Bitcoin Core lets a node replace certain unconfirmed transactions in its local mempool when the replacement meets that node’s fee and policy checks. This is a mempool policy—not a consensus rule—so acceptance by one node does not guarantee relay by other nodes or confirmation. Bitcoin Core’s current default policy does not require the original transaction to signal replaceability under BIP 125.
What transaction replacement means
A transaction conflicts with another when both spend at least one of the same inputs. If a node accepts a replacement, it can remove the conflicting transaction or transactions from its mempool, along with their unconfirmed descendants. The replacement is still subject to the node’s other validation rules.
These decisions concern unconfirmed transactions and a node’s local mempool. Consensus rules determine whether a transaction in a block is valid; mempool policy determines what a node is willing to accept or relay before confirmation. Nodes can use different software versions or settings, so one node’s decision is not a network-wide guarantee. See Bitcoin’s developer guide and Bitcoin Core’s current replacement policy documentation.
What a replacement must satisfy in current Bitcoin Core
Passing the replacement checks is not enough by itself: the candidate must also satisfy all other applicable consensus and local policy rules. Bitcoin Core’s current policy documentation describes these additional replacement conditions:
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- Cover the total fees of the transactions being replaced. The replacement’s absolute fee must be at least the sum of the fees paid by the transactions it evicts. A higher feerate alone does not satisfy this requirement.
- Pay for the replacement’s relay bandwidth. The replacement must add enough fee to cover its own bandwidth at the node’s configured incremental relay feerate. This is distinct from
-minrelaytxfee. The policy documentation illustrates the calculation with 0.1 sat/vB and a 500-vbyte replacement: that example calls for at least 50 additional satoshis. It is an example, not a universal fixed threshold; the setting and transaction size matter. - Stay within the conflict-cluster limit. The conflicts must span no more than 100 distinct clusters. The limit is intended to constrain the CPU work needed to update the mempool.
- Strictly improve the mempool feerate diagram. The replacement must make this diagram better under the policy’s comparison. The diagram informs transaction selection for mining and mempool eviction, so this check is more involved than simply comparing the candidate’s standalone feerate with an original’s.
The rules and the bandwidth example are documented in Bitcoin Core’s replacement policy page. The role of the feerate diagram is explained in the mempool design documentation.
What BIP 125 signaling means—and what it does not
BIP 125 describes opt-in replaceability. A transaction explicitly signals under BIP 125 if any input has an nSequence value below 0xffffffff−1. A transaction can also inherit replaceability while it has an unconfirmed ancestor that signals. This status describes signaling; it should not be confused with the acceptance rule used by current Bitcoin Core defaults. Read BIP 125 for the opt-in framework.
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Current Bitcoin Core default full-RBF policy does not require the conflicting transaction to signal. Bitcoin Core’s policy documentation says signaling is no longer required, while its implementation continues to expose opt-in status where it can inspect a transaction and its ancestors. Whether signaling is required on a particular node depends on its software version and configuration.
How Bitcoin Core’s defaults changed
| Bitcoin Core version | Replacement behavior described in the cited sources |
|---|---|
| 0.12.0 | Introduced opt-in replacement, with signaling and fee conditions enforced. |
| 24.0 | Added the mempoolfullrbf option, allowing replacement without BIP 125 signaling; it was off by default in this release. |
| 28.0 | Changed the default to mempoolfullrbf=1, enabling full-RBF by default. |
These version milestones are covered in the current policy documentation, Bitcoin Core 24.0 release notes, and Bitcoin Core 28.0 release notes. Check the version and settings of the node in question rather than assuming every Bitcoin Core node has identical behavior.
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What this means for recipients of unconfirmed payments
Seeing one version of a transaction first does not make it final. The Bitcoin Core 24.0 release notes state: “The Bitcoin Protocol does not, and cannot, provide any assurance that the first version of an unconfirmed transaction seen by a particular node will be the version that gets confirmed.” If multiple versions are available, the miner that includes a transaction in a block determines which version confirms. The release notes therefore advise merchants and services not to treat unconfirmed transactions as final.
BIP 125 also describes recipient precautions for opt-in transactions, including waiting for confirmation or treating an unconfirmed opt-in payment with added suspicion. More broadly, a transaction’s signaling status or the first version a recipient sees cannot establish that a payment will confirm unchanged. Confirmation is the relevant milestone for reducing that uncertainty.
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