liquid-gas-sponsor

Send any transaction on Base with a wallet that holds USDC but no ETH. Pay the gas per operation in USDC over x402 (from $0.03) through an ERC-4337 paymaster; one endpoint for smart-wallet SDKs and plain wallets. No account, no API key.

Liquid gas sponsor — transact on Base with USDC only

Most agent wallets hold USDC and nothing else, and every on-chain action needs ETH for gas. This skill removes that: POST https://api.liquidagent.ai/v1/gas is an ERC-4337 paymaster you pay per operation in USDC over x402. The 402 is the quote. Pay it, sign what comes back, and the sponsor's deposit pays the gas. No ETH, no account, no API key, no sign-up.

• Chain: Base mainnet (eip155:8453), EntryPoint v0.8, paymaster 0x9676897c3bf08977cdbe78213f84e72a29b844ec
• Price: max($0.03, 1.3 × the operation's gas cap); a simple transfer is $0.03, a heavy DeFi call a few cents more
• The signed sponsorship locks the gas limits and max fee, so the sponsor can never charge more than it quoted
• Fees settle through a public x402 facilitator (PayAI); the exact price and payTo are in the 402

When to use this skill

• Your wallet has USDC but zero ETH and you need to send, swap, deposit, mint, or call any contract on Base
• You built an unsigned transaction with another API (for example Liquid Agent's stock index: POST /v1/buy) and can't broadcast it
• You run a smart-wallet SDK (ERC-4337) and want a paymaster you can pay per op instead of a developer account

One endpoint, the body picks the lane

Plain wallet (any EOA) — two calls to the same URL

bash

# 1. Quote. No payment header -> HTTP 402 with the exact USDC price for THIS operation (valid 120 s).
curl -s -X POST https://api.liquidagent.ai/v1/gas -H 'content-type: application/json' \
  -d '{"sender":"0xYourEOA","calls":[{"to":"0x…","data":"0x…","value":"0"}]}'

# 2. Pay it. Any x402 client (agentcash, x402-fetch) repeats the call with X-PAYMENT and receives:
#    { "userOperation": {...sponsored...}, "typedData": {...}, "authorization": {...} | null, "validUntil": <unix> }
#    - sign typedData with the sender's key (eth_signTypedData_v4) -> put the 65-byte signature in userOperation.signature
#    - if "authorization" is present (first time only): sign it too (EIP-7702; viem: account.signAuthorization)
#    - POST both back to the SAME URL before validUntil:
curl -s -X POST https://api.liquidagent.ai/v1/gas -H 'content-type: application/json' \
  -d '{"userOperation":{...with signature...},"authorization":{...signed, optional...}}'
#    -> { "txHash": "0x…", "success": true, "actualGasCostEth": "…" }   (no further charge)

Your EOA becomes an eth-infinitism Simple7702Account by delegation (owner = you, reversible, no deployment); that is what lets the sponsor pay for it. Only operations the sponsor signed are accepted, once each. Already delegated to another EIP-7702 implementation (MetaMask and friends)? The first call returns 409. Add "redelegate": true to the same body and the sponsor returns an authorization you sign that re-points the wallet to Simple7702Account in the same transaction (reversible), or keep your delegation and use the next section.

Any other smart account — bring your own operation

Already delegated to a different EIP-7702 implementation, or a deployed smart account (Kernel, Safe, Nexus…) on EntryPoint v0.8? Build the operation yourself and the sponsor submits it: no bundler, no stake, no ETH.

bash

# 1. Quote: POST the UNSIGNED EntryPoint v0.8 operation (your sender, calldata and gas limits) -> 402 with the price
curl -s -X POST https://api.liquidagent.ai/v1/gas -H 'content-type: application/json' \
  -d '{"userOperation":{"sender":"0xYourAccount","nonce":"0x…","callData":"0x…","callGasLimit":"0x…","verificationGasLimit":"0x…","preVerificationGas":"0x…","maxFeePerGas":"0x…","maxPriorityFeePerGas":"0x…","signature":"0x"}}'
# 2. Pay it and repeat -> { "userOperation": {...with paymaster fields...}, "typedData": {...}, "validUntil": <unix> }
# 3. Sign it the way YOUR account expects (viem: account.signUserOperation), then POST {"userOperation": <with signature>}
#    to the same URL. The sponsor dry-runs it through the EntryPoint before submitting: a failed dry run returns 400
#    with the reason, costs nothing, and can be retried before validUntil.

Reference: https://github.com/LiquidAgent/liquidagentx402/blob/main/examples/gasless-bring-your-own.js (swap the account constructor for your SDK's). Fresh, never-delegated EOAs should use the plain-wallet lane above.

Smart-wallet SDK (ERC-7677)

Point the SDK's paymaster URL at https://api.liquidagent.ai/v1/gas. pm_getPaymasterStubData is free. An unpaid pm_getPaymasterData returns a JSON-RPC error {code:402, data:<x402 PaymentRequired>} with HTTP 200 (so SDKs don't treat it as a transport failure). Put the x402 payment object in params[3].context.x402 and retry; the result is the standard {paymaster, paymasterData, paymasterVerificationGasLimit, paymasterPostOpGasLimit}. Then POST the signed operation back to the same URL instead of a public bundler: bundlers only relay staked paymasters and this one is not staked yet, so the bring-your-own-operation step above is the last mile.

Paying the 402

x402 v2, scheme exact, network eip155:8453, asset USDC 0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913. The payment is one EIP-3009 TransferWithAuthorization signature (off-chain, no gas). With agentcash: fetch handles it automatically; set maxAmount to a few cents. A complete Node example that buys a tokenized-stock index with a zero-ETH wallet: https://github.com/LiquidAgent/liquidagentx402/blob/main/examples/gasless.js

Reads (free)

• GET https://api.liquidagent.ai/v1/gas — what the sponsor is, price rule, addresses, both flows
• GET https://api.liquidagent.ai/v1/gas/stats — live usage: sponsorships, fees, success rate

Limits and safety

• Per-operation ceiling $5 of gas; past 500 sponsored ops per sender per day the floor rises to $0.04 (never blocked)
• Quote and sponsorship expire together after 120 s; submit before validUntil or get a fresh quote
• The sponsor never sees your key: you sign the operation and the payment locally, it only signs its own sponsorship
• Spent nothing if the operation fails on-chain? The fee is taken at sponsorship time; build and simulate your calls first