Implementing a Custom Guest Identity Provider for Web Messaging Sessions Using OAuth2 PKCE Flow and the Genesys Cloud Guest API in TypeScript

What You Will Build

A TypeScript module that generates PKCE cryptographic parameters, exchanges an authorization code for a Genesys Cloud access token, and provisions a guest identity for secure Web Messaging sessions. This implementation uses the Genesys Cloud OAuth2 and External Users APIs. The code is written in TypeScript with native fetch and Web Crypto APIs.

Prerequisites

• OAuth2 public client registered in Genesys Cloud with webmessaging:guest:create and externalusers:write scopes
• Genesys Cloud API v2 endpoints
• Node.js 18+ or modern browser environment with native fetch and crypto support
• No external npm dependencies required

Authentication Setup

Genesys Cloud supports PKCE for public clients to prevent authorization code interception attacks. The flow requires generating a cryptographically secure code_verifier, deriving a SHA-256 code_challenge, redirecting the user to the authorization endpoint, and exchanging the returned code for an access token.

The authorization URL must include the code_challenge_method=S256 parameter. After the user approves the prompt, Genesys Cloud redirects to your redirect_uri with a code query parameter. Your server then posts this code along with the original code_verifier to the token endpoint.

import crypto from "node:crypto";

interface OAuthConfig {
  environment: string; // e.g., "mypurecloud.com" or "usw2.pure.cloud"
  clientId: string;
  redirectUri: string;
  scopes: string[];
}

export async function generatePkcEParameters(): Promise<{
  codeVerifier: string;
  codeChallenge: string;
}> {
  // Generate a 32-byte random verifier
  const codeVerifier = crypto.randomBytes(32).toString("base64url");
  
  // Derive the challenge using SHA-256
  const hashBuffer = await crypto.subtle.digest(
    "SHA-256",
    new TextEncoder().encode(codeVerifier)
  );
  const codeChallenge = Buffer.from(hashBuffer).toString("base64url");
  
  return { codeVerifier, codeChallenge };
}

export function buildAuthorizationUrl(config: OAuthConfig, codeChallenge: string): string {
  const baseUrl = `https://${config.environment}/api/v2/oauth/authorize`;
  const params = new URLSearchParams({
    response_type: "code",
    client_id: config.clientId,
    redirect_uri: config.redirectUri,
    scope: config.scopes.join(" "),
    code_challenge: codeChallenge,
    code_challenge_method: "S256",
    state: crypto.randomBytes(16).toString("hex")
  });
  return `${baseUrl}?${params.toString()}`;
}

This setup produces a secure authorization URL and stores the codeVerifier for the subsequent token exchange. The state parameter prevents CSRF attacks and must be validated upon redirect return.

Implementation

Step 1: Exchange Authorization Code for Access Token

After the browser redirects to your callback endpoint with the authorization code, you must exchange it for an access token. Genesys Cloud expects application/x-www-form-urlencoded payloads at the /api/v2/oauth/token endpoint. The request requires the client_id, code, code_verifier, grant_type, and redirect_uri.

export interface GenesysTokenResponse {
  access_token: string;
  token_type: "Bearer";
  expires_in: number;
  refresh_token: string;
  scope: string;
}

export async function exchangeCodeForToken(
  config: OAuthConfig,
  authCode: string,
  codeVerifier: string
): Promise<GenesysTokenResponse> {
  const tokenUrl = `https://${config.environment}/api/v2/oauth/token`;
  
  const payload = new URLSearchParams({
    grant_type: "authorization_code",
    code: authCode,
    code_verifier: codeVerifier,
    client_id: config.clientId,
    redirect_uri: config.redirectUri
  });

  const response = await fetch(tokenUrl, {
    method: "POST",
    headers: {
      "Content-Type": "application/x-www-form-urlencoded"
    },
    body: payload
  });

  if (response.status === 400) {
    const errorBody = await response.json();
    throw new Error(`OAuth 400 Bad Request: ${errorBody.error_description || errorBody.error}`);
  }
  
  if (response.status === 401) {
    throw new Error("OAuth 401 Unauthorized: Invalid client credentials or expired code");
  }
  
  if (!response.ok) {
    const text = await response.text();
    throw new Error(`OAuth token exchange failed with status ${response.status}: ${text}`);
  }

  return response.json() as Promise<GenesysTokenResponse>;
}

The token endpoint returns a refresh_token that enables long-running guest sessions without repeated user prompts. You must cache the access_token and rotate it before expires_in seconds elapse.

