diff --git a/src/obicompactvec/src/bitmatrix/builder.rs b/src/obicompactvec/src/bitmatrix/builder.rs
index 1f76d225..4cb85f1b 100644
--- a/src/obicompactvec/src/bitmatrix/builder.rs
+++ b/src/obicompactvec/src/bitmatrix/builder.rs
@@ -23,6 +23,17 @@ impl PersistentBitMatrixBuilder {
         Ok(Self { dir: dir.to_path_buf(), n, n_cols: 0 })
     }
 
+    /// Resume appending columns to a Columnar matrix directory already
+    /// closed by a previous builder session — reads `n`/`n_cols` from
+    /// `meta.json` instead of starting a fresh matrix at `n_cols == 0`.
+    /// Lets callers that build columns incrementally over time (e.g. a
+    /// streamed merge pipeline) reach `add_col`/`close` without knowing the
+    /// on-disk column naming or `MatrixMeta` themselves.
+    pub fn resume(dir: &Path) -> io::Result<Self> {
+        let meta = MatrixMeta::load(dir)?;
+        Ok(Self { dir: dir.to_path_buf(), n: meta.n, n_cols: meta.n_cols })
+    }
+
     pub fn n(&self)      -> usize { self.n }
     pub fn n_cols(&self) -> usize { self.n_cols }
 
diff --git a/src/obicompactvec/src/intmatrix.rs b/src/obicompactvec/src/intmatrix.rs
index 4680bdb5..4a2015ba 100644
--- a/src/obicompactvec/src/intmatrix.rs
+++ b/src/obicompactvec/src/intmatrix.rs
@@ -481,6 +481,18 @@ impl PersistentCompactIntMatrixBuilder {
         fs::create_dir_all(dir)?;
         Ok(Self { dir: dir.to_path_buf(), n, n_cols: 0 })
     }
+
+    /// Resume appending columns to a Columnar matrix directory already
+    /// closed by a previous builder session — reads `n`/`n_cols` from
+    /// `meta.json` instead of starting a fresh matrix at `n_cols == 0`.
+    /// Lets callers that build columns incrementally over time (e.g. a
+    /// streamed merge pipeline) reach `add_col`/`close` without knowing the
+    /// on-disk column naming or `MatrixMeta` themselves.
+    pub fn resume(dir: &Path) -> io::Result<Self> {
+        let meta = MatrixMeta::load(dir)?;
+        Ok(Self { dir: dir.to_path_buf(), n: meta.n, n_cols: meta.n_cols })
+    }
+
     #[inline]
     pub fn n(&self)      -> usize { self.n }
     #[inline]
diff --git a/src/obicompactvec/src/tests/bitmatrix.rs b/src/obicompactvec/src/tests/bitmatrix.rs
index 7600ac39..ca7002db 100644
--- a/src/obicompactvec/src/tests/bitmatrix.rs
+++ b/src/obicompactvec/src/tests/bitmatrix.rs
@@ -62,6 +62,53 @@ fn two_cols_roundtrip() {
     assert_eq!(&*m.row(2), &[true, false]);
 }
 
