Cryptocurrency exchanges are cracking down on brokerages that bundle clients' orders to enjoy lower, VIP trading fees.

In a letter reviewed by CoinDesk, OKX, the second-largest exchange by volume, recently asked prime brokers for details of subaccounts including the names of the entities or individuals that control each subaccount and the jurisdiction in which they are located. OKX said it needs the information by July 17.

"A failure to do so may result in undisclosed subaccounts being restricted from trading and/or subaccount closure," the letter said.

Earlier this month, OKX's larger rival Binance changed its Link Plus interface, effectively closing a loophole that let prime brokers use a multitiered fee system to offer rebates to clients. Binance said the measure was "to uphold compliance and ensure a level-playing field for all users, whether they access Binance directly or via an intermediary." That news was first reported by Bloomberg.

Exchanges offer their biggest customers discounted trading fees, treating them like VIPs to boost the odds they'll stick around. Prime brokerages – firms that provide trading services for professional, and often large, investors – could, in theory, funnel several customers' trading through a single account at an exchange, qualifying for those lower fees.

"This is being done very much for the purpose of disbanding clients under brokers to price them separately," said a person familiar with the prime brokerage industry who asked to remain anonymous.

OKX declined to comment.

Bybit, another large crypto exchange, is "closely monitoring the recent developments regarding the removal of the prime brokerage multi-tiered fee structure by other platforms," said Eugene Cheung, the firm's head of institutions.

"However, we have no plans to make any changes to our fee structure. Our commitment remains steadfast in ensuring compliance and the best interests of our users," Cheung said in an email.