Round-up savings on Arc

Spend the dollars.
Keep the cents.

Every USDC payment rounds up. The change is saved in a non-custodial vault that earns in USYC. Works for people and for AI agents paying over x402.

Try the simulation Follow on X
Example round-ups

How it works

Three steps. One transaction.

Spare sits beside your wallet, not in front of it. The payment you meant to make is never touched.

  1. 01

    Attach a rule

    Pick a round-up threshold of $1, $5 or $10. Add an optional x2 or x5 multiplier and a daily cap. The rule lives on your wallet and can be revoked at any time.

  2. 02

    Pay as usual

    Every outgoing USDC payment goes through untouched. Only the round-up is routed to your vault, in the same transaction.

  3. 03

    Earn and withdraw

    The vault holds USYC, tokenized T-bill yield. Withdraw any time in USDC or EURC through Arc's native StableFX. No lockup, no minimum, no oracle, no liquidation.

Live exampleUSDC on Arc
WalletMerchantVault
$12.40
$4.37+$0.63

Round-up to $5 · x1 · no cap

Simulator

Simulate your round-ups

Generate a month of payments, tune the rule, and see what the vault would hold. Everything runs in your browser.

Demo modeThis is a simulation. Contracts are not deployed.

40–80 everyday purchases per month.

An assumption for the projection, not a promise. Yield varies with T-bill rates.

Payments this month

62

Total spent

$1,882.85

Total saved

$36.15

12-month balance at 4.5%

$444.37

$433.80 saved + $10.57 yield

Performance fee (10% of yield)

$1.06

With $SPARE staked

Fee $0

Staking sets your fee to zero

Last 8 payments
DayPaymentAmountRound-up
30Transit$3.32+$0.68
30Ride$27.73+$0.27
30Takeout$23.07+$0.93
30Gym$28.33+$0.67
29Lunch$15.10+$0.90
29Gym$30.44+$0.56
29Lunch$11.19+$0.81
28Ride$30.03+$0.97

Docs

Protocol documentation

How Spare is meant to work, written before it exists. Everything here is subject to the audit and the final deployment.

Overview

Spare is a round-up savings protocol on Arc, Circle's Layer 1 where USDC is the gas token. It attaches a small, revocable rule to a wallet: every outgoing USDC payment is rounded up to the next dollar, five or ten, and the difference is routed to a non-custodial vault. The vault holds USYC, a tokenized T-bill yield asset, so the change earns while it sits.

The protocol is designed for two kinds of payers. People, who make a few dozen payments a month. And AI agents, which pay for inference, search and storage over x402 in thousands of cent-sized transactions. Both leave change behind; Spare collects it.

Nothing here is deployed yet. This page describes the intended design. Contract addresses, audit reports and admin key details will be published at deployment.

How it works

A Spare rule is a permission attached to your wallet through a policy module. When the wallet sends USDC, the module reads the amount, computes the round-up, and forwards that amount to your vault in the same transaction. The original payment is not modified, delayed or re-routed. If the round-up cannot be funded, for example because of an empty balance, the payment still goes through and the round-up is skipped.

The vault is a standard ERC-4626-style contract. Deposits are swapped into USYC; the vault mints an ERC-20 share token that represents your claim. Shares are transferable. Withdrawals redeem shares for USDC, or for EURC via Arc's native StableFX, at the current share price.

Round-up rules

A rule has three parameters. All of them can be changed or revoked by the wallet owner at any time; no one else can edit a rule.

  • Threshold. Round up to the next $1, $5 or $10. A payment that already lands on the threshold rounds up by a full step: $5.00 at a $5 threshold saves $5.00.
  • Multiplier. Optionally multiply the round-up by x2 or x5. A $0.63 round-up at x5 saves $3.15.
  • Daily cap. An optional ceiling on total round-ups per UTC day. Once reached, later payments that day go through with no round-up.
  • Revocation. Removing the rule is a single transaction from the wallet. Existing vault balances are unaffected and remain withdrawable.

