The documentation for the Send Whisper endpoint (Reference | Twitch Developers) says:
The maximum message lengths are:
500 characters if the user you’re sending the message to hasn’t whispered you before.
10,000 characters if the user you’re sending the message to has whispered you before.
I am sending a message that is longer than 500 characters to a user that has frequently interacted with the bot through whispers over several years. Yet, the message they receive is truncated to exactly 500 characters. Is there some additional precondition I must check before sending long whispers?