Gas abstraction for 402 payments

When an agent hits an HTTP 402 Payment Required and xpense settles on-chain, someone must pay X Layer gas. The agent must not hold native gas tokens — that breaks the "no keys, no ops for the model" guarantee. This note picks how gas gets paid.

The constraint

• The agent signs intent, never a gas-bearing transaction.
• The user's funds are USDC (or another stablecoin), not native gas.
• Settlement runs through xerpaai-go; xpense only emits the signed authorization.

Options

Option	Mechanism	Agent holds gas?	New contract?	Best for
A · EIP-3009 (x402 std)	Agent signs transferWithAuthorization; a facilitator submits it and pays gas	No	No — USDC native	One-shot 402 charges (recommended start)
B · ERC-4337 paymaster	Smart account + paymaster sponsors gas, recouped in USDC	No	Paymaster contract	MPP sessions, streamed vouchers, batched pays
C · Meta-tx forwarder	Custom forwarder; a relayer pays gas and recoups	No	Forwarder contract	Fallback if X Layer lacks A/B infra

Recommended path

1. Start with A. x402 is already built on EIP-3009 — the agent signs an off-chain transferWithAuthorization, and a facilitator (run by xerpaai-go) submits it on X Layer and pays the gas. If X Layer USDC supports transferWithAuthorization, this is zero new on-chain code. This is exactly what OnchainosGateway.x402Sign already targets.
2. Add B when sessions land. For MPP-style streamed vouchers / spend permissions (mpp/charge session lifecycle), move to ERC-4337 with a paymaster so a smart account can sponsor gas and enforce spend limits on-chain.
3. Fall back to C only if forced. If X Layer USDC does not implement EIP-3009 and there is no 4337 bundler, ship a minimal meta-transaction forwarder contract (ZIHAO's call: "if X Layer doesn't support it, we build a contract"). This is the last resort, not the default.

Who eats the gas

The facilitator fronts gas, then recoups it by charging amount + gas markup at settlement, or the platform subsidizes it so the agent sees a clean stablecoin debit. Either way the agent and the model stay gas-unaware.

To verify on X Layer (open)

• [ ] Does X Layer USDC implement EIP-3009 transferWithAuthorization?
• [ ] Is there an ERC-4337 bundler + paymaster available on X Layer?
• [ ] Does the xerpaai-go facilitator already sponsor gas, or is that a backend gap to file?

Until verified, xpense stays correct regardless: it only emits the signed intent — the gas-payment mechanism is entirely a settlement-side (xerpaai-go / X Layer) concern.