+#[test]
+fn resume_continues_n_cols_and_appends_columns() {
+    let dir = tempdir().unwrap();
+    let presence = dir.path().join("presence");
+
+    let mut b = PersistentBitMatrixBuilder::new(3, &presence).unwrap();
+    let mut col0 = b.add_col().unwrap();
+    col0.set(0, true); col0.set(1, false); col0.set(2, true);
+    col0.close().unwrap();
+    b.close().unwrap();
+
+    let mut resumed = PersistentBitMatrixBuilder::resume(&presence).unwrap();
+    assert_eq!(resumed.n(), 3);
+    assert_eq!(resumed.n_cols(), 1);
+    let mut col1 = resumed.add_col().unwrap();
+    col1.set(0, false); col1.set(1, true); col1.set(2, false);
+    col1.close().unwrap();
+    resumed.close().unwrap();
+
+    let m = PersistentBitMatrix::open(dir.path()).unwrap();
+    assert_eq!(m.n_cols(), 2);
+    assert_eq!(&*m.row(0), &[true, false]);
+    assert_eq!(&*m.row(1), &[false, true]);
+    assert_eq!(&*m.row(2), &[true, false]);
+}
+
+#[test]
+fn resume_twice_keeps_appending() {
+    let dir = tempdir().unwrap();
+    let presence = dir.path().join("presence");
+
+    PersistentBitMatrixBuilder::new(2, &presence).unwrap().close().unwrap();
+
+    for v in [true, false, true] {
+        let mut b = PersistentBitMatrixBuilder::resume(&presence).unwrap();
+        let mut col = b.add_col().unwrap();
+        col.set(0, v); col.set(1, !v);
+        col.close().unwrap();
+        b.close().unwrap();
+    }
+
+    let m = PersistentBitMatrix::open(dir.path()).unwrap();
+    assert_eq!(m.n_cols(), 3);
+    assert_eq!(&*m.row(0), &[true, false, true]);
+    assert_eq!(&*m.row(1), &[false, true, false]);
+}
+
 #[test]
 fn col_accessor() {
     let dir = tempdir().unwrap();
diff --git a/src/obicompactvec/src/tests/intmatrix.rs b/src/obicompactvec/src/tests/intmatrix.rs
index 966f6da5..5af4b51e 100644
--- a/src/obicompactvec/src/tests/intmatrix.rs
+++ b/src/obicompactvec/src/tests/intmatrix.rs
@@ -59,6 +59,53 @@ fn two_cols_roundtrip() {
     assert_eq!(&*m.row(2), &[3u32, 30]);
 }
 
+#[test]
+fn resume_continues_n_cols_and_appends_columns() {
+    let dir = tempdir().unwrap();
+    let counts = dir.path().join("counts");
+
+    let mut b = PersistentCompactIntMatrixBuilder::new(3, &counts).unwrap();
+    let mut col0 = b.add_col().unwrap();
+    col0.set(0, 1); col0.set(1, 2); col0.set(2, 3);
+    col0.close().unwrap();
+    b.close().unwrap();
+
+    let mut resumed = PersistentCompactIntMatrixBuilder::resume(&counts).unwrap();
+    assert_eq!(resumed.n(), 3);
+    assert_eq!(resumed.n_cols(), 1);
+    let mut col1 = resumed.add_col().unwrap();
+    col1.set(0, 10); col1.set(1, 20); col1.set(2, 30);
+    col1.close().unwrap();
+    resumed.close().unwrap();
+
+    let m = PersistentCompactIntMatrix::open(dir.path()).unwrap();
+    assert_eq!(m.n_cols(), 2);
+    assert_eq!(&*m.row(0), &[1u32, 10]);
+    assert_eq!(&*m.row(1), &[2u32, 20]);
+    assert_eq!(&*m.row(2), &[3u32, 30]);
+}
+
+#[test]
+fn resume_twice_keeps_appending() {
+    let dir = tempdir().unwrap();
+    let counts = dir.path().join("counts");
+
+    PersistentCompactIntMatrixBuilder::new(2, &counts).unwrap().close().unwrap();
+
+    for v in [1u32, 2, 3] {
+        let mut b = PersistentCompactIntMatrixBuilder::resume(&counts).unwrap();
+        let mut col = b.add_col().unwrap();
+        col.set(0, v); col.set(1, v * 10);
+        col.close().unwrap();
+        b.close().unwrap();
+    }
+
+    let m = PersistentCompactIntMatrix::open(dir.path()).unwrap();
+    assert_eq!(m.n_cols(), 3);
+    assert_eq!(&*m.row(0), &[1u32, 2, 3]);
+    assert_eq!(&*m.row(1), &[10u32, 20, 30]);
+}
+
 #[test]
 fn col_accessor() {
     let dir = tempdir().unwrap();
diff --git a/src/obikpartitionner/src/common.rs b/src/obikpartitionner/src/common.rs
index 99e345ee..2939e0a8 100644
--- a/src/obikpartitionner/src/common.rs
+++ b/src/obikpartitionner/src/common.rs
@@ -1,6 +1,4 @@
-use std::fs;
-use std::io;
-use std::path::{Path, PathBuf};
+use std::path::Path;
 
 use obicompactvec::{PersistentBitVecBuilder, PersistentCompactIntVecBuilder};
 use obilayeredmap::meta::PartitionMeta;
@@ -43,23 +41,6 @@ pub(crate) fn load_meta(dir: &Path, context: &'static str) -> SKResult<Partition
     }
 }
 
