diff --git a/.gitea/workflows/release.yml b/.gitea/workflows/release.yml
index df9362f..810a12e 100644
--- a/.gitea/workflows/release.yml
+++ b/.gitea/workflows/release.yml
@@ -86,7 +86,11 @@ jobs:
   build-macos-arm64:
     needs: create-release
     runs-on: ubuntu-latest
-    container: registry.metabarcoding.org/cibuilder/rustcrossosx:latest
+    container:
+      image: registry.metabarcoding.org/cibuilder/rustcrossosx:latest
+      credentials:
+        username: ${{ github.actor }}
+        password: ${{ secrets.REGISTRYTOKEN }}
     defaults:
       run:
         working-directory: src
diff --git a/src/Cargo.lock b/src/Cargo.lock
index 5cbe11b..02603ed 100644
--- a/src/Cargo.lock
+++ b/src/Cargo.lock
@@ -1704,7 +1704,7 @@ dependencies = [
 
 [[package]]
 name = "obikmer"
-version = "1.1.26"
+version = "1.1.27"
 dependencies = [
  "clap",
  "csv",
diff --git a/src/obikindex/src/numa.rs b/src/obikindex/src/numa.rs
index 42eb48b..d5c368f 100644
--- a/src/obikindex/src/numa.rs
+++ b/src/obikindex/src/numa.rs
@@ -287,8 +287,9 @@ impl PartitionRunner {
             drop(event_tx);
 
             // ── Controller ────────────────────────────────────────────────────
-            activate_tx.send(()).ok();
-            let mut n_active              = 1usize;
+            let initial_workers = n_nodes.min(max_workers).min(n_total);
+            for _ in 0..initial_workers { activate_tx.send(()).ok(); }
+            let mut n_active              = initial_workers;
             let mut cpu_sample            = CpuSample::now();
             let mut eff_at_last_spawn     = 0.0f64; // 0 = no previous spawn to evaluate
             let mut completed             = 0usize;
@@ -361,13 +362,13 @@ fn maybe_activate(
     // Going from k-1 → k workers, the minimum acceptable speedup is (k-1+0.2)/(k-1).
     // For the very first extra worker (n_active == 1, no previous spawn), skip this
     // check: eff_at_last_spawn == 0 acts as the sentinel.
-    let last_spawn_was_beneficial = if *eff_at_last_spawn < 1e-9 {
-        true  // first additional worker: no prior data to evaluate
+    let last_spawn_was_beneficial = if *eff_at_last_spawn < 1e-9 || eff < 1e-9 {
+        true  // first additional worker, or measurement too short: no prior data to evaluate
     } else {
-        let k_before   = (*n_active - 1) as f64;
-        let min_speedup = (k_before + 0.2) / k_before;
-        let actual_speedup = eff / *eff_at_last_spawn;
-        actual_speedup >= min_speedup
+        let k_new = *n_active as f64;  // worker count after the last spawn
+        let min_gain = 0.2 / k_new;
+        let actual_gain = (eff - *eff_at_last_spawn) / eff;
+        actual_gain >= min_gain
     };
 
     if last_spawn_was_beneficial {
diff --git a/src/obikmer/Cargo.toml b/src/obikmer/Cargo.toml
index 4521ec4..6658b67 100644
--- a/src/obikmer/Cargo.toml
+++ b/src/obikmer/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "obikmer"
-version = "1.1.26"
+version = "1.1.27"
 edition = "2024"
 
 [[bin]]