diff --git a/pkg/service/datastore/datastore.go b/pkg/service/datastore/datastore.go
index d0ddda2..8f8376d 100644
--- a/pkg/service/datastore/datastore.go
+++ b/pkg/service/datastore/datastore.go
@@ -3,10 +3,13 @@ package datastore
 
 import (
 	"bytes"
+	"crypto/sha256"
 	"encoding/base64"
+	"encoding/hex"
 	"encoding/json"
 	"encoding/xml"
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
 	"sort"
@@ -1612,22 +1615,61 @@ func (ds *DataStore) GetETagForRecents(account, device string) int64 {
 	return info.ModTime().UnixNano() / int64(time.Millisecond)
 }
 
-// GetETagForAccount returns the highest ETag among presets, sources, and recents for the account and device.
-func (ds *DataStore) GetETagForAccount(account, device string) int64 {
-	e1 := ds.GetETagForPresets(account, device)
-	e2 := ds.GetETagForSources(account, device)
-	e3 := ds.GetETagForRecents(account, device)
+// contentHashForFiles returns a SHA-256 hex digest over the concatenated contents of the given file paths.
+func contentHashForFiles(paths ...string) string {
+	h := sha256.New()
 
-	maxETag := e1
-	if e2 > maxETag {
-		maxETag = e2
+	for _, p := range paths {
+		f, err := os.Open(p)
+		if err != nil {
+			continue
+		}
+
+		_, _ = io.Copy(h, f)
+		_ = f.Close()
 	}
 
-	if e3 > maxETag {
-		maxETag = e3
+	return hex.EncodeToString(h.Sum(nil))
+}
+
+// GetETagForAccount returns a content hash (SHA-256) over presets, sources, and recents for the account and device.
+// If device is empty, it hashes across all devices in the account.
+func (ds *DataStore) GetETagForAccount(account, device string) string {
+	if device != "" {
+		deviceDir := ds.AccountDeviceDir(account, device)
+
+		return contentHashForFiles(
+			filepath.Join(deviceDir, constants.PresetsFile),
+			filepath.Join(deviceDir, constants.SourcesFile),
+			filepath.Join(deviceDir, constants.RecentsFile),
+		)
 	}
 
-	return maxETag
+	devicesDir := ds.AccountDevicesDir(account)
+
+	// Ignore error: missing directory is treated as no devices, producing a
+	// stable non-empty hash rather than "" which would false-match an absent
+	// If-None-Match header and return 304 on the first request.
+	entries, _ := os.ReadDir(devicesDir)
+
+	h := sha256.New()
+
+	for _, entry := range entries {
+		if entry.IsDir() {
+			deviceDir := ds.AccountDeviceDir(account, entry.Name())
+			for _, name := range []string{constants.PresetsFile, constants.SourcesFile, constants.RecentsFile} {
+				f, err := os.Open(filepath.Join(deviceDir, name))
+				if err != nil {
+					continue
+				}
+
+				_, _ = io.Copy(h, f)
+				_ = f.Close()
+			}
+		}
+	}
+
+	return hex.EncodeToString(h.Sum(nil))
 }
 
