What is the most critical diagnostic information an API error code for an `OTP_INVALID` response must convey to the client for immediate and effective troubleshooting?

The `OTP_INVALID` response from an API signifies that the One-Time Password (OTP) provided by the client in an authentication or transaction request was not accepted by the server. While this general status indicates a problem, it is insufficient for "immediate and effective troubleshooting," which is the process of pinpointing and resolving the precise cause of an issue. The most critical diagnostic information an API error code for an `OTP_INVALID` response must convey to the client is the *specific reasonor *underlying conditionthat caused the OTP to be deemed invalid.