How to parse custom scalars in graphql-codegen / apollo client?

Question

The ethers.js library provides the BigNumber type. To create a BigNumber from a string we call BigNumber.from("42")

I have created aGraphQLScalar called GraphQLBignumber on my server:

import { GraphQLScalarType } from 'graphql';
import { BigNumber } from 'ethers';

const EthersBigNumberType = new GraphQLScalarType({
  name: 'BigNumber',
  description: 'A custom scalar type for representing ethers.js BigNumber values',
  parseValue(value: string | number | BigNumber) {
    return BigNumber.from(value);
  },
  serialize(value: BigNumber) {
    // Serialize the BigNumber instance into a string
    return value.toString();
  },
  parseLiteral(ast) {
    // Parse the ast value into a BigNumber instance
    if (ast.kind === Kind.STRING || ast.kind === Kind.INT || ast.kind === Kind.FLOAT) {
      return BigNumber.from(ast.value);
    }
    return null;
  },
});

Now I would like to use this BigNumber scalar on my client using graphql-codegen. In my codegen.yml I add:

plugins:
// other plugins incl typescript etc.
  - 'add':
      content: 'import {BigNumber} from "ethers"'
scalars:
  BigNumber: 'ethers.BigNumber'

Now if I have the query:

  query GetBalance($address: String!) {
    balance(address: $address)
  }

And I use codegen to generate the code, the output of GetBalanceQuery returns an ethers.BigNumber

When I use my application I am not getting any errors (so far) and everything seems to be working just fine.

My question is: How does my client application know how to convert the server response in to a ethers.BigNumber? As far as I can tell the client does not have a copy of the parseValue function yet the server is sending the BigNumber as a string so somewhere BigNumber.from(...) needs to be called...