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1.29.0 • Public • Published

Voiceflow Runtime

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runtime SDK for executing voiceflow projects and conversational state management across different platforms.

⚠️ This repository is still undergoing active development: Major breaking changes may be pushed periodically and the documentation may become outdated - a stable version has not been released

yarn add @voiceflow/runtime

npm i @voiceflow/runtime


ci pipeline

At a very high level, you can think of the whole Voiceflow system like code:

  • The frontend creator system allows conversation designers to "write" in a visual programming language, like an IDE. Each of their diagrams is like a function/instruction.
  • When they press Upload or Export all their flow diagrams are "compiled" into programs
  • The runtime is like the CPU that reads these programs and executes them based on end user input.
  • these programs can be read through Local Project Files or through an Voiceflow API where they are hosted with Voiceflow databases
  • the runtime handles state management and tracks the user's state at each turn of conversation

Where conversational state management differs from traditional code execution is that it is heavily I/O based and always blocked when awaiting user input:

It is important to understand the Conversation Request/Response Webhook Model

client architecture

The end user session storage is determined by the implementation of the runtime, but stores the most sensitive data in this architecture (what the user has said, where they are in the conversation, variables, etc.)

Anatomy of an Interaction (Alexa Example)

As a practical example, here's what happens when a user speaks to a custom skill (app) on their Alexa device. Most other conversational platforms function in this manner - if it is a text/or chat, it would be the exact same, just without a voice-to-text/text-to-voice layer.

  1. user says something to alexa, alexa uses natural language processing to transcribe user intent, then sends it via webhook (along with other metadata i.e. userID) to alexa-runtime, which implements @voiceflow/runtime
  2. fetch user state (JSON format) from end user session storage based on a userID identifier
  3. fetch the current program (flow) that the user is on from Voiceflow API/Local Project File
  4. go through nodes in the flow and update the user state
  5. save the final user state to end user session storage
  6. generate a webhook response based on the final user state, send back to alexa
  7. alexa interprets response and speaks to user

repeat all steps each time a user speaks to the alexa skill, to perform a conversation. (here's Amazon Alexa's documentation)

Implementation Example (Generic Conversation Platform)


import Client, { LocalDataApi } from '@voiceflow/runtime';

import CustomHandlers from './handlers';

const client = new Client({
  handlers: {
  api: new LocalDataApi({ projectSource: PROJECT_SOURCE /* local project file */ }),

// if you want to inject custom handlers during lifecycle events
client.setEvent(EventType.handlerDidCatch, (err, runtime) => {
  throw err;

Client can be initialized with either LocalDataApi, which reads a local file for project and program source, or ServerDataApi, which requires an API key (adminToken) and endpoint (dataEndpoint) and fetches the data remotely.

Handle Interaction Request

// incoming webhook request
const handleRequest = async (userID, versionID, payload) => {
  // retrieve the previous user state
  const rawState = DB.fetchState(userID);
  const runtime = client.createRuntime(versionID, rawState, payload);

  // update the state and generate new runtime (step through the program)

  // save the new user state
  DB.saveState(userID, runtime.getFinalState());

  // generate a response based on trace
  let response = {};
  runtime.trace.forEach((trace) => {
    if (trace.type === 'card') response.card = trace.payload;
    if (trace.type === 'speak') response.speak += trace.payload;

  return response; // the SDK usually handles this

Custom Node Handler

import { HandlerFactory } from '@voiceflow/runtime';

// create a handler for a particular node type
export const CustomNodeHandler: HandlerFactory<Node, typeof utilsObj> = (utils) => ({
  // return a boolean whether or not this handler can handle a particular node
  canHandle: (node) => {
    return node.type === 'CUSTOM_NODE' && typeof node.custom === 'string';
  // perform side effects and return the next node to go to
  handle: (node, runtime, variables) => {
    if (runtime.storage.get('customSetting')) {
      // add something to the trace to help generate the response
        type: 'card',
        payload: node.custom,
      // maybe add something to analytics when this block is triggered

    // if you return null it will immediately end the current flow
    return node.nextId;

Return Types

  • return null: ends the current flow, and pops it off the stack
  • return different nodeID: as long as the nodeID is present in the current flow program, it will attempt to be handled next - if it is not found, same behavior as return null
  • return self nodeID: if the handler's handle() returns the same nodeID as the node it is handling, then the execution of this interaction (runtime.update()) will end in the exact same state and wait for the next user interaction/webhook. The next request will begin on this same node. You would do this to await user input. Example: Choice Node


request: what the end user has done (intent, push button, etc)

runtime: general purpose object that provides an API to manipulate state

platform: alexa, google, IVR, messenger, slack, twilio, etc.

frame: program frame - contains local variables, pointers, etc.

stack: stack of program frames (like code execution stack)

program: flow program object (equivalent to text/code in memory model) (READ ONLY)

node: node metadata - each individual step/block in the program

variables: global variables

storage: object full of things you want to persist between requests

turn: object full of things that only matter during this turn

handlers: array of handlers that handle specific node functionalities


(in progress)

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