The Bugbear has a trait called Long-Limbed that states:
When you make a melee attack on your turn, your reach for it is 5 feet greater than normal.
The Way of the Astral Self monk has a feature called Arms of the Astral Self that gives the monk many benefits, but the ones I want to focus on are:
You can use the spectral arms to
make unarmed strikes.
When you make an unarmed strike with the arms on your turn, your reach for it is 5 feet greater than normal.
Do the reach increases from Long-Limbed and Arms of the Astral Self stack?
Best Answer
Yes, they stack: your reach is 15 feet.†
Arms of the Astral Self and Long-Limbed are game features with different names, so they can stack, even though they provide similar effects. Per DMG errata, emphasis mine:
Additionally, per an unofficial tweet by rules designer Jeremy Crawford:
Relevant Q&As:
† Sidebar: when each feature matters
Long-Limbed applies to melee attacks on your turn, but Arms of the Astral Self applies to unarmed strikes, with the arms, on your turn. They stack only if making unarmed strikes (which are melee attacks) on your turn. Off-turn, neither applies; with any weapon other than your arms (including your legs, head, or tongue), only Long-Limbed applies.