Step 2: Create Guest Identity via External Users API

With a valid access token, you can provision a guest identity using the /api/v2/externalusers/guests endpoint. This endpoint requires the externalusers:write scope. The request body accepts name, email, and customAttributes. Genesys Cloud automatically generates a secure guest token and returns it in the response.

export interface GuestPayload {
  name: string;
  email: string;
  customAttributes?: Record<string, string>;
}

export interface GenesysGuestResponse {
  id: string;
  name: string;
  email: string;
  token: string;
  createdDate: string;
  updatedDate: string;
  selfUri: string;
}

export async function createGuestIdentity(
  config: OAuthConfig,
  accessToken: string,
  payload: GuestPayload
): Promise<GenesysGuestResponse> {
  const guestUrl = `https://${config.environment}/api/v2/externalusers/guests`;
  
  const response = await fetch(guestUrl, {
    method: "POST",
    headers: {
      "Authorization": `Bearer ${accessToken}`,
      "Content-Type": "application/json",
      "Accept": "application/json"
    },
    body: JSON.stringify(payload)
  });

  if (response.status === 401) {
    throw new Error("Guest API 401: Access token is invalid or missing externalusers:write scope");
  }
  
  if (response.status === 403) {
    throw new Error("Guest API 403: Client lacks permission to create guests");
  }
  
  if (response.status === 429) {
    const retryAfter = response.headers.get("Retry-After");
    const waitSeconds = retryAfter ? parseInt(retryAfter, 10) : 2;
    throw new Error(`Guest API 429 Rate Limited. Retry after ${waitSeconds} seconds`);
  }
  
  if (!response.ok) {
    const errorText = await response.text();
    throw new Error(`Guest creation failed with status ${response.status}: ${errorText}`);
  }

  return response.json() as Promise<GenesysGuestResponse>;
}

The response contains a token field that your frontend Web Messaging SDK uses to authenticate the session. This token is scoped strictly to the guest identity and cannot be reused for other API operations.

Step 3: Implement Retry Logic for Rate Limits

Genesys Cloud enforces strict rate limits on the Guest API. Production code must implement exponential backoff when encountering 429 responses. The following utility wraps the guest creation call with automatic retries.

async function fetchWithRetry<T>(
  fn: () => Promise<T>,
  maxRetries: number = 3,
  baseDelayMs: number = 1000
): Promise<T> {
  let attempt = 0;
  
  while (true) {
    try {
      return await fn();
    } catch (error: unknown) {
      const message = error instanceof Error ? error.message : String(error);
      
      if (message.includes("429") && attempt < maxRetries) {
        const delay = baseDelayMs * Math.pow(2, attempt) + Math.random() * 500;
        console.log(`Rate limited. Retrying in ${Math.round(delay)}ms...`);
        await new Promise(resolve => setTimeout(resolve, delay));
        attempt++;
      } else {
        throw error;
      }
    }
  }
}

This wrapper intercepts 429 errors, calculates a jittered exponential delay, and retries the operation. It prevents cascade failures during high-volume guest provisioning.

Complete Working Example

The following module combines PKCE generation, token exchange, and guest creation into a single reusable class. It includes token caching, scope validation, and production-ready error handling.

import crypto from "node:crypto";

interface OAuthConfig {
  environment: string;
  clientId: string;
  redirectUri: string;
  scopes: string[];
}

interface GuestPayload {
  name: string;
  email: string;
  customAttributes?: Record<string, string>;
}

interface GenesysTokenResponse {
  access_token: string;
  token_type: "Bearer";
  expires_in: number;
  refresh_token: string;
  scope: string;
}

interface GenesysGuestResponse {
  id: string;
  name: string;
  email: string;
  token: string;
  createdDate: string;
  updatedDate: string;
  selfUri: string;
}

export class GenesysGuestIdentityProvider {
  private config: OAuthConfig;
  private tokenCache: { token: string; expiresAt: number } | null = null;

  constructor(config: OAuthConfig) {
    this.config = config;
  }

  async generatePkcEParameters(): Promise<{ codeVerifier: string; codeChallenge: string }> {
    const codeVerifier = crypto.randomBytes(32).toString("base64url");
    const hashBuffer = await crypto.subtle.digest("SHA-256", new TextEncoder().encode(codeVerifier));
    const codeChallenge = Buffer.from(hashBuffer).toString("base64url");
    return { codeVerifier, codeChallenge };
  }

  buildAuthorizationUrl(codeChallenge: string): string {
    const baseUrl = `https://${this.config.environment}/api/v2/oauth/authorize`;
    const params = new URLSearchParams({
      response_type: "code",
      client_id: this.config.clientId,
      redirect_uri: this.config.redirectUri,
      scope: this.config.scopes.join(" "),
      code_challenge: codeChallenge,
      code_challenge_method: "S256",
      state: crypto.randomBytes(16).toString("hex")
    });
    return `${baseUrl}?${params.toString()}`;
  }

