diff --git a/include/core/radix_sort.h b/include/core/radix_sort.h
index 8f1e31a4..9e234cd6 100644
--- a/include/core/radix_sort.h
+++ b/include/core/radix_sort.h
@@ -31,4 +31,8 @@ void radix_sort_rows(int *rows, size_t n, const int *sparsity_IDs,
 void parallel_radix_sort_rows(int *rows, size_t n, const int *sparsity_IDs,
                               const int *coeff_hashes, int *aux);
 
+// Single-key LSD radix sort. Sorts indices by keys[indices[i]]
+// ascending (unsigned order). Stable. aux must have space for n ints.
+void radix_sort_by_key(int *indices, size_t n, const int *keys, int *aux);
+
 #endif /* RADIX_SORT_H */
diff --git a/src/core/radix_sort.c b/src/core/radix_sort.c
index fc43c2c2..dffc38bf 100644
--- a/src/core/radix_sort.c
+++ b/src/core/radix_sort.c
@@ -138,6 +138,98 @@ void radix_sort_rows(int *rows, size_t n, const int *sparsity_IDs,
     }
 }
 
+// --- Single-key radix sort ---
+
+static void insertion_sort_by_key(int *indices, size_t n,
+                                  const int *keys)
+{
+    for (size_t i = 1; i < n; i++)
+    {
+        int idx = indices[i];
+        int idx_key = keys[idx];
+        size_t j = i;
+        while (j > 0 && keys[indices[j - 1]] > idx_key)
+        {
+            indices[j] = indices[j - 1];
+            j--;
+        }
+        indices[j] = idx;
+    }
+}
+
+void radix_sort_by_key(int *indices, size_t n,
+                       const int *keys, int *aux)
+{
+    if (n < 256)
+    {
+        insertion_sort_by_key(indices, n, keys);
+        return;
+    }
+
+    size_t counts[256];
+    int *src = indices, *dst = aux;
+
+    for (int pass = 0; pass < 4; pass++)
+    {
+        int shift = pass * 8;
+
+        // histogram
+        memset(counts, 0, 256 * sizeof(size_t));
+        for (size_t i = 0; i < n; i++)
+        {
+            unsigned byte =
+                ((uint32_t) keys[src[i]] >> shift) & 0xFF;
+            counts[byte]++;
+        }
+
+        // skip pass if all values fall in one bucket
+        if (pass > 0)
+        {
+            int skip = 0;
+            for (int b = 0; b < 256; b++)
+            {
+                if (counts[b] == n)
+                {
+                    skip = 1;
+                    break;
+                }
+            }
+            if (skip)
+            {
+                continue;
+            }
+        }
+
+        // prefix sum
+        size_t total = 0;
+        for (int b = 0; b < 256; b++)
+        {
+            size_t c = counts[b];
+            counts[b] = total;
+            total += c;
+        }
+
+        // scatter (forward — stable)
+        for (size_t i = 0; i < n; i++)
+        {
+            unsigned byte =
+                ((uint32_t) keys[src[i]] >> shift) & 0xFF;
+            dst[counts[byte]++] = src[i];
+        }
+
+        // swap src and dst
+        int *tmp = src;
+        src = dst;
+        dst = tmp;
+    }
+
+    // if result ended up in aux, copy back to indices
+    if (src != indices)
+    {
+        memcpy(indices, src, n * sizeof(int));
+    }
+}
+
 // --- Parallel radix sort infrastructure ---
 
 typedef struct
diff --git a/src/explorers/Primal_propagation.c b/src/explorers/Primal_propagation.c
index 73417fbf..6efab08e 100644
--- a/src/explorers/Primal_propagation.c
+++ b/src/explorers/Primal_propagation.c
@@ -28,6 +28,7 @@
 #include "SimpleReductions.h"
 #include "State.h"
 #include "Workspace.h"
+#include "radix_sort.h"
 #include "utils.h"
 
 static PresolveStatus update_lb_within_propagation(double new_lb, double *lb,
@@ -782,27 +783,6 @@ PresolveStatus propagate_primal(Problem *prob, bool finite_bound_tightening)
     return UNCHANGED;
 }
 
