RBF + CPFP

See also ../../protocol/transactions/quick-ref/rbf-cpfp.md for protocol-level details. This SKILL.md focuses on wallet implementation.

Wallet-level RBF flow

1. Build initial tx with sequence 0xfffffffd on every input (the
   BIP125 opt-in signal, and the ecosystem-majority value — see
   "nSequence is a fingerprint" below). Under full-RBF policy this
   does not decide replaceability; it is an anti-fingerprinting
   default.
2. Sign and broadcast.
3. After N blocks of non-confirmation OR explicit user request:
   a. Estimate new fee rate.
   b. Build replacement: same outputs, possibly different change
      amount (smaller, since more goes to fee).
   c. Verify the replacement rules: pays more than the sum of the
      replaced fees, plus incrementalrelayfee × vsize.
   d. Sign and broadcast.
4. Old tx is dropped from mempool; new tx awaits confirmation.

Replacement rules (simplified)

Core's policy has drifted away from BIP125; the live specification is doc/policy/mempool-replacements.md, not the BIP. For replacement R of original set O (the directly conflicting txs plus their in-mempool descendants), as of Bitcoin Core 31.1 (July 2026) — numbered as in that doc: 3. R pays an absolute fee at least the sum paid by O. 4. R pays for its own bandwidth: the fee delta is at least incrementalrelayfee × R.vsize (the default dropped from 1 sat/vB to 0.1 sat/vB in Core 30.0, October 2025). 5. R conflicts with at most 100 distinct clusters. 6. R strictly improves the mempool's feerate diagram. For a singleton — a tx alone in its cluster — a higher fee and a higher feerate is sufficient.

Rules 1 and 2 are both marked "(Removed)" in that doc. Rule 1 was the opt-in signalling requirement; rule 2 was "no new unconfirmed inputs", dropped in Core 31.0 (April 2026), so a replacement may now spend unconfirmed outputs the original did not. Rule 6 was, before 31.0, the Core 0.12 constraint that R beat the feerate of every directly conflicting tx; the feerate-diagram form replaced it with cluster mempool.

Bitcoin Core 28.0 made full-RBF the default — even txs without opt-in signal (sequence ≥ 0xfffffffe) can be replaced. Core 29.0 then removed the -mempoolfullrbf opt-out entirely (PR #30592), so as of Bitcoin Core 31.1 (July 2026) a default-policy node replaces any transaction regardless of its nSequence values.

nSequence is a fingerprint, not a switch

Because default-policy nodes ignore the signal, the nSequence value a wallet picks no longer controls replaceability — it mainly identifies the wallet that built the transaction. rkrux opened Bitcoin Core #35405 in May 2026 to stop the Core wallet signalling by default, using MAX-1 (0xfffffffe, where MAX is 0xffffffff), and took the question — should wallets converge on a single value? — to bitcoin-dev in June 2026. Murch and Electrum contributor SomberNight objected: MAX-2 (0xfffffffd) is already the dominant value — roughly 75–78% of transactions as of June 2026 (mainnet-observer, as read by Optech #410 and by the #35405 thread), and nearly all Electrum transactions — so moving Core to a non-signalling MAX-1 would make Core txs stand out rather than blend in. rkrux closed the PR in light of that feedback; Core's wallet default is unchanged (DEFAULT_WALLET_RBF = true, emitting MAX_BIP125_RBF_SEQUENCE = 0xfffffffd) as of September 2026.

Practical rule: emit 0xfffffffd on every input unless you need nSequence for a BIP68 relative timelock. Choosing a distinctive value to "opt out" of replacement buys nothing under full-RBF policy and costs privacy.

Mempool topology limits (Core 31.0+)

Core 31.0 (April 2026) reimplemented the mempool as a cluster mempool. Ancestor and descendant count/size limits are no longer enforced. In their place, a cluster — any set of mempool transactions connected through parent/child relationships — is capped at 64 transactions and 101 kB of virtual size. Both limits are overridable from the command line (see -help-debug). Wallets that budgeted chain depth against the pre-31.0 defaults (25 ancestors, 25 descendants, 101 kvB each) must budget against the cluster instead.

