Gas price tracker: live gas and priority fees for Ethereum, Base, Arbitrum, Polygon and Solana in one call, from public nodes. Returns base fee, slow/standard/fast tips, max fee, block congestion, and the USD cost of a transfer, an ERC-20 transfer and a swap, plus the cheapest chain. Optional chain. 0.002 USDC (Base/Polygon/Arbitrum/Solana/Avalanche/Sei). Errors are never charged: 400 bad input, 422 nothing found or target unreachable, 503 busy.