diff --git a/Cargo.toml b/Cargo.toml
index 7a976bc9b..c0e3cbe77 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -86,7 +86,7 @@ tower-http = { version = "0.6.10", features = ["catch-panic", "compression-br",
 tower-layer = "0.3"
 tracing = { version = "0.1", default-features = false, features = ["std"] }
 ureq = { version = "3.3.0", features = ["json"] }
-vecdb = { version = "=0.10.3", features = ["derive", "serde_json", "pco", "schemars"] }
+vecdb = { version = "0.10.3", features = ["derive", "serde_json", "pco", "schemars"] }
 # vecdb = { path = "../anydb/crates/vecdb", features = ["derive", "serde_json", "pco", "schemars"] }
 
 [workspace.metadata.release]
diff --git a/crates/brk_mempool/src/chunking.rs b/crates/brk_mempool/src/chunking.rs
new file mode 100644
index 000000000..3ca28c029
--- /dev/null
+++ b/crates/brk_mempool/src/chunking.rs
@@ -0,0 +1,89 @@
+//! Cluster mempool linearization (Core 31's "Single Fee Linearization").
+//!
+//! Given a topologically ordered cluster (parents before children) with
+//! per-tx `(fee, vsize)` and parent edges as local indices, partition the
+//! cluster into chunks ordered by descending feerate, where each chunk is
+//! the highest-rate ancestor-closed set of remaining txs.
+//!
+//! The "lift" merging this implements is what makes CPFP visible at the
+//! cluster level: a child whose rate exceeds its parent's rate gets folded
+//! into a chunk with the parent, and the chunk's rate is the combined
+//! `(parent_fee + child_fee) / (parent_vsize + child_vsize)`. Cascades
+//! upward through any further parents until rates are non-increasing.
+//!
+//! This is the proxy-fallback case; under Core 31+ each tx's `fees.chunk`
+//! / `chunkweight` already encodes the chunked rate, so all members of a
+//! chunk would share that rate. Computing locally from `(fee, vsize)`
+//! gives the same answer either way and works on older Core too.
+//!
+//! Complexity is `O(n^2)` per linearization (n bounded by cluster cap),
+//! matching mempool.space's frontend implementation.
+
+use brk_types::{CpfpClusterChunk, CpfpClusterTxIndex, FeeRate, Sats, VSize};
+use rustc_hash::{FxBuildHasher, FxHashSet};
+
+/// One cluster member: its `(fee, vsize)` and parent edges as
+/// local indices into the same array.
+pub struct ChunkInput<'a> {
+    pub fee: Sats,
+    pub vsize: VSize,
+    pub parents: &'a [CpfpClusterTxIndex],
+}
+
+/// Linearize `items` into chunks. `items` must be in topological order
+/// (parents before children); `parents` indices must point earlier in
+/// the slice. Returns chunks sorted by descending feerate, with each
+/// chunk's `txs` listed in the input topological order.
+pub fn linearize(items: &[ChunkInput<'_>]) -> Vec<CpfpClusterChunk> {
+    let n = items.len();
+    if n == 0 {
+        return Vec::new();
+    }
+    let mut remaining: Vec<bool> = vec![true; n];
+    let mut chunks: Vec<CpfpClusterChunk> = Vec::new();
+
+    while remaining.iter().any(|&r| r) {
+        let mut best: Option<(FeeRate, FxHashSet<u32>)> = None;
+        for i in 0..n {
+            if !remaining[i] {
+                continue;
+            }
+            let mut anc: FxHashSet<u32> =
+                FxHashSet::with_capacity_and_hasher(8, FxBuildHasher);
+            let mut stack: Vec<u32> = vec![i as u32];
+            while let Some(x) = stack.pop() {
+                if !anc.insert(x) {
+                    continue;
+                }
+                for &p in items[x as usize].parents {
+                    let pu: u32 = u32::from(p);
+                    if remaining[pu as usize] && !anc.contains(&pu) {
+                        stack.push(pu);
+                    }
+                }
+            }
+            let mut fee = Sats::ZERO;
+            let mut vsize = VSize::from(0u64);
+            for &x in &anc {
+                fee += items[x as usize].fee;
+                vsize += items[x as usize].vsize;
+            }
+            let rate = FeeRate::from((fee, vsize));
+            match &best {
+                Some((br, _)) if *br >= rate => {}
+                _ => best = Some((rate, anc)),
+            }
+        }
+
+        let (rate, set) = best.expect("at least one remaining tx");
+        let mut indices: Vec<u32> = set.into_iter().collect();
+        indices.sort_unstable();
+        for &x in &indices {
+            remaining[x as usize] = false;
+        }
+        let txs: Vec<CpfpClusterTxIndex> =
+            indices.into_iter().map(CpfpClusterTxIndex::from).collect();
+        chunks.push(CpfpClusterChunk { txs, feerate: rate });
+    }
+    chunks
+}
diff --git a/crates/brk_mempool/src/cpfp.rs b/crates/brk_mempool/src/cpfp.rs
index a681e9b87..f458add19 100644
--- a/crates/brk_mempool/src/cpfp.rs
+++ b/crates/brk_mempool/src/cpfp.rs
@@ -1,11 +1,25 @@
 //! CPFP (Child Pays For Parent) walk over a `Snapshot`'s adjacency.
 //!
 //! The snapshot stores per-tx parent/child edges in `TxIndex` space and
-//! a per-tx `chunk_rate` (Core's `fees.chunk` / `chunkweight` truth, or
-//! the proxy fallback). The walk is a pair of capped DFSes, then the
-//! cluster wire shape is materialized from the visited set.
+//! per-tx `(fee, vsize)` we need for chunking.
+//!
+//! Three independent walks:
+//! - `ancestors_idx`: capped DFS up `parents` only.
+//! - `descendants_idx`: capped DFS down `children` only.
+//! - cluster `members`: capped DFS over `parents ∪ children`, i.e. the
+//!   connected component of the seed in the in-mempool dependency
+//!   graph. Required to match Core 31's cluster mempool semantics:
+//!   siblings (sharing a parent) and cousins (sharing a descendant)
+//!   belong to the same cluster but are missed by ancestor/descendant
+//!   walks alone.
+//!
+//! The cluster is then linearized via `brk_types::linearize` (single fee
+//! linearization) so chunks reflect Core's CPFP "lift": a child whose
+//! rate exceeds its parent's gets folded into a chunk with the parent
+//! at the combined feerate. The seed's chunk feerate is what
+//! `effective_fee_per_vsize` reports.
 
