'Firebase createCustomToken() returns an invalid token in local emulators

I am using firebase authentication to handle user accounts for my web application. When a user enters valid login credentials I am using the firebase admin SDK to generate a JWT in a cloud function, which is sent back to the client and then authenticated with firebase. I am running into an issue where the JWT generated by the firebase admin SDK is invalid. More specifically, when the token is decoded (I used jwt.io), I get the following:

Header: {
  "alg": "none",
  "typ": "JWT"
}

Payload: {
  "aud": "https://identitytoolkit.googleapis.com/google.identity.identitytoolkit.v1.IdentityToolkit",
  "iat": 1614367953,
  "exp": 1614371553,
  "iss": "[email protected]",
  "sub": "[email protected]",
  "uid": "some uid here"
}

The token is not only missing a signature, but the iss and sub values are wrong. I am using admin.auth().createCustomToken(uid) to generate the token and the app is being initialized with a valid service account that has Service Token Creation privileges.

EDIT: I have just figured out that the token is generated incorrectly only while everything is run locally using firebase emulators. When I deploy the app, the token is generated and validated just fine.

Solution 1:^[1]

Make sure you are instrumenting your app to talk to the authentication emulator ... and doing it before the authentication listeners do their thing.

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'Firebase createCustomToken() returns an invalid token in local emulators

Solution 1:[1]

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Solution 1:^[1]