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Architecture & Fees

Architecture

Tessera settles per-second support on Arc Testnet through Circle Gateway. Viewers can fund the Circle SCA on Arc directly, or bridge USDC in with Circle CCTP (Bridge Kit + Forwarding) from Ethereum Sepolia, Base Sepolia, or Arbitrum Sepolia. CCTP mints to the SCA; Tessera then deposits that USDC into Gateway (prepare-deposit).

flowchart LR
    classDef origin fill:#1A1B26,stroke:#7AA2F7,stroke-width:2px,color:#FFFFFF,rx:10,ry:10
    classDef cctp fill:#1F2335,stroke:#9ECE6A,stroke-width:2px,color:#FFFFFF,rx:10,ry:10
    classDef arc fill:#24283B,stroke:#BB9AF7,stroke-width:3px,color:#FFFFFF,rx:10,ry:10,stroke-dasharray: 5 5
    classDef blockchain fill:#24283B,stroke:#F7768E,stroke-width:2px,color:#FFFFFF,rx:10,ry:10
    classDef engine fill:#1F2335,stroke:#7DCFFF,stroke-width:2px,color:#FFFFFF,rx:5,ry:5

    subgraph Origins ["Origin testnets"]
        direction TB
        ETH["Ethereum Sepolia"]:::origin
        BAS["Base Sepolia"]:::origin
        ARB["Arbitrum Sepolia"]:::origin
    end

    CCTP(("Circle CCTP + Forwarding")):::cctp

    subgraph ArcNet ["Arc Testnet"]
        direction TB
        SCA["Viewer Circle SCA"]:::blockchain
        Gateway["Gateway"]:::blockchain
        Engine["Sidecar billing loop"]:::engine
    end
    class ArcNet arc

    ETH -->|"USDC burn"| CCTP
    BAS -->|"USDC burn"| CCTP
    ARB -->|"USDC burn"| CCTP

    CCTP -->|"mint to SCA"| SCA
    SCA -->|"prepare-deposit"| Gateway
    Engine -.->|"GatewayClient.pay each second"| Gateway

Fee Structure

Billing ticks are off-chain. Each second the sidecar signs with the ephemeral Gateway key (GatewayClient.pay to POST /api/core/stream-access). Ticks do not cost Arc gas.

On-chain USDC movement for a typical exclusive session:

  1. Deposit (Gateway funding): SCA USDC into Gateway (prepare-deposit), or leftover ephemeral-wallet USDC via register-session.
  2. Cash-out (optional): Manual withdraw of unused Gateway balance back to the SCA. Leaving the player (sessions/stop) does not withdraw. Creator earnings already moved on each billing tick.

SCA USDC can also go to an external address via quote-external-withdraw / prepare-external-withdraw (Circle challenge, then poll-challenge).

Those Arc transactions use USDC for gas. Tessera keeps a small buffer on the ephemeral wallet (RETAINED_GAS_AMOUNT, default 0.01 USDC) so deposit and cash-out can pay gas. Arc's published target is about $0.01 USDC per transaction.

CCTP (optional): Bridge Kit burns on the source chain and Circle Forwarding mints on Arc. The Forwarding fee is 0.20 USDC. The mint credits the SCA; Gateway funding is still prepare-deposit.

Why Arc Testnet

Tessera bills per second. Settlement runs on Arc Testnet (@circle-fin/x402-batching, eip155:5042002) so ticks can stay off-chain while deposit and cash-out use USDC gas.

  • Native USDC gas: Deposit and cash-out pay gas in USDC. No extra native token.
  • Low per-tx cost: Arc targets about ~$0.01 USDC gas.
  • Gasless ticks: After Gateway is funded, GatewayClient.pay runs once per second with no wallet popup.
  • Why Arc: A 10-minute exclusive session is hundreds of ticks. On-chain gas per tick would dwarf the content rate.

Environment Configuration

  • PUBLIC_URL: Public origin of the sidecar. Circle auth cookies are Secure when this is https://, or when COOKIE_SECURE=true, or when NODE_ENV=production.
  • Gas Buffer: RETAINED_GAS_AMOUNT (default 0.01 USDC) keeps USDC on the ephemeral wallet so on-chain deposit/cash-out can pay Arc gas.

Security & Performance

  • Ingest HMAC: POST /api/core/v1/sessions/start and stop require TESSERA_INGEST_SECRET. Tips and session money routes use Circle wallet proof (userToken + returnAddress).
  • USDC deposits: POST /api/core/circle/prepare-deposit transfers USDC only.
  • Rate limiting: Session routes (register-session, start/stop, tips, end-session, cash-out, topup-session) share an IP limiter. sync-session has its own. Circle routes use a separate IP limiter.
  • Session store: The ephemeral Gateway key survives sessions/stop. cash-out withdraws leftover USDC to the SCA and then clears the record.
  • Dynamic top-up: Viewers can add Gateway balance without leaving the overlay (topup-session).

Observability

  • Healthcheck: GET /health returns sidecar version, a ping to https://api-testnet.circle.com/ping, and active session count.
  • Challenge confirmations: After deposit or cash-out, the overlay calls POST /api/core/circle/poll-challenge until the status is COMPLETE, FAILED, or EXPIRED.