diff --git a/README.md b/README.md
index aafbaa8..373444c 100644
--- a/README.md
+++ b/README.md
@@ -236,7 +236,7 @@ then everyone (you included) uses Join.
 | `--[no-]yt-dlp` | on | read web pages (YouTube, X, …) with yt-dlp when the server fetches a link |
 | `--yt-dlp-dir` | next to the binary | writable folder for the server's yt-dlp (systemd: the unit's `StateDirectory`) |
 | `--yt-dlp-cookies` | none | cookies.txt or cookie-extension JSON export for that yt-dlp (sites that refuse guests); copied into the yt-dlp folder at start, removed from it when not given. **An account login**: keep it private, use a throwaway account for YouTube |
-| `--log-dir` | `logs` next to `--config`, else next to the binary (systemd: the unit's `StateDirectory`) | folder for the log files (`server-YYYY-MM-DD.log`, kept 7 days; passwords, tokens and cookies left out) |
+| `--log-dir` | `logs` next to `--config`, else next to the binary (systemd: the unit's `StateDirectory`) | folder for the log files (`server-YYYY-MM-DD.log` and the session log `session-YYYY-MM-DD.log`, kept 7 days; passwords, tokens and cookies left out) |
 | `--config` | none | YAML or JSON file with the same settings (`port`, `secret`, `name`, `mediaPath`, `advertiseHost`, `ytDlp`, `ytDlpDir`, `ytDlpCookies`); replaces the flags |
 | `--upgrade` | | update the install folder to the latest release ([Install and update](#install-and-update)), then exit |
 | `--check-update` | | print whether a newer release exists, then exit |
@@ -251,6 +251,19 @@ Something not playing? Settings → About → **Copy diagnostics** puts the
 app's recent log (passwords, tokens and cookies left out) on the
 clipboard: paste it to whoever helps you, from both sides if you can.
 
+While you are in a server, the app also sends its log to that server, which
+merges everyone's lines and its own into one session log
+(`session-YYYY-MM-DD.log` next to the server's logs; the hosting app keeps
+it in its own logs folder), so a problem can be read afterwards from every
+side. Chat is never logged anywhere, nor passwords, tokens and cookies.
+When something goes wrong, press **Mark a problem** (Ctrl+Shift+B, also in
+the player's ⚙ and Settings → About) and add a few words if you like: a
+marker with your name goes in the session log at that moment. In a server,
+Copy diagnostics can add the session log of the last 6 hours (any member
+may read it). Don't want to share? Settings → About → **Share diagnostics
+with the server** off: your log stays on your machine (the server only
+notes that you don't share; a problem you mark still goes in).
+
 ## Join
 
 Join in the strip. Servers you got into are listed first (up to 6), with
@@ -281,7 +294,8 @@ overrides it); in the player also the subtitle and audio track. Every
 change applies right away.
 
 Anywhere in a server (unless you are typing): Space = ready / unready,
-Tab = chat & activity (Tab never moves between buttons). Keys while the
+Tab = chat & activity (Tab never moves between buttons). Anywhere:
+Ctrl+Shift+B = mark a problem in the session log. Keys while the
 player fills the window: Left/Right = seek 10 s, Up/Down = volume, Shift+Right = skip 1:30,
 PgUp/PgDn = previous / next chapter (as in mpv), F = fullscreen, Esc =
 close the chat, then fullscreen, then shrink the player to mini; from the
diff --git a/apps/desktop/lib/src/controls/player_settings.dart b/apps/desktop/lib/src/controls/player_settings.dart
index 294069b..e1ce673 100644
--- a/apps/desktop/lib/src/controls/player_settings.dart
+++ b/apps/desktop/lib/src/controls/player_settings.dart
@@ -8,6 +8,7 @@ import '../jellyfin/track_prefs.dart' show MpvTrack;
 import '../playback/playback_controller.dart' show PlaybackController, RenderMode;
 import '../shaders/shader_presets.dart';
 import '../state/session_state.dart';
+import '../shell/mark_problem.dart';
 import '../shell/motion.dart';
 import '../state/settings.dart';
 import '../widgets/app_menu.dart';
@@ -332,9 +333,16 @@ class PlayerSettingsCard extends StatelessWidget {
                 const TrackSetting(type: JellyfinStreamType.audio),
               ],
               const SizedBox(height: 4),
-              Text(
-                'Custom shader files: Settings → Player',
-                style: TextStyle(color: Theme.of(context).colorScheme.onSurfaceVariant, fontSize: 12),
+              Row(
+                children: [
+                  Expanded(
+                    child: Text(
+                      'Custom shader files: Settings → Player',
+                      style: TextStyle(color: Theme.of(context).colorScheme.onSurfaceVariant, fontSize: 12),
+                    ),
+                  ),
+                  const MarkProblemButton(),
+                ],
               ),
             ],
           ),
diff --git a/apps/desktop/lib/src/platform/app_log.dart b/apps/desktop/lib/src/platform/app_log.dart
index c318a17..e9fbace 100644
--- a/apps/desktop/lib/src/platform/app_log.dart
+++ b/apps/desktop/lib/src/platform/app_log.dart
@@ -44,8 +44,10 @@ Future<void> startAppLogging() async {
 }
 
