diff --git a/src/java/org/apache/cassandra/io/sstable/ULIDBasedSSTableId.java b/src/java/org/apache/cassandra/io/sstable/ULIDBasedSSTableId.java
index 543cbca32dd9..69aedc506d9c 100644
--- a/src/java/org/apache/cassandra/io/sstable/ULIDBasedSSTableId.java
+++ b/src/java/org/apache/cassandra/io/sstable/ULIDBasedSSTableId.java
@@ -28,6 +28,7 @@
 import java.util.stream.Stream;
 import javax.annotation.Nonnull;
 
+import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Preconditions;
 
 import de.huxhorn.sulky.ulid.ULID;
@@ -109,11 +110,17 @@ public static class Builder implements SSTableId.Builder<ULIDBasedSSTableId>
     {
         private static final Pattern PATTERN = Pattern.compile("[0-9a-z]{26}", Pattern.CASE_INSENSITIVE);
 
-        public static final Builder instance = new Builder();
+        public static final Builder instance = new Builder(new ULID());
 
-        private static final ULID ulid = new ULID();
+        private final ULID ulid;
         private static final AtomicReference<ULID.Value> prevRef = new AtomicReference<>();
 
+        @VisibleForTesting
+        Builder(ULID ulid)
+        {
+            this.ulid = ulid;
+        }
+
         /**
          * Creates a new ULID based identifiers generator.
          *
@@ -139,7 +146,7 @@ public Supplier<ULIDBasedSSTableId> generator(Stream<SSTableId> existingIdentifi
                     {
                         newVal = ulid.nextValue();
                     }
-                } while (newVal != null && !prevRef.compareAndSet(prevVal, newVal));
+                } while (newVal == null || !prevRef.compareAndSet(prevVal, newVal));
                 return new ULIDBasedSSTableId(newVal);
             };
         }
diff --git a/test/unit/org/apache/cassandra/io/sstable/ULIDBasedSSTableIdGeneratorTest.java b/test/unit/org/apache/cassandra/io/sstable/ULIDBasedSSTableIdGeneratorTest.java
new file mode 100644
index 000000000000..a6af258070cf
--- /dev/null
+++ b/test/unit/org/apache/cassandra/io/sstable/ULIDBasedSSTableIdGeneratorTest.java
@@ -0,0 +1,225 @@
+/*
+ * Copyright DataStax, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.cassandra.io.sstable;
+
+import java.lang.reflect.Field;
+import java.util.HashSet;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.BrokenBarrierException;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.CyclicBarrier;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Supplier;
+import java.util.stream.Stream;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.mockito.Mockito;
+
+import de.huxhorn.sulky.ulid.ULID;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for ULIDBasedSSTableId.Builder.generator() method focusing on the specific
+ * retry logic and state transitions introduced in PR#2175.
+ *<p>
+ * Note: Basic generator functionality (uniqueness, sorting, concurrent access) is already
+ * tested in SSTableIdTest.testULIDBasedIdProperties() and generatorFuzzTest().
+ *<p>
+ * These tests specifically focus on:
+ * - The retry loop when nextStrictlyMonotonicValue() returns empty Optional
+ * - State transitions from null to initialized
+ * - concurrency generation that may trigger retry logic
+ */
+public class ULIDBasedSSTableIdGeneratorTest
+{
+    private AtomicReference<ULID.Value> prevRef;
+
+    @Before
+    public void setUp() throws Exception
+    {
+        Field prevRefField = ULIDBasedSSTableId.Builder.class.getDeclaredField("prevRef");
+        prevRefField.setAccessible(true);
+        prevRef = (AtomicReference<ULID.Value>) prevRefField.get(null);
+        prevRef.set(null);
+    }
+
+    /**
+     * Test initial state: when prevRef is null, generator should call ulid.nextValue()
+     * and successfully set the prevRef.
+     */
+    @Test
+    public void testGeneratorInitialState()
+    {
+        Supplier<ULIDBasedSSTableId> generator = ULIDBasedSSTableId.Builder.instance.generator(Stream.empty());
+
+        ULIDBasedSSTableId id1 = generator.get();
+        assertThat(id1).isNotNull();
+        assertThat(prevRef.get()).isNotNull();
+        assertThat(prevRef.get()).isEqualTo(id1.ulid);
+    }
+
+    /**
+     * Test monotonic progression: subsequent calls should generate strictly monotonic ULIDs.
+     */
+    @Test
+    public void testGeneratorMonotonicProgression()
+    {
+        Supplier<ULIDBasedSSTableId> generator = ULIDBasedSSTableId.Builder.instance.generator(Stream.empty());
+
+        ULIDBasedSSTableId id1 = generator.get();
+        ULIDBasedSSTableId id2 = generator.get();
+        ULIDBasedSSTableId id3 = generator.get();
+
+        assertThat(id1.compareTo(id2)).isLessThan(0);
+        assertThat(id2.compareTo(id3)).isLessThan(0);
+        assertThat(id1.compareTo(id3)).isLessThan(0);
+    }
+
+    @Test
+    public void testGeneratorRetryOnEmptyOptional()
+    {
+        // Use a real ULID just to manufacture realistic ULID.Value instances
+        ULID realUlid = new ULID();
+