-// Static variable to hold col_sizes (for qsort)
-static const int *global_col_sizes;
-
-int compare_col_len(const void *a, const void *b)
-{
-    int idx_a = *(const int *) a;
-    int idx_b = *(const int *) b;
-
-    if (global_col_sizes[idx_a] < global_col_sizes[idx_b])
-    {
-        return -1;
-    }
-    if (global_col_sizes[idx_a] > global_col_sizes[idx_b])
-    {
-        return 1;
-    }
-
-    // if equal len, sort by index
-    return idx_a - idx_b;
-}
-
 void remove_redundant_bounds(Constraints *constraints)
 {
     const Matrix *A = constraints->A;
@@ -830,9 +810,8 @@ void remove_redundant_bounds(Constraints *constraints)
         col_order[ii] = ii;
     }
 
-    global_col_sizes = col_sizes;
-    qsort(col_order, n_cols, sizeof(int), compare_col_len);
-    global_col_sizes = NULL;
+    int *aux = constraints->state->work->radix_aux;
+    radix_sort_by_key(col_order, n_cols, col_sizes, aux);
 
     for (jj = 0; jj < n_cols; jj++)
     {
@@ -863,7 +842,7 @@ void remove_redundant_bounds(Constraints *constraints)
                                    bounds))
             {
                 remove_finite_lb_from_activities(&col, acts, bounds[ii].lb);
-                DEBUG(bounds[ii].lb = -INF);
+                bounds[ii].lb = -INF;
                 UPDATE_TAG(col_tags[ii], C_TAG_LB_INF);
             }
         }
@@ -875,7 +854,7 @@ void remove_redundant_bounds(Constraints *constraints)
                                    bounds))
             {
                 remove_finite_ub_from_activities(&col, acts, bounds[ii].ub);
-                DEBUG(bounds[ii].ub = INF);
+                bounds[ii].ub = INF;
                 UPDATE_TAG(col_tags[ii], C_TAG_UB_INF);
             }
         }
diff --git a/tests/test_radix_sort.h b/tests/test_radix_sort.h
index a6ad1f8c..af7c49df 100644
--- a/tests/test_radix_sort.h
+++ b/tests/test_radix_sort.h
@@ -432,6 +432,141 @@ static char *test_14_radix_sort()
     return 0;
 }
 
