Fantasy Football AI Auction Manager

The problem

During a fantasy football auction (6 to 12 teams, configurable budget) someone has to note by hand who buys which player and for how much, and keep budgets and role slots up to date. I wanted to say or type a sentence like "Dzeko squadraX 150" and get a structured update of the rosters, visible in real time to the other participants. It is a personal project.

What I built

• A FastAPI backend: session creation, updates from text or audio, rosters, available players, player removal with refund, CSV export and a WebSocket for broadcast. Sessions are stored in Redis.

• A Next.js frontend: a control panel for the operator (text input, microphone, role selector, team cards with budget and slots) and a read-only guest page, shared by link, that receives rosters over WebSocket.

How the agent works

This is a single-step LLM pipeline, not an agent with a tool loop. Audio is recorded in the browser, converted to WAV and sent to Google Speech-to-Text with phrase hints made of the player and team names, which biases recognition towards the auction vocabulary. A LangChain chain with GPT-4o-mini at temperature 0 then extracts player, team and price as structured output. The prompt only receives the players still available for the current role and the list of teams, so the model also resolves an imprecise transcript to the canonical name.

Guardrails: the returned player and team must match the provided lists exactly, otherwise it is an error. Then deterministic code checks that the player is not already assigned, the budget is enough, the role has a free slot and the team exists. The model never writes to state directly. The operator chooses the current role, sees the outcome and can remove a purchase with a refund.

Interesting decisions

• Domain rules are validated in code, not by the model.

• After each change the backend broadcasts to the session; a failed broadcast does not block the update, and a new socket gets the current roster immediately.

• CSV export compatible with LegheFantacalcio.