+        // Simulate an already existing previous value so that the generator
+        // takes the nextStrictlyMonotonicValue(prevVal) branch.
+        ULID.Value prevVal = realUlid.nextValue();
+        prevRef.set(prevVal);
+
+        // This is the value we expect to be used after the retry.
+        ULID.Value nextVal = realUlid.nextValue();
+
+        // Mock the ULID that will be injected into the Builder
+        ULID mockUlid = Mockito.mock(ULID.class);
+
+        // First call returns Optional.empty() → used to trigger the retry.
+        // Second call returns nextVal → should be the value that generator uses.
+        Mockito.when(mockUlid.nextStrictlyMonotonicValue(prevVal))
+                        .thenReturn(Optional.empty(), Optional.of(nextVal));
+
+        // Create a Builder that uses our mock ULID instead of the static one
+        ULIDBasedSSTableId.Builder builder = new ULIDBasedSSTableId.Builder(mockUlid);
+        Supplier<ULIDBasedSSTableId> generator = builder.generator(Stream.empty());
+
+        // With the old loop condition (newVal != null && !CAS),
+        // this would have exited after the Optional.empty() with newVal == null,
+        // leading to a NPE in ULIDBasedSSTableId(newVal).
+        ULIDBasedSSTableId id = generator.get();
+
+        // Now we expect a successful retry and a non-null ULID
+        assertThat(id).isNotNull();
+        assertThat(id.ulid).isEqualTo(nextVal);
+        assertThat(prevRef.get()).isEqualTo(nextVal);
+
+        // Ensure we actually hit the retry path (two calls: empty, then present)
+        Mockito.verify(mockUlid, Mockito.times(2)).nextStrictlyMonotonicValue(prevVal);
+    }
+
+    /**
+     * Test the retry loop when nextStrictlyMonotonicValue
+     *  by generating many IDs rapidly in a tight loop, which may
+     * trigger the retry logic when the timestamp hasn't advanced enough for
+     * strictly monotonic values.
+     */
+    @Test
+    public void testGeneratorRetry()
+    {
+        Supplier<ULIDBasedSSTableId> generator = ULIDBasedSSTableId.Builder.instance.generator(Stream.empty());
+
+        // Generate many IDs in rapid succession
+        // This increases the likelihood of hitting the case where nextStrictlyMonotonicValue
+        // returns empty because the timestamp hasn't advanced
+        Set<ULIDBasedSSTableId> ids = new HashSet<>();
+        for (int i = 0; i < 1000; i++)
+        {
+            ULIDBasedSSTableId id = generator.get();
+            assertThat(id).isNotNull();
+            assertThat(ids.add(id)).isTrue(); // Ensure all IDs are unique
+        }
+
+        assertThat(ids).hasSize(1000);
+
+        // Verify monotonic ordering
+        ULIDBasedSSTableId[] sortedIds = ids.toArray(new ULIDBasedSSTableId[0]);
+        java.util.Arrays.sort(sortedIds);
+        for (int i = 1; i < sortedIds.length; i++)
+            assertThat(sortedIds[i - 1].compareTo(sortedIds[i])).isLessThan(0);
+    }
+
+    /**
+     * Test concurrent access: multiple threads generating IDs simultaneously
+     * should produce unique, monotonically increasing IDs without duplicates.
+     */
+    @Test
+    public void testGeneratorConcurrentAccess() throws InterruptedException
+    {
+        final int NUM_THREADS = 20;
+        final int IDS_PER_THREAD = 50;
+
+        ExecutorService executor = Executors.newFixedThreadPool(NUM_THREADS);
+        CyclicBarrier barrier = new CyclicBarrier(NUM_THREADS);
+        Set<ULIDBasedSSTableId> allIds = ConcurrentHashMap.newKeySet();
+        CountDownLatch latch = new CountDownLatch(NUM_THREADS);
+
+        Supplier<ULIDBasedSSTableId> generator = ULIDBasedSSTableId.Builder.instance.generator(Stream.empty());
+
+        for (int i = 0; i < NUM_THREADS; i++)
+        {
+            executor.submit(() -> {
+                try
+                {
+                    barrier.await(); // Synchronize start
+                    for (int j = 0; j < IDS_PER_THREAD; j++)
+                    {
+                        ULIDBasedSSTableId id = generator.get();
+                        assertThat(id).isNotNull();
+                        allIds.add(id);
+                    }
+                }
+                catch (InterruptedException | BrokenBarrierException e)
+                {
+                    throw new RuntimeException(e);
+                }
+                finally
+                {
+                    latch.countDown();
+                }
+            });
+        }
+
+        assertThat(latch.await(30, TimeUnit.SECONDS)).isTrue();
+        executor.shutdown();
+
+        // Verify all IDs are unique
+        assertThat(allIds).hasSize(NUM_THREADS * IDS_PER_THREAD);
+
+        // Verify monotonic ordering
+        ULIDBasedSSTableId[] sortedIds = allIds.toArray(new ULIDBasedSSTableId[0]);
+        java.util.Arrays.sort(sortedIds);
+
+        for (int i = 1; i < sortedIds.length; i++)
+            assertThat(sortedIds[i - 1].compareTo(sortedIds[i])).isLessThan(0);
+    }
+
+}