diff --git a/pkg/caldavtests/sync_test.go b/pkg/caldavtests/sync_test.go
new file mode 100644
index 000000000..0f30f2580
--- /dev/null
+++ b/pkg/caldavtests/sync_test.go
@@ -0,0 +1,254 @@
+// 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"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestETagBehavior(t *testing.T) {
+	t.Run("GET returns ETag header", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		rec := caldavGET(t, e, "/dav/projects/36/uid-caldav-test.ics")
+		assert.Equal(t, http.StatusOK, rec.Code)
+
+		etag := rec.Header().Get("ETag")
+		assert.NotEmpty(t, etag, "GET should return an ETag header")
+		// ETag must be a quoted string per HTTP spec
+		assert.True(t, len(etag) >= 2 && etag[0] == '"' && etag[len(etag)-1] == '"',
+			"ETag must be a quoted string. Got: %s", etag)
+	})
+
+	t.Run("Same resource returns same ETag on repeated GET", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		rec1 := caldavGET(t, e, "/dav/projects/36/uid-caldav-test.ics")
+		rec2 := caldavGET(t, e, "/dav/projects/36/uid-caldav-test.ics")
+
+		etag1 := rec1.Header().Get("ETag")
+		etag2 := rec2.Header().Get("ETag")
+
+		assert.Equal(t, etag1, etag2,
+			"Same resource should return same ETag on consecutive GETs")
+	})
+
+	t.Run("ETag changes after PUT update", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Create a task
+		vtodo := NewVTodo("etag-change-test", "ETag Change Test").Build()
+		rec1 := caldavPUT(t, e, "/dav/projects/36/etag-change-test.ics", vtodo)
+		require.Equal(t, http.StatusCreated, rec1.Code)
+
+		// Get its ETag
+		rec2 := caldavGET(t, e, "/dav/projects/36/etag-change-test.ics")
+		etag1 := rec2.Header().Get("ETag")
+
+		// Update the task
+		vtodoUpdated := NewVTodo("etag-change-test", "ETag Change Test UPDATED").
+			DtStamp(time.Now().Add(time.Second).UTC()).
+			Build()
+		rec3 := caldavPUT(t, e, "/dav/projects/36/etag-change-test.ics", vtodoUpdated)
+		require.True(t, rec3.Code >= 200 && rec3.Code < 300)
+
+		// Get the new ETag
+		rec4 := caldavGET(t, e, "/dav/projects/36/etag-change-test.ics")
+		etag2 := rec4.Header().Get("ETag")
+
+		if etag1 != "" && etag2 != "" {
+			assert.NotEqual(t, etag1, etag2,
+				"ETag should change after task is updated. Before: %s, After: %s", etag1, etag2)
+		}
+	})
+
+	t.Run("PROPFIND ETag matches GET ETag", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Get ETag via GET
+		getResp := caldavGET(t, e, "/dav/projects/36/uid-caldav-test.ics")
+		getETag := getResp.Header().Get("ETag")
+
+		// Get ETag via PROPFIND
+		propfindResp := caldavPROPFIND(t, e, "/dav/projects/36/uid-caldav-test.ics", "0", PropfindResourceProperties)
+		ms := parseMultistatus(t, propfindResp)
+		require.Len(t, ms.Responses, 1)
+		propfindETag := getSuccessfulProp(t, ms.Responses[0]).GetETag
+
+		if getETag != "" && propfindETag != "" {
+			assert.Equal(t, getETag, propfindETag,
+				"ETag from GET and PROPFIND should match")
+		}
+	})
+
+	t.Run("Different tasks have different ETags", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		rec1 := caldavGET(t, e, "/dav/projects/36/uid-caldav-test.ics")
+		rec2 := caldavGET(t, e, "/dav/projects/36/uid-caldav-test-parent-task.ics")
+
+		etag1 := rec1.Header().Get("ETag")
+		etag2 := rec2.Header().Get("ETag")
+
+		if etag1 != "" && etag2 != "" {
+			assert.NotEqual(t, etag1, etag2,
+				"Different tasks should have different ETags")
+		}
+	})
+}
+
+func TestCTagBehavior(t *testing.T) {
+	// Apple CalendarServer getctag extension:
+	// A collection-level tag that changes when any resource within is modified.
+	// Used by DAVx5, Thunderbird, Apple clients for efficient sync.
