Simulate a transaction before signing: runs `eth_estimateGas` for `{ to, from?, data?, value? }` and returns the estimated gas units and estimated fee (current gas price x gas, in wei + ETH). If the node rejects the call it returns `ok:false` with the decoded `revertReason` (why it would fail) instead of erroring. Reads live on-chain state with automatic endpoint fallback.

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