diff --git a/lib/screens/settings/add_jellyfin_screen.dart b/lib/screens/settings/add_jellyfin_screen.dart
index 3c0f7a2b..ae25bcc6 100644
--- a/lib/screens/settings/add_jellyfin_screen.dart
+++ b/lib/screens/settings/add_jellyfin_screen.dart
@@ -443,9 +443,12 @@ class _AddJellyfinScreenState extends State<AddJellyfinScreen> with AsyncFormSta
     return JellyfinConnectionAuthService(clientName: 'Plezy', clientVersion: clientVersion, deviceName: deviceName);
   }
 
+  /// The raw name, not a header-sanitized one: the Jellyfin `MediaBrowser`
+  /// header percent-encodes it, so the device list shows it verbatim.
   Future<String> _resolveDeviceName() async {
     final identity = await DeviceIdentityService.resolve();
-    return sanitizeHeaderValue(identity.deviceName) ?? 'Plezy';
+    final name = identity.deviceName?.trim();
+    return name == null || name.isEmpty ? 'Plezy' : name;
   }
 
   @override
diff --git a/lib/services/jellyfin_auth_header.dart b/lib/services/jellyfin_auth_header.dart
index 624a1b4b..be1821b4 100644
--- a/lib/services/jellyfin_auth_header.dart
+++ b/lib/services/jellyfin_auth_header.dart
@@ -1,14 +1,24 @@
 import '../utils/device_identity.dart';
 
-/// Build the `MediaBrowser` Authorization header value the way the Jellyfin
-/// SDK formats it. Used at auth time and on every authenticated request so
-/// the server sees a consistent client identity.
+/// Build the `MediaBrowser` Authorization header value the way the official
+/// Jellyfin SDK formats it: every field value is percent-encoded, and the
+/// server reverses that with `WebUtility.UrlDecode` while parsing the header.
+/// Used at auth time and on every authenticated request so the server sees a
+/// consistent client identity.
 ///
-/// Unsafe header characters and embedded quotes are removed. Jellyfin requires
-/// non-empty client, device, and version fields when creating a session, so
-/// those values use stable fallbacks. An empty device ID is omitted for
-/// authenticated requests, where Jellyfin can recover it from the token;
-/// unauthenticated entry points must call [requireJellyfinDeviceId].
+/// Encoding is what keeps the header sendable at all. A device name like
+/// `Bjørn PC` cannot travel verbatim: `dart:io` rejects header values above
+/// 0x7F outright, and CFNetwork puts the raw code unit on the wire as a
+/// Latin-1 byte, which Kestrel — the HTTP server hosting Jellyfin — refuses
+/// as a malformed header with 400 before the request is ever routed. It also
+/// removes the grammar hazards the header has no escape for: quotes, commas,
+/// and `=` inside a value.
+///
+/// Jellyfin requires non-empty client, device, and version fields when
+/// creating a session, so those values use stable fallbacks. An empty device
+/// ID is omitted for authenticated requests, where Jellyfin can recover it
+/// from the token; unauthenticated entry points must call
+/// [requireJellyfinDeviceId].
 String buildJellyfinAuthHeader({
   required String clientName,
   required String clientVersion,
@@ -16,27 +26,33 @@ String buildJellyfinAuthHeader({
   required String deviceId,
   String? accessToken,
 }) {
-  String clean(String value) => (sanitizeHeaderValue(value) ?? '').replaceAll('"', '');
+  String field(String name, String value) => '$name="${Uri.encodeComponent(value)}"';
 
-  final client = clean(clientName);
+  final client = _meaningful(clientName);
   final effectiveClient = client.isEmpty ? 'Plezy' : client;
-  final device = clean(deviceName);
-  final effectiveDevice = device.isEmpty ? effectiveClient : device;
-  final id = clean(deviceId);
-  final version = clean(clientVersion);
-  final token = accessToken == null ? '' : clean(accessToken);
-  String quoted(String value) => '"$value"';
+  final device = _meaningful(deviceName);
+  final version = _meaningful(clientVersion);
+  final id = _meaningful(deviceId);
+  final token = _meaningful(accessToken ?? '');
 
   final parts = <String>[
-    'Client=${quoted(effectiveClient)}',
-    'Device=${quoted(effectiveDevice)}',
-    if (id.isNotEmpty) 'DeviceId=${quoted(id)}',
-    'Version=${quoted(version.isEmpty ? '1.0' : version)}',
-    if (token.isNotEmpty) 'Token=${quoted(token)}',
+    field('Client', effectiveClient),
+    field('Device', device.isEmpty ? effectiveClient : device),
+    if (id.isNotEmpty) field('DeviceId', id),
+    field('Version', version.isEmpty ? '1.0' : version),
+    if (token.isNotEmpty) field('Token', token),
   ];
   return 'MediaBrowser ${parts.join(', ')}';
 }
 
