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Crossmint is the wallet infrastructure behind Lobster Cash and other agent payment products. Like AgentCash, Sponge, and ATXP, it’s an agent side wallet — Remlo doesn’t depend on it. Crossmint’s distinguishing feature is the non custodial delegated model: the user owns the wallet, the agent gets scoped signer permissions (spend cap, allowed counterparties, time window) enforced on chain.

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Why Crossmint

Three things matter for paying Remlo:
  1. Non custodial wallets with on chain delegation. The user keeps custody. The agent is added as a signer with a permission set the user signed. Anything outside the delegated scope is rejected at the wallet layer. Crossmint and the developer never take custody of funds.
  2. Server agent wallets. For autonomous agents with no human in the loop, Crossmint also offers backend only wallets where your server is the root signer.
  3. Standard libs. Crossmint’s x402 path uses @x402/core + @x402/evm, the canonical Coinbase libraries. The MPP path uses mppx/client. Crossmint contributes the wallet; the protocol layer is upstream.

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Setup

You need: 
npm i @crossmint/wallets-sdk @x402/core @x402/evm viem@^2.22
viem@^2.22 is the minimum version that exports tempoModerato from viem/chains. Older versions don’t include the chain definition. For MPP on Tempo, also install: 
npm i mppx/client

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Calling Remlo via x402 (Base, Solana)

import { CrossmintWallets, createCrossmint } from "@crossmint/wallets-sdk"
import { EVMWallet } from "@crossmint/client-sdk-react-ui"
import { x402Client, wrapFetchWithPayment } from "@x402/core/client"
import { ExactEvmScheme } from "@x402/evm/exact/client"
import type { Hex } from "viem"

const crossmint = createCrossmint({
  apiKey: process.env.CROSSMINT_SERVER_API_KEY!,
})

const crossmintWallets = CrossmintWallets.from(crossmint)

const wallet = await crossmintWallets.getWallet(
  process.env.CROSSMINT_WALLET_ADDRESS!,
  { chain: "base" },
)

await wallet.useSigner({
  type: "server",
  secret: process.env.CROSSMINT_SIGNER_SECRET!,
})

const evmWallet = EVMWallet.from(wallet)

const x402Signer = {
  address: evmWallet.address as `0x${string}`,
  async signTypedData(typedData: any) {
    const { signature } = await evmWallet.signTypedData({
      ...typedData,
      chain: "base",
    })
    return signature as Hex
  },
}

const client = new x402Client()
client.register("eip155:*", new ExactEvmScheme(x402Signer))

const fetchWithPayment = wrapFetchWithPayment(fetch, client)

const res = await fetchWithPayment(
  "https://www.remlo.xyz/api/mpp/treasury/yield-rates",
  { method: "GET" },
)
wrapFetchWithPayment handles the 402 → sign → retry loop. Your agent code calls a normal fetch. For principal bound endpoints, include the Tier 1 or Tier 2 headers on the request: 
const res = await fetchWithPayment(
  "https://www.remlo.xyz/api/mpp/agent/pay",
  {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      "X-Agent-Identifier": "erc8004:tempo:42",
      "X-Agent-Timestamp": String(Date.now()),
      "X-Agent-Signature": "0x...",
    },
    body: JSON.stringify({ /* charge body */ }),
  },
)

​
Calling Remlo via MPP (Tempo)

import { CrossmintWallets, createCrossmint } from "@crossmint/wallets-sdk"
import { EVMWallet } from "@crossmint/client-sdk-react-ui"
import {
  createWalletClient,
  custom,
  http,
  MethodNotFoundRpcError,
  type EIP1193RequestFn,
} from "viem"
import { tempoModerato } from "viem/chains"
import { Mppx, tempo } from "mppx/client"

const crossmint = createCrossmint({
  apiKey: process.env.CROSSMINT_SERVER_API_KEY!,
})

const crossmintWallets = CrossmintWallets.from(crossmint)
const wallet = await crossmintWallets.getWallet(
  process.env.CROSSMINT_WALLET_ADDRESS!,
  { chain: "base" },
)
await wallet.useSigner({
  type: "server",
  secret: process.env.CROSSMINT_SIGNER_SECRET!,
})

const evmWallet = EVMWallet.from(wallet)
const httpTransport = http()

const request: EIP1193RequestFn = async ({ method, params }) => {
  if (method === "wallet_sendCalls") {
    throw new MethodNotFoundRpcError(new Error("not implemented"))
  }
  if (method === "eth_sendTransaction") {
    const [tx] = params as Array<{
      to: `0x${string}`
      data?: `0x${string}`
      value?: `0x${string}`
    }>
    const sent = await evmWallet.sendTransaction({
      to: tx.to,
      data: tx.data ?? "0x",
      value: tx.value ? BigInt(tx.value) : undefined,
    })
    return sent.hash as `0x${string}`
  }
  return httpTransport.request({ method, params } as any)
}

const walletClient = createWalletClient({
  account: evmWallet.address as `0x${string}`,
  chain: tempoModerato,
  transport: custom({ request }),
})

const mppx = Mppx.create({
  polyfill: false,
  methods: [tempo({ mode: "push", getClient: () => walletClient as any })],
})

const res = await mppx.fetch(
  "https://www.remlo.xyz/api/mpp/treasury/yield-rates",
)
Same pattern as x402 — mppx.fetch handles the negotiation and retry transparently.

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What Crossmint isn’t

  • It isn’t a dependency for Remlo. We don’t ship code that imports the Crossmint SDK.
  • It isn’t a card processor for Remlo. Crossmint also offers virtual cards (Visa VIC + Mastercard Agent Pay) for agents to spend at traditional websites. Remlo receives stablecoins, not cards; the card story is unrelated to how an agent pays our endpoints.
  • It isn’t an authentication system. Principal bound endpoints use the standard Remlo Tier 1 / Tier 2 headers, signed independently of Crossmint.
  • It isn’t required to use @x402/core or mppx/client directly. Crossmint is one wallet flavor; you can pair @x402/core with any EVM signer.