TERM_DEF // TRANSACTION / CPFP_CARVE_OUT
CPFP CARVE
OUT
OUT
cpfp-carve-out/">CPFP Carve Out. A relay exception letting one extra small high-fee child evade mempool descendant limits, enabling reliable bumping.
This page sits in the Transaction section — How money moves: inputs, outputs, fees, signatures, sighash flags, and the formats that wrap them. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
This page sits in the Transaction section — How money moves: inputs, outputs, fees, signatures, sighash flags, and the formats that wrap them. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
WHAT_CPFP_CARVE_OUT_IS
CPFP Carve Out — at a glance
TRANSACTION
CPFP Carve Out is part of the FEE BUMPING sub-family of transaction-level concepts. Child-Pays-For-Parent: a high-fee child tx pulls a low-fee parent into the next block by ancestor-feerate accounting. A relay exception letting one extra small high-fee child evade mempool descendant limits, enabling reliable bumping.
Why it exists
DESIGN
The UTXO model is what makes Bitcoin transactions auditable, parallel-verifiable, and resistant to replay attacks. Every input directly references a specific previous output by txid + index. There is no shared state to lock, no "account version" to clash on, no oracle to consult — each input is a self-contained spending proof, and nodes can verify thousands of them in parallel.
HOW_IT_WORKS
Mechanism
HOW IT WORKS
A transaction has one or more inputs (each pointing at a UTXO being spent) and one or more outputs (each creating a new UTXO). Inputs supply the unlocking data; outputs specify the new locks. The fee is the difference between input sum and output sum, paid to the miner who includes the tx in a block. Once mined and buried under enough confirmations, the spend is for all practical purposes irreversible — the UTXOs it consumed are gone, the ones it created are spendable.
1. Wallet selects UTXOs whose total value covers the spend amount + estimated fee (coin selection).
2. Wallet builds the transaction body: version, inputs (each with prev txid + vout + sequence), outputs (each with value + scriptPubKey), locktime.
3. Wallet computes the sighash for each input (which parts of the tx the signature commits to — controlled by the SIGHASH flag).
4. Wallet signs each input with the right private key. Witness/scriptSig is populated with the resulting signatures + pubkeys.
5. Tx is broadcast to peers. Mempool propagation: tens of seconds globally.
6. A miner includes it in a block. Confirmation count grows by 1 per block; after ~6 the tx is effectively final.
WORKED_EXAMPLE
CPFP Carve Out — FEE BUMPING
EXAMPLE
Setup : Parent tx has fee F_p, child tx (spending parent's output) has fee F_c
Accounting : Miners select by (F_p + F_c) / (size_p + size_c) — ancestor feerate
Result : Parent + child mined together if the combined feerate clears the block tip
Use case : Recover from a stuck low-fee parent without RBF; receiver-driven fee bump
KEY_PROPERTIES
ANCESTOR FEERATE
Miners select transactions by the highest ancestor feerate. A high-fee child can drag a low-fee parent into the next block.
ATOMIC
A transaction is either fully accepted into a block or fully rejected. There is no partial spend.
IMMUTABLE INPUTS
A UTXO can only ever be spent once. After that, it is permanently consumed.
NO BALANCES
Bitcoin tracks UTXOs, not balances. Your wallet computes a balance by summing the UTXOs it controls.
COMMON_PITFALLS
Things that catch people out
PITFALLS
- CPFP requires the child to spend an output you control — receivers can only CPFP-bump txs paying to them.
- address-reuse/">Address reuse degrades privacy — every reuse links more of your UTXOs together publicly. Modern wallets generate a fresh address per receive.
- Fee estimation matters: under-pay and your tx sits in the mempool for hours; over-pay and you tip the miner more than necessary. Use a fee estimator.
- "Change outputs" must go back to a fresh address you control. A missing change output sends the difference to the miner as fee — a known footgun.
RELATED_CONCEPTS
Other terms from Transaction — click any to read its page:
TERMINOLOGY_INDEX
TERMINOLOGY
CPFP Carve Out
A relay exception letting one extra small high-fee child evade mempool descendant limits, enabling reliable bumping.
Transaction (Tx)
A signed payload spending one or more UTXOs and creating new ones; every state change in Bitcoin is a tx.
Raw Transaction
The hex-serialized bytes of a transaction, ready to broadcast or analyze.
Transaction ID (TXID)
wTXID (Witness TXID)
HASH256 of the full transaction including witness data; commits to signatures and used in the witness commitment.
Input
A reference to a previous output being spent, plus the data (scriptSig/witness) authorizing the spend.
Output
An (amount, scriptPubKey) pair created by a transaction; spendable later by a tx whose input references it.
UTXO (Unspent Transaction Output)
An output that hasn't been spent yet; your "balance" is the sum of UTXOs you can sign for.