diff --git a/pkg/caldavtests/bugs_test.go b/pkg/caldavtests/bugs_test.go
new file mode 100644
index 000000000..a75b28128
--- /dev/null
+++ b/pkg/caldavtests/bugs_test.go
@@ -0,0 +1,51 @@
+// 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"
+)
+
+// TestBugs contains tests that reproduce specific bugs reported by users.
+// Each test references the GitHub issue it reproduces.
+// These tests are expected to FAIL until the bug is fixed.
+//
+// To add a new bug reproduction test:
+// 1. Create a new t.Run with the issue number in the name
+// 2. Reproduce the exact CalDAV request sequence from the bug report
+// 3. Assert what the correct behavior SHOULD be (not what it currently does)
+// 4. The test will fail until the bug is fixed — this is expected and good
+
+func TestBugs(t *testing.T) {
+	// Template for adding bug reproductions:
+	//
+	// t.Run("GitHub_Issue_NNNN_short_description", func(t *testing.T) {
+	//     e := setupTestEnv(t)
+	//
+	//     // Reproduce the steps from the issue...
+	//     vtodo := NewVTodo("issue-NNNN", "...").Build()
+	//     rec := caldavPUT(t, e, "/dav/projects/36/issue-NNNN.ics", vtodo)
+	//
+	//     // Assert the expected (correct) behavior
+	//     assert.Equal(t, 201, rec.Code)
+	// })
+
+	t.Run("placeholder_no_bugs_yet", func(t *testing.T) {
+		// Remove this placeholder once real bug tests are added
+		t.Skip("No bug reproductions added yet")
+	})
+}
diff --git a/pkg/caldavtests/client_compat_test.go b/pkg/caldavtests/client_compat_test.go
new file mode 100644
index 000000000..f7a103e87
--- /dev/null
+++ b/pkg/caldavtests/client_compat_test.go
@@ -0,0 +1,215 @@
+// 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 (
+	"net/http"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestClientDAVx5Flow(t *testing.T) {
+	t.Run("Full DAVx5 sync flow", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Step 1: Discover principal
+		// DAVx5 sends PROPFIND to the server root or well-known URL
+		rec := caldavPROPFIND(t, e, "/dav/", "0", PropfindCurrentUserPrincipal)
+		assert.True(t, rec.Code == 207 || rec.Code == 301,
+			"Step 1: PROPFIND /dav/ should return 207 or redirect. Got %d", rec.Code)
+
+		// Step 2: Get calendar-home-set from principal
+		rec = caldavPROPFIND(t, e, "/dav/principals/user15/", "0", PropfindCalendarHomeSet)
+		assertResponseStatus(t, rec, 207)
+		assert.Contains(t, rec.Body.String(), "calendar-home-set",
+			"Step 2: Principal should advertise calendar-home-set")
+
+		// Step 3: List calendars
+		rec = caldavPROPFIND(t, e, "/dav/projects", "1", PropfindCalendarCollectionProperties)
+		assertResponseStatus(t, rec, 207)
+		ms := parseMultistatus(t, rec)
+		assert.GreaterOrEqual(t, len(ms.Responses), 2,
+			"Step 3: Should list calendars")
+
+		// Step 4: Check CTag for a specific calendar
+		rec = caldavPROPFIND(t, e, "/dav/projects/36", "0", PropfindCalendarCollectionProperties)
+		assertResponseStatus(t, rec, 207)
+
+		// Step 5: Full sync — calendar-query to get all task ETags
+		rec = caldavREPORT(t, e, "/dav/projects/36", ReportCalendarQuery)
+		assertResponseStatus(t, rec, 207)
+		ms = parseMultistatus(t, rec)
+		assert.Greater(t, len(ms.Responses), 0,
+			"Step 5: calendar-query should return tasks")
+
+		// Collect hrefs for multiget
+		var hrefs []string
+		for _, r := range ms.Responses {
+			if strings.HasSuffix(r.Href, ".ics") {
+				hrefs = append(hrefs, r.Href)
+			}
+		}
+
+		// Step 6: Multiget to fetch specific tasks
+		if len(hrefs) > 0 {
+			body := ReportCalendarMultiget(hrefs[:1]...) // Just fetch first task
+			rec = caldavREPORT(t, e, "/dav/projects/36", body)
+			assertResponseStatus(t, rec, 207)
+			ms = parseMultistatus(t, rec)
+			assert.Len(t, ms.Responses, 1,
+				"Step 6: multiget should return requested task")
+		}
+
+		// Step 7: Push a local change via PUT
+		vtodo := NewVTodo("davx5-sync-test", "DAVx5 Synced Task").
