PROOFMARK

A real 32 KB Game Boy cartridge. Free browser demo below — days one and two, no download.

You are the assay master at a mint. Specimens are struck and brought to your bench, and each day a hidden rule decides what “genuine” means. It is a rule about what you can see on the coin: its shape, how its field is divided, which charge is struck on it, how many of them, whether it carries a rim.

You are never told the rule. You work it out.

The loop

STUDY. Verdicts are given to you free. They cost nothing, and the phase runs exactly as long as it needs to — the day card tells you how many specimens that is before the day begins.

ASSAY. Eight specimens, graded. Mistakes cost standing, and you only have five.

REVEAL. The rule is stated in plain words, next to a coin that satisfies it. The shape you half-saw becomes a sentence.

The promise the cartridge makes

By the time the first graded specimen reaches your bench, the evidence you have already been shown narrows the rule to exactly one possibility. There is never a day where the honest answer is a guess. If you reason from the ledger, you cannot be wrongfooted.

That is a guarantee rather than an intention, and it is why the study phase is variable-length instead of a fixed four: all 192 days in this cartridge were checked against it before it shipped, and a sweep of 1,200 generated days passes it 1,200 times.

An induction game whose rule is not pinned down by the evidence is not hard, it is arbitrary — the player reasons correctly, is told they are wrong, and concludes the game is broken. This one is built so that cannot happen.

The ledger is on screen so the day is not in your head

Every specimen you have seen today sits down the left of the bench with its five features and its verdict beside it. Read across the passes for what they share, and across the breaks for what they do not. That is the whole game.

Why this needs 5,760 coins and not a handful

The ROM carries 960 bytes of authored mark silhouettes — forty of them, drawn by hand at 12×12 in eight families — plus four blank masks. There is not one pre-rendered specimen in the cartridge. No sprite sheet, no atlas, no shaded pixel data at all: a 4 MHz CPU composes the shape, the field, the charges and the rim while you play, and derives the four-shade relief by one lighting rule applied identically to every mark.

Baking in a fixed set would have been smaller and simpler. It would also have been a different game, because the thing you are inducing a rule about is the art. Take the generator out and there is nothing left to deduce.

The families are a mechanic, not a filing system: a rule may be “the charge is a BEAST” rather than naming one mark, so you have to see that a horse and a boar belong together before you can rule anything out.

Six day tiers. Then the rules start combining.

Day one is meant to be solved on sight. By day four you are dealing with AND, OR, NOT and AND NOT — two clauses at once — and the free study phase grows to match, because the promise above still has to hold.

Measured against its own bots: a player who eliminates candidate rules from the study specimens scores 100%. One who just guesses the base rate scores 63%. The gap between those two numbers is the game — if they were close, this would not be a deduction game at all.

What you get

• 192 distinct days — six tiers, 32 certified variants each.
• 5,760 specimens, composed on the hardware.
• Forty hand-authored marks in eight families.
• Music and sound on the Game Boy’s own four channels. A mute toggle that is honoured.
• A real .gb file — plays on an EverDrive, an EZ-Flash, an Analogue Pocket, a Super Game Boy, or any accurate emulator. No mapper, no save chip, no BIOS needed.

The free demo is days one and two, playable right here in the browser. The download is the whole cartridge.

Controls

A — PASS, stamp it genuine · B — BREAK, stamp it false · START — begin, and continue · SELECT — mute

That is the whole control scheme. There is no menu.

Honest notes

• It is a turn-based logic game on a 4 MHz CPU. There is no action in it, and if you do not enjoy being handed evidence and asked what follows, this is not for you.
• The screen is 160×144 in four shades of grey, because it is a Game Boy game rather than a game styled after one.
• Made by Core Systems Asset Factory. AI-assisted and tagged truthfully: code yes, graphics yes, text yes, sounds no — every sound is the DMG’s own synthesiser driven by register writes, with no sample and no AI audio anywhere in the chain.

Also on a Game Boy cartridge from CSAF: ANATOMIST — a roguelike where a monster’s shape is its stat block. Same pipeline, opposite mechanic: ANATOMIST’s art is a legend you learn once; PROOFMARK’s is a dataset you reason over.