As part of the ongoing simplification of our API and trading system architecture, the api-nonce header will no longer be supported starting on Tuesday 12th March 2019. BitMEX will not check for increasing nonces as part of validation. The nonce scheme will still be valid for generating signatures used to authenticate with the API. Your requests are still safe from replay attacks thanks to TLS. Please see https://www.bitmex.com/app/apiKeysUsage for more information.