore(v, s);
    })(document, 'script');
  </script>
</body>
</html>

How the Data Flows

Step	Where	What Happens
1	Agent flow	Name and email collected into variables
2	Function	Emits `user_form` trace with `{ name, email }` payload; sets `listen: true`
3	Webchat	Extension matches trace, renders form pre-filled with name and email
4	Webchat	User fills in phone and company, clicks Submit
5	Webchat	`interact({ type: 'form_submitted', payload: { name, email, phone, company } })`
6	Function	`listen` receives `form_submitted` event, routes to `submitted` path
7	Agent flow	Code step or Function reads `last_event.payload` to populate variables

Key Concepts

`Why listen: true?`

Setting listen: true in the function’s next command pauses the agent at the function step. Without it, the agent would immediately continue down the defaultTo path before the user has a chance to fill out the form.

Event Matching

The to array uses MongoDB-style queries (via sift.js) to match incoming events. { 'event.type': 'form_submitted' } matches any interact() call where type equals 'form_submitted'. You can also match on payload properties:

to: [
  {
    on: {
      'event.type': 'form_submitted',
      'event.payload.company': { $exists: true },
    },
    dest: 'submitted',
  },
]

Accessing Submitted Data

After the listen resolves, the event that triggered it is available as last_event in downstream Code steps:

// last_event structure
{
  type: 'form_submitted',
  payload: {
    name: 'John Doe',
    email: 'john@example.com',
    phone: '(555) 123-4567',
    company: 'Acme Inc.',
  },
}

Extension Cleanup

The render function can return a cleanup function that runs when the message is removed from the chat (e.g., on session reset):

render: ({ trace, element }) => {
  const form = buildForm(trace.payload);
  element.appendChild(form);

  return () => {
    form.remove();
  };
},

More examples

For additional extension examples including video embeds, maps, input field controls, and confetti animations, visit the sample extensions repository

Example: building a form extension

This example walks through creating a form that collects name, email, and phone number from users.

Step 1: Create the extension

Add this script above your web chat widget snippet, before the closing </body> tag:

<script>
const FormExtension = {
  name: 'Forms',
  type: 'response',
  match: ({ trace }) =>
    trace.type === 'Custom_Form' || trace.payload?.name === 'Custom_Form',
  render: ({ trace, element }) => {
    const formContainer = document.createElement('form');

    formContainer.innerHTML = `
      <style>
        label {
          font-size: 0.8em;
          color: #888;
        }
        input[type="text"], input[type="email"], input[type="tel"] {
          width: 100%;
          border: none;
          border-bottom: 0.5px solid rgba(0, 0, 0, 0.1);
          background: transparent;
          margin: 5px 0;
          outline: none;
          padding: 8px 0;
        }
        .phone {
          width: 150px;
        }
        .invalid {
          border-color: red;
        }
        .submit {
          background: linear-gradient(to right, #2e6ee1, #2e7ff1);
          border: none;
          color: white;
          padding: 10px;
          border-radius: 5px;
          width: 100%;
          cursor: pointer;
        }
      </style>

      <label for="name">Name</label>
      <input type="text" class="name" name="name" required><br><br>

      <label for="email">Email</label>
      <input type="email" class="email" name="email" required pattern="[a-z0-9._%+-]+@[a-z0-9.-]+\\.[a-z]{2,}$" title="Invalid email address"><br><br>

      <label for="phone">Phone Number</label>
      <input type="tel" class="phone" name="phone" required pattern="\\d+" title="Invalid phone number, please enter only numbers"><br><br>

