BillEarn: The STRK-Powered Subscription Savings Account

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  Landing Page

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  Dashboard View

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  Add new subscription

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  Merchant View

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  Merchant SDK for subscriptions

What it does

BillEarn is the first subscription savings account on Starknet. Users deposit STRK tokens for their annual subscription budget, and BillEarn automatically optimizes their funds:

80% supplied to Vesu lending pools earning 4.2% APY 20% kept liquid for immediate payment availability Automatic payment execution when subscriptions are due Real-time yield tracking in user dashboard

Example User Journey:

Alice deposits 3,000 STRK for her annual subscriptions BillEarn supplies 2,400 STRK to Vesu (earning yield), keeps 600 STRK liquid Alice creates subscriptions: Netflix (50 STRK/month), Spotify (35 STRK/month) Each month, BillEarn automatically pays merchants from the liquid buffer After 1 year, Alice earned 126 STRK in yield—equivalent to 1.5 free months of her subscriptions

Merchant Integration: Merchants embed a simple "Pay with BillEarn" button using our SDK. When clicked, it displays a QR code that users scan with their Starknet wallet to create the subscription—no manual setup needed.

Challenges we ran into

1. Understanding Vesu's Architecture Initial approach: We assumed Vesu had simple deposit() and withdraw() functions. Reality: Vesu uses a sophisticated modify_position system with signed integers (i257), amount types (Delta vs Target), and denomination types (Assets vs Native). Solution: Deep-dived into the Vesu v1 GitHub repository (github.com/vesuxyz/vesu-v1). Studied their vToken implementation to understand:

How to construct ModifyPositionParams correctly Using i257_new(amount, false) for deposits (positive) Using i257_new(amount, true) for withdrawals (negative) Proper handling of collateral without debt positions

1. Balancing Liquidity and Yield Challenge: How much to keep liquid vs. supply to Vesu? Too conservative (50% liquid) = lower yield, less competitive Too aggressive (95% in Vesu) = payment failures if withdrawal delayed Solution:

Settled on 80/20 split after modeling payment patterns Keeper withdraws from Vesu 24 hours before payment due Added buffer safety checks to prevent over-withdrawal If Vesu position insufficient, payment fails gracefully with clear error

What we learned

• Learnt Cairo smart contract development and how contracts are deployed.

What's next for BillEarn: The STRK-Powered Subscription Savings Account

• Support for multiple tokens (ETH, USDC, USDT)
• Dynamic APY optimization (auto-switch to highest yield)
• Subscription management (pause, modify amounts)
• Payment scheduling (custom dates)
• Mobile-responsive optimization
• Multi-pool yield strategies (diversification)
• Automatic budget adjustments

Built With

• cairo
• nextjs
• sdk
• starknet