-use std::cmp::Reverse;
+use std::collections::VecDeque;
 
 use brk_types::{
     CpfpCluster, CpfpClusterChunk, CpfpClusterTx, CpfpClusterTxIndex, CpfpEntry, CpfpInfo, FeeRate,
@@ -13,13 +27,22 @@ use brk_types::{
 };
 use rustc_hash::{FxBuildHasher, FxHashMap, FxHashSet};
 
-use crate::Mempool;
-use crate::steps::{SnapTx, TxIndex};
+use crate::{
+    Mempool,
+    chunking::{ChunkInput, linearize},
+    steps::{SnapTx, TxIndex},
+};
 
 /// Cap matches Bitcoin Core's default mempool ancestor/descendant
 /// chain limits and mempool.space's truncation.
 const MAX: usize = 25;
 
+/// Cluster cap matches Bitcoin Core 31's `MAX_CLUSTER_COUNT_LIMIT`
+/// (max txs in a single cluster-mempool cluster). Sized large enough
+/// to hold the whole connected component for any policy-conformant
+/// cluster, then truncated.
+const MAX_CLUSTER: usize = 64;
+
 impl Mempool {
     /// CPFP info for a live mempool tx. Returns `None` only when the
     /// tx isn't in the mempool, so callers can fall through to the
@@ -46,30 +69,21 @@ fn build_cpfp_info(
     seed: &SnapTx,
     sigops: SigOps,
 ) -> CpfpInfo {
-    let ancestors_idx = walk(txs, seed_idx, |t| &t.parents);
-    let descendants_idx = walk(txs, seed_idx, |t| &t.children);
-
-    let ancestors: Vec<CpfpEntry> = ancestors_idx
-        .iter()
-        .filter_map(|&i| txs.get(i.as_usize()).map(CpfpEntry::from))
-        .collect();
-    let descendants: Vec<CpfpEntry> = descendants_idx
-        .iter()
-        .filter_map(|&i| txs.get(i.as_usize()).map(CpfpEntry::from))
-        .collect();
+    let ancestors = collect_entries(txs, seed_idx, |t| &t.parents);
+    let descendants = collect_entries(txs, seed_idx, |t| &t.children);
     let best_descendant = descendants
         .iter()
         .max_by_key(|e| FeeRate::from((e.fee, e.weight)))
         .cloned();
 
-    let cluster = build_cluster(txs, seed_idx, &ancestors_idx, &descendants_idx);
+    let (cluster, effective_fee_per_vsize) = build_cluster(txs, seed_idx, seed);
     let vsize = VSize::from(seed.weight);
 
     CpfpInfo {
         ancestors,
         best_descendant,
         descendants,
-        effective_fee_per_vsize: seed.chunk_rate,
+        effective_fee_per_vsize,
         sigops,
         fee: seed.fee,
         vsize,
@@ -78,17 +92,30 @@ fn build_cpfp_info(
     }
 }
 
+/// Walk the graph from `seed` along `next` and lift the visited indices
+/// into wire-shape `CpfpEntry`s in one go.
+fn collect_entries(
+    txs: &[SnapTx],
+    seed: TxIndex,
+    next: impl Fn(&SnapTx) -> &[TxIndex],
+) -> Vec<CpfpEntry> {
+    walk(txs, seed, next)
+        .iter()
+        .filter_map(|&i| txs.get(i.as_usize()).map(CpfpEntry::from))
+        .collect()
+}
+
 /// Capped DFS from `seed` (exclusive), following the neighbors yielded
 /// by `next`. Used for both the ancestor and descendant walks.
 fn walk(txs: &[SnapTx], seed: TxIndex, next: impl Fn(&SnapTx) -> &[TxIndex]) -> Vec<TxIndex> {
+    let Some(seed_node) = txs.get(seed.as_usize()) else {
+        return Vec::new();
+    };
     let mut visited: FxHashSet<TxIndex> =
         FxHashSet::with_capacity_and_hasher(MAX + 1, FxBuildHasher);
     visited.insert(seed);
     let mut out: Vec<TxIndex> = Vec::with_capacity(MAX);
-    let mut stack: Vec<TxIndex> = txs
-        .get(seed.as_usize())
-        .map(|t| next(t).to_vec())
-        .unwrap_or_default();
+    let mut stack: Vec<TxIndex> = next(seed_node).to_vec();
     while let Some(idx) = stack.pop() {
         if out.len() >= MAX {
             break;
@@ -104,84 +131,175 @@ fn walk(txs: &[SnapTx], seed: TxIndex, next: impl Fn(&SnapTx) -> &[TxIndex]) ->
     out
 }
 
-/// Wire-shape `CpfpCluster`. Members are emitted in `[ancestors..., seed,
-/// descendants...]` order so the seed's index inside the cluster is
-/// `ancestors.len()`. Chunks group txs by exact `chunk_rate` value: under
-/// Core 31 this matches Core's actual chunks; under proxy fallback it
-/// produces a fine-grained but consistent breakdown.
+/// Wire-shape `CpfpCluster` plus the seed's chunk feerate. Members are
+/// the connected component of the seed in the dependency graph, then
+/// topologically sorted (parents before children) so wire indices and
+/// chunk-internal ordering are valid for client-side reconstruction.
+/// Returns `(None, seed_per_tx_rate)` for singletons (matches
+/// mempool.space, which omits `cluster` when no relations exist).
 fn build_cluster(
     txs: &[SnapTx],
     seed_idx: TxIndex,
-    ancestors: &[TxIndex],
-    descendants: &[TxIndex],
-) -> CpfpCluster {
-    let members: Vec<TxIndex> = ancestors
-        .iter()
-        .copied()
-        .chain(std::iter::once(seed_idx))
-        .chain(descendants.iter().copied())
-        .collect();
+    seed: &SnapTx,
+) -> (Option<CpfpCluster>, FeeRate) {
+    let seed_per_tx_rate = FeeRate::from((seed.fee, seed.vsize));
+    let component = walk_cluster(txs, seed_idx);
+    if component.len() <= 1 {
+        return (None, seed_per_tx_rate);
+    }
 
-    let local_of: FxHashMap<TxIndex, CpfpClusterTxIndex> = members
+    let members = topo_sort(txs, &component);
+    let local_of = build_local_index(&members);
+    let (cluster_txs, vsizes) = collect_cluster_members(txs, &members, &local_of);
+    let chunks = linearize_cluster(&cluster_txs, &vsizes);
+    let (chunk_index, seed_chunk_rate) =
+        locate_seed_chunk(local_of[&seed_idx], &chunks, seed_per_tx_rate);
+
+    (
+        Some(CpfpCluster {
+            txs: cluster_txs,
+            chunks,
+            chunk_index,
+        }),
+        seed_chunk_rate,
+    )
+}
+
+/// `members[i]`'s wire index, keyed by snapshot `TxIndex`. Built once
+/// so per-tx parent edges can be remapped without a linear scan.
+fn build_local_index(members: &[TxIndex]) -> FxHashMap<TxIndex, CpfpClusterTxIndex> {
+    members
         .iter()
         .enumerate()
         .map(|(i, &idx)| (idx, CpfpClusterTxIndex::from(i as u32)))
-        .collect();
+        .collect()
+}
 