 /// The text "Copy diagnostics" gives: versions, then the last [since] of
-/// the app log (its hosted server's records included), already redacted.
-Future<String> diagnosticsText({Duration since = const Duration(hours: 6)}) async {
+/// the app log (its hosted server's records included), already redacted;
+/// with [sessionLog], the server's session log of the same hours after it
+/// (every member's shared lines and the server's, merged).
+Future<String> diagnosticsText({Duration since = const Duration(hours: 6), bool sessionLog = false}) async {
   final b = StringBuffer()
     ..writeln('$appName ${displayVersion()}')
     ..writeln('OS: ${Platform.operatingSystem} ${Platform.operatingSystemVersion}')
@@ -68,6 +70,13 @@ Future<String> diagnosticsText({Duration since = const Duration(hours: 6)}) asyn
   b.writeln('Log: ${files?.currentPath ?? 'none (console only)'}');
   b.writeln('--- last ${since.inHours} h of the log ---');
   b.write(files?.recent(since) ?? '');
+  if (sessionLog) {
+    final server = await SessionState.instance.coordinator.sessionLog(hours: since.inHours);
+    b
+      ..writeln()
+      ..writeln("--- last ${since.inHours} h of the server's session log ---")
+      ..write(server ?? "(none: the server keeps no session log, or it couldn't be reached)");
+  }
   return redactLog(b.toString());
 }
 
diff --git a/apps/desktop/lib/src/screens/home/about_page.dart b/apps/desktop/lib/src/screens/home/about_page.dart
index 7c83bb2..992bfcc 100644
--- a/apps/desktop/lib/src/screens/home/about_page.dart
+++ b/apps/desktop/lib/src/screens/home/about_page.dart
@@ -1,7 +1,12 @@
 import 'package:flutter/material.dart';
+import 'package:signals/signals_flutter.dart';
 
