r402

x402 Payment Protocol SDK for Rust. Protocol version 2 only.

[dependencies]
r402 = { version = "0.20", features = ["evm", "http"] }
# r402 = { version = "0.20", features = ["evm", "svm", "http"] }
# r402 = { version = "0.20", features = ["full"] }

default = ["evm", "http"]. Chain crates are feature-gated re-exports (svm, near, xrpl, hedera, avm, aptos, keeta, tvm, stellar, concordium). full is every production chain plus HTTP and MCP. tron / casper are opt-in, not in full.

Protect a Route (Server)

use alloy_primitives::address;
use axum::{Router, routing::get};
use r402::evm::{Eip155Exact, USDC};
use r402::http::server::X402Middleware;

let x402 = X402Middleware::try_new("https://facilitator.example.com")?
    .with_base_url("https://api.example.com".parse()?)
    .with_scheme("eip155:*".parse()?, Eip155Exact);

let app = Router::new().route(
    "/paid-content",
    get(handler).layer(
        x402.with_price_tag(Eip155Exact::price_tag(
            address!("0xYourPayToAddress"),
            USDC::base().amount(1_000_000u64),
            None,
        ))?,
    ),
);

Send Payments (Client)

use alloy_signer_local::PrivateKeySigner;
use r402::evm::Eip155ExactClient;
use r402::http::{WithPayments, X402Client};
use std::sync::Arc;

let signer = Arc::new("0x...".parse::<PrivateKeySigner>()?);
let client = reqwest::Client::new().with_payments(
    X402Client::new().register(Eip155ExactClient::new(signer)),
);
let res = client.get("https://api.example.com/paid").send().await?;

Settlement Modes

SettlementMode is not a paymentFlow. Spec paymentFlow (authorization / upfront / escrow) is the on-wire ordering of verify and settle around the handler. Sequential / Concurrent / Background are a resource-server scheduler for the after-handler settle of authorization: whether this HTTP response waits for that settle. Facilitators and clients never see the knob; it is not written into PAYMENT-REQUIRED.

• Sequential (default) is spec authorization: verify → handler → settle → respond with Payment-Response.
• Concurrent and Background overlap that after-handler settle with the handler (async I/O). They are not a fourth flow. upfront / escrow reject them (IncompatibleSettlementMode) because those flows already settle before the handler.

Configurable via with_settlement_mode() after with_price_tag.

Sequential (default)

Verify → execute → settle. Spec authorization. On-chain settlement only after the handler succeeds. Payment-Response is attached.

sequenceDiagram
    participant C as Client
    participant S as Server
    participant F as Facilitator
    participant H as Handler

    C->>S: HTTP Request + Payment-Signature
    S->>F: verify(payment)
    F-->>S: VerifyResponse ✓
    S->>H: execute request
    Note over S,H: Balance verified but NOT locked —<br/>handler executing (variable latency)
    H-->>S: response body
    S->>F: settle(payment)
    Note over S,F: On-chain transfer (2–5 s)
    F-->>S: SettleResponse (tx_hash)
    S-->>C: 200 OK + Payment-Response header

Concurrent

Verify → (settle ∥ execute) → await both. Still authorization on the wire. Overlaps the after-handler settle RPC with the handler. The response still waits and still carries Payment-Response. On handler error the settlement task is detached — the payer may be charged even if the handler failed.

sequenceDiagram
    participant C as Client
    participant S as Server
    participant F as Facilitator
    participant H as Handler

    C->>S: HTTP Request + Payment-Signature
    S->>F: verify(payment)
    F-->>S: VerifyResponse ✓
    par settle ∥ execute
        S->>F: settle(payment)
        Note over S,F: On-chain transfer
        F-->>S: SettleResponse (tx_hash)
    and
        S->>H: execute request
        H-->>S: response body
    end
    S-->>C: 200 OK + Payment-Response header

Background

Verify → spawn settle → execute → return. Still authorization on the wire. No Payment-Response (headers leave before settle finishes). For SSE / LLM streams.

sequenceDiagram
    participant C as Client
    participant S as Server
    participant F as Facilitator
    participant H as Handler

    C->>S: HTTP Request + Payment-Signature
    S->>F: verify(payment)
    F-->>S: VerifyResponse ✓
    S-)F: settle(payment) [fire-and-forget]
    S->>H: execute request
    H-->>S: response body (or stream)
    S-->>C: 200 OK (no Payment-Response header)
    Note over S,F: Settlement completes asynchronously
    F-)S: SettleResponse (logged)

Comparison

Applies only to paymentFlow = authorization. upfront / escrow stay Sequential.

Note: UptoActualAmount is honoured only by SettlementMode::Sequential. Concurrent and Background start settlement before the handler returns and therefore charge the signed maximum. They cannot meter a stream.

License

Licensed under either of MIT or Apache-2.0, at your option.