-    let cluster_txs: Vec<CpfpClusterTx> = members
+/// Materialize wire-shape `CpfpClusterTx`s for every member with parent
+/// edges remapped to local indices, plus the parallel `vsize` column the
+/// linearizer needs (not carried on `CpfpClusterTx`, which only stores
+/// weight).
+fn collect_cluster_members(
+    txs: &[SnapTx],
+    members: &[TxIndex],
+    local_of: &FxHashMap<TxIndex, CpfpClusterTxIndex>,
+) -> (Vec<CpfpClusterTx>, Vec<VSize>) {
+    let mut cluster_txs: Vec<CpfpClusterTx> = Vec::with_capacity(members.len());
+    let mut vsizes: Vec<VSize> = Vec::with_capacity(members.len());
+    for &idx in members {
+        let Some(t) = txs.get(idx.as_usize()) else {
+            continue;
+        };
+        let parents: Vec<CpfpClusterTxIndex> = t
+            .parents
+            .iter()
+            .filter_map(|p| local_of.get(p).copied())
+            .collect();
+        cluster_txs.push(CpfpClusterTx {
+            txid: t.txid,
+            weight: t.weight,
+            fee: t.fee,
+            parents,
+        });
+        vsizes.push(t.vsize);
+    }
+    (cluster_txs, vsizes)
+}
+
+/// Single-fee-linearize the cluster, borrowing parents from the
+/// already-built `cluster_txs` so no re-allocation is needed.
+fn linearize_cluster(cluster_txs: &[CpfpClusterTx], vsizes: &[VSize]) -> Vec<CpfpClusterChunk> {
+    let inputs: Vec<ChunkInput<'_>> = cluster_txs
         .iter()
-        .filter_map(|&idx| {
-            let t = txs.get(idx.as_usize())?;
-            Some(CpfpClusterTx {
-                txid: t.txid,
-                weight: t.weight,
-                fee: t.fee,
-                parents: t
-                    .parents
-                    .iter()
-                    .filter_map(|p| local_of.get(p).copied())
-                    .collect(),
-            })
+        .zip(vsizes)
+        .map(|(c, &vsize)| ChunkInput {
+            fee: c.fee,
+            vsize,
+            parents: &c.parents,
         })
         .collect();
-
-    let chunks = chunk_groups(&members, txs, &local_of);
-    let seed_local = local_of[&seed_idx];
-    let chunk_index = chunks
-        .iter()
-        .position(|ch| ch.txs.contains(&seed_local))
-        .unwrap_or(0) as u32;
-
-    CpfpCluster {
-        txs: cluster_txs,
-        chunks,
-        chunk_index,
-    }
+    linearize(&inputs)
 }
 
-/// Group cluster members into chunks by descending `chunk_rate`. Cluster
-/// size is bounded by `2 * MAX + 1` so a sort-then-fold is cheaper and
-/// simpler than a hashmap keyed on `f64` bits.
-fn chunk_groups(
-    members: &[TxIndex],
-    txs: &[SnapTx],
-    local_of: &FxHashMap<TxIndex, CpfpClusterTxIndex>,
-) -> Vec<CpfpClusterChunk> {
-    let mut entries: Vec<(FeeRate, CpfpClusterTxIndex)> = members
+/// Find the chunk containing the seed and return its index plus rate.
+/// Falls back to `(0, seed_per_tx_rate)` when the seed isn't in any
+/// chunk - shouldn't happen but keeps the wire shape valid.
+fn locate_seed_chunk(
+    seed_local: CpfpClusterTxIndex,
+    chunks: &[CpfpClusterChunk],
+    seed_per_tx_rate: FeeRate,
+) -> (u32, FeeRate) {
+    chunks
         .iter()
-        .filter_map(|&idx| Some((txs.get(idx.as_usize())?.chunk_rate, local_of[&idx])))
-        .collect();
-    entries.sort_by_key(|e| Reverse(e.0));
+        .enumerate()
+        .find(|(_, ch)| ch.txs.contains(&seed_local))
+        .map(|(i, ch)| (i as u32, ch.feerate))
+        .unwrap_or((0, seed_per_tx_rate))
+}
 
-    let mut chunks: Vec<CpfpClusterChunk> = Vec::new();
-    for (rate, local) in entries {
-        match chunks.last_mut() {
-            Some(last) if last.feerate == rate => last.txs.push(local),
-            _ => chunks.push(CpfpClusterChunk {
-                txs: vec![local],
-                feerate: rate,
-            }),
+/// Capped DFS over the undirected dependency graph (`parents ∪
+/// children`) starting from `seed`. Returns the connected component
+/// truncated to `MAX_CLUSTER`, with `seed` at index 0.
+fn walk_cluster(txs: &[SnapTx], seed: TxIndex) -> Vec<TxIndex> {
+    if txs.get(seed.as_usize()).is_none() {
+        return Vec::new();
+    }
+    let mut visited: FxHashSet<TxIndex> =
+        FxHashSet::with_capacity_and_hasher(MAX_CLUSTER, FxBuildHasher);
+    visited.insert(seed);
+    let mut out: Vec<TxIndex> = Vec::with_capacity(MAX_CLUSTER);
+    out.push(seed);
+    let mut stack: Vec<TxIndex> = vec![seed];
+    while let Some(idx) = stack.pop() {
+        let Some(t) = txs.get(idx.as_usize()) else {
+            continue;
+        };
+        for &n in t.parents.iter().chain(t.children.iter()) {
+            if out.len() >= MAX_CLUSTER {
+                return out;
+            }
+            if visited.insert(n) {
+                out.push(n);
+                stack.push(n);
+            }
         }
     }
-    chunks
+    out
+}
+
+/// Kahn's topological sort over the connected component, restricted to
+/// in-cluster parent edges. Returns members in an order where every tx
+/// follows all its in-cluster parents.
+fn topo_sort(txs: &[SnapTx], component: &[TxIndex]) -> Vec<TxIndex> {
+    let n = component.len();
+    let pos: FxHashMap<TxIndex, usize> = component
+        .iter()
+        .enumerate()
+        .map(|(i, &x)| (x, i))
+        .collect();
+    let mut indeg: Vec<u32> = vec![0; n];
+    let mut children: Vec<Vec<usize>> = vec![Vec::new(); n];
+    for (i, &idx) in component.iter().enumerate() {
+        let Some(t) = txs.get(idx.as_usize()) else {
+            continue;
+        };
+        indeg[i] = t.parents.iter().filter(|p| pos.contains_key(p)).count() as u32;
+        for &c in t.children.iter() {
+            if let Some(&ci) = pos.get(&c) {
+                children[i].push(ci);
+            }
+        }
+    }
+    let mut queue: VecDeque<usize> = (0..n).filter(|&i| indeg[i] == 0).collect();
+    let mut out: Vec<TxIndex> = Vec::with_capacity(n);
+    while let Some(i) = queue.pop_front() {
+        out.push(component[i]);
+        for &c in &children[i] {
+            indeg[c] -= 1;
+            if indeg[c] == 0 {
+                queue.push_back(c);
+            }
+        }
+    }
+    out
 }
diff --git a/crates/brk_mempool/src/lib.rs b/crates/brk_mempool/src/lib.rs
index 0bc21857e..7d94eeda8 100644
--- a/crates/brk_mempool/src/lib.rs
+++ b/crates/brk_mempool/src/lib.rs
@@ -11,14 +11,15 @@
 //!    `TxsPulled { added, removed }`. Pure CPU.
 //! 3. [`steps::applier::Applier`] - apply the diff to
 //!    [`state::State`] under a single write lock.
-//! 4. [`prevouts::fill`] - fills `prevout: None` inputs in one pass,
-//!    using same-cycle in-mempool parents directly and the
+//! 4. [`steps::Prevouts::fill`] - fills `prevout: None` inputs in one
+//!    pass, using same-cycle in-mempool parents directly and the
 //!    caller-supplied resolver (default: `getrawtransaction`) for
 //!    confirmed parents.
 //! 5. [`steps::rebuilder::Rebuilder`] - throttled rebuild of the
 //!    projected-blocks `Snapshot` from the same-cycle GBT and min fee.
 