-// ── path helpers ──────────────────────────────────────────────────────────────
-
-pub(crate) fn col_path_bit(dir: &Path, col: usize) -> PathBuf {
-    dir.join(format!("col_{col:06}.pbiv"))
-}
-
-pub(crate) fn col_path_int(dir: &Path, col: usize) -> PathBuf {
-    dir.join(format!("col_{col:06}.pciv"))
-}
-
-pub(crate) fn write_matrix_meta(dir: &Path, n: usize, n_cols: usize) -> io::Result<()> {
-    fs::write(
-        dir.join("meta.json"),
-        format!("{{\"n\":{n},\"n_cols\":{n_cols}}}\n"),
-    )
-}
-
 // ── ColBuilder ────────────────────────────────────────────────────────────────
 
 pub(crate) enum ColBuilder {
diff --git a/src/obikpartitionner/src/merge_layer/mod.rs b/src/obikpartitionner/src/merge_layer/mod.rs
index a822c87c..c02011b5 100644
--- a/src/obikpartitionner/src/merge_layer/mod.rs
+++ b/src/obikpartitionner/src/merge_layer/mod.rs
@@ -8,7 +8,8 @@
 //! builder setup, and pass 2), not a set of independently callable steps.
 
 use std::fs;
-use std::path::PathBuf;
+use std::io;
+use std::path::{Path, PathBuf};
 use std::sync::{Arc, Mutex};
 
 use tracing::debug;
@@ -18,16 +19,13 @@ use obipipeline::{
     make_sink, make_source, make_transform,
 };
 
-use obicompactvec::{
-    PersistentBitMatrix, PersistentBitMatrixBuilder, PersistentBitVecBuilder,
-    PersistentCompactIntMatrix, PersistentCompactIntMatrixBuilder, PersistentCompactIntVecBuilder,
-};
+use obicompactvec::{PersistentBitMatrixBuilder, PersistentCompactIntMatrixBuilder};
 use obikseq::CanonicalKmer;
 use obilayeredmap::meta::PartitionMeta;
 use obilayeredmap::{IndexMode, Layer, LayeredMap, MphfOnly};
 use obiskio::{SKError, SKResult, UnitigFileReader};
 
-use crate::common::{ColBuilder, col_path_bit, col_path_int, load_meta, olm_to_sk, write_matrix_meta};
+use crate::common::{ColBuilder, load_meta, olm_to_sk};
 use crate::graph_pipeline::{build_graph, materialize_layer};
 use crate::partition::KmerPartition;
 
