Feature: Add Stripe Shared Payment Token (SPT) adapter

[up2itnow0822]

Context

Stripe, Klarna, and Affirm just launched the Shared Payment Token (SPT) standard -- a unified agent payment rail that lets AI agents pay across multiple providers using a single tokenized payment method. This is part of the broader agent payment infrastructure war that is accelerating in Q1 2026.

Why this matters for agent-wallet-sdk

agent-wallet-sdk currently supports x402 native payments on 17 chains, CCTP cross-chain USDC transfers, and Jupiter swaps on Solana. The x402 implementation positions us well since Stripe is actively building on x402 for Base.

However, SPT is a separate standard targeting the custodial payment layer -- credit cards, BNPL, fiat rails -- that x402 does not cover. Agents that need to pay for services priced in fiat (SaaS subscriptions, API credits billed monthly, marketplace purchases) cannot use x402 today.

Proposed adapter

Add a StripeSPTAdapter that:

1. Accepts a Stripe SPT (shared payment token) as a payment method in the wallet configuration
2. Routes fiat-priced payments through the SPT instead of on-chain
3. Falls back to x402/on-chain when SPT is not accepted by the merchant
4. Maintains the same wallet.fetchWithPayment() interface so agent code does not need to change

const wallet = new AgentWallet({
  privateKey: process.env.AGENT_KEY,
  paymentAdapters: [
    new StripeSPTAdapter({ sharedPaymentToken: process.env.STRIPE_SPT }),
    new X402Adapter({ network: "base" })
  ]
});

// Wallet picks the right adapter based on merchant response
const result = await wallet.fetchWithPayment("https://api.example.com/data", {
  maxAmountUSDC: "0.01"
});

Evaluation needed

• Review Stripe SPT API docs and token lifecycle
• Assess Klarna and Affirm SPT compatibility (same token format?)
• Define fallback priority order (SPT vs x402 vs direct on-chain)
• Security review: SPT token storage, rotation, and revocation
• Determine if SPT is agent-addressable (agent holds token vs. user holds token)

References

• Stripe x402 announcement on Base
• agent-wallet-sdk x402 support (v0.5.5)
• Circle CCTP integration (v0.5.5)

Priority

High -- the agent payment rail war is moving fast. x402 covers the crypto-native side. SPT covers the fiat/custodial side. Both rails will likely coexist. We should support both.