Can a player concede in the middle of resolving a spell to prevent its effects from happening to them?

A player may concede the game at any time, even in the middle of resolving a spell. This is explicitly covered by CR 104.3a, which states that a player can concede at any time. Upon conceding, that player immediately loses the game (CR 104.3a).

However, conceding does not retroactively stop a spell from resolving or cancel its effects mid-resolution. When a spell is resolving, it finishes resolving according to its instructions (CR 608.2). The effects have already begun happening; conceding simply ends your participation in the game, it does not function like a counterspell or an undo button.

Practically speaking, if you concede while a spell targeting you is resolving, the game ends for you, but the spell still fully resolves in that game state. In a multiplayer game, this means the spell continues to resolve and may still affect other players. Any effects that would have targeted you specifically may become illegal targets at that point, but that is a consequence of you leaving the game, not a benefit you chose (CR 800.4).

Concrete example: Your opponent casts a spell that deals 10 damage to each player. While it is resolving and the damage is being dealt, you concede. You still lose the game — the concession does not prevent the 10 damage from being dealt to you in that final game state. You simply lose via concession rather than from the damage itself.

So while conceding is always a legal action (CR 104.3a), it is not a valid strategy to 'dodge' spell effects mid-resolution, since the spell will still resolve completely before the game state changes meaningfully for you.