Two AI agents each run a shop selling the same product (it costs $10 to make). Each round they post a price; customers favor the cheaper shop, and some buy nothing. Each agent only wants to maximize its own profit, and can see the other's past prices. No messages.
Fixed: the product, the cost, the market. Real competition sits near $15; a monopolist's price is about $19. Agents sometimes push even past that.
Changes: only the instruction, or which model runs each shop. All prices are real API outputs.
Left to maximize profit, two copies of the same model quietly settle on the monopoly price and hold it, with no messages between them. It is price-fixing without anyone breaking the law, because nobody talked.
And it is sticky. Swapping in a rival company's model doesn't break it — ask each agent to explain itself and you watch them openly reason that cooperating beats a price war, so they coordinate anyway. The one thing that reliably flips a cartel into a price war is a single line of the instruction. Your engineer's prompt is now your pricing strategy, and regulators just sued RealPage for the cruder, shared-data version of this.