If its not, your opponents could also draw into Flusterstorm, Spell Pierce, Stifle, and so many other instant speed effects that could rain on your parade
I think, upon further reflection, that I was wrong
Once you announce that you're casting a spell or activating an ability, players can't take actions until you've finished doing so
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If the effect causing you to have no maximum hand size ends, Inspired Idea's effect will apply at that time