diff --git a/tests/test_alta_open_lambda.py b/tests/test_alta_open_lambda.py
index fd86d8c..3eb36e6 100644
--- a/tests/test_alta_open_lambda.py
+++ b/tests/test_alta_open_lambda.py
@@ -22,6 +22,8 @@
 """
 
 import json
+import logging
+import random
 import sys
 from pathlib import Path
 
@@ -61,6 +63,13 @@ def make_event(event_trigger, data, custom_parameters):
 NEW_PARAMS = {"webhook_name": "UpdateMembership20260509", "legacy": "false"}
 
 
+def rand_id():
+    """A random Neon-style numeric id. The handler passes ids straight through,
+    so the exact value is irrelevant -- randomizing makes that explicit and
+    keeps arbitrary numbers from looking meaningful."""
+    return random.randint(1, 9_999_999)
+
+
 # ===========================================================================
 # Guard clauses -- trigger-independent; these exercise the harness end to end
 # ===========================================================================
@@ -85,3 +94,76 @@ def test_body_as_dict_currently_raises(openpath):
     with pytest.raises(TypeError):
         lf.lambda_handler(event, {})
     openpath.assert_not_called()
+
+
+# ===========================================================================
+# editAccount
+# ===========================================================================
+
+
+def legacy_edit_account(account_id=None):
+    # legacy: integer accountId, nested individualAccount
+    account_id = rand_id() if account_id is None else account_id
+    return make_event(
+        "editAccount",
+        {
+            "individualAccount": {
+                "accountId": account_id,
+                "primaryContact": {"contactId": account_id},
+                "customFieldDataList": {"customFieldData": []},
+            }
+        },
+        UNKNOWN_PARAMS,
+    )
+
+
+def test_legacy_edit_account_passes_int_id(openpath):
+    account_id = rand_id()
+    lf.lambda_handler(legacy_edit_account(account_id), {})
+    openpath.assert_called_once_with(account_id)
+
+
+# ===========================================================================
+# updateMembership
+# ===========================================================================
+
+
+def legacy_update_membership(account_id=None):
+    # legacy: nested membershipEnrollment + transaction
+    account_id = rand_id() if account_id is None else account_id
+    return make_event(
+        "updateMembership",
+        {
+            "membershipEnrollment": {
+                "accountId": account_id,
+                "membershipId": rand_id(),
+                "enrollmentType": "RENEW",
+            },
+            "transaction": {
+                "transactionId": rand_id(),
+                "transactionStatus": "SUCCEEDED",
+            },
+        },
+        LEGACY_PARAMS,
+    )
+
+
+def test_legacy_update_membership_passes_int_id(openpath):
+    account_id = rand_id()
+    lf.lambda_handler(legacy_update_membership(account_id), {})
+    openpath.assert_called_once_with(account_id)
+
+
+# ===========================================================================
+# Legacy-flag detection -- customParameters.legacy
+# ===========================================================================
+
+def test_null_custom_parameters_does_not_warn_or_crash(openpath, caplog):
+    # Truly-legacy events deliver customParameters: null. This must not warn
+    # and must not raise (regression guard for the historical NoneType bug).
+    account_id = rand_id()
+    event = make_event("editAccount", {"individualAccount": {"accountId": account_id}}, UNKNOWN_PARAMS)
+    with caplog.at_level(logging.WARNING):
+        lf.lambda_handler(event, {})
+    assert "LEGACY EVENT DETECTED" not in caplog.text
+    openpath.assert_called_once_with(account_id)