A real-money prediction market, built end to end

Tennis Expert is a real-money prediction market for professional tennis, live on Polygon. Fans back the exact set score of a match with USDC in one signature, and everyone who picked the winning score splits the pool, minus a 3% rake. The house never quotes odds, so it never takes price risk. Around that core sit tournament brackets with on-chain escrow, free matches, achievement badges, a leaderboard with prizes, private leagues and a daily quiz.

Real money on a chain that can reorganise, run by a very small team. Every write had to be safe to repeat, results had to become final without paying for an oracle, and the off-chain database could never quietly become the source of truth. The first version, which minted an NFT ticket per prediction, also had to give way to a simpler pool model.

• The database is a cache

  Supabase holds a read model rebuilt by replaying on-chain events. Pages load fast and nothing off-chain can disagree with the contracts.

  Trade-off Data lags the chain slightly, covered by optimistic UI rather than more polling.

• Payouts are pushed, not claimed

  A permissionless payout function pays winners automatically, and keeps working even while the contract is paused.

  Trade-off The platform pays gas for every winner, so a manual claim stays as a fallback.

• Propose, then challenge, instead of an oracle

  Results are proposed and open to challenge for at least an hour before anyone can finalise them. A market is never voided automatically for missing data.

  Trade-off Settlement waits for the challenge window.

• Settlement terms fixed at creation

  The rake is snapshotted when a market opens, voids take no rake, and the rake can never exceed the losers' pool, so no winner gets back less than their stake.

  Trade-off Changing fees only affects new markets.

• Observability on a budget

  Full APM was turned off on purpose, roughly halving the monitoring bill; custom metrics still drive 35 monitors.

  Trade-off Less automatic tracing; new hot paths need explicit metrics.

1. Reorgs versus sync speed

   The worker trusts the chain's finalized head only when it is fresher than a fixed confirmation depth. Polygon sync dropped from about 42 s to 6 s, and another chain was saved from a silent 20-minute stall.

2. Writes that are safe to repeat

   Events are stored under a unique chain, transaction and log key, jobs are de-duplicated by id, and a distributed lock keeps two worker instances from doing the same work.

3. One-signature bets

   Bets use a USDC permit so there is no separate approval step, with a fallback to the existing allowance when a permit is front-run.

Agents work inside hard rules: an AGENTS.md with constraints, a 21-principle constitution, a domain glossary, a guard hook that blocks edits to secrets and retired contracts, a security-review agent on every contract change, and a nightly headless run that reviews production logs. 522 commits are co-authored with Claude.