      <input type="submit" class="submit" value="Submit">
    `;

    formContainer.addEventListener('input', function () {
      const name = formContainer.querySelector('.name');
      const email = formContainer.querySelector('.email');
      const phone = formContainer.querySelector('.phone');

      if (name.checkValidity()) name.classList.remove('invalid');
      if (email.checkValidity()) email.classList.remove('invalid');
      if (phone.checkValidity()) phone.classList.remove('invalid');
    });

    formContainer.addEventListener('submit', function (event) {
      event.preventDefault();

      const name = formContainer.querySelector('.name');
      const email = formContainer.querySelector('.email');
      const phone = formContainer.querySelector('.phone');

      if (
        !name.checkValidity() ||
        !email.checkValidity() ||
        !phone.checkValidity()
      ) {
        name.classList.add('invalid');
        email.classList.add('invalid');
        phone.classList.add('invalid');
        return;
      }

      formContainer.querySelector('.submit').remove();

      window.voiceflow.chat.interact({
        type: 'complete',
        payload: { name: name.value, email: email.value, phone: phone.value },
      });
    });

    element.appendChild(formContainer);
  },
};
</script>

Step 2: Register the extension

Update your chat.load() configuration to include the extension:

window.voiceflow.chat.load({
  verify: { projectID: 'YOUR_PROJECT_ID' },
  url: 'https://general-runtime.voiceflow.com',
  assistant: {
    extensions: [FormExtension]
  }
});

Step 3: Trigger the extension from your workflow

Add a Custom action step to your workflow with the name Custom_Form. Create a path called complete and enable Stop on action so the agent waits for the form submission before continuing. When triggered, your widget receives a trace that activates the extension:

{
  "type": "Custom_Form",
  "payload": "{}",
  "defaultPath": 0,
  "paths": [
    { "event": { "type": "complete" } }
  ]
}

Step 4: Retrieve the submitted values

After the form is submitted, the values are stored in the last_event system variable. Access them in a JavaScript step:

name = last_event.payload.name
email = last_event.payload.email
phone = last_event.payload.phone

More examples

For additional extension examples including video embeds, maps, input field controls, and confetti animations, visit the sample extensions repository.

Build

Deploy

Measure

Account

Web chat extensions

Extension types

How extensions work

Response extension structure

Effect extension structure

Registering extensions

Example: building a form extension

Architecture

Step 1: The Voiceflow Function

Function Configuration

Function Code

Step 2: The Webchat Extension

Extension Code

Register the Extension

Step 3: Capture the Data in the Agent Flow

Step 4: Wire the Agent Flow

Complete Working Example

`form.js`

`index.html`

How the Data Flows

Key Concepts

`Why listen: true?`

Event Matching

Accessing Submitted Data

Extension Cleanup

More examples

Example: building a form extension

Step 1: Create the extension

Step 2: Register the extension

Step 3: Trigger the extension from your workflow

Step 4: Retrieve the submitted values

More examples

Build

Deploy

Measure

Account

Documentation Index

​Extension types

​How extensions work

​Response extension structure

​Effect extension structure

​Registering extensions

​Example: building a form extension

​Architecture

​Step 1: The Voiceflow Function

​Function Configuration

​Function Code

​Step 2: The Webchat Extension

​Extension Code

​Register the Extension

​Step 3: Capture the Data in the Agent Flow

​Step 4: Wire the Agent Flow

​Complete Working Example

​form.js

​index.html

​How the Data Flows

​Key Concepts

​Why listen: true?

​Event Matching

​Accessing Submitted Data

​Extension Cleanup

​More examples

​Example: building a form extension

​Step 1: Create the extension

​Step 2: Register the extension

​Step 3: Trigger the extension from your workflow

​Step 4: Retrieve the submitted values

​More examples

Extension types

How extensions work

Response extension structure

Effect extension structure

Registering extensions

Example: building a form extension

Architecture

Step 1: The Voiceflow Function

Function Configuration

Function Code

Step 2: The Webchat Extension

Extension Code

Register the Extension

Step 3: Capture the Data in the Agent Flow

Step 4: Wire the Agent Flow

Complete Working Example

`form.js`

`index.html`

How the Data Flows

Key Concepts

`Why listen: true?`

Event Matching

Accessing Submitted Data

Extension Cleanup

More examples

Example: building a form extension

Step 1: Create the extension

Step 2: Register the extension

Step 3: Trigger the extension from your workflow

Step 4: Retrieve the submitted values

More examples