diff --git a/hindsight-clients/python/hindsight_client/hindsight_client.py b/hindsight-clients/python/hindsight_client/hindsight_client.py
index b4e4441f..c88768a9 100644
--- a/hindsight-clients/python/hindsight_client/hindsight_client.py
+++ b/hindsight-clients/python/hindsight_client/hindsight_client.py
@@ -204,7 +204,7 @@ class Hindsight:
 
         request_obj = retain_request.RetainRequest(
             items=memory_items,
-            async_=retain_async,
+            var_async=retain_async,
             document_tags=document_tags,
         )
 
@@ -618,7 +618,7 @@ class Hindsight:
 
         request_obj = retain_request.RetainRequest(
             items=memory_items,
-            async_=retain_async,
+            var_async=retain_async,
             document_tags=document_tags,
         )
 
diff --git a/hindsight-clients/python/tests/test_retain_request_async.py b/hindsight-clients/python/tests/test_retain_request_async.py
new file mode 100644
index 00000000..afe53c38
--- /dev/null
+++ b/hindsight-clients/python/tests/test_retain_request_async.py
@@ -0,0 +1,42 @@
+"""
+Test that RetainRequest correctly serializes the async field.
+
+Regression test for a bug where the client passed async_=True (invalid kwarg)
+instead of var_async=True, causing async mode to be silently ignored.
+"""
+
+from hindsight_client_api.models.memory_item import MemoryItem
+from hindsight_client_api.models.retain_request import RetainRequest
+
+
+def _make_item():
+    return MemoryItem(content="test content")
+
+
+def test_retain_request_async_true_serialized():
+    """var_async=True must appear as 'async': True in the serialized dict."""
+    req = RetainRequest(items=[_make_item()], var_async=True)
+    d = req.to_dict()
+    assert d["async"] is True
+
+
+def test_retain_request_async_false_serialized():
+    """var_async=False (default) must appear as 'async': False."""
+    req = RetainRequest(items=[_make_item()], var_async=False)
+    d = req.to_dict()
+    assert d["async"] is False
+
+
+def test_retain_request_default_is_sync():
+    """Omitting var_async should default to synchronous (async=False)."""
+    req = RetainRequest(items=[_make_item()])
+    d = req.to_dict()
+    assert d["async"] is False
+
+
+def test_retain_request_async_json_roundtrip():
+    """async=True must survive a JSON serialization roundtrip."""
+    req = RetainRequest(items=[_make_item()], var_async=True)
+    json_str = req.to_json()
+    restored = RetainRequest.from_json(json_str)
+    assert restored.var_async is True