Does a commander that is tucked into a library go to the command zone instead as a replacement effect?

In the Commander format, a special rule states that if a commander would be put into a library, hand, graveyard, or exile from anywhere, its owner may choose instead to put it into the command zone. This is a replacement effect applied by the player, not the game automatically — the choice is optional (CR 903.9).

This means 'tuck' effects — cards like Chaos Warp or Hinder that shuffle a commander into a library — no longer guarantee a commander stays tucked. The owner simply exercises the replacement effect and redirects it to the command zone instead.

Importantly, this replacement effect can only be applied once per zone-change event (CR 614.1). The commander doesn't bounce between zones; it simply ends up in the command zone as a replacement for going to the library.

Example: Your opponent casts Chaos Warp targeting your commander. Normally it would be shuffled into your library. Instead, you announce you are applying the Commander replacement effect — your commander goes directly to the command zone, and you may recast it later by paying its commander tax.

Note: This rule was officially adopted by the Commander Rules Committee, and is reflected in the MTG Comprehensive Rules under CR 903.9, which covers the Commander format's specific replacement-effect rules for commander zone changes.