Custom code to support the x402 payment protocol in the SDK fetch/websockets

[rishabluthra]

Summary

Adds built-in support for the x402 payment protocol so HTTP API calls and WebSocket connections automatically handle 402 Payment Required responses.

• Pass an x402Client to AgentMailClient — all API calls and WebSocket connections are payment-aware with zero extra code
• Zero new hard dependencies — @x402/fetch is dynamically imported only when x402 option is provided
• Uses Fern's custom code pattern: wrapper classes in src/wrapper/ extend generated classes,
  protected via .fernignore

Usage

HTTP

All REST API calls automatically handle 402 responses — the SDK intercepts the payment requirement, signs a USDC payment, and retries the request with a PAYMENT-SIGNATURE
header.

import { AgentMailClient, AgentMailEnvironment } from "agentmail";
import { x402Client } from "@x402/fetch";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { registerExactSvmScheme } from "@x402/svm/exact/client";
import { privateKeyToAccount } from "viem/accounts";
import { createKeyPairSignerFromBytes } from "@solana/kit";
import { base58 } from "@scure/base";

// Create signers
const evmSigner = privateKeyToAccount("0x...");
const svmSigner = await createKeyPairSignerFromBytes(
  base58.decode("your-solana-private-key"),
);

// Create x402 client with EVM + SVM schemes
const x402 = new x402Client();
registerExactEvmScheme(x402, { signer: evmSigner });
registerExactSvmScheme(x402, { signer: svmSigner });

// Pass x402 client — all API calls handle payments automatically
const client = new AgentMailClient({
  environment: AgentMailEnvironment.ProdX402,
  x402,
});

// These just work — 402 responses are handled transparently
const inboxes = await client.inboxes.list();
const inbox = await client.inboxes.create({ username: "my-agent" });
await client.inboxes.messages.send(inbox.inboxId, {
  to: ["recipient@example.com"],
  subject: "Hello",
  text: "Sent via x402",
});

WebSocket

WebSocket endpoints don't natively support 402 responses, so the SDK probes the endpoint over HTTP first, signs the payment, then opens the WebSocket with the payment headers.

  // Same client from above — websockets are also payment-aware
  const socket = await client.websockets.connect();

  socket.on("message", (msg) => {
    console.log("Received:", msg);
  });

  socket.sendSubscribe({
    type: "subscribe",
    inboxIds: ["my-agent@agentmail.to"],
  });

Without x402

Existing usage is unchanged — if you don't pass x402, the client behaves exactly as before.

  const client = new AgentMailClient({
    environment: AgentMailEnvironment.Prod,
    apiKey: "your-api-key",
  });

Peer dependencies

When using x402, install the relevant packages:

  # Required
  npm install @x402/fetch

  # EVM (Base, Ethereum)
  npm install @x402/evm viem

  # SVM (Solana)
  npm install @x402/svm @solana/kit @scure/base

---

Summary by cubic

Adds built-in x402 payment support for both HTTP and WebSockets using a single x402 option on AgentMailClient. All 402 flows are handled automatically; existing usage is unchanged if you don’t pass x402.

• New Features

  • Pass x402 to AgentMailClient — HTTP and WebSockets handle 402 automatically; no fetch override needed.
  • HTTP: SDK wraps fetch internally and pay-then-retries on 402.
  • WebSockets: probe over HTTP, sign payment, then connect with payment headers.
  • No API key required when x402 is provided; the SDK defaults it internally.
  • No hard dependency added; @x402/fetch is imported only when x402 is used.

• Dependencies

  • Required: @x402/fetch
  • EVM: @x402/evm, viem
  • SVM: @x402/svm, @solana/kit, @scure/base

^{Written for commit d957f2c. Summary will update on new commits.}

[cubic-dev-ai]

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[cubic-dev-ai]

No issues found across 7 files