 // Settings represents the global service settings.
diff --git a/pkg/service/handlers/handlers_etag_account_test.go b/pkg/service/handlers/handlers_etag_account_test.go
new file mode 100644
index 0000000..5c800ec
--- /dev/null
+++ b/pkg/service/handlers/handlers_etag_account_test.go
@@ -0,0 +1,235 @@
+package handlers
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/gesellix/bose-soundtouch/pkg/service/datastore"
+)
+
+// TestMargeAccountETagNoDevice tests ETag correctness on the account-level endpoints
+// when no ?device= query parameter is provided (the "no-device" code path in GetETagForAccount).
+//
+// These tests are specifically for the bug where:
+//   - GetETagForAccount returns "" when the devices directory does not exist
+//   - A missing If-None-Match header also produces "" via Header.Get
+//   - The equality check "" == "" causes a false 304 on the very first request
+func TestMargeAccountETagNoDevice(t *testing.T) {
+	newServer := func(t *testing.T) (ts *httptest.Server, tempDir string) {
+		t.Helper()
+		tempDir, _ = os.MkdirTemp("", "st-etag-nodev-*")
+		t.Cleanup(func() { _ = os.RemoveAll(tempDir) })
+		ds := datastore.NewDataStore(tempDir)
+		r, _ := setupRouter("http://localhost:8001", ds)
+		ts = httptest.NewServer(r)
+		t.Cleanup(ts.Close)
+		return ts, tempDir
+	}
+
+	writeDeviceFiles := func(t *testing.T, tempDir, account, deviceID string) string {
+		t.Helper()
+		deviceDir := filepath.Join(tempDir, "accounts", account, "devices", deviceID)
+		_ = os.MkdirAll(deviceDir, 0755)
+		_ = os.WriteFile(filepath.Join(deviceDir, "Presets.xml"), []byte("<presets/>"), 0644)
+		_ = os.WriteFile(filepath.Join(deviceDir, "Sources.xml"), []byte("<sources/>"), 0644)
+		_ = os.WriteFile(filepath.Join(deviceDir, "Recents.xml"), []byte("<recents/>"), 0644)
+		return filepath.Join(deviceDir, "Presets.xml")
+	}
+
+	// Bug: GetETagForAccount returns "" when the devices directory does not exist.
+	// Handler then checks: r.Header.Get("If-None-Match") == "" which is true on any
+	// request without the header, causing a 304 before the client has any cached content.
+	t.Run("first request without If-None-Match never returns 304 when devices dir is missing", func(t *testing.T) {
+		ts, _ := newServer(t)
+		// Deliberately no directories created for this account.
+		res, err := http.Get(ts.URL + "/marge/accounts/no-devices-account/full")
+		if err != nil {
+			t.Fatal(err)
+		}
+		_ = res.Body.Close()
+
+		if res.StatusCode == http.StatusNotModified {
+			t.Error("got 304 on first request without If-None-Match — empty ETag matched empty header value")
+		}
+		if res.StatusCode != http.StatusOK {
+			t.Errorf("expected 200, got %v", res.Status)
+		}
+	})
+
+	t.Run("ETag is non-empty even when devices dir does not exist", func(t *testing.T) {
+		ts, _ := newServer(t)
+		res, err := http.Get(ts.URL + "/marge/accounts/no-devices-account/full")
+		if err != nil {
+			t.Fatal(err)
+		}
+		_ = res.Body.Close()
+
+		if etag := res.Header.Get("ETag"); etag == "" {
+			t.Error("ETag must not be empty string — empty ETag causes false 304 on first request")
+		}
+	})
+
+	t.Run("ETag is non-empty when devices dir exists but contains no device subdirs", func(t *testing.T) {
+		ts, tempDir := newServer(t)
+		account := "empty-devices-account"
+		_ = os.MkdirAll(filepath.Join(tempDir, "accounts", account, "devices"), 0755)
+
+		res, err := http.Get(ts.URL + "/marge/accounts/" + account + "/full")
+		if err != nil {
+			t.Fatal(err)
+		}
+		_ = res.Body.Close()
+
+		if etag := res.Header.Get("ETag"); etag == "" {
+			t.Error("ETag must not be empty string when devices dir exists but is empty")
+		}
+	})
+
+	t.Run("ETag is stable across multiple requests when data does not change", func(t *testing.T) {
+		ts, tempDir := newServer(t)
+		account := "stable-etag-account"
+		writeDeviceFiles(t, tempDir, account, "DEV1")
+		url := ts.URL + "/marge/accounts/" + account + "/full"
+
+		res1, err := http.Get(url)
+		if err != nil {
+			t.Fatal(err)
+		}
+		etag1 := res1.Header.Get("ETag")
+		_ = res1.Body.Close()
+
+		res2, err := http.Get(url)
+		if err != nil {
+			t.Fatal(err)
+		}
+		etag2 := res2.Header.Get("ETag")
+		_ = res2.Body.Close()
+
+		if etag1 == "" {
+			t.Fatal("ETag must not be empty")
+		}
+		if etag1 != etag2 {
+			t.Errorf("ETag changed between identical requests: %q → %q", etag1, etag2)
+		}
+	})
+
+	t.Run("304 flow works correctly with no device param", func(t *testing.T) {
+		ts, tempDir := newServer(t)
+		account := "304-flow-account"
+		writeDeviceFiles(t, tempDir, account, "DEV1")
