TabMates — the roommate ledger on Monad

Split the bill. Actually settle it.

Live app: https://lynnmeanslight.github.io/tabmates/ · Contract: 0xc294C7E608F79e9FfCF4eDB85e36A91E4CCBAdB9 (Monad Mainnet, verified)

The problem (a personal one)

I live with two roommates. We track shared groceries, internet, and 3am pizza in a notes app, and the "ledger" always ends the same way: everyone knows who owes what, and nobody ever actually pays. Splitwise-style apps stop at the IOU — settling means opening a bank app, typing an amount, forgetting, being nagged, repeat. The ledger and the money live in different worlds.

The solution

TabMates puts the ledger and the money in the same place. It's a shared-expenses tracker where the settle button is a real value transfer:

• Sign in like a normal app — email, Google, or any wallet, via Privy. No wallet? An embedded one is created for you.
• Open a tab with any group (roommates, a trip, the lunch crew) and give everyone a human name, stored onchain — the ledger reads "Maya owes Lynn", not 0xf6A5… owes 0xA858….
• Log expenses — equal split between whoever you pick. Every entry is an onchain transaction, viable only because Monad confirms in well under a second and charges a fraction of a cent.
• Debts net automatically in the contract. If I owe you 1 MON from groceries and you owe me 0.5 from the cab, the ledger holds a single 0.5 edge — never two opposing IOUs (per pair, at most one direction is ever non-zero, enforced by _addDebt).
• Settle = pay. One click sends MON through the contract straight to your roommate's wallet and clears the debt in the same transaction. Partial payments welcome. The contract holds zero custody — it never keeps funds.

No indexer, no backend, no database: the UI reads everything from contract view functions (getAllDebts, netBalance, ranged feeds) every few seconds.

Run it in 3 minutes

Sign in with email/Google (Privy creates an embedded wallet for you) or a browser wallet. TabMates runs on Monad mainnet — settling moves real MON (logging expenses only costs gas, a fraction of a cent).

Just use it: open the live app, sign in, open a tab (name your mates!), log an expense. To see the full loop, add a second account as a member and settle from it.

Or run locally:

cd web
npm install
npm run dev          # http://localhost:5173, talks to the live mainnet contract

Contract dev (Foundry):

cd contracts
forge test           # 17 tests: splitting, netting, settlement, naming, auth, dust, caps
forge build

Live end-to-end check (creates a real tab on mainnet with two wallets, logs expenses, settles, prints the resulting state — spends a little real MON):

DEPLOYER_KEY=0x<funded key> node scripts/smoke.mjs

How it's built

Layer	Stack
Contract	Solidity 0.8.28, Foundry, zero dependencies, ~400 lines
Auth	Privy — email / Google / wallet login, embedded wallets on Monad
Frontend	Vite + React + TypeScript, wagmi v2 / viem, hand-rolled CSS
Chain	Monad Mainnet (chain id 143) — 400ms blocks make every UI action feel instant

Contract design notes

• debt[groupId][debtor][creditor] with automatic pairwise netting — opposing debts cancel on write, so the "who owes who" list stays minimal.
• Member names live onchain (memberName[groupId][member], ≤ 32 bytes): set when adding a mate, editable by any group member (tabs are trust-scoped — the people who share your fridge can fix your typo).
• settle() forwards msg.value to the creditor in the same tx (checks-effects-interactions + reentrancy guard). The contract balance is always zero; there is nothing to drain.
• Integer-division dust on splits (< n wei) is absorbed by the payer — at 18 decimals that's nothing.
• View functions are designed so a frontend needs no indexer: groupsOf(address), getAllDebts, netBalance, getGroup (returns labels too), and ranged getExpenses/getSettlements readers.

Repo layout

contracts/   Foundry project — src/TabMates.sol, test/TabMates.t.sol
web/         Vite React app (Privy + wagmi/viem)
scripts/     smoke.mjs — live e2e against the deployed contract

Security model

Tabs are trust-scoped: members are people who already share a fridge or a hotel room. Any member can add expenses/members and edit labels within their tab; the contract enforces membership, split validity, settlement caps (can't overpay a debt), and holds no funds. It is not designed for adversarial strangers — it replaces the notes app, not the courts.

Earlier deployments live on Monad Testnet: v1 (pre-rename, no member names) at 0x698EBb78528e2a55B14ccf3c33171CcBF8f6392f, v2 at 0x6B7DF3C263E495c319b3841c658A23E5E361d110. v3 (mainnet) adds an expense cap (MAX_AMOUNT) and a hardened _addDebt after a pre-mainnet security review.

License

MIT