Vault & USYC

Each user's savings sit in a shared vault contract that holds USYC. USYC is a tokenized money-market instrument backed by short-dated U.S. Treasury bills; its price accrues yield over time, so the vault does not need to harvest or compound anything. There is no oracle: the vault reads the USYC redemption rate from the issuer's on-chain contract.

Share token. Deposits mint an sUSDC-style ERC-20 receipt. The share price equals total USYC value divided by total shares, so yield shows up as a rising share price rather than as a growing balance. Shares can be transferred like any token.

Withdrawals. Redeem shares any time for USDC, or for EURC through StableFX, Arc's native stablecoin FX layer. There is no lockup, no minimum balance, no withdrawal window and no liquidation mechanism, because nothing is borrowed against the vault.

Agents

x402 lets a client pay for an HTTP resource in-band with a stablecoin. Agents that use it make an enormous number of tiny payments: an inference call for $0.02, a search for $0.05, a storage read for $0.01. Rounding a $0.02 payment up to $1.00 saves $0.98, so at agent volumes round-ups are not pocket change. They are a meaningful fraction of spend, which is why caps and multipliers exist.

Agent wallets are policy-controlled. The operator sets the rule, the cap and the withdrawal address; the agent cannot change them. This makes Spare a simple way to give an autonomous system a savings buffer that its own actions fund, without giving it the keys to the buffer.

Fees

  • Deposits: 0. Round-ups enter the vault in full.
  • Withdrawals: 0 from Spare. StableFX conversion to EURC uses Arc's native rate.
  • Performance fee: 10% of USYC yield, taken only on yield, never on principal. Half of every fee buys back and burns $SPARE; the other half goes to the protocol treasury.
  • With $SPARE staked: the performance fee is 0.

Security model

  • Non-custodial. Only the wallet owner can revoke a rule or redeem shares. The protocol cannot move principal.
  • No oracle. The vault reads USYC's on-chain redemption rate directly. There is no external price feed to manipulate.
  • No liquidation. The vault has no debt, so there is nothing to liquidate.
  • No lockup. Withdrawals are always available at the current share price.
  • Upgrade and admin keys. Which contracts are upgradeable, who holds the keys, and the timelock on changes will be published at deployment together with the audit report. Until then, assume nothing about them.

Roadmap

  1. 1Site liveDone
  2. 2Contracts auditNext
  3. 3Protocol + $SPARE launch
  4. 4Agent SDK

FAQ

Is the protocol live?
No. The contracts are not deployed and $SPARE does not exist. This site is a showcase with a client-side simulation.
Does Spare ever touch my payment?
No. The payment you send goes through exactly as you sent it. Only the round-up, a separate amount, is routed to the vault.
What if I don't have enough USDC to cover the round-up?
The payment still goes through. The round-up is skipped for that transaction.
Can I lose money?
The vault holds USYC, whose value tracks short-dated T-bills. Spare adds no leverage and no liquidation. Smart-contract risk and issuer risk on USYC remain, and yield can go down.
Can I use Spare without $SPARE?
Yes. The token is optional. Without it you pay the 10% performance fee on yield; with it staked, you pay nothing.
Where do I follow the launch?
On X at @SpareOnArc. Any contract address you see before an announcement there is fake.
Not launched yet

Token

$SPARE

The protocol's utility and governance token. It is not live. There is no contract, no presale and no airdrop to claim.

Fee-free staking

Stake $SPARE and your performance fee drops to 0. You keep every basis point of USYC yield your vault earns.

Governance

Holders vote on round-up thresholds, the list of accepted yield assets, and which agent integrations the protocol supports.

Buyback & burn

50% of all performance fees buy $SPARE on the open market and burn it. The other 50% funds the treasury.

Distribution details will be published before launch.

There is no $SPARE contract. Any address claiming to be $SPARE is fake.

Follow @SpareOnArc for the launch