01. The Problem: The Web3 Freelance Billing Friction
Independent software engineers, auditors, and digital creators operating globally face constant hurdles when attempting to get paid in stablecoins. While traditional invoicing solutions require banking rails with heavy FX conversions and chargeback risks, crypto-native invoicing tools force clients into rigid token constraints:
- Ecosystem Fragmentation: A freelancer billing on Solana often deals with clients whose treasury resides strictly on Ethereum, Arbitrum, or Base, causing days of friction over manual bridging.
- Reconciliation Hell: Clients send irregular tokens or round up amounts incorrectly, leaving developers to manually parse block explorers to prove an invoice was settled.
- Centralized Custody Risk: Legacy invoicing platforms hold user balances in proprietary database ledgers, introducing single-point-of-failure counterparty risk.
02. Architectural Solution: Multi-Chain Routing & SVM Settlement
InvoiceChain pairs multi-chain payment routers with an Anchor smart contract state machine on Solana. The settlement lifecycle proceeds deterministically:
// Step 1: Invoice InitializationThe freelancer connects a Phantom wallet and initializes an on-chain Invoice account on Solana, defining the counterparty, invoice ID, and requested amount in USD value.
// Step 2: Payment Link GenerationThe Next.js 16 backend invokes KIRAPAY via POST /api/link/generate, establishing a multi-chain checkout link tagged with the Solana invoice ID.
// Step 3: Multi-Chain Liquidity IngestionThe client accesses the checkout link and pays with any arbitrary token (ETH, USDT, DAI) across supported EVM networks. KIRAPAY handles cross-chain swaps and routes settled USDC directly to the receiver's Base address.
// Step 4: Cryptographic Webhook & State MutationUpon transaction finality, KIRAPAY dispatches a cryptographically verified transaction.succeeded webhook. The backend relayer executes the Anchor instruction mark_paid, locking the invoice state on Solana permanently.
“Freelancers should not care which chain their client’s treasury is locked in. The client pays with whatever token they have; the contractor gets USDC, and the final state is verified immutably in Solana bytecode.”— Vinicius Pontual, Project Architecture
04. Smart Contract Mechanics: Anchor Account Constraints
State guarantees are enforced through deterministic account derivations:
["invoice", authority.key(), invoice_id], preventing account collisions across different contractors.mark_paid instruction is strictly restricted to the authorized relayer keypair, preventing malicious actors from artificially settling unfulfilled invoices.05. Webhook Security & Replay Prevention
Because payment settlement spans two distinct networks (Base and Solana), the relayer infrastructure enforces defensive boundaries:
- HMAC Signature Verification: Inbound webhooks from KIRAPAY require strict SHA-256 signature verification matching the
WEBHOOK_SECRET. Spoofed HTTP requests are rejected before hitting the database. - Idempotent Event Ingestion: In the event of webhook retries, the backend checks on-chain state via Solana RPC before constructing the transaction, eliminating redundant network fees and transaction reversions.
- Key Isolation: The backend signing key (
BACKEND_WALLET_PRIVATE_KEY) is held entirely in serverless runtime memory and never exposed to the client bundle.
06. Specifications & Deployments
On-chain program and deployment configuration:
| Parameter | Value / Address |
|---|---|
| Solana Program ID | 4wy52jbYZop2pWWBtBmVZKUMFMrj86qrork9StcypSu7 |
| Target Network | Solana Devnet (Settlement) / Base (Receivables) |
| Frontend Framework | Next.js 16 App Router + TypeScript + Tailwind CSS |
| Gateway Integration | KIRAPAY Merchant API (POST /api/link/generate) |
anchor build && anchor deploy --provider.cluster devnethttps://api.kira-pay.com
03. Public Verification: Announcement & Demo on X
Technical demonstration published on X documenting the end-to-end payment loop, cross-chain checkout execution, and instant Solana state synchronization:
“On-chain invoice system for freelancers. Create a payment link, share with your client, get paid in USDC via KIRAPAY — payment confirmed on Solana.”