diff --git a/src/Cargo.lock b/src/Cargo.lock
index 39706b2..cfe335d 100644
--- a/src/Cargo.lock
+++ b/src/Cargo.lock
@@ -1507,6 +1507,7 @@ dependencies = [
 name = "obikindex"
 version = "0.1.0"
 dependencies = [
+ "crossbeam-channel",
  "indicatif",
  "ndarray",
  "obicompactvec",
diff --git a/src/obikindex/Cargo.toml b/src/obikindex/Cargo.toml
index d670480..3f89bb0 100644
--- a/src/obikindex/Cargo.toml
+++ b/src/obikindex/Cargo.toml
@@ -11,7 +11,8 @@ obisys           = { path = "../obisys" }
 obicompactvec    = { path = "../obicompactvec" }
 obilayeredmap    = { path = "../obilayeredmap" }
 ndarray          = "0.16"
-rayon            = "1"
+rayon             = "1"
+crossbeam-channel = "0.5"
 serde            = { version = "1", features = ["derive"] }
 serde_json       = "1"
 indicatif        = "0.17"
diff --git a/src/obikindex/src/merge.rs b/src/obikindex/src/merge.rs
index 0380e98..fa990d8 100644
--- a/src/obikindex/src/merge.rs
+++ b/src/obikindex/src/merge.rs
@@ -2,8 +2,10 @@ use std::collections::HashMap;
 use std::fs;
 use std::io;
 use std::path::Path;
+use std::time::{Duration, Instant};
 
-use obisys::{Reporter, Stage, progress_bar, spinner};
+use crossbeam_channel::unbounded;
+use obisys::{CpuSample, Reporter, Stage, progress_bar, spinner};
 use tracing::{debug, info};
 
 use obilayeredmap::IndexMode;
@@ -191,48 +193,97 @@ impl KmerIndex {
             let mut order: Vec<usize> = (0..n_partitions).collect();
             order.sort_unstable_by_key(|&i| std::cmp::Reverse(partition_sizes[i]));
 
-            // ── First partition (largest) ─────────────────────────────────────
-            let worst_id    = order[0];
-            let worst_bytes = partition_sizes[worst_id];
+            // ── Adaptive worker pool ──────────────────────────────────────────
+            // Start with 1 worker thread. After each completed partition,
+            // measure CPU efficiency (via getrusage delta). If efficiency is
+            // below the spawn threshold and more partitions remain, spawn one
+            // additional worker. Workers share a crossbeam channel of partition
+            // IDs; each reports (id, g_len, duration) on a result channel.
+            const SPAWN_THRESHOLD: f64 = 0.95; // spawn when >5% capacity idle
+            let n_cores = std::thread::available_parallelism()
+                .map(|n| n.get()).unwrap_or(1);
+            let max_workers = (n_cores / 2).max(1);
+            let _ = budget_fraction; // kept in signature for CLI compatibility
 
-            let worst_g_len = dst_partition
-                .merge_partition(worst_id, &srcs, mode, n_dst_genomes, block_bits, &evidence)
-                .map_err(OKIError::Partition)?;
-            pb.inc(1);
+            let (part_tx, part_rx) = unbounded::<usize>();
+            let (result_tx, result_rx) =
+                unbounded::<(usize, Result<usize, obiskio::SKError>, Duration)>();
+            // activate_tx: controller sends () to wake the next dormant worker.
+            // Dropping activate_tx closes the channel; dormant workers exit.
+            let (activate_tx, activate_rx) = unbounded::<()>();
 
-            info!(
-                "merge_partitions: first partition {} — {} unitig bytes → {} new kmers",
-                worst_id, fmt_bytes(worst_bytes), worst_g_len,
-            );
+            for &i in &order {
+                part_tx.send(i).ok();
+            }
+            drop(part_tx);
 
             let mut part_stats: Vec<PartStat> = Vec::with_capacity(n_partitions);
-            part_stats.push(PartStat {
-                id:           worst_id,
-                unitig_bytes: worst_bytes,
-                g_len:        worst_g_len,
-            });
+            let mut n_workers = 0usize;
+            let mut cpu_sample = CpuSample::now();
 
-            // ── Sequential remainder ──────────────────────────────────────────
-            // One partition at a time; each partition uses an internal pipeline
-            // (obipipeline) to parallelise file I/O and dst_map filtering.
-            let _ = budget_fraction; // kept in signature for CLI compatibility
-            for &i in &order[1..] {
-                let ubytes = partition_sizes[i];
-                debug!("partition {i}: start — {} unitig bytes", fmt_bytes(ubytes));
+            // Shadow as references so closures can capture them by copy.
+            let srcs = &srcs;
+            let evidence = &evidence;
 
-                let g_len = dst_partition
-                    .merge_partition(i, &srcs, mode, n_dst_genomes, block_bits, &evidence)
-                    .map_err(OKIError::Partition)?;
-                pb.inc(1);
+            std::thread::scope(|s| -> OKIResult<()> {
+                // Pre-spawn max_workers threads; each waits for an activation
+                // signal before consuming from part_rx.
+                for _ in 0..max_workers {
+                    let prx = part_rx.clone();
+                    let rtx = result_tx.clone();
+                    let arx = activate_rx.clone();
+                    s.spawn(move || {
+                        if arx.recv().is_ok() {
+                            for i in &prx {
+                                let t = Instant::now();
+                                let r = dst_partition.merge_partition(
+                                    i, srcs, mode, n_dst_genomes, block_bits, evidence,
+                                );
+                                rtx.send((i, r, t.elapsed())).ok();
+                            }
+                        }
+                    });
+                }
+                drop(result_tx);
 
