BidSim

BidSim is the game I wish I had when I was learning how auctions actually behave.

It is a multi-unit ascending auction, the same family used to sell mobile spectrum. Teams are given home bands, marginal valuations per block, per-band caps, deployment costs and a churn model. Then the rounds open and they bid against each other in real time.

Why it works as a teaching tool

Auction theory reads as clean until you sit in a room where four teams are two minutes from a round close and someone realises they are one block short of a contiguous holding. The interesting behaviour is all in that moment: exposure risk, the temptation to chase a block you do not need to deny it to a rival, and the slow realisation that the winner is often whoever made the fewest expensive mistakes.

Score is surplus, not spend. Every session someone wins the most spectrum and loses the game.

How it is built

Deliberately small. FastAPI, Jinja2 templates, WebSockets for the live round state and four dependencies in total. One auction lives in memory at a time and every completed run is appended to a JSON lines file, which is the entire persistence layer.

That constraint is not laziness. It means the whole thing starts in under a second, runs comfortably on the cheapest server available, and there is no database to go wrong halfway through a session with twelve people watching.

There are two consoles for whoever is running the session and two team screens, and it supports being mounted under a URL prefix so it can sit alongside other apps on one host.