+			Due(time.Date(2024, 6, 15, 12, 0, 0, 0, time.UTC)).
+			Build()
+		rec = caldavPUT(t, e, "/dav/projects/36/davx5-sync-test.ics", vtodo)
+		assert.Equal(t, http.StatusCreated, rec.Code,
+			"Step 7: PUT should create the task")
+	})
+}
+
+func TestClientThunderbirdFlow(t *testing.T) {
+	t.Run("Thunderbird discovery and initial sync", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Step 1: Thunderbird starts with OPTIONS to check DAV support
+		rec := caldavOPTIONS(t, e, "/dav/")
+		assert.Equal(t, http.StatusOK, rec.Code,
+			"Step 1: OPTIONS should succeed")
+		davHeader := rec.Header().Get("DAV")
+		assert.NotEmpty(t, davHeader,
+			"Step 1: Should have DAV header")
+
+		// Step 2: PROPFIND on well-known for principal
+		rec = caldavRequest(t, e, "PROPFIND", "/.well-known/caldav", PropfindCurrentUserPrincipal, map[string]string{
+			"Depth": "0",
+		})
+		assert.True(t, rec.Code == 207 || rec.Code == 301 || rec.Code == 302,
+			"Step 2: well-known should respond. Got %d", rec.Code)
+
+		// Step 3: PROPFIND principal for calendar-home-set
+		rec = caldavPROPFIND(t, e, "/dav/principals/user15/", "0", PropfindCalendarHomeSet)
+		assertResponseStatus(t, rec, 207)
+
+		// Step 4: Thunderbird checks current-user-privilege-set to know if it can write
+		// RFC 3744 §5.4 (rfc3744.txt line 1158)
+		rec = caldavPROPFIND(t, e, "/dav/projects/36", "0", PropfindCurrentUserPrivilegeSet)
+		// This may return 207 with or without the property — document the behavior
+		assert.True(t, rec.Code == 207 || rec.Code == 200,
+			"Step 4: PROPFIND for privileges should not error. Got %d", rec.Code)
+
+		// Step 5: List calendars
+		rec = caldavPROPFIND(t, e, "/dav/projects", "1", PropfindCalendarCollectionProperties)
+		assertResponseStatus(t, rec, 207)
+
+		// Step 6: Sync via calendar-query
+		rec = caldavREPORT(t, e, "/dav/projects/36", ReportCalendarQuery)
+		assertResponseStatus(t, rec, 207)
+	})
+}
+
+func TestClientTasksOrgSubtasks(t *testing.T) {
+	t.Run("Tasks.org subtask sync: child-only RELATED-TO", func(t *testing.T) {
+		// Tasks.org behavior:
+		// - Child tasks include RELATED-TO;RELTYPE=PARENT:<parent-uid>
+		// - Parent tasks have NO RELATED-TO at all
+		// - Tasks may arrive in any order
+		// - On re-sync, parent is sent again without RELATED-TO
+
+		e := setupTestEnv(t)
+
+		// Round 1: Initial sync — parent first, then children
+		parent := NewVTodo("tasks-org-parent", "Buy groceries").Build()
+		rec := caldavPUT(t, e, "/dav/projects/36/tasks-org-parent.ics", parent)
+		require.Equal(t, 201, rec.Code)
+
+		child1 := NewVTodo("tasks-org-child-1", "Buy milk").
