Transmitting category codes to payment instruments in proxy card transactions comprises receiving a first payment request to authorize a proxy card transaction, the first payment request identifying a proxy account of a user for payment of the transaction and a merchant category code associated with the merchant; selecting a financial account associated with the proxy account to fund the transaction; communicating a second payment request to authorize the transaction to a financial account system associated with the selected financial account, the second payment request comprising the merchant category code associated with the merchant; receiving an authorization for the transaction from the financial account system to fund the transaction using the selected financial account; and communicating an authorization for the transaction to the merchant computing device in response to receiving the authorization for the transaction from the financial account system to fund the transaction using the selected financial account.