can I respond to thousand tail watcher play effect?

Yes, you can respond to Thousand-Tailed Watcher's "When you play me" ability.

According to the RiftJudge FAQ (#93, #4730, and #6628), Thousand-Tailed Watcher has a triggered ability that uses the chain, which means it can be responded to with [Reaction] speed cards just like any other triggered or activated ability.

How it works:

1. Playing the Unit: When you play Thousand-Tailed Watcher, it enters the battlefield. This action does not use the chain and cannot be responded to.
2. Triggered Ability: Its "When you play me" ability immediately goes on the chain. This puts the game into a Closed State.
3. Responding: Because the ability is on the chain, you (and your opponent) may play [Reaction] cards to interact with the game state before the ability resolves.

Important Considerations:

• No "Fizzling": The ability will resolve even if the Thousand-Tailed Watcher is removed from the battlefield in response (see FAQ #3100).
• Blanket Effect: Because the effect applies to all enemy units simultaneously rather than choosing specific targets, you cannot use cards like Not So Fast to counter it, as those cards require the ability to "choose" or target a specific unit (see FAQ #9386, #9052, #8449).
• Resolution: The ability will affect all enemy units present at the moment the ability resolves, not just those that were on the field when the Watcher was played (see FAQ #2236).