Cobalt Star Idle

What was hard about it

The hard part of an idle game is not the idling, it is the arithmetic that happens while nobody is watching. Cobalt Star accrues resources offline, which means the game has to reconcile “how long was I away” against “what did I already pay you for” every time it wakes.

That reconciliation is where the real defect lived: a storage race that could double-count accrual, crediting the same elapsed window twice when two contexts woke close together. It is invisible in ordinary play and obvious in a headless simulation that fast-forwards days of play in milliseconds, which is how it was found.

The lesson generalised past this game. Any accrual that reads a timestamp, computes a delta, and writes a new timestamp is a read-modify-write, and read-modify-write across two contexts is a race unless you make it one.