X402 Kit

A fully modular, framework-agnostic, easy-to-extend SDK for building complex X402 payment integrations.

📦 Crates

Crate	Description
x402-kit	Main SDK with network definitions, payment schemes, and facilitator client
x402-signer	Buyer-side signing SDK with reqwest middleware for automatic x402 payments
x402-core	Core traits, types, and transport mechanisms for the X402 protocol
x402-paywall	Framework-agnostic HTTP paywall middleware

📚 Developer Docs

Docs are available at docs.rs

💡 Core Pain Points Solved

X402-kit is not a facilitator — it's a composable SDK for buyers (signers) and sellers (servers) to build custom business logic. Future support for modular facilitator components is planned.

Beyond Static Pricing and Payment Gateway Middlewares

Existing X402 SDKs only support static prices per API route. X402-kit's fully modular architecture enables complex, dynamic pricing logic while maximizing code reuse.

Complete Modularity

All internal fields and methods are public by design. Compose and extend functionality freely without fighting the framework.

Layered Type Safety

• Transport Layer: Uses generalized String types to prevent serialization failures and ensure service availability
• Network + Scheme Layer: Leverages traits and generics for compile-time type checking without runtime overhead

Ship New Networks Without PRs

Implement a new asset, network, or scheme entirely in your codebase and plug it into the SDK immediately—no upstream pull request or waiting period required thanks to trait-driven extension points.

However, we still recommend contributing back any useful implementations to the main repository to help grow the ecosystem!

Production-Ready Design

Minimize runtime errors through compile-time guarantees while maintaining the flexibility needed for real-world business logic.

🧪 Usage Examples

Using x402-paywall with Axum

The x402-paywall crate (re-exported via x402-kit) provides a composable PayWall that handles the complete X402 payment flow. Run the example:

FACILITATOR_URL=https://your-facilitator.example \
  cargo run -p x402-kit --example axum_seller

Standard Payment Flow

The handle_payment method provides a complete flow: update accepts from facilitator, verify payment, run handler, and settle on success.

use alloy::primitives::address;
use axum::{extract::{Request, State}, middleware::Next, response::{IntoResponse, Response}};
use url_macro::url;
use x402_kit::{
    core::Resource,
    facilitator_client::FacilitatorClient,
    networks::evm::assets::UsdcBaseSepolia,
    paywall::paywall::PayWall,
    schemes::exact_evm::ExactEvm,
};

async fn paywall_middleware(State(state): State<AppState>, req: Request, next: Next) -> Response {
    let paywall = PayWall::builder()
        .facilitator(state.facilitator)
        .accepts(
            ExactEvm::builder()
                .amount(1000)
                .asset(UsdcBaseSepolia)
                .pay_to(address!("0x3CB9B3bBfde8501f411bB69Ad3DC07908ED0dE20"))
                .build(),
        )
        .resource(
            Resource::builder()
                .url(url!("https://example.com/resource"))
                .description("X402 payment protected resource")
                .mime_type("application/json")
                .build(),
        )
        .build();

    paywall
        .handle_payment(req, |req| next.run(req))
        .await
        .unwrap_or_else(|err| err.into_response())
}

Multiple Payment Options

Accept payments on multiple networks (EVM and SVM):

use x402_kit::{
    networks::{evm::assets::UsdcBaseSepolia, svm::assets::UsdcSolanaDevnet},
    schemes::{exact_evm::ExactEvm, exact_svm::ExactSvm},
    transport::Accepts,
};

let paywall = PayWall::builder()
    .facilitator(facilitator)
    .accepts(
        Accepts::new()
            .push(
                ExactEvm::builder()
                    .amount(1000)
                    .asset(UsdcBaseSepolia)
                    .pay_to(address!("0x3CB9B3bBfde8501f411bB69Ad3DC07908ED0dE20"))
                    .build(),
            )
            .push(
                ExactSvm::builder()
                    .amount(1000)
                    .asset(UsdcSolanaDevnet)
                    .pay_to(pubkey!("Ge3jkza5KRfXvaq3GELNLh6V1pjjdEKNpEdGXJgjjKUR"))
                    .build(),
            ),
    )
    .resource(/* ... */)
    .build();

