sequenceDiagram participant Agent as 🤖 AI Agent participant API as 📚 Book402 API participant MW as 🔒 x402 Middleware participant Fac as ⚡ Facilitator participant Base as ⛓️ Base L2 rect rgb(20, 20, 40) Note over Agent,API: Phase 1 — Discovery Agent->>API: GET /search/hybrid?q=stoicism API->>MW: Check if route is paid MW-->>API: Yes — $0.01 USDC on eip155:8453 API-->>Agent: 402 Payment Required Note right of API: PAYMENT-REQUIRED header
contains: price, asset,
payTo address, network end rect rgb(20, 30, 20) Note over Agent: Phase 2 — Sign Payment Note over Agent: 1. Parse PAYMENT-REQUIRED header Note over Agent: 2. Create EIP-3009 authorization Note over Agent: 3. Sign with wallet private key Note over Agent: (No gas needed — off-chain signature!) end rect rgb(20, 20, 40) Note over Agent,Base: Phase 3 — Verify & Deliver Agent->>API: GET /search/hybrid?q=stoicism
+ X-PAYMENT: {signed payload} API->>MW: Has payment header MW->>Fac: POST /verify {payment payload} Fac->>Fac: Validate EIP-3009 signature Fac->>Fac: Check USDC balance Fac->>Fac: Verify nonce unused Fac-->>MW: ✅ Payment valid MW-->>API: Proceed to handler API->>API: Execute hybrid search API-->>Agent: 200 OK + search results end rect rgb(30, 20, 20) Note over API,Base: Phase 4 — Settlement (async) API->>MW: Response sent, settle payment MW->>Fac: POST /settle {payment payload} Fac->>Base: Submit transferWithAuthorization tx Base-->>Fac: ✅ USDC transferred Fac-->>MW: Settlement confirmed Note over Base: $0.01 USDC moved from
Agent wallet → Book402 wallet end

graph TD subgraph Traditional["Traditional ERC-20 Transfer"] A1[Sender] -->|"1. approve()"| B1[USDC Contract] A1 -->|"2. transfer() + gas"| B1 B1 -->|"3. Move tokens"| C1[Receiver] end subgraph EIP3009["EIP-3009 (x402 uses this)"] A2[Agent] -->|"1. Sign authorization
(off-chain, free)"| D[Signed Message] D -->|"2. Send signature"| E[Book402] E -->|"3. Submit to chain"| F[USDC Contract] F -->|"4. Move tokens"| G[Book402 Wallet] end style EIP3009 fill:#0a1a0a,stroke:#33aa33 style Traditional fill:#1a0a0a,stroke:#aa3333

stateDiagram-v2 [*] --> Created: Agent signs authorization Created --> Verified: Facilitator validates signature Created --> Rejected: Invalid signature / insufficient balance Verified --> Delivered: Server returns content Delivered --> Settling: Facilitator submits tx Settling --> Settled: On-chain confirmation Settling --> Failed: Settlement error (retried) Settled --> [*] Rejected --> [*] Failed --> Settling: Auto-retry