Timing issue ?

ok here's the problem. It's my opponent's turn and he activates MST. I chain it and activate CotH and summon Stratos onto the field.

So can i use its eff to get a hero to hand? Because im hearing its illegal somehow? Also what if i summon Sangan onto the field Would Its eff activate or not? :?:

Stratos effect is optional. Optional effect must be the last thing that happened during a chain so they can work. Sangan effect is not optional thats why u can activate the effect. But stratos effect is optional so it won't work.

Stratos has an optional effect which means the last thing to happen needs to be him being summoned. In your situation he is summoned while a chain is still resolving so you won't receive his effect.