+			RelatedToParent("tasks-org-parent").Build()
+		rec = caldavPUT(t, e, "/dav/projects/36/tasks-org-child-1.ics", child1)
+		require.Equal(t, 201, rec.Code)
+
+		child2 := NewVTodo("tasks-org-child-2", "Buy eggs").
+			RelatedToParent("tasks-org-parent").Build()
+		rec = caldavPUT(t, e, "/dav/projects/36/tasks-org-child-2.ics", child2)
+		require.Equal(t, 201, rec.Code)
+
+		// Verify parent shows children
+		rec = caldavGET(t, e, "/dav/projects/36/tasks-org-parent.ics")
+		body := rec.Body.String()
+		assert.Contains(t, body, "tasks-org-child-1")
+		assert.Contains(t, body, "tasks-org-child-2")
+
+		// Round 2: Re-sync — parent updated (title change), still no RELATED-TO
+		parentUpdated := NewVTodo("tasks-org-parent", "Buy groceries (updated list)").Build()
+		rec = caldavPUT(t, e, "/dav/projects/36/tasks-org-parent.ics", parentUpdated)
+		require.True(t, rec.Code >= 200 && rec.Code < 300)
+
+		// Verify children are still linked after parent re-sync
+		rec = caldavGET(t, e, "/dav/projects/36/tasks-org-parent.ics")
+		body = rec.Body.String()
+		assert.Contains(t, body, "Buy groceries (updated list)",
+			"Parent title should be updated")
+		assert.Contains(t, body, "tasks-org-child-1",
+			"Child 1 relation should survive parent re-sync")
+		assert.Contains(t, body, "tasks-org-child-2",
+			"Child 2 relation should survive parent re-sync")
+
+		// Round 3: Complete child via PUT with STATUS:COMPLETED
+		child1Done := NewVTodo("tasks-org-child-1", "Buy milk").
+			RelatedToParent("tasks-org-parent").
+			Status("COMPLETED").
+			Completed(time.Now().UTC()).
+			Build()
+		rec = caldavPUT(t, e, "/dav/projects/36/tasks-org-child-1.ics", child1Done)
+		require.True(t, rec.Code >= 200 && rec.Code < 300)
+
+		// Verify child is completed
+		rec = caldavGET(t, e, "/dav/projects/36/tasks-org-child-1.ics")
+		assert.Contains(t, rec.Body.String(), "STATUS:COMPLETED")
+	})
+
+	t.Run("Tasks.org subtask sync: children arrive before parent", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Children arrive first (reverse order)
+		child := NewVTodo("tasks-rev-child", "Subtask").
+			RelatedToParent("tasks-rev-parent").Build()
+		rec := caldavPUT(t, e, "/dav/projects/36/tasks-rev-child.ics", child)
+		require.Equal(t, 201, rec.Code)
+
+		// Parent arrives later — no RELATED-TO
+		parent := NewVTodo("tasks-rev-parent", "Main Task").Build()
+		rec = caldavPUT(t, e, "/dav/projects/36/tasks-rev-parent.ics", parent)
+		require.Equal(t, 201, rec.Code)
+
+		// Verify bidirectional relations
+		rec = caldavGET(t, e, "/dav/projects/36/tasks-rev-parent.ics")
+		assert.Contains(t, rec.Body.String(), "SUMMARY:Main Task",
+			"Parent should have real title, not DUMMY")
+		assert.Contains(t, rec.Body.String(), "tasks-rev-child",
+			"Parent should show child relation")
+
+		rec = caldavGET(t, e, "/dav/projects/36/tasks-rev-child.ics")
+		assert.Contains(t, rec.Body.String(), "RELATED-TO;RELTYPE=PARENT:tasks-rev-parent")
+	})
+}