-                debug!("partition {i}: done — {} new kmers", g_len);
-                part_stats.push(PartStat { id: i, unitig_bytes: ubytes, g_len });
-            }
+                // Activate first worker immediately.
+                activate_tx.send(()).ok();
+                n_workers = 1;
+
+                let mut completed = 0usize;
+                while completed < n_partitions {
+                    let (i, r, dur) = result_rx.recv()
+                        .map_err(|_| OKIError::Io(io::Error::new(
+                            io::ErrorKind::UnexpectedEof, "worker channel closed")))?;
+                    let g_len = r.map_err(OKIError::Partition)?;
+                    pb.inc(1);
+                    debug!("partition {i}: done in {:.1}s — {} new kmers",
+                        dur.as_secs_f64(), g_len);
+                    part_stats.push(PartStat {
+                        id: i, unitig_bytes: partition_sizes[i], g_len,
+                    });
+                    completed += 1;
+
+                    if n_workers < max_workers && completed < n_partitions {
+                        let eff = cpu_sample.cpu_efficiency(n_cores);
+                        if eff < SPAWN_THRESHOLD {
+                            activate_tx.send(()).ok();
+                            n_workers += 1;
+                            cpu_sample = CpuSample::now();
+                            debug!("activated worker {n_workers} — efficiency {:.0}%",
+                                eff * 100.0);
+                        }
+                    }
+                }
+                // Close activate_tx: dormant workers exit cleanly.
+                drop(activate_tx);
+                Ok(())
+            })?;
 
             pb.finish_and_clear();
 
             // ── Diagnostic report ─────────────────────────────────────────────
-            print_merge_partition_report(&part_stats);
+            print_merge_partition_report(&part_stats, n_workers, max_workers);
 
             rep.push(t.stop());
         }
@@ -254,7 +305,7 @@ impl KmerIndex {
 
 // ── Diagnostic report ─────────────────────────────────────────────────────────
 
-fn print_merge_partition_report(stats: &[PartStat]) {
+fn print_merge_partition_report(stats: &[PartStat], n_workers: usize, max_workers: usize) {
     let total_new: usize = stats.iter().map(|s| s.g_len).sum();
     let non_empty = stats.iter().filter(|s| s.unitig_bytes > 0).count();
 
@@ -268,6 +319,9 @@ fn print_merge_partition_report(stats: &[PartStat]) {
         "  {} partition(s) processed, {} total new kmers",
         non_empty, total_new,
     );
+    info!(
+        "  workers spawned: {n_workers} / {max_workers} (max)",
+    );
 
     // Top 8 partitions by new-kmer count
     let mut by_new: Vec<&PartStat> = stats.iter().filter(|s| s.g_len > 0).collect();
diff --git a/src/obisys/src/lib.rs b/src/obisys/src/lib.rs
index 5f8a3d3..aeccb1b 100644
--- a/src/obisys/src/lib.rs
+++ b/src/obisys/src/lib.rs
@@ -212,6 +212,39 @@ fn rss_to_bytes(ru: &rusage) -> u64 { ru.ru_maxrss as u64 * 1024 }
 // Monotonically increasing counters — negative delta would be a kernel bug.
 fn delta(end: i64, start: i64) -> u64 { (end - start).max(0) as u64 }
 
+// ── CpuSample ─────────────────────────────────────────────────────────────────
+
+/// Snapshot of process-wide CPU time + wall clock at a point in time.
+/// Use [`cpu_efficiency`](Self::cpu_efficiency) to measure the fraction of
+/// available cores used since the snapshot was taken.
+pub struct CpuSample {
+    wall:      Instant,
+    user_secs: f64,
+    sys_secs:  f64,
+}
+
+impl CpuSample {
+    pub fn now() -> Self {
+        let ru = get_rusage();
+        Self {
+            wall:      Instant::now(),
+            user_secs: tv_to_secs(ru.ru_utime),
+            sys_secs:  tv_to_secs(ru.ru_stime),
+        }
+    }
+
+    /// (user_delta + sys_delta) / (wall_delta × n_cores) since this snapshot.
+    /// Returns 0.0 if less than 100 ms have elapsed (too noisy).
+    pub fn cpu_efficiency(&self, n_cores: usize) -> f64 {
+        let ru   = get_rusage();
+        let wall = self.wall.elapsed().as_secs_f64();
+        if wall < 0.1 { return 0.0; }
+        let cpu = (tv_to_secs(ru.ru_utime) - self.user_secs)
+                + (tv_to_secs(ru.ru_stime) - self.sys_secs);
+        cpu / (wall * n_cores as f64)
+    }
+}
+
 // ── public API ────────────────────────────────────────────────────────────────
 
 /// Snapshot taken at the start of a pipeline stage.