Callout — Gasless Prediction Markets for Friends

Bet STRK tokens on anything with your friend group. No wallet setup, no gas fees, no friction.

Live Demo: calloutzap.vercel.app
GitHub: github.com/ivedmohan/Callout

Built on Starknet with Starkzap SDK.

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What is Callout?

Callout is a social prediction market where friends bet on anything — sports results, personal dares, pop culture outcomes — with automatic settlement. Users sign in with Google or email, stake STRK tokens gaslessly, and winners get paid automatically.

No seed phrases. No gas fees. No manual payouts.

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Deployed Contract

Parameter	Value
Network	Starknet Sepolia
Contract Address	0x04d5da31a4ebf09166864035f0f02feb02085224204a0fccc44ab7b1d9d6c02a
STRK Token	0x04718f5a0fc34cc1af16a1cdee98ffb20c31f5cd61d6ab07201858f4287c938d
Paymaster	AVNU Paymaster (Sepolia)

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Architecture

graph TB
    subgraph Client["Frontend (Next.js)"]
        UI[React UI]
        Privy[Privy Auth]
        SDK[Starkzap SDK]
    end

    subgraph API["API Routes (Next.js)"]
        WalletAPI["/api/wallet/starknet"]
        SignAPI["/api/wallet/sign"]
        PaymasterProxy["/api/paymaster"]
    end

    subgraph Starknet["Starknet Sepolia"]
        Contract[BetEscrow Contract]
        STRK[STRK Token]
        Paymaster[AVNU Paymaster]
    end

    subgraph External["External Services"]
        PrivyCloud[Privy Cloud]
        AVNU[AVNU Paymaster API]
    end

    UI -->|Social Login| Privy
    Privy -->|Access Token| SDK
    SDK -->|Onboard| WalletAPI
    SDK -->|Sign Tx| SignAPI
    SDK -->|Gasless Tx| PaymasterProxy

    WalletAPI -->|Create/Get Wallet| PrivyCloud
    SignAPI -->|rawSign| PrivyCloud
    PaymasterProxy -->|Sponsor Gas| AVNU

    SDK -->|Execute Calls| Contract
    Contract -->|Transfer| STRK
    Contract -->|Fee| Paymaster

Loading

---

User Flow

sequenceDiagram
    actor User
    participant App as Callout App
    participant Privy as Privy Auth
    participant SDK as Starkzap SDK
    participant PM as AVNU Paymaster
    participant SC as BetEscrow Contract

    User->>App: Sign in (Google/Email)
    App->>Privy: Authenticate
    Privy-->>App: Access Token
    App->>SDK: Auto-onboard wallet
    SDK-->>App: Wallet ready (gasless)

    User->>App: Create bet (title, options, stake)
    App->>SDK: approve STRK + create_bet
    SDK->>PM: Sponsor transaction
    PM-->>SDK: Signed paymaster tx
    SDK->>SC: Execute (gasless)
    SC-->>SDK: Bet created (ID)

    User->>App: Share link with friends
    Note over User,App: /bet/[id]

    actor Friend
    Friend->>App: Open link & sign in
    App->>SDK: approve STRK + join_bet
    SDK->>PM: Sponsor transaction
    SDK->>SC: Execute (gasless)

    User->>App: Settle bet (pick winner)
    App->>SDK: settle_bet
    SDK->>SC: Execute (gasless)

    Friend->>App: Claim winnings
    App->>SDK: claim_payout
    SDK->>SC: Transfer STRK to winner

Loading

---

Smart Contract: BetEscrow

Interface

#[starknet::interface]
trait IBetEscrow<TContractState> {
    fn create_bet(title: felt252, option_a: felt252, option_b: felt252,
                  stake_amount: u256, deadline: u64) -> felt252;
    fn join_bet(bet_id: felt252, option: u8);
    fn settle_bet(bet_id: felt252, winning_option: u8);
    fn claim_payout(bet_id: felt252);
    fn get_bet(bet_id: felt252) -> Bet;
    fn get_participant(bet_id: felt252, index: u32) -> Participant;
    fn get_bet_count() -> felt252;
}

