Passwordless Login Fails with Error: "Client Authentication is Required"

Last Updated: Feb 20, 2026

Overview

Passwordless SMS logins are failing with an error message:

Client authentication is required.

Tenant Log entry example:

Cause

This error will occur if a client_id is omitted from the POST request to https://{{auth0_domain}}/passwordless/start, or if the login is for a Regular Web App and the client_secret is incorrect or omitted and the application does not have the Authentication Method set to None.

Solution

Make sure that the POST request to https://{{auth0_domain}}/passwordless/start is including a client_id.

If the login is to a Regular Web App that is using an Authentication Method other than None, make sure that the client_secret is also included and accurate.