Bitcoin Core’s replacement rules decide whether a node will accept a new unconfirmed transaction that conflicts with one already in its mempool. They are local mempool policy—not consensus rules—and today’s Bitcoin Core default permits replacement without the original transaction signaling for it. A replacement still has to meet several fee and mempool conditions, and no node’s decision guarantees that a miner will confirm it.
What transaction replacement means
A transaction conflict occurs when two transactions spend one or more of the same inputs. A node that accepts a replacement may remove the conflicting transaction and its in-mempool descendants, then consider the new transaction under its replacement policy.
These are mempool rules: they govern whether a node accepts or relays an unconfirmed transaction. Consensus rules determine whether transactions in a block are valid. Nodes can use different software versions or settings, so acceptance or relay by one node does not guarantee acceptance by every node—or confirmation by a miner. See Bitcoin Core’s opt-in RBF documentation and the current Bitcoin Core replacement policy.
What a replacement must satisfy in current Bitcoin Core
Passing the replacement checks is not enough by itself: the candidate must also pass all other consensus and local policy checks. Bitcoin Core’s current policy documentation describes these replacement-specific requirements:
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- Pay at least the total fees of the transactions it replaces. The comparison is in absolute fees, not just fee rate. A higher fee rate alone does not satisfy this condition if the candidate’s total fee is too low.
- Add enough fee to cover its own relay bandwidth. The replacement must pay an increment at least equal to its size multiplied by the node’s incremental relay feerate. This setting is distinct from
-minrelaytxfee. The documentation’s example uses an incremental relay feerate of 0.1 sat/vB and a 500-vbyte replacement, requiring 50 satoshis in additional fee. That is an illustration, not a fixed threshold for every node or transaction. - Stay within the conflict-cluster limit. The conflicting transactions cannot span more than 100 distinct clusters. The limit helps constrain the CPU work required to update the mempool.
- Strictly improve the mempool feerate diagram. This diagram informs transaction selection for mining and mempool eviction ordering. The check is more involved than simply asking whether the candidate has a higher standalone fee rate than the transaction it conflicts with.
The absolute-fee and bandwidth-increment requirements address different concerns: the first ensures the replacement pays at least as much in total as the transactions being removed; the second compensates for relaying the replacement itself. Meeting both does not bypass the cluster or feerate-diagram tests.
How BIP 125 signaling fits in
BIP 125 defines an opt-in framework. Under its signaling convention, a transaction explicitly signals replaceability if any input has an nSequence value below 0xffffffff−1. A transaction can also inherit replaceability while it has an unconfirmed ancestor that signals. The BIP 125 specification describes this signaling model and discusses precautions for recipients of opt-in, unconfirmed payments.
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Signaling status and replacement acceptance are not the same thing. Current Bitcoin Core default full-RBF policy does not require BIP 125 signaling before accepting a replacement. The implementation can still expose opt-in status when it can inspect the transaction and its ancestors; that status does not establish that every node will treat the transaction identically.
How Bitcoin Core’s defaults changed
| Bitcoin Core version | Replacement behavior described in the release or policy documentation |
|---|---|
| 0.12.0 | Introduced opt-in replacement, which required signaling under the original approach. |
| 24.0 | Added the mempoolfullrbf option so nodes could accept replacements without BIP 125 signaling; it was off by default in this release. |
| 28.0 | Changed the mempoolfullrbf default to 1. |
| Current Bitcoin Core policy documentation | States that signaling is no longer required for replacement. |
The version matters when describing a particular node: older releases and customized settings may behave differently from the current default. See the Bitcoin Core 24.0 release notes and Bitcoin Core 28.0 release notes.
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What this means for an unconfirmed payment
Seeing a transaction first is not a guarantee that it will be the version ultimately confirmed. Bitcoin Core’s 24.0 release notes explain that when multiple versions are available, the miner that includes a transaction in a block determines which version confirms. The notes advise merchants and services not to treat unconfirmed transactions as final. A recipient should therefore account for replacement and confirmation risk rather than relying only on first-seen status or an RBF label.
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