Payout Logic

Payout per winner = Total Pot / Number of Winners

• All staked STRK goes into a shared pot
• Only the creator can settle (creator = oracle for MVP)
• Winners split the pot equally
• Losers receive nothing

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Features

• Social Login — Sign in with Google or email via Privy
• Gasless Transactions — AVNU Paymaster sponsors all gas fees
• Create Bets — Pick a topic, set two options, choose a stake amount
• Shareable Links — Send /bet/[id] to friends to join
• Auto Payout — Smart contract distributes STRK to winners proportionally
• Dashboard — Track all your active and settled bets
• Probability Bars — Polymarket-style horizontal vote split visualization
• Donut Chart — Visual ring chart showing option distribution
• Earnings Potential — See projected payout, profit & ROI before betting
• Friendly Errors — Human-readable error messages, not raw RPC JSON

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Tech Stack

Layer	Technology
Frontend	Next.js 16 (TypeScript), Tailwind CSS v4, React 19
Blockchain SDK	Starkzap — social login, gasless txs, STRK ops
Smart Contracts	Cairo (Starknet) — BetEscrow contract
Auth	Privy via Starkzap (Google/email login)
Gas	AVNU Paymaster via Starkzap (free for users)
Wallet Storage	Privy User Custom Metadata (serverless-compatible)
Deployment	Vercel
Contract Tooling	cairo-coder.com

---

Project Structure

callout/
├── contracts/
│   └── bet_escrow.cairo       # Cairo smart contract
├── src/
│   ├── app/
│   │   ├── api/               # Next.js API routes (wallet, sign, paymaster)
│   │   ├── layout.tsx         # Root layout with providers
│   │   ├── page.tsx           # Landing page
│   │   ├── create/page.tsx    # Create a bet
│   │   ├── bet/[id]/page.tsx  # View / join a bet
│   │   └── dashboard/page.tsx # My bets dashboard
│   ├── components/
│   │   ├── Navbar.tsx         # Navigation bar
│   │   ├── BetCard.tsx        # Bet card with probability bar
│   │   ├── ProbabilityBar.tsx # Horizontal vote split bar
│   │   ├── DonutChart.tsx     # SVG donut chart
│   │   ├── EarningsPotential.tsx # Projected payout card
│   │   ├── CalloutLogo.tsx    # SVG logo component
│   │   ├── Button.tsx         # Reusable button component
│   │   └── LoadingSpinner.tsx # Loading states
│   ├── hooks/
│   │   └── useWallet.ts       # Wallet connection + auto-onboard
│   ├── lib/
│   │   ├── starkzap.ts        # Starkzap SDK init & Privy onboarding
│   │   ├── contract.ts        # Contract interaction helpers
│   │   ├── constants.ts       # Network/contract constants
│   │   └── errors.ts          # Human-friendly error parser
│   └── types/
│       └── index.ts           # TypeScript type definitions
├── public/
│   └── logo.svg               # Geometric logo (SVG)
├── .env.local                 # Environment variables (not committed)
├── claude.md                  # AI context file
└── README.md

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Quick Start

Prerequisites

• Node.js 18+
• A Privy app (App ID + Secret)
• An AVNU Paymaster API key (funded with STRK)

1. Install Dependencies

npm install

2. Configure Environment

Create .env.local:

NEXT_PUBLIC_NETWORK=sepolia
NEXT_PUBLIC_CONTRACT_ADDRESS=0x04d5da31a4ebf09166864035f0f02feb02085224204a0fccc44ab7b1d9d6c02a
NEXT_PUBLIC_PRIVY_APP_ID=your-privy-app-id
PRIVY_APP_SECRET=your-privy-secret
PAYMASTER_API_KEY=your-avnu-key

3. Start Development Server

npm run dev

Runs on http://localhost:3000.

4. Deploy Smart Contract

cd contracts
scarb build
# Deploy with Starknet CLI or snforge

Or vibe-code at cairo-coder.com.

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Resources

• Starkzap Docs
• Starkzap Repo
• Awesome Starkzap
• Starkzap Step-by-step Tutorial
• Cairo Coder (Vibe-code contracts)
• Starknet Faucet (Sepolia)

License

MIT