 use std::{
+    any::Any,
     cmp::Reverse,
     panic::{AssertUnwindSafe, catch_unwind},
     sync::Arc,
@@ -35,17 +36,18 @@ use brk_types::{
 use parking_lot::{RwLock, RwLockReadGuard};
 use tracing::error;
 
+pub mod chunking;
 mod cpfp;
 mod diagnostics;
-mod prevouts;
 mod rbf;
 mod state;
 pub(crate) mod steps;
 pub(crate) mod stores;
 
+pub use chunking::{ChunkInput, linearize};
 pub use diagnostics::MempoolStats;
 pub use rbf::{RbfForTx, RbfNode};
-use steps::{Applier, Fetched, Fetcher, Preparer, Rebuilder};
+use steps::{Applier, Fetched, Fetcher, Preparer, Prevouts, Rebuilder};
 pub use steps::{BlockStats, RecommendedFees, Snapshot, TxEntry, TxRemoval};
 pub use stores::{TxGraveyard, TxStore, TxTombstone};
 
@@ -166,23 +168,19 @@ impl Mempool {
         let Some(entry) = state.addrs.get(addr) else {
             return vec![];
         };
-        let mut ordered: Vec<(Timestamp, &Txid)> = entry
+        let mut ordered: Vec<(Timestamp, &Transaction)> = entry
             .txids
             .iter()
-            .map(|txid| {
-                let first_seen = state
-                    .txs
-                    .entry(txid)
-                    .map(|e| e.first_seen)
-                    .unwrap_or_default();
-                (first_seen, txid)
+            .filter_map(|txid| {
+                let record = state.txs.record_by_prefix(&TxidPrefix::from(txid))?;
+                Some((record.entry.first_seen, &record.tx))
             })
             .collect();
         ordered.sort_unstable_by_key(|b| Reverse(b.0));
         ordered
             .into_iter()
-            .filter_map(|(_, txid)| state.txs.get(txid).cloned())
             .take(limit)
+            .map(|(_, tx)| tx.clone())
             .collect()
     }
 
@@ -238,7 +236,7 @@ impl Mempool {
     /// confirmed-parent prevouts via the default `getrawtransaction`
     /// resolver; requires bitcoind started with `txindex=1`.
     pub fn start(&self) {
-        self.start_with(prevouts::rpc_resolver(self.0.client.clone()));
+        self.start_with(Prevouts::rpc_resolver(self.0.client.clone()));
     }
 
     /// Variant of `start` that uses a caller-supplied resolver for
@@ -256,14 +254,7 @@ impl Mempool {
                 }
             }));
             if let Err(payload) = outcome {
-                let msg = if let Some(s) = payload.downcast_ref::<&'static str>() {
-                    (*s).to_string()
-                } else if let Some(s) = payload.downcast_ref::<String>() {
-                    s.clone()
-                } else {
-                    "<non-string panic payload>".to_string()
-                };
-                error!("mempool update panicked, continuing loop: {msg}");
+                error!("mempool update panicked, continuing loop: {}", panic_msg(&payload));
             }
             thread::sleep(Duration::from_secs(1));
         }
@@ -273,7 +264,7 @@ impl Mempool {
     /// `update_with(rpc_resolver)`. Standalone consumers (Core +
     /// `txindex=1`) get a one-line driver loop.
     pub fn update(&self) -> Result<()> {
-        self.update_with(prevouts::rpc_resolver(self.0.client.clone()))
+        self.update_with(Prevouts::rpc_resolver(self.0.client.clone()))
     }
 
     /// One sync cycle: fetch, prepare, apply, fill prevouts, maybe
@@ -301,9 +292,17 @@ impl Mempool {
         };
         let pulled = Preparer::prepare(entries_info, new_raws, state);
         let changed = Applier::apply(state, pulled);
-        prevouts::fill(state, resolver);
+        Prevouts::fill(state, resolver);
         rebuilder.tick(state, changed, &gbt, min_fee);
 
