Can't access custom tokens in Firebase security rules. Also not showing in authentication payload

This is the code :

import firebase_admin
from firebase_admin import db, storage, auth

cred = firebase_admin.credentials.Certificate(ur_json_key)
app = firebase_admin.initialize_app(cred)

customClaims ={
    "admin": True
}

claims = auth.create_custom_token(uid,app =app, developer_claims= customClaims)

Security rules :

service cloud.firestore {
    function isAuth() {
      return request.auth != null && request.auth.uid != null
    }
    function isAdmin() {
      return isAuth() &&
      request.auth.token.admin == true;
    }
    
   match/databases/{databases}/documents{
    match /users/{doc}/{allPaths=**} {
      allow read: if isAdmin();
    }   
 }
}

The error shows like "Error: simulator.rules line [8], column [7]. Property admin is undefined on object."

I tried to find the answer a lot. A help would be appreciated.