EIP-3651, also known as "Warm COINBASE", is an Ethereum Improvement Proposal that aims to change the COINBASE address from a cold address to a hot address as part of the Shanghai hard fork of the Shapella Upgrade. This is expected to lower the cost of access to COINBASE and make it more affordable for network contributors known as builders. 
On July 12th, 2021, EIP-3651 was created to improve accessibility and reduce the cost of Ethereum blockchain transactions. Builders are responsible for packaging Ethereum transactions into blocks, which are then forwarded to validators for proper ordering in the blockchain. Builders are paid by traders to arrange transactions in a certain order, with higher fees charged for earlier validation. Flashbots is currently the largest builder in the Ethereum ecosystem, accounting for 80% of relayed blocks. 
Currently, COINBASE direct transactions are becoming more popular due to the benefits they offer, such as conditional payments, which enable implicit cancellation of transactions. However, the high price of access to COINBASE is a hindrance for network participants because COINBASE was introduced under the access list framework to carry out gas calculations based on the cost of access to cold addresses, which is relatively higher than the cost of access to hot addresses. 
EIP-3651 proposes to keep the Coinbase software warm to begin with, which will lower the gas fee required to access it. This is expected to significantly reduce network fees for builders and traders who use builders to execute their trades. Furthermore, traders will no longer have to pay transaction fees for failed trades, as is currently the case, because miners need to confirm all transactions to the chain, whether they succeed or fail. 
- William Morriss
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