+final RegExp _controlCharacters = RegExp(r'[\x00-\x1f\x7f-\x9f]');
+
+/// Percent-encoding makes any byte transportable, so the only values worth
+/// filtering are the ones that carry no identity at all — a name of control
+/// characters would otherwise reach Jellyfin's device list as `%00` noise
+/// instead of falling back to a readable label.
+String _meaningful(String value) => value.replaceAll(_controlCharacters, '').trim();
+
 /// Validates the stable device identity required by unauthenticated Jellyfin
 /// session creation. Never substitute a placeholder: Jellyfin keys sessions
 /// and access tokens by this value, so a shared fallback would collide across
diff --git a/lib/services/jellyfin_client.dart b/lib/services/jellyfin_client.dart
index a34205cf..7e3dbbf0 100644
--- a/lib/services/jellyfin_client.dart
+++ b/lib/services/jellyfin_client.dart
@@ -141,9 +141,11 @@ class JellyfinClient
     } catch (_) {
       // Tests / non-platform contexts — keep the fallback version.
     }
+    // Raw, not header-sanitized: [buildJellyfinAuthHeader] percent-encodes it.
     String? deviceName;
     try {
-      deviceName = sanitizeHeaderValue((await DeviceIdentityService.resolve()).deviceName);
+      final resolved = (await DeviceIdentityService.resolve()).deviceName?.trim();
+      if (resolved != null && resolved.isNotEmpty) deviceName = resolved;
     } catch (_) {
       // Tests / non-platform contexts — keep the fallback name.
     }
diff --git a/lib/utils/device_identity.dart b/lib/utils/device_identity.dart
index 28187f60..22aea2dc 100644
--- a/lib/utils/device_identity.dart
+++ b/lib/utils/device_identity.dart
@@ -2,6 +2,7 @@ import 'dart:io';
 
 import 'package:device_info_plus/device_info_plus.dart';
 import 'package:flutter/foundation.dart';
+import 'package:unorm_dart/unorm_dart.dart';
 
 import 'app_logger.dart';
 import 'platform_detector.dart';
@@ -98,13 +99,54 @@ class DeviceIdentityService {
   }
 }
 
-/// Makes a free-form device name safe to send as an HTTP header value:
-/// drops HTTP control characters and code units above 0xFF (dart:io's
-/// HttpHeaders rejects them), trims, and returns null when nothing usable
-/// remains.
+/// Makes a free-form device name safe to send as an HTTP header value on
+/// every transport Plezy uses: folds Latin letters to their base form
+/// (`Bjørn PC` → `Bjorn PC`), drops whatever is still outside printable
+/// ASCII, trims, and returns null when nothing usable remains.
+///
+/// The ASCII restriction is not cosmetic. `dart:io` rejects header values
+/// containing anything above 0x7F with a `FormatException`, and CFNetwork
+/// puts the raw code unit on the wire as a Latin-1 byte, which HTTP servers
+/// decoding headers as UTF-8 (Kestrel, hosting Jellyfin) reject as a
+/// malformed request. Headers with a documented percent-encoded wire format
+/// carry the name intact instead — see `buildJellyfinAuthHeader`.
 String? sanitizeHeaderValue(String? value) {
   if (value == null) return null;
-  final filtered = String.fromCharCodes(value.codeUnits.where((unit) => unit >= 0x20 && unit != 0x7F && unit <= 0xFF));
-  final trimmed = filtered.trim();
+  final buffer = StringBuffer();
+  for (final unit in nfd(_foldNonDecomposableLatin(value)).codeUnits) {
+    if (unit >= 0x20 && unit < 0x7F) buffer.writeCharCode(unit);
+  }
+  final trimmed = buffer.toString().trim();
   return trimmed.isEmpty ? null : trimmed;
 }
+
+/// Latin letters NFD leaves alone because they are single code points rather
+/// than base + combining mark. Without this, Nordic and Central European
+/// device names lose whole letters instead of being transliterated.
+const Map<String, String> _nonDecomposableLatin = {
+  'æ': 'ae',
+  'Æ': 'AE',
+  'œ': 'oe',
+  'Œ': 'OE',
+  'ø': 'o',
+  'Ø': 'O',
+  'ß': 'ss',
+  'đ': 'd',
+  'Đ': 'D',
+  'ð': 'd',
+  'Ð': 'D',
+  'þ': 'th',
+  'Þ': 'Th',
+  'ł': 'l',
+  'Ł': 'L',
+  'ħ': 'h',
+  'Ħ': 'H',
+  'ı': 'i',
+  'ŧ': 't',
+  'Ŧ': 'T',
+};
+
+final RegExp _nonDecomposableLatinPattern = RegExp('[${_nonDecomposableLatin.keys.join()}]');
+
+String _foldNonDecomposableLatin(String value) =>
+    value.replaceAllMapped(_nonDecomposableLatinPattern, (match) => _nonDecomposableLatin[match[0]]!);
diff --git a/test/services/jellyfin_auth_header_test.dart b/test/services/jellyfin_auth_header_test.dart
index 1da891fe..cf310fa6 100644
--- a/test/services/jellyfin_auth_header_test.dart
+++ b/test/services/jellyfin_auth_header_test.dart
@@ -1,6 +1,19 @@
 import 'package:flutter_test/flutter_test.dart';
 import 'package:plezy/services/jellyfin_auth_header.dart';
 
