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Bitcoin Core’s transaction replacement rules decide whether a node will accept a new, conflicting unconfirmed transaction into its own mempool and relay it. They are local mempool policy, not Bitcoin consensus rules: nodes can use different settings, and acceptance or relay by one node does not guarantee acceptance elsewhere or confirmation.
What counts as a replacement?
Two transactions conflict when they spend one or more of the same inputs. A replacement can displace those transactions and their unconfirmed descendants from a node’s mempool, provided the replacement passes the node’s applicable checks. The node may then relay it, but the eventual block contents are determined by the miner that constructs and publishes the block.
That distinction matters for recipients: seeing one version first does not make it final. Bitcoin Core 24.0 release notes explain that the protocol cannot assure a node that the first version it sees will be the one confirmed. The notes advise merchants and services not to treat unconfirmed transactions as final (Bitcoin Core 24.0 release notes).
What a replacement must satisfy in current Bitcoin Core policy
The candidate must pass all other consensus and local policy checks as well as the replacement-specific conditions. The current Bitcoin Core policy documentation describes these principal requirements:
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- Cover the originals’ total fees. The replacement’s absolute fee must be at least the combined fees of the transactions it replaces. A higher fee rate alone is not enough if the replacement’s total fee falls short.
- Pay for the replacement’s relay bandwidth. It must add fees sufficient to cover its own bandwidth at or above the node’s configured incremental relay feerate. The documentation’s example uses 0.1 sat/vB and a 500-vbyte replacement, which requires at least 50 additional satoshis under those example conditions. This is an illustration, not a universal fixed threshold; the configured rate and transaction size matter.
- Stay within the conflict-cluster limit. The affected transactions may not span more than 100 distinct clusters. Bitcoin Core documents this cap as a way to limit the CPU work needed to update the mempool.
- Strictly improve the feerate diagram. The replacement must make the mempool’s feerate diagram strictly better. This is not simply a test of whether the replacement has a higher standalone feerate; the diagram informs transaction ordering for mining and eviction (replacement policy documentation; mempool design documentation).
These checks are applied by an individual node under its software and configuration. A transaction that qualifies under one node’s policy can still be rejected by another node, and passing policy does not guarantee that a miner will include it.
Does a transaction have to signal for replacement?
That depends on which concept is meant. BIP 125 defines an opt-in signaling scheme: 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. BIP 125 is therefore useful for identifying opt-in status, but signaling is no longer required by Bitcoin Core’s current default full-RBF policy (BIP 125; Bitcoin Core replacement policy).
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Historical defaults explain why older descriptions may say signaling is required. Bitcoin Core 0.12.0 introduced opt-in replacement. Version 24.0 added the mempoolfullrbf option, which allowed replacement without BIP 125 signaling but was off by default. Version 28.0 changed the default to mempoolfullrbf=1. For a specific node, check its version and configuration rather than assuming every installation has the same policy (Bitcoin Core 24.0 release notes; Bitcoin Core 28.0 release notes).
Opt-in RBF and full-RBF compared
| Approach | What it means | Signaling required? |
|---|---|---|
| Opt-in RBF (BIP 125) | Replacement eligibility is indicated explicitly by an input sequence value or inherited from an unconfirmed signaling ancestor. | Yes, for the BIP 125 opt-in status. |
| Bitcoin Core full-RBF default, since 28.0 | The node’s default policy can consider conflicting transactions for replacement without requiring BIP 125 signaling, subject to replacement and other policy checks. | No. |
This comparison describes Bitcoin Core’s documented behavior, not a network-wide guarantee. Nodes outside Bitcoin Core, older versions, or differently configured nodes may behave differently.
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Policy is not consensus
Consensus rules determine whether transactions in a block are valid under the protocol. Mempool policy governs what a particular node accepts into its unconfirmed-transaction pool and may relay. Replacement is in the latter category. A node’s mempool decision does not bind other nodes or miners, and policy acceptance does not itself make a transaction valid for a block if it violates consensus rules.
There is no adoption percentage established here for how many nodes enforce each replacement policy. The documented limits and fee conditions are Bitcoin Core policy details, not measurements of network-wide behavior.
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