         Ok(())
     }
 }
+
+fn panic_msg(payload: &(dyn Any + Send)) -> &str {
+    payload
+        .downcast_ref::<&'static str>()
+        .copied()
+        .or_else(|| payload.downcast_ref::<String>().map(String::as_str))
+        .unwrap_or("<non-string panic payload>")
+}
diff --git a/crates/brk_mempool/src/prevouts.rs b/crates/brk_mempool/src/prevouts.rs
deleted file mode 100644
index 6c0b53444..000000000
--- a/crates/brk_mempool/src/prevouts.rs
+++ /dev/null
@@ -1,145 +0,0 @@
-//! Prevout fill plumbing.
-//!
-//! A fresh tx can land in the store with `prevout: None` on some
-//! inputs when the Preparer can't see the parent (parent arrived in
-//! the same cycle as the child, or parent is confirmed and we don't
-//! have an indexer hooked up). [`fill`] runs after each successful
-//! `Applier::apply` and closes both gaps in one pass:
-//!
-//! 1. Snapshot under a read guard, walking `txs.unresolved()` once.
-//!    For each hole, if the parent is also in the live pool we record
-//!    a fill directly (cheap, lock-local). Otherwise we record the
-//!    hole for external resolution.
-//! 2. Drop the read guard. Call `resolver` on the remaining holes
-//!    (typically `getrawtransaction` or an indexer lookup); failures
-//!    are simply skipped and retried next cycle.
-//! 3. Take the write guard once and fold both fill batches into the
-//!    `TxStore` via `apply_fills` -> `add_input`. Idempotent: each
-//!    fill checks `prevout.is_none()` and bails if the tx was already
-//!    removed or filled between phases.
-
-use std::sync::atomic::{AtomicBool, Ordering};
-
-use brk_rpc::Client;
-use brk_types::{TxOut, Txid, TxidPrefix, Vin, Vout};
-use parking_lot::RwLock;
-use tracing::warn;
-
-use crate::{State, stores::TxStore};
-
-/// Default resolver: per-call `getrawtransaction` against the bitcoind
-/// RPC client `Mempool` already holds. Requires `txindex=1`. On any
-/// failure logs once with a hint, then returns `None`; the next cycle
-/// retries automatically.
-pub(crate) fn rpc_resolver(client: Client) -> impl Fn(&Txid, Vout) -> Option<TxOut> {
-    let warned = AtomicBool::new(false);
-    move |txid, vout| {
-        let bt: &bitcoin::Txid = txid.into();
-        match client.get_raw_transaction(bt, None as Option<&bitcoin::BlockHash>) {
-            Ok(tx) => tx
-                .output
-                .get(usize::from(vout))
-                .map(|o| TxOut::from((o.script_pubkey.clone(), o.value.into()))),
-            Err(_) => {
-                if !warned.swap(true, Ordering::Relaxed) {
-                    warn!(
-                        "mempool: getrawtransaction missed for {txid}; ensure bitcoind is running with txindex=1"
-                    );
-                }
-                None
-            }
-        }
-    }
-}
-
-type Fills = Vec<(Vin, TxOut)>;
-type Holes = Vec<(Vin, Txid, Vout)>;
-type FillBatch = Vec<(TxidPrefix, Txid, Fills)>;
-type HoleBatch = Vec<(TxidPrefix, Txid, Holes)>;
-
-/// Fill every unfilled prevout the cycle can resolve. Same-cycle
-/// in-mempool parents are filled lock-locally; the remainder go
-/// through `resolver` outside any lock. Returns true iff anything
-/// was written.
-pub(crate) fn fill<F>(lock: &RwLock<State>, resolver: F) -> bool
-where
-    F: Fn(&Txid, Vout) -> Option<TxOut>,
-{
-    let (in_mempool, holes) = {
-        let state = lock.read();
-        gather(&state.txs)
-    };
-    let external = resolve_external(holes, resolver);
-
-    if in_mempool.is_empty() && external.is_empty() {
-        return false;
-    }
-
-    let mut state = lock.write();
-    write_fills(&mut state, in_mempool);
-    write_fills(&mut state, external);
-    true
-}
-
-/// Single pass over `txs.unresolved()`: bucket each hole into a
-/// same-cycle in-mempool fill (parent is live) or an external hole
-/// (parent is confirmed or unknown).
-fn gather(txs: &TxStore) -> (FillBatch, HoleBatch) {
-    if txs.unresolved().is_empty() {
-        return (Vec::new(), Vec::new());
-    }
-    let mut filled: FillBatch = Vec::new();
-    let mut holes: HoleBatch = Vec::new();
-    for prefix in txs.unresolved() {
-        let Some(record) = txs.record_by_prefix(prefix) else {
-            continue;
-        };
-        let mut tx_fills: Fills = Vec::new();
-        let mut tx_holes: Holes = Vec::new();
-        for (i, txin) in record.tx.input.iter().enumerate() {
-            if txin.prevout.is_some() {
-                continue;
-            }
-            let vin = Vin::from(i);
-            if let Some(parent) = txs.get(&txin.txid)
-                && let Some(out) = parent.output.get(usize::from(txin.vout))
-            {
-                tx_fills.push((vin, out.clone()));
-            } else {
-                tx_holes.push((vin, txin.txid, txin.vout));
-            }
-        }
-        let txid = record.entry.txid;
-        if !tx_fills.is_empty() {
-            filled.push((*prefix, txid, tx_fills));
-        }
-        if !tx_holes.is_empty() {
-            holes.push((*prefix, txid, tx_holes));
-        }
-    }
-    (filled, holes)
-}
-
-fn resolve_external<F>(holes: HoleBatch, resolver: F) -> FillBatch
-where
-    F: Fn(&Txid, Vout) -> Option<TxOut>,
-{
-    holes
-        .into_iter()
-        .filter_map(|(prefix, txid, holes)| {
-            let fills: Fills = holes
-                .into_iter()
-                .filter_map(|(vin, prev_txid, vout)| resolver(&prev_txid, vout).map(|o| (vin, o)))
-                .collect();
-            (!fills.is_empty()).then_some((prefix, txid, fills))
-        })
-        .collect()
-}
-
-fn write_fills(state: &mut State, fills: FillBatch) {
-    for (prefix, txid, tx_fills) in fills {
-        for prevout in state.txs.apply_fills(&prefix, tx_fills) {
-            state.addrs.add_input(&txid, &prevout);
-        }
-    }
-}
diff --git a/crates/brk_mempool/src/rbf.rs b/crates/brk_mempool/src/rbf.rs
index d0066fdf4..264635562 100644
--- a/crates/brk_mempool/src/rbf.rs
+++ b/crates/brk_mempool/src/rbf.rs
@@ -103,8 +103,7 @@ fn resolve_node<'a>(
     txs: &'a TxStore,
     graveyard: &'a TxGraveyard,
 ) -> Option<(&'a Transaction, &'a TxEntry)> {
-    if let Some(record) = txs.record_by_prefix(&TxidPrefix::from(txid)) {
-        return Some((&record.tx, &record.entry));
-    }
-    graveyard.get(txid).map(|tomb| (&tomb.tx, &tomb.entry))
+    txs.record_by_prefix(&TxidPrefix::from(txid))
+        .map(|r| (&r.tx, &r.entry))
+        .or_else(|| graveyard.get(txid).map(|t| (&t.tx, &t.entry)))
 }
diff --git a/crates/brk_mempool/src/steps/mod.rs b/crates/brk_mempool/src/steps/mod.rs
index 3228e7956..4808b70e4 100644
--- a/crates/brk_mempool/src/steps/mod.rs
+++ b/crates/brk_mempool/src/steps/mod.rs
@@ -1,11 +1,13 @@
-//! The four pipeline steps. See the crate-level docs for the cycle.
+//! The five pipeline steps. See the crate-level docs for the cycle.
 
 mod applier;
 mod fetcher;
 pub(crate) mod preparer;
+mod prevouts;
 pub(crate) mod rebuilder;
 
 pub use applier::Applier;
 pub use fetcher::{Fetched, Fetcher};
 pub use preparer::{Preparer, TxEntry, TxRemoval};
+pub use prevouts::Prevouts;
 pub use rebuilder::{BlockStats, Rebuilder, RecommendedFees, SnapTx, Snapshot, TxIndex};
diff --git a/crates/brk_mempool/src/steps/preparer/mod.rs b/crates/brk_mempool/src/steps/preparer/mod.rs
index f28be4330..5338a5f8a 100644
--- a/crates/brk_mempool/src/steps/preparer/mod.rs
+++ b/crates/brk_mempool/src/steps/preparer/mod.rs
@@ -41,20 +41,16 @@ impl Preparer {
     ) -> TxsPulled {
         let state = lock.read();
 
-        let live = Self::live_set(&entries_info);
+        let live: FxHashSet<TxidPrefix> = entries_info
+            .iter()
+            .map(|info| TxidPrefix::from(&info.txid))
+            .collect();
         let added = Self::classify_additions(entries_info, new_raws, &state.txs, &state.graveyard);
         let removed = TxRemoval::classify(&live, &added, &state.txs);
 
