diff --git a/erpnext/accounts/doctype/account_closing_balance/test_account_closing_balance.py b/erpnext/accounts/doctype/account_closing_balance/test_account_closing_balance.py
index a39bd00579e..ca9bfd5731d 100644
--- a/erpnext/accounts/doctype/account_closing_balance/test_account_closing_balance.py
+++ b/erpnext/accounts/doctype/account_closing_balance/test_account_closing_balance.py
@@ -1,10 +1,56 @@
-# Copyright (c) 2023, Frappe Technologies Pvt. Ltd. and Contributors
+# Copyright (c) 2026, Frappe Technologies Pvt. Ltd. and Contributors
 # See license.txt
 
-# import frappe
-
+from erpnext.accounts.doctype.account_closing_balance.account_closing_balance import (
+	aggregate_with_last_account_closing_balance,
+	generate_key,
+)
 from erpnext.tests.utils import ERPNextTestSuite
 
 
+def entry(**overrides):
+	row = {"debit": 0, "credit": 0, "debit_in_account_currency": 0, "credit_in_account_currency": 0}
+	row.update(overrides)
+	return row
+
+
 class TestAccountClosingBalance(ERPNextTestSuite):
-	pass
+	"""The closing-balance snapshot is built by merging this period's entries with the
+	previous period's. These lock the merge/key logic that drives that carry-forward."""
+
+	def test_matching_entries_are_summed(self):
+		# this is how a prior-period balance carries forward into the current one
+		merged = aggregate_with_last_account_closing_balance(
+			[
+				entry(account="Cash - _TC", debit=100, debit_in_account_currency=100),
+				entry(
+					account="Cash - _TC",
+					debit=50,
+					credit=20,
+					debit_in_account_currency=50,
+					credit_in_account_currency=20,
+				),
+			],
+			[],
+		)
+		self.assertEqual(len(merged), 1)
+		row = next(iter(merged.values()))
+		self.assertEqual(row["debit"], 150)
+		self.assertEqual(row["credit"], 20)
+
+	def test_entries_are_kept_separate_per_dimension(self):
+		merged = aggregate_with_last_account_closing_balance(
+			[
+				entry(account="Cash - _TC", cost_center="CC1", debit=100, debit_in_account_currency=100),
+				entry(account="Cash - _TC", cost_center="CC2", debit=40, debit_in_account_currency=40),
+			],
+			[],
+		)
+		self.assertEqual(len(merged), 2)
+
+	def test_period_closing_flag_is_part_of_the_key(self):
+		# a P&L reversal (flag 0) and a closing-account entry (flag 1) for the same
+		# account must not merge, so the flag has to distinguish their keys
+		key_reversal, _ = generate_key(entry(account="Sales - _TC", is_period_closing_voucher_entry=0), [])
+		key_closing, _ = generate_key(entry(account="Sales - _TC", is_period_closing_voucher_entry=1), [])
+		self.assertNotEqual(key_reversal, key_closing)