Advent of Chess 2025 Day 12 (!!! Spoiler Alert !!!)

Welcome to the halfway mark of the Advent Calendar! While first eleven days challenged our tactical and solving abilities, Day 12 of the Advent of Chess 2025 demanded a completely different and perhaps, more artistic skill; Construction.

The task was wonderfully specific and instantly intriguing: Construct a full game, starting from the initial position, that concludes with the move 6. B x Rc4#. This is not a problem about finding the best move; it's about finding the unique six-move sequence that fullfil the strange requirement. 

Reverse engineer the position

To solve a problem this precise, you can't rely on luck; you must reverse-engineer the position. The construction is a delicate balancing act where every move is essential. Success hinges on satisfying three objectives:

1. Objective 1: Piece Placement: Successfully orchestrate Black's moves to position a Black Rook on c4 within five moves.
2. Objective 2: White's Attacker: Strategically maneuver white piedces to a position where the bishop can deliver the final blow on c4.
3. Objective 3: The Black King's Role: Ensure the Black King is positioned and trapped such that the capture 6. B x Rc4 forces checkmate.
   
   

The Path Not Taken: Why Logical Sequences Fail

This problem requires a counter-intuitive sequence of moves, which is why most logical, human-like attempts at construction quickly fall apart. When starting from the initial position, our natural inclination is to move pieces in a way that seems helpful. Let's look at some tempting, yet ultimately flawed, attempts and see exactly where the common-sense approach leads us astray.

One of my first attempts was a direct, four-move plan to get the Black Rook to c4:

This plan is very logical, however, it just takes too many moves to place the rook on c4. White even has to waste a tempo(6. a3) just to wait for the rook to land on c4. Let's look at another strategy that involves using white's pawns to prevent the black king from escaping via g6:

The conclusion we must draw from these two examples is that any sequence that uses multiple moves to manually walk the Black Rook to c4, or that requires White to make lengthy preparatory moves to take space, inevitably consumes too much time. Since we only have six moves total, we cannot afford to waste moves with white and the black Rook must reach c4 in fewer than four moves.

The Brilliant Breakthrough: A Counter-Intuitive Discovery

After exploring the logical dead ends, we arrive at the elegant and counter-intuitive solution—as expected from the Advent of Chess! The actual breakthrough lies in a single, critical realization: the Bishop is not the piece that delivers the check!. The condition 6. B x Rc4 means the Bishop makes the final move, but the check itself is a discovered check delivered by another piece—the White Queen— by clearing a file or rank to attack the Black King. Although my first attempt to deliver the discovered checkmate was with the Queen on h8 and Bishop on g8 (followed by B x Rc4 checkmating the king on e8) proved too slow, grasping the concept of the Queen's discovery was the key. As usual with Advent of Chess problems, once you find out the idea, the solution immediately becames clear:

I hope you enjoyed dissecting this wonderfully tricky construction problem from the Advent of Chess 2025! Day 12 truly tested our ability to look past 'normal' chess and appreciate the extreme precision of composition. If you managed to solve the 6. B x Rc4 # sequence, congratulations—you mastered move economy! Thank you for reading, and be sure to check back tomorrow for the solution to Day 13!