diff --git a/pkg/caldavtests/relations_test.go b/pkg/caldavtests/relations_test.go
new file mode 100644
index 000000000..0d09ee898
--- /dev/null
+++ b/pkg/caldavtests/relations_test.go
@@ -0,0 +1,235 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package caldavtests
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRelationsBasic(t *testing.T) {
+	// RFC 5545 §3.8.4.5 (rfc5545.txt line 6391):
+	// "This property is used to represent a relationship or reference
+	//  between one calendar component and another."
+
+	t.Run("Parent with RELTYPE=CHILD and child with RELTYPE=PARENT", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Create parent (no relations)
+		parent := NewVTodo("rel-parent-1", "Parent Task").Build()
+		rec := caldavPUT(t, e, "/dav/projects/36/rel-parent-1.ics", parent)
+		require.Equal(t, 201, rec.Code)
+
+		// Create child referencing parent
+		child := NewVTodo("rel-child-1", "Child Task").
+			RelatedToParent("rel-parent-1").
+			Build()
+		rec = caldavPUT(t, e, "/dav/projects/36/rel-child-1.ics", child)
+		require.Equal(t, 201, rec.Code)
+
+		// GET child — should have RELATED-TO;RELTYPE=PARENT
+		rec = caldavGET(t, e, "/dav/projects/36/rel-child-1.ics")
+		assert.Contains(t, rec.Body.String(), "RELATED-TO;RELTYPE=PARENT:rel-parent-1",
+			"Child should have RELATED-TO pointing to parent")
+
+		// GET parent — should have RELATED-TO;RELTYPE=CHILD (inverse)
+		rec = caldavGET(t, e, "/dav/projects/36/rel-parent-1.ics")
+		assert.Contains(t, rec.Body.String(), "RELATED-TO;RELTYPE=CHILD:rel-child-1",
+			"Parent should have inverse RELATED-TO pointing to child")
+	})
+
+	t.Run("Grandchild chain: parent -> child -> grandchild", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Create in order: parent, child, grandchild
+		parent := NewVTodo("rel-gp-parent", "Grandparent").Build()
+		caldavPUT(t, e, "/dav/projects/36/rel-gp-parent.ics", parent)
+
+		child := NewVTodo("rel-gp-child", "Parent").
+			RelatedToParent("rel-gp-parent").
+			Build()
+		caldavPUT(t, e, "/dav/projects/36/rel-gp-child.ics", child)
+
+		grandchild := NewVTodo("rel-gp-grandchild", "Child").
+			RelatedToParent("rel-gp-child").
+			Build()
+		caldavPUT(t, e, "/dav/projects/36/rel-gp-grandchild.ics", grandchild)
+
+		// Verify middle node has both parent and child relations
+		rec := caldavGET(t, e, "/dav/projects/36/rel-gp-child.ics")
+		body := rec.Body.String()
+		assert.Contains(t, body, "RELATED-TO;RELTYPE=PARENT:rel-gp-parent")
+		assert.Contains(t, body, "RELATED-TO;RELTYPE=CHILD:rel-gp-grandchild")
+	})
+}
+
+func TestRelationsReverseOrder(t *testing.T) {
+	t.Run("Child arrives before parent (Tasks.org pattern)", func(t *testing.T) {
+		// This is the most common real-world scenario:
+		// Tasks.org sends child with RELATED-TO;RELTYPE=PARENT but the parent
+		// has NO RELATED-TO at all. The child may arrive before the parent.
+
+		e := setupTestEnv(t)
+
+		// Step 1: Child arrives first
+		child := NewVTodo("rev-child-first", "Child First").
+			RelatedToParent("rev-parent-late").
+			Build()
+		rec := caldavPUT(t, e, "/dav/projects/36/rev-child-first.ics", child)
+		require.Equal(t, 201, rec.Code)
+
+		// Step 2: Parent arrives later (no RELATED-TO)
+		parent := NewVTodo("rev-parent-late", "Parent Late").Build()
+		rec = caldavPUT(t, e, "/dav/projects/36/rev-parent-late.ics", parent)
+		require.Equal(t, 201, rec.Code)
+
+		// Step 3: Verify parent has correct title (not DUMMY-UID)
+		rec = caldavGET(t, e, "/dav/projects/36/rev-parent-late.ics")
+		assert.Contains(t, rec.Body.String(), "SUMMARY:Parent Late",
+			"Parent should have its real title, not DUMMY-UID")
+		assert.NotContains(t, rec.Body.String(), "DUMMY",
+			"DUMMY placeholder should be replaced")
+
+		// Step 4: Verify child still has parent relation
+		rec = caldavGET(t, e, "/dav/projects/36/rev-child-first.ics")
+		assert.Contains(t, rec.Body.String(), "RELATED-TO;RELTYPE=PARENT:rev-parent-late",
+			"Child should still have parent relation after parent arrives")
+	})
+
+	t.Run("Multiple children before parent", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Two children arrive before parent
+		child1 := NewVTodo("rev-mc1", "Multi Child 1").
+			RelatedToParent("rev-mparent").Build()
+		caldavPUT(t, e, "/dav/projects/36/rev-mc1.ics", child1)
+
+		child2 := NewVTodo("rev-mc2", "Multi Child 2").