 import '../../platform/app_log.dart';
+import '../../shell/mark_problem.dart';
+import '../../state/session_state.dart';
+import '../../state/settings.dart';
 import '../../update/update_controller.dart';
+import '../../widgets/app_menu.dart';
 import '../../widgets/copy_open.dart';
 import 'settings_page.dart';
 import 'ui.dart';
@@ -45,18 +50,24 @@ class AboutPanel extends StatelessWidget {
               ),
               const SizedBox(height: kGap),
               Text('Something went wrong? Copy diagnostics and send them to whoever helps you: '
-                  'versions and the last hours of the log, passwords, tokens and cookies left out.',
+                  'versions and the last hours of the log, passwords, tokens and cookies left out. '
+                  'In a session it can add the server\'s session log, everyone\'s lines merged. '
+                  'Mark a problem (Ctrl+Shift+B) when it happens so the spot is easy to find.',
                   style: t.bodySmall?.copyWith(color: Colors.grey)),
               const SizedBox(height: 8),
               Wrap(
                 spacing: 8,
                 runSpacing: 8,
                 children: [
-                  FilledButton.tonalIcon(
-                    onPressed: _copyDiagnostics,
-                    icon: const Icon(Icons.bug_report_outlined, size: 18),
-                    label: const Text('Copy diagnostics'),
+                  Builder(
+                    builder: (context) => FilledButton.tonalIcon(
+                      key: const ValueKey('copy-diagnostics'),
+                      onPressed: () => _copyDiagnostics(context),
+                      icon: const Icon(Icons.bug_report_outlined, size: 18),
+                      label: const Text('Copy diagnostics'),
+                    ),
                   ),
+                  const MarkProblemButton(outlined: true),
                   OutlinedButton.icon(
                     onPressed: appLogDir == null ? null : () => _openLogs(),
                     icon: const Icon(Icons.folder_open, size: 18),
@@ -64,13 +75,36 @@ class AboutPanel extends StatelessWidget {
                   ),
                 ],
               ),
+              const SizedBox(height: 8),
+              SignalBuilder(
+                builder: (context) => SwitchListTile(
+                  key: const ValueKey('share-diagnostics'),
+                  contentPadding: EdgeInsets.zero,
+                  title: const Text('Share diagnostics with the server'),
+                  subtitle: const Text('In a session, this app\'s log goes to the server\'s session log, '
+                      'merged with everyone\'s, so a problem can be read afterwards. Never chat; '
+                      'passwords and tokens left out. Off: only a problem you mark is sent.'),
+                  value: AppSettings.instance.shareDiagnostics.value,
+                  onChanged: AppSettings.instance.setShareDiagnostics,
+                ),
+              ),
             ],
           ),
         );
   }
 
-  static Future<void> _copyDiagnostics() async {
-    final text = await diagnosticsText();
+  /// In a session it asks whether to add the server's session log.
+  static Future<void> _copyDiagnostics(BuildContext context) async {
+    var withServer = false;
+    if (SessionState.instance.route.peek() == AppRoute.session) {
+      final pick = await showAppMenu<bool>(context, items: const [
+        AppMenuItem(value: false, label: "This app's log"),
+        AppMenuItem(value: true, label: "This app's and the server's session log"),
+      ]);
+      if (pick == null) return;
+      withServer = pick;
+    }
+    final text = await diagnosticsText(sessionLog: withServer);
     final saved = await saveDiagnostics(text);
     await copyToClipboard(text,
         done: saved == null ? 'Diagnostics copied' : 'Diagnostics copied (also saved in the logs folder)');
diff --git a/apps/desktop/lib/src/screens/home/settings_page.dart b/apps/desktop/lib/src/screens/home/settings_page.dart
index a3fe6e5..b8605f3 100644
--- a/apps/desktop/lib/src/screens/home/settings_page.dart
+++ b/apps/desktop/lib/src/screens/home/settings_page.dart
@@ -23,6 +23,7 @@ const sessionShortcuts = [
   ('Ctrl+1…6', 'Shaders: Anime4K mode A, B, C, A+A, B+B, C+A'),
   ('Ctrl+0', 'Shaders off'),
   ('Esc', 'Close the panel or fullscreen, shrink the player, then ask to leave'),
+  ('Ctrl+Shift+B', 'Mark a problem in the session log (anywhere)'),
 ];
 
