NextAuth: how to return custom errors without redirection

I have been looking at implementing NextAuth with username and password credentials and I am not finding a way to return custom errors to the client side. It seems that I can only return a 200 ok or a redirection to an error page from the authorize method, where then you could add a custom error to a query string. However that is not a proper solution to my case as I need the client side to simply receive a custom error code or message from the sigIn call.

How do you return custom errors like

"Invalid email address"

"Account blocked"

"Invalid password"

Or any other customization required?

Thanks

I found the solution that I needed by implementing a signIn callback. The authorize method can return a custom object and the signIn callback catches it and handles it accordingly.

See the documentation here

// [...nextauth].js
providers: [
// credentials provider
CredentialsProvider({
    type: 'credentials',
    credentials: {},

    // authorization function
    async authorize(credentials, req) {
        const { credential, password } = credentials

        if (myCustomCondition) {
          return { error: 'my custom error' };
        }
        return data // success
    }
 })
],
callbacks: {
   async signIn({ user, account, profile, email, credentials }) 
      {
      if(user?.error === 'my custom error') {
         throw new Error('custom error to the client')
      }
      return true
   }
}

On the client side we can evaluate the error message and act accordingly:

const res = await signIn('credentials', {
    credential: data.credential,
    password: data.password,
    redirect: false
  })
  if (res?.status == 200) {
    push('/')
  } else if(res?.error === 'custom error to the client') {
    // handle this particular error
  } else {
    // handle generic error
  }