Does the draw step end immediately after you draw or can you cast instants before your main phase?

The draw step, like all steps, has a structured sequence: first the active player draws a card (a turn-based action requiring no priority), and then priority is passed. Once the draw resolves, the active player receives priority before the step ends, meaning players can cast instants, activate abilities, or take other actions with priority before moving to the main phase.

Under CR 117.3b, after a turn-based action (like drawing in the draw step), the active player receives priority. Under CR 500.4, a step or phase only ends when all players pass priority in succession with an empty stack. So the draw step continues until that condition is met.

This means if you draw a card you want to use immediately — or if your opponent wants to respond to something before your main phase — that window exists during the draw step after the draw has occurred.

Concrete example: You draw a Lightning Bolt during your draw step. Your opponent has a creature with a 'at the beginning of your main phase' trigger they'd rather not deal with. They could cast an instant like Giant Growth on their creature during the draw step after you draw, and you could respond by casting that Lightning Bolt — all before the main phase even begins.

In short, the draw step ends only when both players pass priority consecutively with an empty stack (CR 500.4), not the moment the card is drawn.