 /// One titled card of the Settings tab (rounded Material, so list tiles
diff --git a/apps/desktop/lib/src/session/session_coordinator.dart b/apps/desktop/lib/src/session/session_coordinator.dart
index 72be63c..1b9d315 100644
--- a/apps/desktop/lib/src/session/session_coordinator.dart
+++ b/apps/desktop/lib/src/session/session_coordinator.dart
@@ -17,6 +17,7 @@ import '../playback/playback_controller.dart';
 import '../platform/attention.dart';
 import '../playback/yt_dlp_app.dart';
 import '../schedule/anilist.dart';
+import '../screens/home/ui.dart' show appName, displayVersion;
 import '../screens/home/picked_media.dart';
 import '../sfx/sfx.dart';
 import '../state/settings.dart';
@@ -174,6 +175,32 @@ class SessionCoordinator {
   /// The app's yt-dlp and link probes (tests pass their own).
   final AppYtDlp ytDlp;
   final SyncClient sync = SyncClient();
+  /// This app's log lines to the server's session log (Settings: "Share
+  /// diagnostics with the server"), and "Mark a problem".
+  late final DiagUploader diag = DiagUploader(
+      target: _diagTarget, version: '$appName ${displayVersion()} on ${Platform.operatingSystem}');
+
+  DiagTarget? _diagTarget() {
+    final uri = sync.wsUri;
+    if (sync.token.isEmpty || uri == null || !sync.clockSync.hasSample) return null;
+    return (base: DiagUploader.baseOf(uri), token: sync.token, offsetMs: sync.clockSync.offsetMs);
+  }
+
+  /// "Mark a problem": a marker with [note] in the server's session log
+  /// (and this app's log). False when not in a session: then only this
+  /// app's log has it.
+  Future<bool> markProblem(String note) async {
+    if (!diag.running) {
+      createLogger(DiagUploader.markLogger).warning('=== PROBLEM marked${note.trim().isEmpty ? '' : ': ${note.trim()}'} ===');
+      return false;
+    }
+    await diag.markProblem(note);
+    return true;
+  }
+
+  /// The server's session log of the last [hours], null when not in a
+  /// session or the server has none.
+  Future<String?> sessionLog({int hours = 6}) => diag.fetchSessionLog(hours: hours);
   /// Up next: the server's queue, adding to it, and putting items up.
   late final QueueController queue = QueueController(this);
   /// The server has a password (it fetches links only then).
@@ -498,6 +525,9 @@ class SessionCoordinator {
     }
     markInput();
     _subscribe();
+    diag
+      ..share = AppSettings.instance.shareDiagnostics.peek()
+      ..start();
     feedLog.value = [];
     toasts.value = [];
     await initPlayer();
@@ -1201,6 +1231,9 @@ class SessionCoordinator {
         toasts.peek().where((t) => now.difference(t.at).inSeconds < 4).toList();
     final r = serverState.peek();
     if (sync.token.isNotEmpty) sync.sendPing();
+    diag
+      ..share = AppSettings.instance.shareDiagnostics.peek()
+      ..tick();
     if (sync.token.isNotEmpty && r != null && _playerReady) {
       _sendReport(r);
       if (!_lastOut) {
@@ -2235,6 +2268,8 @@ class SessionCoordinator {
     try {
       await playback.dispose();
     } catch (_) {}
+    // The last lines go before the token does.
+    await diag.stop();
     try {
       await sync.disconnect();
     } catch (_) {}
diff --git a/apps/desktop/lib/src/shell/mark_problem.dart b/apps/desktop/lib/src/shell/mark_problem.dart
new file mode 100644
index 0000000..3bef1c7
--- /dev/null
+++ b/apps/desktop/lib/src/shell/mark_problem.dart
@@ -0,0 +1,113 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:sync_relay_core/core.dart' show RelayConstants;
+
+import '../state/session_state.dart';
+import '../widgets/app_menu.dart' show AppGlass;
+import 'motion.dart';
+
+// "Mark a problem" (Ctrl+Shift+B, the player's ⚙, Settings → About): a
+// marker line in the server's session log at this moment, with an
+// optional short note, so whoever reads the log finds the spot.
+
+/// The key that opens it.
+const markProblemKey = SingleActivator(LogicalKeyboardKey.keyB, control: true, shift: true);
+
+/// Asks for the note, then marks; the toast says where it went.
+Future<void> showMarkProblem(BuildContext context) async {
+  final note = await showPopover<String>(context, at: Alignment.center, child: const MarkProblemCard());
+  if (note == null) return;
+  final coord = SessionState.instance.coordinator;
+  final inSession = await coord.markProblem(note);
+  coord.showToast(inSession ? 'Marked in the session log' : "Marked in this app's log");
+}
+
+/// The small card: a one-line note and Mark. Pops with the note (maybe
+/// empty), or nothing on Cancel / Esc.
