A code-breaking deduction puzzle where you crack a hidden four-dial combination blind, and a paid informant tells you only how many dials you got right.
A secret four-dial code is set, and you can't see it. You make guesses, and an informant sells you feedback, chiefly how many of your dials are correct, letting you narrow the possibilities through logic. Each clue costs you, so the goal is to deduce the exact code in as few purchased hints and guesses as possible.
Guessing a hidden combination and narrowing it from feedback is Mastermind, invented in 1970 by Mordecai Meirowitz, and before it the pencil game Bulls and Cows.
The original layer is that feedback is sold. An informant charges for each clue, so the count of correct dials is something you buy rather than receive.
Mastermind's feedback is free and complete, which makes optimal play a solved information problem. Charging for it makes the real question economic — how little you can afford to know before committing — and a cheap guess that eliminates a lot becomes worth more than an expensive certainty.
Tipline is an original free browser game in the code-breaking deduction genre, where buying limited feedback about a hidden combination turns guessing into a tight logic puzzle.