Use before sending a transaction or to tell a user what it will cost: reads eth_feeHistory over the last 20 blocks (10th/50th/90th percentile tips), the next base fee and gas price, on Base also the L1 data fee upper bound from the OP-stack GasPriceOracle, and prices typical actions (ETH transfer 21k gas, ERC-20 transfer 65k, DEX swap 180k) in ETH and USD (Chainlink ETH/USD). Paid tool: x402 USDC on Base, $0.001 per successful call; call without payment to receive the PaymentRequired object.