diff --git a/sdk/src/test/java/software/amazon/lambda/durable/operation/ParallelOperationTest.java b/sdk/src/test/java/software/amazon/lambda/durable/operation/ParallelOperationTest.java
index a7693591..4aa32d60 100644
--- a/sdk/src/test/java/software/amazon/lambda/durable/operation/ParallelOperationTest.java
+++ b/sdk/src/test/java/software/amazon/lambda/durable/operation/ParallelOperationTest.java
@@ -19,6 +19,7 @@
 import software.amazon.awssdk.services.lambda.model.OperationAction;
 import software.amazon.awssdk.services.lambda.model.OperationStatus;
 import software.amazon.awssdk.services.lambda.model.OperationType;
+import software.amazon.awssdk.services.lambda.model.OperationUpdate;
 import software.amazon.lambda.durable.DurableConfig;
 import software.amazon.lambda.durable.TestUtils;
 import software.amazon.lambda.durable.TypeToken;
@@ -44,14 +45,21 @@ class ParallelOperationTest {
 
     private DurableContextImpl durableContext;
     private ExecutionManager executionManager;
+    // Thread-safe backing store for getOperationAndUpdateReplayState.
+    // Tests pre-populate this; doAnswer writes here before firing onCheckpointComplete,
+    // guaranteeing visibility to any thread that reads after the future unblocks.
+    private ConcurrentHashMap<String, Operation> operationStore;
 
     @BeforeEach
     void setUp() {
         durableContext = mock(DurableContextImpl.class);
         executionManager = mock(ExecutionManager.class);
+        operationStore = new ConcurrentHashMap<>();
 
         when(executionManager.getCurrentThreadContext()).thenReturn(new ThreadContext(null, ThreadType.CONTEXT));
-        when(executionManager.getOperationAndUpdateReplayState(anyString())).thenReturn(null);
+        // Delegate to operationStore so all reads see the latest write, regardless of thread.
+        when(executionManager.getOperationAndUpdateReplayState(anyString()))
+                .thenAnswer(inv -> operationStore.get(inv.getArgument(0)));
 
         var childContext = mock(DurableContextImpl.class);
         when(childContext.getExecutionManager()).thenReturn(executionManager);
@@ -77,9 +85,10 @@ void setUp() {
                 .when(executionManager)
                 .registerOperation(any());
 
-        // Simulate the real backend for all sendOperationUpdate calls:
-        // - For SUCCEED on the parallel op: update the stub and fire onCheckpointComplete to unblock join().
-        // - For everything else (START, child checkpoints): just return a completed future.
+        // Simulate the real backend for all sendOperationUpdate calls.
+        // For SUCCEED on the parallel op: write to operationStore first (establishes happens-before
+        // via ConcurrentHashMap's volatile semantics), then fire onCheckpointComplete to unblock join().
+        // This ordering guarantees getOperationAndUpdateReplayState() never returns null after unblocking.
         var succeededParallelOp = Operation.builder()
                 .id(OPERATION_ID)
                 .name("test-parallel")
@@ -88,11 +97,11 @@ void setUp() {
                 .status(OperationStatus.SUCCEEDED)
                 .build();
         doAnswer(inv -> {
-                    var update = (software.amazon.awssdk.services.lambda.model.OperationUpdate) inv.getArgument(0);
+                    var update = (OperationUpdate) inv.getArgument(0);
 
                     if (OPERATION_ID.equals(update.id()) && update.action() == OperationAction.SUCCEED) {
-                        when(executionManager.getOperationAndUpdateReplayState(OPERATION_ID))
-                                .thenReturn(succeededParallelOp);
+                        // Write before completing the future — ConcurrentHashMap guarantees visibility.
+                        operationStore.put(OPERATION_ID, succeededParallelOp);
                         var op = registeredOps.get(OPERATION_ID);
                         if (op != null) {
                             op.onCheckpointComplete(succeededParallelOp);