Within the mempool, transactions are ordered by chunk feerate — the feerate of the set that would be mined together, e.g. a parent and its child. That ordering drives block template selection, eviction and relay announcement, and it is what makes CPFP work. New in 31.0: getmempoolcluster and getmempoolfeeratediagram; getmempoolentry now also reports chunk size and chunk fees.

The CPFP carveout — one extra child, at most 10 kvB with exactly one ancestor, allowed past the descendant limit — was removed in 31.0. Nothing bypasses the cluster count limit; TRUC plus sibling eviction is the replacement mechanism for those use cases.

Wallet UX for RBF

User-facing controls:

Slider: target confirmation time (10 min, 1 hour, next day).
"Bump fee" button on pending txs.
Display fee bump cost: extra fee + new total.
Warning: "Replacing this transaction will invalidate the previous one. Recipient may not see it."

Wallet-level CPFP flow

When sender's tx is stuck and CPFP is the only option (e.g., tx is non-RBF or is from someone else):

1. Identify a UTXO from the stuck tx that you control (your change
   output, or a received output).
2. Build a child tx spending that UTXO.
3. Compute child fee such that:
   package_fee_rate = (parent_fee + child_fee) / (parent_vsize + child_vsize)
   ≥ target_rate
4. Broadcast child via submitpackage (Core 26.0+, December 2023), or
   via sendrawtransaction and rely on opportunistic 1p1c relay
   (Core 28.0+).

Receiver-side CPFP

If you're the receiver of an unconfirmed payment that's stuck:

Option A: spend the received output to yourself with high fee. Now both txs in mempool; package CPFP includes parent.
Option B: wait. Sender may RBF on their side.

Package CPFP (`submitpackage`)

submitpackage RPC accepts a [parent, child] array atomically; it landed in Bitcoin Core 26.0 (December 2023). Before it, a child alone was rejected if the parent was below mempool min. Now the package effective rate is computed.

This is Core's own RPC, not an implementation of BIP331. BIP331 ("Ancestor Package Relay") is still Status: Draft as of September 2026 and its sendpackages / pkgtxns P2P messages do not exist in Core 31.1. What Core relays over P2P is a narrower, opportunistic 1-parent-1-child (1p1c) scheme carried on the existing transaction relay protocol: added in 28.0 (October 2024), and widened in 31.0 (April 2026) so the parent of a non-TRUC 1p1c package may also pay below -minrelaytxfee, even 0 fee.

bitcoin-cli submitpackage '["<parent_hex>","<child_hex>"]'

TRUC v3 (BIP431)

Strict version=3 transactions for Lightning-style use:

≤ 1 unconfirmed ancestor / descendant.
≤ 10 kvB tx size.
Sibling eviction: a v3 child can replace a sibling without paying for the sibling's bandwidth.

Combined with ephemeral anchors (value=0 anchor outputs that must be spent in the same package), gives non-pinnable Lightning fee bumping.

Lightning-specific fee bumping

Anchor commitment (BOLT-3 update): small value (330 sats) output spendable by either party for CPFP.
TRUC v3 + ephemeral anchor: zero-value anchor, must be spent in same package. Mitigates pinning.

LSP and self-custodial Lightning wallets implement these via:

LND: chantools transient-fee-bumper (legacy) or built-in fee bumping for anchor channels.
CLN: feebumper plugin or core support.
LDK: OnchainTxHandler orchestrates child txs.

Common bugs

Reusing the same input in both old and new tx but for different outputs → not a valid replacement (input set must overlap to conflict).
Assuming an RBF replacement may never add a new unconfirmed input → that was BIP125 rule 2, removed in Core 31.0 (April 2026). Default policy now allows it; only code targeting pre-31.0 nodes still needs to avoid it.
Building child that spends a parent output that's already fully spent in another descendant → rejected as conflict.
Computing fee rate against size instead of vsize → underpays for SegWit txs.
Setting nSequence to 0xffffffff or 0xfffffffe believing it makes the tx non-replaceable → it does not (full-RBF), and it marks the transaction as coming from a minority wallet.

bitcoin-rbf-cpfp