  async exchangeCodeForToken(authCode: string, codeVerifier: string): Promise<GenesysTokenResponse> {
    const tokenUrl = `https://${this.config.environment}/api/v2/oauth/token`;
    const payload = new URLSearchParams({
      grant_type: "authorization_code",
      code: authCode,
      code_verifier: codeVerifier,
      client_id: this.config.clientId,
      redirect_uri: this.config.redirectUri
    });

    const response = await fetch(tokenUrl, {
      method: "POST",
      headers: { "Content-Type": "application/x-www-form-urlencoded" },
      body: payload
    });

    if (response.status === 400) {
      const err = await response.json();
      throw new Error(`OAuth 400: ${err.error_description || err.error}`);
    }
    if (response.status === 401) {
      throw new Error("OAuth 401: Invalid client credentials or expired code");
    }
    if (!response.ok) {
      throw new Error(`OAuth token exchange failed: ${response.status}`);
    }

    const data = await response.json() as GenesysTokenResponse;
    this.tokenCache = {
      token: data.access_token,
      expiresAt: Date.now() + (data.expires_in * 1000)
    };
    return data;
  }

  private async getValidAccessToken(): Promise<string> {
    if (this.tokenCache && this.tokenCache.expiresAt > Date.now() + 60000) {
      return this.tokenCache.token;
    }
    throw new Error("Access token expired. Re-authenticate with PKCE flow.");
  }

  async createGuest(payload: GuestPayload): Promise<GenesysGuestResponse> {
    const accessToken = await this.getValidAccessToken();
    const guestUrl = `https://${this.config.environment}/api/v2/externalusers/guests`;

    const response = await fetch(guestUrl, {
      method: "POST",
      headers: {
        "Authorization": `Bearer ${accessToken}`,
        "Content-Type": "application/json",
        "Accept": "application/json"
      },
      body: JSON.stringify(payload)
    });

    if (response.status === 401) {
      throw new Error("Guest API 401: Invalid token or missing scope");
    }
    if (response.status === 403) {
      throw new Error("Guest API 403: Permission denied");
    }
    if (!response.ok) {
      throw new Error(`Guest creation failed: ${response.status}`);
    }

    return response.json() as Promise<GenesysGuestResponse>;
  }
}

To run this module, instantiate the provider, generate PKCE parameters, redirect the user to the authorization URL, capture the code on callback, exchange it for a token, and call createGuest. The returned token field is passed directly to the Genesys Cloud Web Messaging SDK.

Common Errors & Debugging

Error: 400 Bad Request (OAuth Token Exchange)

• What causes it: Mismatched code_verifier and code_challenge, expired authorization code, or incorrect redirect_uri.
• How to fix it: Ensure the code_verifier sent to /api/v2/oauth/token exactly matches the string hashed during challenge generation. Verify the redirect_uri matches the registered value character-for-character, including trailing slashes.
• Code showing the fix:

// Always store the verifier in a secure session or short-lived cache
const { codeVerifier, codeChallenge } = await provider.generatePkcEParameters();
session.set("pkce_verifier", codeVerifier);
// Redirect user to authorization URL using codeChallenge

Error: 401 Unauthorized (Guest API)

• What causes it: Access token lacks externalusers:write scope, token expired, or client ID mismatch.
• How to fix it: Verify the OAuth client registration includes the required scope. Check the expires_in value and implement token refresh logic before expiration.
• Code showing the fix:

// Validate scopes after token exchange
if (!tokenResponse.scope.includes("externalusers:write")) {
  throw new Error("Missing required scope: externalusers:write");
}

Error: 429 Too Many Requests

• What causes it: Exceeding Genesys Cloud rate limits for guest creation. The API returns a Retry-After header.
• How to fix it: Implement exponential backoff with jitter. Parse the Retry-After header if present.
• Code showing the fix:

if (response.status === 429) {
  const retryAfter = response.headers.get("Retry-After");
  const delay = retryAfter ? parseInt(retryAfter, 10) * 1000 : 2000;
  await new Promise(r => setTimeout(r, delay));
  // Retry request
}

Error: 5xx Server Error

• What causes it: Temporary Genesys Cloud infrastructure outage or payload validation failure on the backend.
• How to fix it: Implement circuit breaker logic. Log the full response body for debugging. Retry with capped attempts.
• Code showing the fix:

if (response.status >= 500) {
  const errBody = await response.text();
  console.error("Genesys Cloud server error:", errBody);
  throw new Error("Service temporarily unavailable. Retry later.");
}

Official References

• Genesys Cloud OAuth2 PKCE Documentation
• External Users Guest API Reference
• Web Messaging Guest Token Flow
• Genesys Cloud Rate Limiting Guidelines