diff --git a/src/obidebruinj/src/debruijn.rs b/src/obidebruinj/src/debruijn.rs
index 1bef858..f96958c 100644
--- a/src/obidebruinj/src/debruijn.rs
+++ b/src/obidebruinj/src/debruijn.rs
@@ -222,9 +222,7 @@ impl GraphDeBruijn {
 
     pub fn set_visited(&self, kmer: CanonicalKmer) {
         if let Some(a) = self.nodes.get(&kmer) {
-            let mut node = Node(a.load(Ordering::Relaxed));
-            node.set_visited();
-            a.store(node.0, Ordering::Relaxed);
+            a.fetch_or(0b0000_0100, Ordering::AcqRel);
         }
     }
 
@@ -296,9 +294,9 @@ impl GraphDeBruijn {
             return None;
         }
 
-        let mut updated = next_node;
-        updated.set_visited();
-        atomic.store(updated.0, Ordering::Relaxed);
+        if !try_claim(atomic) {
+            return None;
+        }
         Some(oriented)
     }
 
@@ -333,6 +331,113 @@ impl GraphDeBruijn {
         self.nodes.extend(other.nodes);
     }
 
+    /// Build one unitig starting at `start`, whose node flags are `node` (pre-claim value).
+    /// The caller has already atomically claimed `start`; this method claims subsequent nodes.
+    ///
+    /// The first right extension is always included (mirroring the original `start_iter`
+    /// behaviour). Branching checks for subsequent extensions are handled inside
+    /// `iter_unitig_kmers` → `next_unitig_kmer`.
+    fn collect_from_start(&self, start: CanonicalKmer, node: Node, k: usize) -> Unitig {
+        let mut nucs: Vec<u8> = (0..k).map(|i| start.nucleotide(i)).collect();
+        if node.can_extend_right() {
+            let next_c = start.into_kmer().push_right(node.right_nuc()).canonical();
+            if let Some(next_a) = self.nodes.get(&next_c) {
+                let old = next_a.fetch_or(0b0000_0100, Ordering::AcqRel);
+                if old & 0b0000_0100 == 0 {
+                    let oriented = oriented_next(start.into_kmer(), next_c);
+                    nucs.push(oriented.nucleotide(k - 1));
+                    for kmer in self.iter_unitig_kmers(oriented) {
+                        nucs.push(kmer.nucleotide(k - 1));
+                    }
+                }
+            }
+        }
+        Unitig::from_nucleotides(&nucs)
+    }
+
+    /// Returns `true` if `start` is a unitig start node:
+    /// - no unique left predecessor (`!can_extend_left`), or
+    /// - unique left predecessor exists but cannot extend right
+    ///   (i.e., no chain traversal from the left can reach `start`).
+    fn is_start(&self, start: CanonicalKmer, node: Node) -> bool {
+        if !node.can_extend_left() {
+            return true;
+        }
+        let pred = start.into_kmer().push_left(node.left_nuc()).canonical();
+        self.nodes.get(&pred)
+            .map(|a| !Node(a.load(Ordering::Acquire)).can_extend_right())
+            .unwrap_or(false)
+    }
+
+    /// Call `f` once per unitig.
+    ///
+    /// Uses the extended start definition: a node is a start if it has no unique
+    /// left predecessor, or if its left predecessor cannot extend right (so no chain
+    /// traversal from the left could ever claim it).
+    ///
+    /// Two-step execution to avoid races between start claiming and chain extension:
+    ///   1. Claim all starts atomically (parallel).
+    ///   2. Build and emit chains from claimed starts (parallel).
+    ///
+    /// Parallel in production builds; sequential in test builds.
+    /// Must be called before [`for_each_remaining_unitig`].
+    pub fn par_for_each_chain_unitig(&self, f: impl Fn(Unitig) + Sync) {
+        let k = k();
+
+        #[cfg(not(any(test, feature = "test-utils")))]
+        {
+            // Step 1 — claim all starts in parallel.
+            let starts: Vec<CanonicalKmer> = self.nodes
+                .par_iter()
+                .filter_map(|(&start, atomic)| {
+                    let node = Node(atomic.load(Ordering::Acquire));
+                    if node.is_visited() || !self.is_start(start, node) {
+                        return None;
+                    }
+                    let old = atomic.fetch_or(0b0000_0100, Ordering::AcqRel);
+                    (old & 0b0000_0100 == 0).then_some(start)
+                })
+                .collect();
+
+            // Step 2 — build chains in parallel.
+            starts.into_par_iter().for_each(|start| {
+                let node = Node(self.nodes[&start].load(Ordering::Acquire));
+                f(self.collect_from_start(start, node, k));
+            });
+        }
+
+        #[cfg(any(test, feature = "test-utils"))]
+        {
+            let starts: Vec<CanonicalKmer> = self.nodes
+                .iter()
+                .filter_map(|(&start, atomic)| {
+                    let node = Node(atomic.load(Ordering::Acquire));
+                    if node.is_visited() || !self.is_start(start, node) {
+                        return None;
+                    }
+                    let old = atomic.fetch_or(0b0000_0100, Ordering::AcqRel);
+                    (old & 0b0000_0100 == 0).then_some(start)
+                })
+                .collect();
+
+            for start in starts {
+                let node = Node(self.nodes[&start].load(Ordering::Acquire));
+                f(self.collect_from_start(start, node, k));
+            }
+        }
+    }
+
+    /// Call `f` for each node still unvisited after [`par_for_each_chain_unitig`].
+    /// Handles true cycles and rare deeply-nested junctions. Always sequential.
+    pub fn for_each_remaining_unitig(&self, f: impl Fn(Unitig)) {
+        let k = k();
+        for (&kmer, atomic) in &self.nodes {
+            let old = atomic.fetch_or(0b0000_0100, Ordering::AcqRel);
+            if old & 0b0000_0100 != 0 { continue; }
+            f(self.collect_from_start(kmer, Node(old), k));
+        }
+    }
+
     pub fn len(&self) -> usize {
         self.nodes.len()
     }
@@ -448,6 +553,12 @@ impl Iterator for UnitigIter<'_> {
 