         TxsPulled { added, removed }
     }
 
-    fn live_set(entries_info: &[MempoolEntryInfo]) -> FxHashSet<TxidPrefix> {
-        entries_info
-            .iter()
-            .map(|info| TxidPrefix::from(&info.txid))
-            .collect()
-    }
-
     fn classify_additions(
         entries_info: Vec<MempoolEntryInfo>,
         mut new_raws: FxHashMap<Txid, RawTx>,
diff --git a/crates/brk_mempool/src/steps/prevouts.rs b/crates/brk_mempool/src/steps/prevouts.rs
new file mode 100644
index 000000000..25f266761
--- /dev/null
+++ b/crates/brk_mempool/src/steps/prevouts.rs
@@ -0,0 +1,152 @@
+//! Prevout fill plumbing.
+//!
+//! A fresh tx can land in the store with `prevout: None` on some
+//! inputs when the Preparer can't see the parent (parent arrived in
+//! the same cycle as the child, or parent is confirmed and we don't
+//! have an indexer hooked up). [`Prevouts::fill`] runs after each
+//! successful `Applier::apply` and closes both gaps in one pass:
+//!
+//! 1. Snapshot under a read guard, walking `txs.unresolved()` once.
+//!    For each hole, if the parent is also in the live pool we record
+//!    a fill directly (cheap, lock-local). Otherwise we record the
+//!    hole for external resolution.
+//! 2. Drop the read guard. Call `resolver` on the remaining holes
+//!    (typically `getrawtransaction` or an indexer lookup); failures
+//!    are simply skipped and retried next cycle.
+//! 3. Take the write guard once and fold both fill batches into the
+//!    `TxStore` via `apply_fills` -> `add_input`. Idempotent: each
+//!    fill checks `prevout.is_none()` and bails if the tx was already
+//!    removed or filled between phases.
+
+use std::sync::atomic::{AtomicBool, Ordering};
+
+use brk_rpc::Client;
+use brk_types::{TxOut, Txid, TxidPrefix, Vin, Vout};
+use parking_lot::RwLock;
+use tracing::warn;
+
+use crate::{State, stores::TxStore};
+
+pub struct Prevouts;
+
+type Fills = Vec<(Vin, TxOut)>;
+type Holes = Vec<(Vin, Txid, Vout)>;
+type FillBatch = Vec<(Txid, Fills)>;
+type HoleBatch = Vec<(Txid, Holes)>;
+
+impl Prevouts {
+    /// Fill every unfilled prevout the cycle can resolve. Same-cycle
+    /// in-mempool parents are filled lock-locally; the remainder go
+    /// through `resolver` outside any lock. Returns true iff anything
+    /// was written.
+    pub fn fill<F>(lock: &RwLock<State>, resolver: F) -> bool
+    where
+        F: Fn(&Txid, Vout) -> Option<TxOut>,
+    {
+        let (in_mempool, holes) = {
+            let state = lock.read();
+            Self::gather(&state.txs)
+        };
+        let external = Self::resolve_external(holes, resolver);
+
+        if in_mempool.is_empty() && external.is_empty() {
+            return false;
+        }
+
+        let mut state = lock.write();
+        Self::write_fills(&mut state, in_mempool);
+        Self::write_fills(&mut state, external);
+        true
+    }
+
+    /// Default resolver: per-call `getrawtransaction` against the
+    /// bitcoind RPC client `Mempool` already holds. Requires
+    /// `txindex=1`. On any failure logs once with a hint, then returns
+    /// `None`; the next cycle retries automatically.
+    pub fn rpc_resolver(client: Client) -> impl Fn(&Txid, Vout) -> Option<TxOut> {
+        let warned = AtomicBool::new(false);
+        move |txid, vout| {
+            let bt: &bitcoin::Txid = txid.into();
+            match client.get_raw_transaction(bt, None as Option<&bitcoin::BlockHash>) {
+                Ok(tx) => tx
+                    .output
+                    .get(usize::from(vout))
+                    .map(|o| TxOut::from((o.script_pubkey.clone(), o.value.into()))),
+                Err(_) => {
+                    if !warned.swap(true, Ordering::Relaxed) {
+                        warn!(
+                            "mempool: getrawtransaction missed for {txid}; ensure bitcoind is running with txindex=1"
+                        );
+                    }
+                    None
+                }
+            }
+        }
+    }
+
+    /// Single pass over `txs.unresolved()`: bucket each hole into a
+    /// same-cycle in-mempool fill (parent is live) or an external hole
+    /// (parent is confirmed or unknown).
+    fn gather(txs: &TxStore) -> (FillBatch, HoleBatch) {
+        if txs.unresolved().is_empty() {
+            return (Vec::new(), Vec::new());
+        }
+        let mut filled: FillBatch = Vec::new();
+        let mut holes: HoleBatch = Vec::new();
+        for prefix in txs.unresolved() {
+            let Some(record) = txs.record_by_prefix(prefix) else {
+                continue;
+            };
+            let mut tx_fills: Fills = Vec::new();
+            let mut tx_holes: Holes = Vec::new();
+            for (i, txin) in record.tx.input.iter().enumerate() {
+                if txin.prevout.is_some() {
+                    continue;
+                }
+                let vin = Vin::from(i);
+                if let Some(parent) = txs.get(&txin.txid)
+                    && let Some(out) = parent.output.get(usize::from(txin.vout))
+                {
+                    tx_fills.push((vin, out.clone()));
+                } else {
+                    tx_holes.push((vin, txin.txid, txin.vout));
+                }
+            }
+            let txid = record.entry.txid;
+            if !tx_fills.is_empty() {
+                filled.push((txid, tx_fills));
+            }
+            if !tx_holes.is_empty() {
+                holes.push((txid, tx_holes));
+            }
+        }
+        (filled, holes)
+    }
+
+    fn resolve_external<F>(holes: HoleBatch, resolver: F) -> FillBatch
+    where
+        F: Fn(&Txid, Vout) -> Option<TxOut>,
+    {
+        holes
+            .into_iter()
+            .filter_map(|(txid, holes)| {
+                let fills: Fills = holes
+                    .into_iter()
+                    .filter_map(|(vin, prev_txid, vout)| {
+                        resolver(&prev_txid, vout).map(|o| (vin, o))
+                    })
+                    .collect();
+                (!fills.is_empty()).then_some((txid, fills))
+            })
+            .collect()
+    }
+
+    fn write_fills(state: &mut State, fills: FillBatch) {
+        for (txid, tx_fills) in fills {
+            let prefix = TxidPrefix::from(&txid);
+            for prevout in state.txs.apply_fills(&prefix, tx_fills) {
+                state.addrs.add_input(&txid, &prevout);
+            }
+        }
+    }
+}
diff --git a/crates/brk_mempool/src/steps/rebuilder/partition.rs b/crates/brk_mempool/src/steps/rebuilder/partition.rs
index 24f384acc..136ecce2c 100644
--- a/crates/brk_mempool/src/steps/rebuilder/partition.rs
+++ b/crates/brk_mempool/src/steps/rebuilder/partition.rs
@@ -52,7 +52,7 @@ impl Partitioner {
 }
 
 fn sorted_indices(txs: &[SnapTx], excluded: &FxHashSet<TxIndex>) -> Vec<(TxIndex, VSize)> {
-    let mut cands: Vec<(TxIndex, VSize, brk_types::FeeRate)> = txs
+    let mut cands: Vec<(TxIndex, VSize, _)> = txs
         .iter()
         .enumerate()
         .filter_map(|(i, t)| {
@@ -61,8 +61,5 @@ fn sorted_indices(txs: &[SnapTx], excluded: &FxHashSet<TxIndex>) -> Vec<(TxIndex
         })
         .collect();
     cands.sort_by_key(|(_, _, rate)| Reverse(*rate));
-    cands
-        .into_iter()
-        .map(|(idx, vsize, _)| (idx, vsize))
-        .collect()
+    cands.into_iter().map(|(i, v, _)| (i, v)).collect()
 }
diff --git a/crates/brk_mempool/src/steps/rebuilder/snapshot/builder.rs b/crates/brk_mempool/src/steps/rebuilder/snapshot/builder.rs
index e7b26762e..848ce4da7 100644
--- a/crates/brk_mempool/src/steps/rebuilder/snapshot/builder.rs
+++ b/crates/brk_mempool/src/steps/rebuilder/snapshot/builder.rs
@@ -23,10 +23,6 @@ use super::{SnapTx, TxIndex};
 pub type PrefixIndex = FxHashMap<TxidPrefix, TxIndex>;
 