@@ -43,6 +41,152 @@ pub enum MergeMode {
     Count,
 }
 
+// ── MatrixBuilder ─────────────────────────────────────────────────────────────
+//
+// Wraps whichever matrix builder `mode` calls for, so the merge pipeline never
+// has to know the on-disk column naming (`col_NNNNNN.pbiv`/`.pciv`) or the
+// matrix `meta.json` schema itself — both stay private to obicompactvec.
+// `resume` reopens a matrix directory already closed by a previous builder
+// session (an existing destination layer), continuing from its current
+// `n_cols` instead of starting a fresh matrix at 0.
+
+enum MatrixBuilder {
+    Bit(PersistentBitMatrixBuilder),
+    Int(PersistentCompactIntMatrixBuilder),
+}
+
+impl MatrixBuilder {
+    fn new(mode: MergeMode, n: usize, dir: &Path) -> io::Result<Self> {
+        Ok(match mode {
+            MergeMode::Presence => MatrixBuilder::Bit(PersistentBitMatrixBuilder::new(n, dir)?),
+            MergeMode::Count    => MatrixBuilder::Int(PersistentCompactIntMatrixBuilder::new(n, dir)?),
+        })
+    }
+
+    fn resume(mode: MergeMode, dir: &Path) -> io::Result<Self> {
+        Ok(match mode {
+            MergeMode::Presence => MatrixBuilder::Bit(PersistentBitMatrixBuilder::resume(dir)?),
+            MergeMode::Count    => MatrixBuilder::Int(PersistentCompactIntMatrixBuilder::resume(dir)?),
+        })
+    }
+
+    /// Add a column with no data written (all-zero/false) — for genome
+    /// columns absent from this source (e.g. dst genomes in a new layer).
+    fn add_absent_col(&mut self) -> io::Result<()> {
+        match self {
+            MatrixBuilder::Bit(b) => b.add_col()?.close(),
+            MatrixBuilder::Int(b) => b.add_col()?.close(),
+        }
+    }
+
+    fn add_col(&mut self) -> io::Result<ColBuilder> {
+        Ok(match self {
+            MatrixBuilder::Bit(b) => ColBuilder::Bit(b.add_col()?),
+            MatrixBuilder::Int(b) => ColBuilder::Int(b.add_col()?),
+        })
+    }
+
+    fn close(self) -> io::Result<()> {
+        match self {
+            MatrixBuilder::Bit(b) => b.close(),
+            MatrixBuilder::Int(b) => b.close(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod matrix_builder_tests {
+    use tempfile::tempdir;
+
+    use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix};
+
+    use super::{ColBuilder, MatrixBuilder, MergeMode};
+
+    /// Mirrors `merge_partition`'s "new layer" setup: absent (dst-genome)
+    /// columns get no data, then source columns are filled — all through one
+    /// continuous `MatrixBuilder` session, closed once at the end.
+    #[test]
+    fn new_layer_absent_then_source_columns_presence() {
+        let dir = tempdir().unwrap();
+        let data_dir = dir.path().join("presence");
+        let mut mb = MatrixBuilder::new(MergeMode::Presence, 3, &data_dir).unwrap();
+
+        // Two absent (dst-genome) columns.
+        mb.add_absent_col().unwrap();
+        mb.add_absent_col().unwrap();
+
+        // One source column, filled like pass 2 would.
+        let mut col = mb.add_col().unwrap();
+        match &mut col {
+            ColBuilder::Bit(b) => { b.set(0, true); b.set(1, false); b.set(2, true); }
+            ColBuilder::Int(_) => unreachable!(),
+        }
+        col.close().unwrap();
+        mb.close().unwrap();
+
+        let m = PersistentBitMatrix::open(dir.path()).unwrap();
+        assert_eq!(m.n_cols(), 3);
+        assert_eq!(&*m.row(0), &[false, false, true]);
+        assert_eq!(&*m.row(1), &[false, false, false]);
+        assert_eq!(&*m.row(2), &[false, false, true]);
+    }
+
+    #[test]
+    fn new_layer_absent_then_source_columns_count() {
+        let dir = tempdir().unwrap();
+        let data_dir = dir.path().join("counts");
+        let mut mb = MatrixBuilder::new(MergeMode::Count, 2, &data_dir).unwrap();
+
+        mb.add_absent_col().unwrap();
+
+        let mut col = mb.add_col().unwrap();
+        match &mut col {
+            ColBuilder::Int(b) => { b.set(0, 7); b.set(1, 42); }
+            ColBuilder::Bit(_) => unreachable!(),
+        }
+        col.close().unwrap();
+        mb.close().unwrap();
+
+        let m = PersistentCompactIntMatrix::open(dir.path()).unwrap();
+        assert_eq!(m.n_cols(), 2);
+        assert_eq!(&*m.row(0), &[0u32, 7]);
+        assert_eq!(&*m.row(1), &[0u32, 42]);
+    }
+
+    /// Mirrors `merge_partition`'s "existing dst layer" setup: an
+    /// already-closed matrix (from a previous merge) gets more columns
+    /// appended via `resume`, without callers ever seeing `col_path`/
+    /// `MatrixMeta` — `n` itself is read back from the matrix directory.
+    #[test]
+    fn resume_appends_source_columns_to_existing_layer() {
+        let dir = tempdir().unwrap();
+        let data_dir = dir.path().join("presence");
+
+        // Previous merge: one dst-genome column already on disk.
+        let mut mb0 = MatrixBuilder::new(MergeMode::Presence, 3, &data_dir).unwrap();
+        mb0.add_absent_col().unwrap();
+        mb0.close().unwrap();
+
+        // This merge: resume and append two more source columns.
+        let mut mb = MatrixBuilder::resume(MergeMode::Presence, &data_dir).unwrap();
+        for vals in [[true, false, true], [false, true, false]] {
+            let mut col = mb.add_col().unwrap();
+            match &mut col {
+                ColBuilder::Bit(b) => for (slot, v) in vals.into_iter().enumerate() { b.set(slot, v); },
+                ColBuilder::Int(_) => unreachable!(),
+            }
+            col.close().unwrap();
+        }
+        mb.close().unwrap();
+
+        let m = PersistentBitMatrix::open(dir.path()).unwrap();
+        assert_eq!(m.n_cols(), 3);
+        assert_eq!(&*m.row(0), &[false, true, false]);
+        assert_eq!(&*m.row(1), &[false, false, true]);
+        assert_eq!(&*m.row(2), &[false, true, false]);
+    }
+}
+
 // ── helpers ───────────────────────────────────────────────────────────────────
 
