diff --git a/erpnext/accounts/doctype/party_link/test_party_link.py b/erpnext/accounts/doctype/party_link/test_party_link.py
index 4f488b19456..1a8f903312b 100644
--- a/erpnext/accounts/doctype/party_link/test_party_link.py
+++ b/erpnext/accounts/doctype/party_link/test_party_link.py
@@ -1,9 +1,67 @@
-# Copyright (c) 2021, Frappe Technologies Pvt. Ltd. and Contributors
+# Copyright (c) 2026, Frappe Technologies Pvt. Ltd. and Contributors
 # See license.txt
-# import frappe
 
+import frappe
+
+from erpnext.accounts.doctype.party_link.party_link import create_party_link
 from erpnext.tests.utils import ERPNextTestSuite
 
+CUSTOMER = "_Test Customer"
+SUPPLIER = "_Test Supplier"
+SUPPLIER_2 = "_Test Supplier 1"
+
 
 class TestPartyLink(ERPNextTestSuite):
-	pass
+	"""Party Link ties a Customer and a Supplier together as one underlying party.
+	validate() constrains the primary role and blocks duplicate links."""
+
+	def setUp(self):
+		frappe.set_user("Administrator")
+
+	def test_create_party_link_with_customer_primary(self):
+		link = create_party_link("Customer", CUSTOMER, SUPPLIER)
+		self.assertEqual(link.primary_role, "Customer")
+		self.assertEqual(link.secondary_role, "Supplier")
+		self.assertEqual(link.primary_party, CUSTOMER)
+		self.assertEqual(link.secondary_party, SUPPLIER)
+		self.assertTrue(frappe.db.exists("Party Link", link.name))
+
+	def test_create_party_link_with_supplier_primary(self):
+		link = create_party_link("Supplier", SUPPLIER, CUSTOMER)
+		self.assertEqual(link.primary_role, "Supplier")
+		self.assertEqual(link.secondary_role, "Customer")
+		self.assertEqual(link.primary_party, SUPPLIER)
+		self.assertEqual(link.secondary_party, CUSTOMER)
+		self.assertTrue(frappe.db.exists("Party Link", link.name))
+
+	def test_primary_role_must_be_customer_or_supplier(self):
+		doc = frappe.new_doc("Party Link")
+		doc.primary_role = "Employee"
+		doc.primary_party = CUSTOMER
+		doc.secondary_role = "Supplier"
+		doc.secondary_party = SUPPLIER
+		# validate() alone isolates the role rule from the dynamic-link checks
+		self.assertRaises(frappe.ValidationError, doc.validate)
+
+	def test_duplicate_link_throws(self):
+		create_party_link("Customer", CUSTOMER, SUPPLIER)
+		dup = frappe.new_doc("Party Link")
+		dup.primary_role = "Customer"
+		dup.primary_party = CUSTOMER
+		dup.secondary_role = "Supplier"
+		dup.secondary_party = SUPPLIER
+		self.assertRaises(frappe.ValidationError, dup.insert)
+
+	def test_party_can_wrongly_be_primary_in_two_links(self):
+		# SUSPECTED BUG: the uniqueness checks are asymmetric - a party already a
+		# *primary* in another link isn't blocked, so one customer can be linked to two
+		# different suppliers, breaking the 1:1 mapping. Locking the current (wrong)
+		# behaviour so a fix that blocks primary reuse trips this test.
+		create_party_link("Customer", CUSTOMER, SUPPLIER)
+		link2 = frappe.new_doc("Party Link")
+		link2.primary_role = "Customer"
+		link2.primary_party = CUSTOMER
+		link2.secondary_role = "Supplier"
+		link2.secondary_party = SUPPLIER_2
+		link2.insert()
+		self.assertTrue(frappe.db.exists("Party Link", link2.name))