+// ---- Tests for radix_sort_by_key ----
+
+// helper: check indices sorted ascending by keys[indices[i]]
+static int is_sorted_by_single_key(const int *indices, int n, const int *keys)
+{
+    for (int i = 1; i < n; i++)
+    {
+        unsigned int prev = (unsigned int) keys[indices[i - 1]];
+        unsigned int curr = (unsigned int) keys[indices[i]];
+        if (prev > curr)
+        {
+            return 0;
+        }
+    }
+    return 1;
+}
+
+/* Test 15: basic ascending sort by single key */
+static char *test_15_radix_sort()
+{
+    int n = 5;
+    int indices[] = {0, 1, 2, 3, 4};
+    int keys[] = {30, 10, 50, 20, 40};
+    int aux[5];
+
+    radix_sort_by_key(indices, (size_t) n, keys, aux);
+    mu_assert("should be sorted by key", is_sorted_by_single_key(indices, n, keys));
+    mu_assert("should be permutation", is_permutation(indices, n));
+    // expected: 1(10), 3(20), 0(30), 4(40), 2(50)
+    mu_assert("pos 0", indices[0] == 1);
+    mu_assert("pos 1", indices[1] == 3);
+    mu_assert("pos 2", indices[2] == 0);
+    mu_assert("pos 3", indices[3] == 4);
+    mu_assert("pos 4", indices[4] == 2);
+    return 0;
+}
+
+/* Test 16: already-sorted input */
+static char *test_16_radix_sort()
+{
+    int n = 5;
+    int indices[] = {0, 1, 2, 3, 4};
+    int keys[] = {1, 2, 3, 4, 5};
+    int aux[5];
+
+    radix_sort_by_key(indices, (size_t) n, keys, aux);
+    mu_assert("should be sorted", is_sorted_by_single_key(indices, n, keys));
+    for (int i = 0; i < n; i++)
+    {
+        mu_assert("order preserved", indices[i] == i);
+    }
+    return 0;
+}
+
+/* Test 17: reverse-sorted input */
+static char *test_17_radix_sort()
+{
+    int n = 5;
+    int indices[] = {0, 1, 2, 3, 4};
+    int keys[] = {50, 40, 30, 20, 10};
+    int aux[5];
+
+    radix_sort_by_key(indices, (size_t) n, keys, aux);
+    mu_assert("should be sorted", is_sorted_by_single_key(indices, n, keys));
+    mu_assert("first", indices[0] == 4);
+    mu_assert("last", indices[4] == 0);
+    return 0;
+}
+
+/* Test 18: duplicate keys — stability preserves input order */
+static char *test_18_radix_sort()
+{
+    int n = 6;
+    int indices[] = {0, 1, 2, 3, 4, 5};
+    int keys[] = {5, 5, 5, 10, 10, 10};
+    int aux[6];
+
+    radix_sort_by_key(indices, (size_t) n, keys, aux);
+    mu_assert("should be sorted", is_sorted_by_single_key(indices, n, keys));
+    mu_assert("should be permutation", is_permutation(indices, n));
+    // stable: group key=5 keeps order 0,1,2; group key=10 keeps 3,4,5
+    mu_assert("stable 0", indices[0] == 0);
+    mu_assert("stable 1", indices[1] == 1);
+    mu_assert("stable 2", indices[2] == 2);
+    mu_assert("stable 3", indices[3] == 3);
+    mu_assert("stable 4", indices[4] == 4);
+    mu_assert("stable 5", indices[5] == 5);
+    return 0;
+}
+
+/* Test 19: large input (n=300) forces radix path */
+static char *test_19_radix_sort()
+{
+    int n = 300;
+    int indices[300];
+    int keys[300];
+    int aux[300];
+
+    srand(54321);
+    for (int i = 0; i < n; i++)
+    {
+        indices[i] = i;
+        keys[i] = rand() % 100;
+    }
+
+    radix_sort_by_key(indices, (size_t) n, keys, aux);
+    mu_assert("should be sorted", is_sorted_by_single_key(indices, n, keys));
+    mu_assert("should be permutation", is_permutation(indices, n));
+    return 0;
+}
+
+/* Test 20: large input with all identical keys (skip-pass opt) */
+static char *test_20_radix_sort()
+{
+    int n = 300;
+    int indices[300];
+    int keys[300];
+    int aux[300];
+
+    for (int i = 0; i < n; i++)
+    {
+        indices[i] = i;
+        keys[i] = 42;
+    }
+
+    radix_sort_by_key(indices, (size_t) n, keys, aux);
+    mu_assert("should be sorted", is_sorted_by_single_key(indices, n, keys));
+    // stable: identical keys preserve input order 0,1,...,n-1
+    for (int i = 0; i < n; i++)
+    {
+        mu_assert("stable order", indices[i] == i);
+    }
+    return 0;
+}
+
 static const char *all_tests_radix_sort()
 {
     mu_run_test(test_1_radix_sort, counter_radix_sort);
@@ -448,6 +583,12 @@ static const char *all_tests_radix_sort()
     mu_run_test(test_12_radix_sort, counter_radix_sort);
     mu_run_test(test_13_radix_sort, counter_radix_sort);
     mu_run_test(test_14_radix_sort, counter_radix_sort);
+    mu_run_test(test_15_radix_sort, counter_radix_sort);
+    mu_run_test(test_16_radix_sort, counter_radix_sort);
+    mu_run_test(test_17_radix_sort, counter_radix_sort);
+    mu_run_test(test_18_radix_sort, counter_radix_sort);
+    mu_run_test(test_19_radix_sort, counter_radix_sort);
+    mu_run_test(test_20_radix_sort, counter_radix_sort);
 
     return 0;
 }