Is a player required to proactively announce Master Yi's bonus when defending alone, or is it the opponent's responsibility to recognize it?

Ruling: Master Yi is a passive ability, not a trigger. You are not required to proactively announce it, but you cannot lie about it if asked. It is the opponent's responsibility to account for Yi's ability.

Nuances:

• Yi and similar abilities like Shield/Assault apply automatically when their conditions are met (being the Attacker or Defender)
• If an opponent mistakenly moves a weaker unit without accounting for Yi's ability, that is on them
• Better etiquette to announce it, but not required until it matters (damage, ordering triggers, being asked, or anything signifying acknowledgement)