 pub fn build_txs(txs: &TxStore) -> (Vec<SnapTx>, PrefixIndex) {
-    if txs.is_empty() {
-        return (Vec::new(), PrefixIndex::default());
-    }
-
     let (prefix_to_idx, ordered) = compact_index(txs);
     let mut snap_txs: Vec<SnapTx> = ordered.iter().map(|e| live_tx(e, &prefix_to_idx)).collect();
 
diff --git a/crates/brk_mempool/src/steps/rebuilder/snapshot/stats.rs b/crates/brk_mempool/src/steps/rebuilder/snapshot/stats.rs
index fde786653..bbde8c9ba 100644
--- a/crates/brk_mempool/src/steps/rebuilder/snapshot/stats.rs
+++ b/crates/brk_mempool/src/steps/rebuilder/snapshot/stats.rs
@@ -11,6 +11,17 @@ const CORE_PERCENTILES: [f64; 7] = [0.0, 0.10, 0.25, 0.50, 0.75, 0.90, 1.00];
 /// columns of an otherwise tightly clustered fee tier.
 const PROJECTED_PERCENTILES: [f64; 7] = [0.05, 0.10, 0.25, 0.50, 0.75, 0.90, 0.95];
 
+/// Per-block aggregate stats for a projected block.
+///
+/// `block_stats[0]` mirrors Bitcoin Core's `getblocktemplate` - the
+/// node's actual next-block selection. `fee_range` spans the full
+/// 0..100 percentiles.
+///
+/// `block_stats[1..]` are a coarse greedy-packed projection by
+/// descending chunk rate, useful as a client-facing fee-tier gradient
+/// but not a prediction of what miners will include. Their `fee_range`
+/// is clipped to 5..95 percentiles so a single stale-GBT leftover or
+/// CPFP orphan doesn't dominate the min/max columns.
 #[derive(Debug, Clone, Default)]
 pub struct BlockStats {
     pub tx_count: u32,
diff --git a/crates/brk_mempool/src/stores/addr_tracker/mod.rs b/crates/brk_mempool/src/stores/addr_tracker/mod.rs
index 37f479f0c..7423c4bd1 100644
--- a/crates/brk_mempool/src/stores/addr_tracker/mod.rs
+++ b/crates/brk_mempool/src/stores/addr_tracker/mod.rs
@@ -17,37 +17,27 @@ pub struct AddrTracker(FxHashMap<AddrBytes, AddrEntry>);
 impl AddrTracker {
     pub fn add_tx(&mut self, tx: &Transaction, txid: &Txid) {
         for txin in &tx.input {
-            let Some(prevout) = txin.prevout.as_ref() else {
-                continue;
-            };
-            let Some(bytes) = prevout.addr_bytes() else {
-                continue;
-            };
-            self.apply_add(bytes, txid, |stats| stats.sending(prevout));
+            if let Some(prevout) = txin.prevout.as_ref() {
+                self.add_input(txid, prevout);
+            }
         }
         for txout in &tx.output {
-            let Some(bytes) = txout.addr_bytes() else {
-                continue;
-            };
-            self.apply_add(bytes, txid, |stats| stats.receiving(txout));
+            if let Some(bytes) = txout.addr_bytes() {
+                self.apply_add(bytes, txid, |stats| stats.receiving(txout));
+            }
         }
     }
 
     pub fn remove_tx(&mut self, tx: &Transaction, txid: &Txid) {
         for txin in &tx.input {
-            let Some(prevout) = txin.prevout.as_ref() else {
-                continue;
-            };
-            let Some(bytes) = prevout.addr_bytes() else {
-                continue;
-            };
-            self.apply_remove(bytes, txid, |stats| stats.sent(prevout));
+            if let Some(prevout) = txin.prevout.as_ref() {
+                self.remove_input(txid, prevout);
+            }
         }
         for txout in &tx.output {
-            let Some(bytes) = txout.addr_bytes() else {
-                continue;
-            };
-            self.apply_remove(bytes, txid, |stats| stats.received(txout));
+            if let Some(bytes) = txout.addr_bytes() {
+                self.apply_remove(bytes, txid, |stats| stats.received(txout));
+            }
         }
     }
 
@@ -67,8 +57,9 @@ impl AddrTracker {
 
     /// Fold a single newly-resolved input into the per-address stats.
     /// Called by the prevout-fill paths after a prevout that was
-    /// previously `None` has been filled. Inputs whose prevout doesn't
-    /// resolve to an addr are no-ops.
+    /// previously `None` has been filled, and by `add_tx` for each
+    /// resolved input. Inputs whose prevout doesn't resolve to an addr
+    /// are no-ops.
     pub fn add_input(&mut self, txid: &Txid, prevout: &TxOut) {
         let Some(bytes) = prevout.addr_bytes() else {
             return;
@@ -76,6 +67,13 @@ impl AddrTracker {
         self.apply_add(bytes, txid, |stats| stats.sending(prevout));
     }
 
+    fn remove_input(&mut self, txid: &Txid, prevout: &TxOut) {
+        let Some(bytes) = prevout.addr_bytes() else {
+            return;
+        };
+        self.apply_remove(bytes, txid, |stats| stats.sent(prevout));
+    }
+
     fn apply_add(
         &mut self,
         bytes: AddrBytes,
diff --git a/crates/brk_query/src/impl/cpfp.rs b/crates/brk_query/src/impl/cpfp.rs
index a9af72bcf..6e030b1dd 100644
--- a/crates/brk_query/src/impl/cpfp.rs
+++ b/crates/brk_query/src/impl/cpfp.rs
@@ -10,9 +10,10 @@
 //! carries the same chunk-rate semantics the live mempool produces.
 
 use brk_error::{Error, OptionData, Result};
+use brk_mempool::{ChunkInput, linearize};
 use brk_types::{
-    CpfpCluster, CpfpClusterChunk, CpfpClusterTx, CpfpClusterTxIndex, CpfpEntry, CpfpInfo, FeeRate,
-    Height, Sats, TxInIndex, TxIndex, Txid, TxidPrefix, VSize, Weight,
+    CpfpCluster, CpfpClusterTx, CpfpClusterTxIndex, CpfpEntry, CpfpInfo, FeeRate, Height, Sats,
+    TxInIndex, TxIndex, Txid, TxidPrefix, VSize, Weight,
 };
 use rustc_hash::{FxBuildHasher, FxHashMap};
 use smallvec::SmallVec;
@@ -323,65 +324,52 @@ fn build_cpfp_info(
         .max_by_key(|e| FeeRate::from((e.fee, e.weight)))
         .cloned();
 