 // ── helpers ───────────────────────────────────────────────────────────────────
 
+/// Atomically set the visited bit. Returns `true` iff this call claimed the node.
+#[inline]
+fn try_claim(atomic: &AtomicU8) -> bool {
+    atomic.fetch_or(0b0000_0100, Ordering::AcqRel) & 0b0000_0100 == 0
+}
+
 fn oriented_next(from: Kmer, to: CanonicalKmer) -> Kmer {
     if from.is_overlapping(to.into_kmer()) {
         to.into_kmer()
diff --git a/src/obikmer/src/cmd/unitig.rs b/src/obikmer/src/cmd/unitig.rs
index 4a81703..79f6ed2 100644
--- a/src/obikmer/src/cmd/unitig.rs
+++ b/src/obikmer/src/cmd/unitig.rs
@@ -1,5 +1,7 @@
 use std::io::{self, BufWriter, Write};
 use std::path::PathBuf;
+use std::sync::Mutex;
+use std::sync::atomic::{AtomicUsize, Ordering};
 
 use clap::Args;
 use obidebruinj::GraphDeBruijn;
@@ -68,24 +70,32 @@ pub fn run(args: UnitigArgs) {
     g.compute_degrees();
     rep.push(stage.stop());
 
-    // ── Phase 3 : enumerate unitigs and write as FASTA ────────────────────────
-    let pb = spinner("unitig");
-    let stdout = io::stdout();
-    let mut out = BufWriter::new(stdout.lock());
+    // ── Phase 3 : enumerate unitigs and write as FASTA ───────────────────────
+    let pb  = spinner("unitig");
+    let out = Mutex::new(BufWriter::new(io::stdout()));
+    let j   = AtomicUsize::new(0);
 
-    let stage = Stage::start("enumerate unitigs");
-    for (j, unitig) in g.iter_unitig().enumerate() {
-        write_unitig(&unitig, k, 0, j, &mut out).unwrap_or_else(|e| {
+    let write_unitig_fn = |unitig: obikseq::unitig::Unitig| {
+        let idx = j.fetch_add(1, Ordering::Relaxed);
+        let mut w = out.lock().unwrap();
+        write_unitig(&unitig, k, 0, idx, &mut *w).unwrap_or_else(|e| {
             eprintln!("write error: {e}");
             std::process::exit(1);
         });
-        if j % 10_000 == 0 {
-            pb.set_message(format!("{j} unitigs written"));
+        if idx % 10_000 == 0 {
+            pb.set_message(format!("{idx} unitigs written"));
         }
-    }
+    };
+
+    let stage = Stage::start("chain unitigs");
+    g.par_for_each_chain_unitig(&write_unitig_fn);
+    rep.push(stage.stop());
+
+    let stage = Stage::start("remaining unitigs");
+    g.for_each_remaining_unitig(&write_unitig_fn);
     pb.finish_and_clear();
     rep.push(stage.stop());
 
-    out.flush().expect("flush error");
+    out.into_inner().unwrap().flush().expect("flush error");
     rep.print();
 }