+class MarkProblemCard extends StatefulWidget {
+  const MarkProblemCard({super.key});
+
+  @override
+  State<MarkProblemCard> createState() => _MarkProblemCardState();
+}
+
+class _MarkProblemCardState extends State<MarkProblemCard> {
+  final _note = TextEditingController();
+
+  @override
+  void dispose() {
+    _note.dispose();
+    super.dispose();
+  }
+
+  void _mark() => Navigator.of(context).pop(_note.text.trim());
+
+  @override
+  Widget build(BuildContext context) {
+    final muted = Theme.of(context).colorScheme.onSurfaceVariant;
+    return AppGlass(
+      key: const ValueKey('mark-problem-card'),
+      child: SizedBox(
+        width: 380,
+        child: Padding(
+          padding: const EdgeInsets.fromLTRB(16, 12, 16, 10),
+          child: Column(
+            mainAxisSize: MainAxisSize.min,
+            crossAxisAlignment: CrossAxisAlignment.stretch,
+            children: [
+              const Text('Mark a problem', style: TextStyle(fontWeight: FontWeight.w600, fontSize: 14)),
+              const SizedBox(height: 4),
+              Text("Puts a marker in the server's session log right now, so whoever reads it finds the moment.",
+                  style: TextStyle(color: muted, fontSize: 12)),
+              const SizedBox(height: 8),
+              TextField(
+                key: const ValueKey('mark-problem-note'),
+                controller: _note,
+                autofocus: true,
+                maxLength: RelayConstants.maxProblemNoteLength,
+                decoration: const InputDecoration(
+                  isDense: true,
+                  hintText: 'What happened? (optional)',
+                  counterText: '',
+                ),
+                onSubmitted: (_) => _mark(),
+              ),
+              const SizedBox(height: 8),
+              Row(
+                mainAxisAlignment: MainAxisAlignment.end,
+                children: [
+                  TextButton(onPressed: () => Navigator.of(context).pop(), child: const Text('Cancel')),
+                  const SizedBox(width: 8),
+                  FilledButton.tonal(
+                    key: const ValueKey('mark-problem-go'),
+                    onPressed: _mark,
+                    child: const Text('Mark'),
+                  ),
+                ],
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+/// "Mark a problem" as a quiet row button (the ⚙ card, Settings → About).
+class MarkProblemButton extends StatelessWidget {
+  final bool outlined;
+  const MarkProblemButton({super.key, this.outlined = false});
+
+  @override
+  Widget build(BuildContext context) {
+    const icon = Icon(Icons.flag_outlined, size: 18);
+    const label = Text('Mark a problem');
+    void open() => showMarkProblem(context);
+    return Tooltip(
+      message: 'Ctrl+Shift+B',
+      child: outlined
+          ? OutlinedButton.icon(key: const ValueKey('mark-problem'), onPressed: open, icon: icon, label: label)
+          : TextButton.icon(key: const ValueKey('mark-problem'), onPressed: open, icon: icon, label: label),
+    );
+  }
+}
diff --git a/apps/desktop/lib/src/shell/session_player.dart b/apps/desktop/lib/src/shell/session_player.dart
index 3e041bc..ca49041 100644
--- a/apps/desktop/lib/src/shell/session_player.dart
+++ b/apps/desktop/lib/src/shell/session_player.dart
@@ -29,6 +29,7 @@ import '../widgets/app_keys.dart';
 import '../widgets/text_focus.dart';
 import '../widgets/window_buttons.dart';
 import 'avatar.dart';
+import 'mark_problem.dart';
 import 'motion.dart';
 import 'now_playing_title.dart';
 import 'player_status.dart';
@@ -627,6 +628,7 @@ class _SessionPlayerState extends State<SessionPlayer> {
             (title: 'Change video', hint: null, run: widget.onChangeVideo),
             (title: 'Chat & activity', hint: 'Tab', run: _toggleChat),
             (title: 'Reload video', hint: 'Ctrl+R', run: _reloadVideo),
+            (title: 'Mark a problem', hint: 'Ctrl+Shift+B', run: () => showMarkProblem(context)),
             (title: 'Mark everyone ready', hint: null, run: coord.forcePlayAll),
             (title: 'Fullscreen', hint: 'F', run: widget.onToggleFullscreen),
             (title: 'Leave server', hint: 'Esc', run: widget.onLeave),
diff --git a/apps/desktop/lib/src/state/server_isolate.dart b/apps/desktop/lib/src/state/server_isolate.dart
index 937efd4..ab038b0 100644
--- a/apps/desktop/lib/src/state/server_isolate.dart
+++ b/apps/desktop/lib/src/state/server_isolate.dart
@@ -33,10 +33,12 @@ class ServerIsolate {
 