-    let cluster_txs: Vec<CpfpClusterTx> = members
-        .iter()
-        .map(|m| CpfpClusterTx {
-            txid: m.txid,
-            weight: m.weight,
-            fee: m.fee,
-            parents: m.parents.iter().copied().collect(),
-        })
-        .collect();
-    let chunks = chunk_groups(members);
-    let chunk_index = chunks
-        .iter()
-        .position(|ch| ch.txs.contains(&seed_local))
-        .map(|i| i as u32)
-        .unwrap_or(0);
+    let (cluster, effective_fee_per_vsize) = if members.len() <= 1 {
+        (None, seed.rate)
+    } else {
+        let inputs: Vec<ChunkInput<'_>> = members
+            .iter()
+            .map(|m| ChunkInput {
+                fee: m.fee,
+                vsize: m.vsize,
+                parents: m.parents.as_slice(),
+            })
+            .collect();
+        let chunks = linearize(&inputs);
+        let (chunk_index, seed_rate) = chunks
+            .iter()
+            .enumerate()
+            .find(|(_, ch)| ch.txs.contains(&seed_local))
+            .map(|(i, ch)| (i as u32, ch.feerate))
+            .unwrap_or((0, seed.rate));
+        let cluster_txs: Vec<CpfpClusterTx> = members
+            .iter()
+            .map(|m| CpfpClusterTx {
+                txid: m.txid,
+                weight: m.weight,
+                fee: m.fee,
+                parents: m.parents.iter().copied().collect(),
+            })
+            .collect();
+        (
+            Some(CpfpCluster {
+                txs: cluster_txs,
+                chunks,
+                chunk_index,
+            }),
+            seed_rate,
+        )
+    };
 
     CpfpInfo {
         ancestors,
         best_descendant,
         descendants,
-        effective_fee_per_vsize: seed.rate,
+        effective_fee_per_vsize,
         sigops,
         fee: seed.fee,
         vsize: seed.vsize,
         adjusted_vsize: sigops.adjust_vsize(seed.vsize),
-        cluster: CpfpCluster {
-            txs: cluster_txs,
-            chunks,
-            chunk_index,
-        },
+        cluster,
     }
 }
-
-fn chunk_groups(members: &[Member]) -> Vec<CpfpClusterChunk> {
-    let mut groups: FxHashMap<u64, (FeeRate, SmallVec<[CpfpClusterTxIndex; 4]>)> =
-        FxHashMap::with_capacity_and_hasher(members.len(), FxBuildHasher);
-    let mut order: Vec<u64> = Vec::new();
-    for (i, m) in members.iter().enumerate() {
-        let key = f64::from(m.rate).to_bits();
-        let local = CpfpClusterTxIndex::from(i as u32);
-        groups
-            .entry(key)
-            .and_modify(|(_, v)| v.push(local))
-            .or_insert_with(|| {
-                order.push(key);
-                let mut v: SmallVec<[CpfpClusterTxIndex; 4]> = SmallVec::new();
-                v.push(local);
-                (m.rate, v)
-            });
-    }
-    order.sort_by_key(|k| std::cmp::Reverse(groups[k].0));
-    order
-        .into_iter()
-        .map(|k| {
-            let (rate, txs) = groups.remove(&k).unwrap();
-            CpfpClusterChunk {
-                txs: txs.into_vec(),
-                feerate: rate,
-            }
-        })
-        .collect()
-}
diff --git a/crates/brk_types/src/cpfp/info.rs b/crates/brk_types/src/cpfp/info.rs
index 5e0f8e438..a31dc75b6 100644
--- a/crates/brk_types/src/cpfp/info.rs
+++ b/crates/brk_types/src/cpfp/info.rs
@@ -16,6 +16,8 @@ pub struct CpfpInfo {
     /// Descendant transactions in the CPFP chain.
     pub descendants: Vec<CpfpEntry>,
     /// Effective fee rate considering CPFP relationships (sat/vB).
+    /// This is the seed's chunk feerate after lift-merging, i.e. the
+    /// rate Core/mempool.space would surface for this tx.
     pub effective_fee_per_vsize: FeeRate,
     /// BIP-141 sigop cost for the seed tx (witness sigops count as 1,
     /// legacy and P2SH-redeem sigops count as 4).
@@ -27,6 +29,8 @@ pub struct CpfpInfo {
     /// Policy-adjusted virtual size: `max(vsize, sigops * 5)`.
     pub adjusted_vsize: VSize,
     /// Cluster the seed belongs to: full tx list, SFL-linearized chunks,
-    /// and the seed's chunk index.
-    pub cluster: CpfpCluster,
+    /// and the seed's chunk index. Omitted when the seed has no
+    /// ancestors and no descendants (matches mempool.space).
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub cluster: Option<CpfpCluster>,
 }
diff --git a/modules/brk-client/index.js b/modules/brk-client/index.js
index db6fde3e6..2002352a8 100644
--- a/modules/brk-client/index.js
+++ b/modules/brk-client/index.js
@@ -421,13 +421,16 @@ Matches mempool.space/bitcoin-cli behavior.
  * @property {(CpfpEntry|null)=} bestDescendant - Best (highest fee rate) descendant, if any.
  * @property {CpfpEntry[]} descendants - Descendant transactions in the CPFP chain.
  * @property {FeeRate} effectiveFeePerVsize - Effective fee rate considering CPFP relationships (sat/vB).
+This is the seed's chunk feerate after lift-merging, i.e. the
+rate Core/mempool.space would surface for this tx.
  * @property {SigOps} sigops - BIP-141 sigop cost for the seed tx (witness sigops count as 1,
 legacy and P2SH-redeem sigops count as 4).
  * @property {Sats} fee - Transaction fee (sats).
  * @property {VSize} vsize - Virtual size of the seed tx (vbytes).
  * @property {VSize} adjustedVsize - Policy-adjusted virtual size: `max(vsize, sigops * 5)`.
- * @property {CpfpCluster} cluster - Cluster the seed belongs to: full tx list, SFL-linearized chunks,
-and the seed's chunk index.
+ * @property {(CpfpCluster|null)=} cluster - Cluster the seed belongs to: full tx list, SFL-linearized chunks,
+and the seed's chunk index. Omitted when the seed has no
+ancestors and no descendants (matches mempool.space).
  */
 /**
  * Range parameters with output format for API query parameters.
diff --git a/packages/brk_client/brk_client/__init__.py b/packages/brk_client/brk_client/__init__.py
index b34eb5d69..560b95fc9 100644
--- a/packages/brk_client/brk_client/__init__.py
+++ b/packages/brk_client/brk_client/__init__.py
@@ -716,13 +716,16 @@ class CpfpInfo(TypedDict):
         bestDescendant: Best (highest fee rate) descendant, if any.
         descendants: Descendant transactions in the CPFP chain.
         effectiveFeePerVsize: Effective fee rate considering CPFP relationships (sat/vB).
+This is the seed's chunk feerate after lift-merging, i.e. the
+rate Core/mempool.space would surface for this tx.
         sigops: BIP-141 sigop cost for the seed tx (witness sigops count as 1,
 legacy and P2SH-redeem sigops count as 4).
         fee: Transaction fee (sats).
         vsize: Virtual size of the seed tx (vbytes).
         adjustedVsize: Policy-adjusted virtual size: `max(vsize, sigops * 5)`.
         cluster: Cluster the seed belongs to: full tx list, SFL-linearized chunks,
-and the seed's chunk index.
+and the seed's chunk index. Omitted when the seed has no
+ancestors and no descendants (matches mempool.space).
     """
     ancestors: List[CpfpEntry]
     bestDescendant: Union[CpfpEntry, None]
@@ -732,7 +735,7 @@ and the seed's chunk index.
     fee: Sats
     vsize: VSize
     adjustedVsize: VSize
-    cluster: CpfpCluster
+    cluster: Union[CpfpCluster, None]
 
 class DataRangeFormat(TypedDict):
     """