+			RelatedToParent("rev-mparent").Build()
+		caldavPUT(t, e, "/dav/projects/36/rev-mc2.ics", child2)
+
+		// Parent arrives
+		parent := NewVTodo("rev-mparent", "Multi Parent").Build()
+		caldavPUT(t, e, "/dav/projects/36/rev-mparent.ics", parent)
+
+		// Verify parent shows both children
+		rec := caldavGET(t, e, "/dav/projects/36/rev-mparent.ics")
+		body := rec.Body.String()
+		assert.Contains(t, body, "RELATED-TO;RELTYPE=CHILD:rev-mc1")
+		assert.Contains(t, body, "RELATED-TO;RELTYPE=CHILD:rev-mc2")
+	})
+}
+
+func TestRelationsCrossProject(t *testing.T) {
+	t.Run("Parent in project 36, child in project 38", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		parent := NewVTodo("xp-parent", "Cross-Project Parent").Build()
+		rec := caldavPUT(t, e, "/dav/projects/36/xp-parent.ics", parent)
+		require.Equal(t, 201, rec.Code)
+
+		child := NewVTodo("xp-child", "Cross-Project Child").
+			RelatedToParent("xp-parent").Build()
+		rec = caldavPUT(t, e, "/dav/projects/38/xp-child.ics", child)
+		require.Equal(t, 201, rec.Code)
+
+		// Verify parent in project 36 knows about child
+		rec = caldavGET(t, e, "/dav/projects/36/xp-parent.ics")
+		assert.Contains(t, rec.Body.String(), "RELATED-TO;RELTYPE=CHILD:xp-child",
+			"Parent should have cross-project child relation")
+
+		// Verify child in project 38 knows about parent
+		rec = caldavGET(t, e, "/dav/projects/38/xp-child.ics")
+		assert.Contains(t, rec.Body.String(), "RELATED-TO;RELTYPE=PARENT:xp-parent",
+			"Child should have cross-project parent relation")
+	})
+
+	t.Run("Pre-existing cross-project relations from fixtures", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Task 45 (project 36) and task 46 (project 38) have cross-project relations in fixtures
+		rec := caldavGET(t, e, "/dav/projects/36/uid-caldav-test-parent-task-another-list.ics")
+		assert.Contains(t, rec.Body.String(), "RELATED-TO;RELTYPE=CHILD:uid-caldav-test-child-task-another-list")
+
+		rec = caldavGET(t, e, "/dav/projects/38/uid-caldav-test-child-task-another-list.ics")
+		assert.Contains(t, rec.Body.String(), "RELATED-TO;RELTYPE=PARENT:uid-caldav-test-parent-task-another-list")
+	})
+}
+
+func TestRelationsDeletion(t *testing.T) {
+	t.Run("Deleting child removes relation from parent", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Task 41 is parent of task 43 (from fixtures)
+		rec := caldavDELETE(t, e, "/dav/projects/36/uid-caldav-test-child-task.ics")
+		assert.Equal(t, 204, rec.Code)
+
+		// Parent should no longer reference deleted child
+		rec = caldavGET(t, e, "/dav/projects/36/uid-caldav-test-parent-task.ics")
+		assert.NotContains(t, rec.Body.String(), "RELATED-TO;RELTYPE=CHILD:uid-caldav-test-child-task\r\n",
+			"Parent should not reference deleted child")
+	})
+
+	t.Run("Deleting parent removes relation from child", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Delete parent task 41
+		rec := caldavDELETE(t, e, "/dav/projects/36/uid-caldav-test-parent-task.ics")
+		assert.Equal(t, 204, rec.Code)
+
+		// Child should no longer reference deleted parent
+		rec = caldavGET(t, e, "/dav/projects/36/uid-caldav-test-child-task.ics")
+		assert.NotContains(t, rec.Body.String(), "RELATED-TO;RELTYPE=PARENT:uid-caldav-test-parent-task",
+			"Child should not reference deleted parent")
+	})
+}
+
+func TestRelationsResync(t *testing.T) {
+	t.Run("Parent re-sync without RELATED-TO preserves child relations", func(t *testing.T) {
+		// This is the DAVx5 behavior: parent is updated (e.g., title change)
+		// and re-synced without any RELATED-TO. The child-declared relations
+		// should survive.
+
+		e := setupTestEnv(t)
+
+		// Create parent
+		parent := NewVTodo("resync-parent", "Original Parent").Build()
+		caldavPUT(t, e, "/dav/projects/36/resync-parent.ics", parent)
+
+		// Create child with parent relation
+		child := NewVTodo("resync-child", "Child").
+			RelatedToParent("resync-parent").Build()
+		caldavPUT(t, e, "/dav/projects/36/resync-child.ics", child)
+
+		// Re-sync parent with updated title but NO RELATED-TO
+		parentUpdated := NewVTodo("resync-parent", "Updated Parent Title").Build()
+		caldavPUT(t, e, "/dav/projects/36/resync-parent.ics", parentUpdated)
+
+		// Verify relations survived
+		rec := caldavGET(t, e, "/dav/projects/36/resync-parent.ics")
+		body := rec.Body.String()
+		assert.Contains(t, body, "Updated Parent Title", "Title should be updated")
+		assert.Contains(t, body, "RELATED-TO;RELTYPE=CHILD:resync-child",
+			"Child relation should survive parent re-sync without RELATED-TO")
+
+		rec = caldavGET(t, e, "/dav/projects/36/resync-child.ics")
+		assert.Contains(t, rec.Body.String(), "RELATED-TO;RELTYPE=PARENT:resync-parent",
+			"Parent relation on child should survive parent re-sync")
+	})
+}