Can you cast a split card and choose which half to cast after announcing it?

When you cast a split card, you must choose which half you are casting at the very moment you announce it. This choice is made as the first step of casting a spell, as part of identifying the spell you are putting on the stack. You cannot wait until later in the process to decide.

According to CR 708.2, split cards have two halves, each with its own name, mana cost, color, type, subtype, and abilities. Only one half is cast at a time; the half you choose becomes the spell on the stack. The other half is treated as if it does not exist while the card is on the stack (CR 708.3).

The process of casting a spell is governed by CR 601.2. Step one (601.2a) requires you to move the card to the stack and identify it — which means declaring which half of the split card you are casting. Costs are calculated and paid in later steps, so there is no room to 'decide later.'

The only exception is Fuse cards (CR 708.4), which allow you to cast both halves simultaneously by paying both costs. But even then, the decision to fuse must be made when you announce the spell, not partway through paying costs.

Example: You have Fire // Ice in hand. When you announce you are casting it, you must say 'I'm casting Fire' or 'I'm casting Ice' right then. You cannot announce the card, look at your available mana, and then pick whichever half suits you — the choice locks in immediately at announcement.