+
+	t.Run("PROPFIND on collection returns getctag", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		rec := caldavPROPFIND(t, e, "/dav/projects/36", "0", PropfindCalendarCollectionProperties)
+
+		assertResponseStatus(t, rec, 207)
+		body := rec.Body.String()
+
+		// Check if getctag is present (it may not be — this documents the behavior)
+		assert.Contains(t, body, "getctag",
+			"PROPFIND on collection should include getctag property.\n"+
+				"If this fails, getctag is not implemented — clients will sync less efficiently.\n"+
+				"Body:\n%s", body)
+	})
+
+	t.Run("CTag changes after task is added", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Get initial CTag
+		rec1 := caldavPROPFIND(t, e, "/dav/projects/36", "0", PropfindCalendarCollectionProperties)
+		assertResponseStatus(t, rec1, 207)
+		ms1 := parseMultistatus(t, rec1)
+		ctag1 := getSuccessfulProp(t, ms1.Responses[0]).GetCTag
+
+		// Add a task
+		vtodo := NewVTodo("ctag-test-add", "CTag Test Add").Build()
+		addRec := caldavPUT(t, e, "/dav/projects/36/ctag-test-add.ics", vtodo)
+		require.True(t, addRec.Code >= 200 && addRec.Code < 300)
+
+		// Get new CTag
+		rec2 := caldavPROPFIND(t, e, "/dav/projects/36", "0", PropfindCalendarCollectionProperties)
+		assertResponseStatus(t, rec2, 207)
+		ms2 := parseMultistatus(t, rec2)
+		ctag2 := getSuccessfulProp(t, ms2.Responses[0]).GetCTag
+
+		if ctag1 != "" && ctag2 != "" {
+			assert.NotEqual(t, ctag1, ctag2,
+				"CTag should change after a task is added. Before: %s, After: %s", ctag1, ctag2)
+		}
+	})
+
+	t.Run("CTag changes after task is deleted", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Get initial CTag
+		rec1 := caldavPROPFIND(t, e, "/dav/projects/36", "0", PropfindCalendarCollectionProperties)
+		assertResponseStatus(t, rec1, 207)
+		ms1 := parseMultistatus(t, rec1)
+		ctag1 := getSuccessfulProp(t, ms1.Responses[0]).GetCTag
+
+		// Delete a task
+		delRec := caldavDELETE(t, e, "/dav/projects/36/uid-caldav-test.ics")
+		require.Equal(t, http.StatusNoContent, delRec.Code)
+
+		// Get new CTag
+		rec2 := caldavPROPFIND(t, e, "/dav/projects/36", "0", PropfindCalendarCollectionProperties)
+		assertResponseStatus(t, rec2, 207)
+		ms2 := parseMultistatus(t, rec2)
+		ctag2 := getSuccessfulProp(t, ms2.Responses[0]).GetCTag
+
+		if ctag1 != "" && ctag2 != "" {
+			assert.NotEqual(t, ctag1, ctag2,
+				"CTag should change after a task is deleted. Before: %s, After: %s", ctag1, ctag2)
+		}
+	})
+}
+
+func TestConditionalRequests(t *testing.T) {
+	// RFC 4918 requires support for conditional requests using ETags.
+	// If-Match prevents lost updates (optimistic concurrency).
+	// If-None-Match prevents unnecessary downloads.
+
+	t.Run("PUT with matching If-Match succeeds", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Create a task and get its ETag
+		vtodo := NewVTodo("if-match-test", "If-Match Test").Build()
+		caldavPUT(t, e, "/dav/projects/36/if-match-test.ics", vtodo)
+
+		getRec := caldavGET(t, e, "/dav/projects/36/if-match-test.ics")
+		etag := getRec.Header().Get("ETag")
+		require.NotEmpty(t, etag, "Need an ETag for this test")
+
+		// Update with correct If-Match
+		vtodoUpdated := NewVTodo("if-match-test", "If-Match Test Updated").Build()
+		rec := caldavRequest(t, e, http.MethodPut, "/dav/projects/36/if-match-test.ics", vtodoUpdated, map[string]string{
+			"Content-Type": "text/calendar; charset=utf-8",
+			"If-Match":     etag,
+		})
+
+		assert.True(t, rec.Code >= 200 && rec.Code < 300,
+			"PUT with matching If-Match should succeed. Got %d", rec.Code)
+	})
+
+	t.Run("PUT with non-matching If-Match returns 412", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Create a task
+		vtodo := NewVTodo("if-match-fail", "If-Match Fail Test").Build()
+		caldavPUT(t, e, "/dav/projects/36/if-match-fail.ics", vtodo)
+
+		// Try to update with wrong ETag
+		vtodoUpdated := NewVTodo("if-match-fail", "Should Not Update").Build()
+		rec := caldavRequest(t, e, http.MethodPut, "/dav/projects/36/if-match-fail.ics", vtodoUpdated, map[string]string{
+			"Content-Type": "text/calendar; charset=utf-8",
+			"If-Match":     `"wrong-etag"`,
+		})
+
+		assert.Equal(t, http.StatusPreconditionFailed, rec.Code,
+			"PUT with non-matching If-Match should return 412 Precondition Failed. Got %d.\n"+
+				"If this fails, the server doesn't support conditional PUT — a common CalDAV bug.", rec.Code)
+	})
+
+	t.Run("GET with matching If-None-Match returns 304", func(t *testing.T) {
+		e := setupTestEnv(t)
+
+		// Get the task and its ETag
+		rec1 := caldavGET(t, e, "/dav/projects/36/uid-caldav-test.ics")
+		etag := rec1.Header().Get("ETag")
+		require.NotEmpty(t, etag, "Need an ETag for this test")
+
+		// Request again with If-None-Match
+		rec2 := caldavRequest(t, e, http.MethodGet, "/dav/projects/36/uid-caldav-test.ics", "", map[string]string{
+			"If-None-Match": etag,
+		})
+
+		assert.Equal(t, http.StatusNotModified, rec2.Code,
+			"GET with matching If-None-Match should return 304 Not Modified. Got %d.\n"+
+				"If this fails, the server doesn't support conditional GET — clients re-download unnecessarily.", rec2.Code)
+	})
+}