Custom Payment Flow

For fine-grained control, use the step-by-step API:

// Skip verification, settle before running handler
let response = paywall
    .process_request(req)?
    .settle()
    .await?
    .run_handler(|req| next.run(req))
    .await?
    .response();

Access Payment State in Handlers

The PayWall injects PaymentState into request extensions:

use axum::{Extension, Json};
use x402_kit::paywall::processor::PaymentState;

async fn handler(Extension(payment_state): Extension<PaymentState>) -> Json<Value> {
    Json(json!({
        "message": "Premium content accessed!",
        "payer": payment_state.verified.map(|v| v.payer),
        "transaction": payment_state.settled.map(|s| s.transaction),
    }))
}

Custom Facilitator Client

Customize request/response types for your facilitator.

Note: The default facilitator client uses Url::join("verify") etc. to construct endpoint URLs from the base URL. This means trailing slashes matter. For example, when using the Coinbase facilitator:

export FACILITATOR_URL=https://www.x402.org/facilitator/

use x402_kit::facilitator_client::{FacilitatorClient, IntoVerifyResponse, IntoSettleResponse};

#[derive(Serialize, Deserialize)]
struct CustomSettleRequest { /* ... */ }

#[derive(Deserialize)]
struct CustomSettleResponse { /* ... */ }

impl IntoSettleResponse for CustomSettleResponse {
    fn into_settle_response(self) -> SettleResult { /* ... */ }
}

let facilitator = FacilitatorClient::from_url(facilitator_url)
    .with_settle_request_type::<CustomSettleRequest>()
    .with_settle_response_type::<CustomSettleResponse>();

Buyer-Side Signing with x402-signer

The x402-signer crate provides a reqwest middleware that automatically handles the x402 payment flow: intercept HTTP 402 → sign payment → retry with payment header.

You need some testnet USDC on Solana devnet or Base Sepolia to run signer examples. You can get some at Circle testnet faucet

EVM Client

EVM_PRIVATE_KEY=0x... RESOURCE_URL=http://localhost:3000/resource/standard \
  cargo run -p x402-signer --example evm_client

use alloy::signers::local::PrivateKeySigner;
use reqwest_middleware::ClientBuilder;
use x402_signer::{X402Client, evm::EvmPaymentSigner, middleware::X402PaymentMiddleware};

let wallet: PrivateKeySigner = "0x...".parse().unwrap();
let signer = EvmPaymentSigner::new(wallet);
let middleware = X402PaymentMiddleware::new(X402Client::new(signer));

let client = ClientBuilder::new(reqwest::Client::new())
    .with(middleware)
    .build();

// Any 402 response is automatically signed and retried
let response = client.post("http://localhost:3000/resource/standard").send().await?;

SVM Client

SOLANA_PRIVATE_KEY=<base58> SOLANA_RPC_URL=https://api.devnet.solana.com \
  RESOURCE_URL=http://localhost:3000/resource/multi_payments \
  cargo run -p x402-signer --example svm_client

use solana_keypair::Keypair;
use solana_rpc_client::nonblocking::rpc_client::RpcClient;
use reqwest_middleware::ClientBuilder;
use x402_signer::{X402Client, svm::SvmPaymentSigner, middleware::X402PaymentMiddleware};

let keypair = Keypair::from_base58_string("<base58-private-key>");
let rpc = RpcClient::new("https://api.devnet.solana.com".to_string());
let signer = SvmPaymentSigner::new(keypair, rpc);
let middleware = X402PaymentMiddleware::new(X402Client::new(signer));

let client = ClientBuilder::new(reqwest::Client::new())
    .with(middleware)
    .build();

let response = client.post("http://localhost:3000/resource/multi_payments").send().await?;

🚀 Next Steps

• More networks / assets / schemes
• MCP / A2A transport support
• Facilitator components support

🤝 Contributing

We welcome all contributions to x402-kit! Here's how you can get involved:

• ⭐ Star this repository
• 🐛 Open issues to report bugs or suggest features
• 🔧 Submit PRs to improve the codebase

Contributors will receive priority access and rewards at AIMO Network's Beta launch (coming soon)!