+/// Every field value Jellyfin reads back out of the header, mirroring the
+/// server's own parse: split on the top-level commas, strip the quotes, then
+/// `UrlDecode` (`WebUtility.UrlDecode` in `AuthorizationContext.GetParts`).
+Map<String, String> parseAsJellyfinWould(String header) {
+  expect(header, startsWith('MediaBrowser '));
+  return {
+    for (final part in header.substring('MediaBrowser '.length).split(', '))
+      part.substring(0, part.indexOf('=')): Uri.decodeComponent(
+        part.substring(part.indexOf('=') + 1).replaceAll('"', ''),
+      ),
+  };
+}
+
 void main() {
   group('buildJellyfinAuthHeader', () {
     test('formats the SDK-style MediaBrowser header', () {
@@ -13,7 +26,7 @@ void main() {
       );
       expect(
         header,
-        'MediaBrowser Client="Plezy", Device="Living Room TV", DeviceId="dev-1", Version="1.2.3", Token="tok"',
+        'MediaBrowser Client="Plezy", Device="Living%20Room%20TV", DeviceId="dev-1", Version="1.2.3", Token="tok"',
       );
     });
 
@@ -30,15 +43,41 @@ void main() {
       }
     });
 
-    test('strips embedded quotes so a device name cannot corrupt the header', () {
+    // Regression: 2.9.0 started sending the real device name verbatim, so a
+    // non-ASCII one made dart:io reject the header outright and made CFNetwork
+    // emit a Latin-1 byte that Jellyfin's host rejects with 400 before the
+    // login request is routed.
+    test('keeps a non-ASCII device name on the wire as ASCII the server decodes back', () {
+      const deviceName = 'Bjørn stue-TV 客厅 📺';
       final header = buildJellyfinAuthHeader(
         clientName: 'Plezy',
-        clientVersion: '1.2.3',
-        deviceName: 'My "cool" TV',
+        clientVersion: '2.10.0',
+        deviceName: deviceName,
         deviceId: 'dev-1',
         accessToken: 'tok',
       );
-      expect(header, contains('Device="My cool TV"'));
+
+      // dart:io's own header-value rule: printable ASCII only.
+      expect(header, matches(RegExp(r'^[\x20-\x7e]+$')));
+      expect(parseAsJellyfinWould(header)['Device'], deviceName);
+    });
+
+    test('keeps a device name that would corrupt the header grammar intact', () {
+      const deviceName = 'My "cool", TV = 1+2 100%';
+      final header = buildJellyfinAuthHeader(
+        clientName: 'Plezy',
+        clientVersion: '1.2.3',
+        deviceName: deviceName,
+        deviceId: 'dev-1',
+        accessToken: 'tok',
+      );
+
+      final parsed = parseAsJellyfinWould(header);
+      expect(parsed['Device'], deviceName);
+      expect(parsed['Client'], 'Plezy');
+      expect(parsed['DeviceId'], 'dev-1');
+      expect(parsed['Version'], '1.2.3');
+      expect(parsed['Token'], 'tok');
     });
 
     test('uses non-empty fallbacks for required session identity fields', () {
diff --git a/test/utils/device_identity_test.dart b/test/utils/device_identity_test.dart
index e52d7fd8..b39e8df6 100644
--- a/test/utils/device_identity_test.dart
+++ b/test/utils/device_identity_test.dart
@@ -3,12 +3,20 @@ import 'package:plezy/utils/device_identity.dart';
 
 void main() {
   group('sanitizeHeaderValue', () {
-    test('passes plain latin-1 names through trimmed', () {
+    test('passes plain ASCII names through trimmed', () {
       expect(sanitizeHeaderValue('  Living Room TV '), 'Living Room TV');
-      expect(sanitizeHeaderValue("Édouard's Mac"), "Édouard's Mac");
     });
 
-    test('strips code units above latin-1 (emoji would break dart:io headers)', () {
+    // Regression: header values above 0x7F make dart:io throw and CFNetwork
+    // emit a Latin-1 byte that UTF-8 header parsers reject as a bad request.
+    test('folds accented Latin letters instead of emitting them', () {
+      expect(sanitizeHeaderValue("Édouard's Mac"), "Edouard's Mac");
+      expect(sanitizeHeaderValue('Bjørn stue-TV'), 'Bjorn stue-TV');
+      expect(sanitizeHeaderValue('Håkons Æra Straße'), 'Hakons AEra Strasse');
+      expect(sanitizeHeaderValue('Łukasz Đorđe'), 'Lukasz Dorde');
+    });
+
+    test('drops code units that have no ASCII equivalent', () {
       expect(sanitizeHeaderValue('📱 Bob\'s iPhone'), "Bob's iPhone");
       expect(sanitizeHeaderValue('电视'), isNull);
     });