Jin-Gitaxias, Core Augur

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2017-11-17
  Official Ruling
  Unless another spell or ability is affecting your opponent's maximum hand size, Jin-Gitaxias's ability will result in your opponent having a maximum hand size of zero. They will discard each card from their hand during their cleanup step.
• WWizards of the Coast2017-11-17
  Official Ruling
  If a player has more cards in their hand than their maximum hand size during the cleanup step of that player's turn, that player discards until they have that many cards. A player's maximum hand size isn't checked at any time other than their own cleanup step.
• WWizards of the Coast2017-11-17
  Official Ruling
  Jin-Gitaxias doesn't affect the maximum hand size of its controller. Because the cleanup step is after the end step, its controller may have to discard some of the cards that were just drawn.

Rulings via Scryfall