 const INDEX_SUBDIR: &str = "index";
@@ -170,89 +314,48 @@ impl KmerPartition {
         debug!("partition {i}: MPHF open in {:.3}s", t_open.elapsed().as_secs_f64());
 
         // ── Prepare matrix directories for the new layer ──────────────────────
-        // Absent columns (dst genomes) are written via append_column (all-zero/false).
-        // Source-genome columns are created as mutable builders for pass 2.
-        let new_src_builders: Vec<ColBuilder> = if any_new {
+        // Absent columns (dst genomes) get an all-zero/false column. Source-genome
+        // columns are created as mutable builders for pass 2. `new_mb` is kept
+        // open (not `close`d) until pass 2 has filled every source column, so its
+        // `n_cols` bookkeeping stays in sync with the columns actually created.
+        let (new_src_builders, new_mb): (Vec<ColBuilder>, Option<MatrixBuilder>) = if any_new {
             let data_dir = match mode {
                 MergeMode::Presence => new_layer_dir.join("presence"),
                 MergeMode::Count => new_layer_dir.join("counts"),
             };
             fs::create_dir_all(&data_dir)?;
-            match mode {
-                MergeMode::Presence => {
-                    PersistentBitMatrixBuilder::new(n_new, &data_dir)
-                        .map_err(SKError::Io)?
-                        .close()
-                        .map_err(SKError::Io)?;
-                    for _ in 0..n_dst_genomes {
-                        PersistentBitMatrix::append_column(&data_dir, |_| false)
-                            .map_err(SKError::Io)?;
-                    }
-                    (0..n_src_total)
-                        .map(|g| -> SKResult<ColBuilder> {
-                            let b = PersistentBitVecBuilder::new(
-                                n_new,
-                                &col_path_bit(&data_dir, n_dst_genomes + g),
-                            )?;
-                            Ok(ColBuilder::Bit(b))
-                        })
-                        .collect::<SKResult<_>>()?
-                }
-                MergeMode::Count => {
-                    PersistentCompactIntMatrixBuilder::new(n_new, &data_dir)
-                        .map_err(SKError::Io)?
-                        .close()
-                        .map_err(SKError::Io)?;
-                    for _ in 0..n_dst_genomes {
-                        PersistentCompactIntMatrix::append_column(&data_dir, |_| 0)
-                            .map_err(SKError::Io)?;
-                    }
-                    (0..n_src_total)
-                        .map(|g| -> SKResult<ColBuilder> {
-                            let b = PersistentCompactIntVecBuilder::new(
-                                n_new,
-                                &col_path_int(&data_dir, n_dst_genomes + g),
-                            )?;
-                            Ok(ColBuilder::Int(b))
-                        })
-                        .collect::<SKResult<_>>()?
-                }
+            let mut mb = MatrixBuilder::new(mode, n_new, &data_dir).map_err(SKError::Io)?;
+            for _ in 0..n_dst_genomes {
+                mb.add_absent_col().map_err(SKError::Io)?;
             }
+            let cols = (0..n_src_total)
+                .map(|_| mb.add_col().map_err(SKError::Io))
+                .collect::<SKResult<Vec<_>>>()?;
+            (cols, Some(mb))
         } else {
-            vec![]
+            (vec![], None)
         };
 