+		url := ts.URL + "/marge/accounts/" + account + "/full"
+
+		res1, err := http.Get(url)
+		if err != nil {
+			t.Fatal(err)
+		}
+		etag := res1.Header.Get("ETag")
+		_ = res1.Body.Close()
+
+		if etag == "" {
+			t.Fatal("expected non-empty ETag from first request")
+		}
+
+		req, _ := http.NewRequest("GET", url, nil)
+		req.Header.Set("If-None-Match", etag)
+		res2, err := http.DefaultClient.Do(req)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer func() { _ = res2.Body.Close() }()
+
+		if res2.StatusCode != http.StatusNotModified {
+			t.Errorf("expected 304 with valid ETag, got %v", res2.Status)
+		}
+	})
+
+	t.Run("ETag changes when file content changes", func(t *testing.T) {
+		ts, tempDir := newServer(t)
+		account := "changing-data-account"
+		presetsFile := writeDeviceFiles(t, tempDir, account, "DEV1")
+		url := ts.URL + "/marge/accounts/" + account + "/full"
+
+		res1, err := http.Get(url)
+		if err != nil {
+			t.Fatal(err)
+		}
+		etag1 := res1.Header.Get("ETag")
+		_ = res1.Body.Close()
+
+		_ = os.WriteFile(presetsFile, []byte(`<presets><preset id="1"/></presets>`), 0644)
+
+		res2, err := http.Get(url)
+		if err != nil {
+			t.Fatal(err)
+		}
+		etag2 := res2.Header.Get("ETag")
+		_ = res2.Body.Close()
+
+		if etag1 == etag2 {
+			t.Errorf("ETag did not change after file modification: %q", etag1)
+		}
+	})
+
+	t.Run("stale ETag returns 200 after data change", func(t *testing.T) {
+		ts, tempDir := newServer(t)
+		account := "stale-etag-account"
+		presetsFile := writeDeviceFiles(t, tempDir, account, "DEV1")
+		url := ts.URL + "/marge/accounts/" + account + "/full"
+
+		res1, err := http.Get(url)
+		if err != nil {
+			t.Fatal(err)
+		}
+		etag1 := res1.Header.Get("ETag")
+		_ = res1.Body.Close()
+
+		_ = os.WriteFile(presetsFile, []byte(`<presets><preset id="1"/></presets>`), 0644)
+
+		req, _ := http.NewRequest("GET", url, nil)
+		req.Header.Set("If-None-Match", etag1)
+		res2, err := http.DefaultClient.Do(req)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer func() { _ = res2.Body.Close() }()
+
+		if res2.StatusCode != http.StatusOK {
+			t.Errorf("expected 200 for stale ETag after data change, got %v", res2.Status)
+		}
+	})
+
+	// The /full, /sources, and /devices endpoints all call GetETagForAccount(account, "")
+	// so they should return the same ETag for the same account state.
+	t.Run("ETag is consistent across full, sources, and devices endpoints", func(t *testing.T) {
+		ts, tempDir := newServer(t)
+		account := "consistent-etag-account"
+		writeDeviceFiles(t, tempDir, account, "DEV1")
+
+		get := func(path string) string {
+			res, err := http.Get(ts.URL + path)
+			if err != nil {
+				t.Fatal(err)
+			}
+			etag := res.Header.Get("ETag")
+			_ = res.Body.Close()
+			return etag
+		}
+
+		etagFull := get("/marge/accounts/" + account + "/full")
+		etagSources := get("/marge/accounts/" + account + "/sources")
+		etagDevices := get("/marge/accounts/" + account + "/devices")
+
+		if etagFull == "" {
+			t.Fatal("ETag from /full must not be empty")
+		}
+		if etagFull != etagSources {
+			t.Errorf("/full ETag %q != /sources ETag %q", etagFull, etagSources)
+		}
+		if etagFull != etagDevices {
+			t.Errorf("/full ETag %q != /devices ETag %q", etagFull, etagDevices)
+		}
+	})
+}
diff --git a/pkg/service/handlers/handlers_marge.go b/pkg/service/handlers/handlers_marge.go
index 6da5fbb..8be7d5f 100644
--- a/pkg/service/handlers/handlers_marge.go
+++ b/pkg/service/handlers/handlers_marge.go
@@ -151,7 +151,7 @@ func (s *Server) HandleMargeAccountFull(w http.ResponseWriter, r *http.Request)
 
 	device := r.URL.Query().Get("device")
 
-	etag := strconv.FormatInt(s.ds.GetETagForAccount(account, device), 10)
+	etag := s.ds.GetETagForAccount(account, device)
 	if r.Header.Get("If-None-Match") == etag {
 		w.WriteHeader(http.StatusNotModified)
 		return
@@ -174,7 +174,7 @@ func (s *Server) HandleMargeAccountSources(w http.ResponseWriter, r *http.Reques
 
 	device := r.URL.Query().Get("device")
 
-	etag := strconv.FormatInt(s.ds.GetETagForAccount(account, device), 10)
+	etag := s.ds.GetETagForAccount(account, device)
 	if r.Header.Get("If-None-Match") == etag {
 		w.WriteHeader(http.StatusNotModified)
 		return
@@ -197,7 +197,7 @@ func (s *Server) HandleMargeAccountDevices(w http.ResponseWriter, r *http.Reques
 
 	device := r.URL.Query().Get("device")
 
-	etag := strconv.FormatInt(s.ds.GetETagForAccount(account, device), 10)
+	etag := s.ds.GetETagForAccount(account, device)
 	if r.Header.Get("If-None-Match") == etag {
 		w.WriteHeader(http.StatusNotModified)
 		return