Child Pays for Parent (CPFP)

Overview

Child Pays for Parent is a fee-bumping technique used in UTXO networks such as Bitcoin. A wallet creates a child transaction spending an output from a low-fee unconfirmed parent and attaches enough fee to make the combined package attractive to miners.

Miners must include the parent before the child, so they evaluate the package’s aggregate fee rate under mempool and policy limits. CPFP requires a spendable parent output and may fail when package size, ancestor limits, dust, or local relay policy prevents acceptance. Wallets should calculate fees across all relevant ancestors, verify that the child output is controlled, and monitor both transaction IDs. Payment systems must not treat the child as confirmed independently; settlement occurs only when the package is mined into the canonical chain.

Wallet defaults, quoted caps, and minimum protocol charges are different values, so interfaces should label each field and avoid presenting a maximum authorization as the amount ultimately paid. An estimate should identify the fee unit, network, block context, and transaction parameters used to calculate it. Operational handling of Child Pays for Parent (CPFP) should account for congestion, replacement rules, rejected fee settings, and software-version differences. Calculations should use integer base units and preserve the original fields submitted by the signer, because rounded display values are insufficient for reconciliation or incident review. A pending quote can change before inclusion, and a transaction that fails at execution may still consume a network fee. Payment systems should store the canonical transaction identifier, inclusion block, execution result, consumed resources, and effective fee reported by the final receipt.