         let t_builders = std::time::Instant::now();
-        // Builders for existing layers: n_src_total per layer.
-        // Columns n_dst_genomes .. n_dst_genomes + n_src_total - 1.
-        let exist_builders: Vec<Vec<ColBuilder>> = (0..n_dst_layers)
-            .map(|l| {
-                let layer_dir = dst_index_dir.join(format!("layer_{l}"));
-                let n = dst_map.layer(l).n();
-                (0..n_src_total)
-                    .map(|src_g| -> SKResult<ColBuilder> {
-                        match mode {
-                            MergeMode::Presence => {
-                                let data_dir = layer_dir.join("presence");
-                                let b = PersistentBitVecBuilder::new(
-                                    n,
-                                    &col_path_bit(&data_dir, n_dst_genomes + src_g),
-                                )?;
-                                Ok(ColBuilder::Bit(b))
-                            }
-                            MergeMode::Count => {
-                                let data_dir = layer_dir.join("counts");
-                                let b = PersistentCompactIntVecBuilder::new(
-                                    n,
-                                    &col_path_int(&data_dir, n_dst_genomes + src_g),
-                                )?;
-                                Ok(ColBuilder::Int(b))
-                            }
-                        }
-                    })
-                    .collect::<SKResult<_>>()
-            })
-            .collect::<SKResult<_>>()?;
+        // Builders for existing layers: n_src_total per layer, resumed from
+        // each layer's own matrix directory (already holding n_dst_genomes
+        // columns from a previous merge). Columns land at
+        // n_dst_genomes .. n_dst_genomes + n_src_total - 1.
+        let mut exist_mbs: Vec<MatrixBuilder> = Vec::with_capacity(n_dst_layers);
+        let mut exist_builders: Vec<Vec<ColBuilder>> = Vec::with_capacity(n_dst_layers);
+        for l in 0..n_dst_layers {
+            let layer_dir = dst_index_dir.join(format!("layer_{l}"));
+            let data_dir = match mode {
+                MergeMode::Presence => layer_dir.join("presence"),
+                MergeMode::Count => layer_dir.join("counts"),
+            };
+            let mut mb = MatrixBuilder::resume(mode, &data_dir).map_err(SKError::Io)?;
+            let cols = (0..n_src_total)
+                .map(|_| mb.add_col().map_err(SKError::Io))
+                .collect::<SKResult<Vec<_>>>()?;
+            exist_mbs.push(mb);
+            exist_builders.push(cols);
+        }
 
         debug!("partition {i}: builders ready in {:.3}s", t_builders.elapsed().as_secs_f64());
 
@@ -407,21 +510,15 @@ impl KmerPartition {
 
         let t_close = std::time::Instant::now();
         // ── Close builders and update metadata ────────────────────────────────
-        for (l, builders) in exist_locked.into_iter().enumerate() {
-            let layer_dir = dst_index_dir.join(format!("layer_{l}"));
+        for (mb, builders) in exist_mbs.into_iter().zip(exist_locked.into_iter()) {
             for b in builders {
                 Arc::try_unwrap(b)
-                    .unwrap_or_else(|_| panic!("pass2: exist_builder[{l}] not uniquely owned"))
+                    .unwrap_or_else(|_| panic!("pass2: exist_builder not uniquely owned"))
                     .into_inner()
                     .unwrap_or_else(|e| e.into_inner())
                     .close()?;
             }
-            let n = dst_map.layer(l).n();
-            let data_dir = match mode {
-                MergeMode::Presence => layer_dir.join("presence"),
-                MergeMode::Count => layer_dir.join("counts"),
-            };
-            write_matrix_meta(&data_dir, n, n_dst_genomes + n_src_total).map_err(SKError::Io)?;
+            mb.close().map_err(SKError::Io)?;
         }
 
         for b in new_locked {
@@ -431,13 +528,8 @@ impl KmerPartition {
                 .unwrap_or_else(|e| e.into_inner())
                 .close()?;
         }
-        if any_new {
-            let data_dir = match mode {
-                MergeMode::Presence => new_layer_dir.join("presence"),
-                MergeMode::Count => new_layer_dir.join("counts"),
-            };
-            write_matrix_meta(&data_dir, n_new, n_dst_genomes + n_src_total)
-                .map_err(SKError::Io)?;
+        if let Some(mb) = new_mb {
+            mb.close().map_err(SKError::Io)?;
 
             let mut part_meta = PartitionMeta::load(&dst_index_dir).map_err(|e| olm_to_sk(e, "merge"))?;
             part_meta.n_layers = new_layer_idx + 1;