[firebase/auth] Auth action links ignore linkDomain param

[jorgeroncero]

Describe your environment

• Operating System version: macOS Sequoia
• Firebase SDK version: 13.1.0
• Firebase Product: auth
• Node.js version: v22.14.0
• NPM version: v10.9.2

Describe the problem

Steps to reproduce:

Whenever we make a call to admin.auth.generateSignInWithEmailLink or any other actionLink-related method, the linkDomain is ignored and the firebase project default url is used instead.

The firebase-admin SDK calls the Identity Toolkit API, which is the actor that is probably ignoring the linkDomain param.

Relevant Code:

var actionCodeSettings = {
      url: `${appDomain}/mobile/emailLinkSignIn`,
      handleCodeInApp: false,
      linkDomain: '<other-site>.firebaseapp.com, // Other Hosting site: Does not work, uses default instead
      // linkDomain: '<custom-domain>, // Custom domain registered in Hosting: Does not work, uses default instead
};

const signInWithEmailLink = await admin.auth.generateSignInWithEmailLink(email, actionCodeSettings);

console.log(signInWithEmailLink) // Returns "https://<project-id>.firebaseapp.com/__/auth/action?..."

[robmcilroy1992]

We're also facing the same issue with generateSignInWithEmailLink.

Firebase SDK version: 13.2.0
Firebase Product: auth
Node.js version: v20.11.0
PNPM version: v8

Our domain is connected on Firebase hosting and has been added to authorised domains whitelist.