sk_test_ and pk_test_ prefixes.
In the documentation pages associated with the sandbox, you will find the credentials that the sandbox uses to allow you to test every possible flow your users might follow without breaking a sweat 😎.
As of right now, you can use the test mode for Movements, Payment Initiation, Direct Debit and Transfers.
Test mode versus live mode
All Fintoc API requests occur in either test mode or live mode. API objects in one mode aren’t accessible to the other.
The Test mode toggle in the Dashboard doesn’t affect your integration code. Your test and live mode API keys are what ultimately affect the behavior of your code.