   /// Spawns the server and waits until it listens. Throws a
   /// [StateError] with the server's message if it cannot start.
-  /// [loopbackOnly]: 127.0.0.1 only (the solo session).
+  /// [loopbackOnly]: 127.0.0.1 only (the solo session). [logDir]: where
+  /// the server writes its session log (none when null).
   static Future<ServerIsolate> start(
     AppConfig config, {
     bool loopbackOnly = false,
+    String? logDir,
     required void Function(String line) onLog,
     required void Function(String error) onCrash,
     Duration timeout = const Duration(seconds: 15),
@@ -85,7 +87,7 @@ class ServerIsolate {
     final Isolate isolate;
     try {
       isolate = await Isolate.spawn(
-          _serverMain, ({...config.toMap(), if (loopbackOnly) 'loopbackOnly': true}, events.sendPort),
+          _serverMain, ({...config.toMap(), if (loopbackOnly) 'loopbackOnly': true, 'logDir': ?logDir}, events.sendPort),
           errorsAreFatal: false,
           onError: errors.sendPort,
           onExit: exit.sendPort,
diff --git a/apps/desktop/lib/src/state/session_state.dart b/apps/desktop/lib/src/state/session_state.dart
index 937a6b4..89c2e70 100644
--- a/apps/desktop/lib/src/state/session_state.dart
+++ b/apps/desktop/lib/src/state/session_state.dart
@@ -10,6 +10,7 @@ import '../jellyfin/accounts.dart';
 import '../jellyfin/session_media.dart';
 import '../screens/home/health_probe.dart';
 import '../screens/home/picked_media.dart';
+import '../platform/app_log.dart' show appLogDir;
 import '../playback/yt_dlp_app.dart';
 import '../sfx/sfx.dart';
 import '../sfx/sound_bank.dart';
@@ -379,6 +380,9 @@ class SessionState {
             ytDlpDir: await _sharedYtDlpDir(),
           ),
           loopbackOnly: soloSession,
+          // Its session log (every member's shared diagnostics and its own
+          // records) goes next to the app's; not for a solo session.
+          logDir: soloSession ? null : appLogDir,
           // Its records join the app's log as `server.<logger>`.
           onLog: (line) => relogLine('server', line),
           onCrash: (cause) => _onServerCrash(server, cause),
diff --git a/apps/desktop/lib/src/state/settings.dart b/apps/desktop/lib/src/state/settings.dart
index 1f08114..4f7a66d 100644
--- a/apps/desktop/lib/src/state/settings.dart
+++ b/apps/desktop/lib/src/state/settings.dart
@@ -28,6 +28,7 @@ const _keySoundPresetFor = 'soundPresetFor';
 const _keyUnreadyOnSwap = 'unreadyOnSwap';
 const _keyIdleSitsOut = 'idleSitsOut';
 const _keyChatPopups = 'chatPopups';
+const _keyShareDiagnostics = 'shareDiagnostics';
 
 enum HwdecMode { auto, off, custom }
 
@@ -70,6 +71,9 @@ class AppSettings {
   /// A 2 px progress line at the bottom of the video while the controls
   /// are hidden (both designs). Applies live.
   final progressLine = signal(true);
+  /// Send this app's log lines to the server's session log while in a
+  /// session (never chat). Off: nothing is sent but a note that it is off.
+  final shareDiagnostics = signal(true);
   /// Chat message toasts (default: only when paused). Applies live.
   final chatPopups = signal(ChatPopups.paused);
   /// App sounds (`sfx/`): 0..1, 0 = silent.
@@ -129,6 +133,8 @@ class AppSettings {
       if (hideRe is bool) hideReReleases.value = hideRe;
       final line = box.get(_keyProgressLine);
       if (line is bool) progressLine.value = line;
+      final share = box.get(_keyShareDiagnostics);
+      if (share is bool) shareDiagnostics.value = share;
       final popups = box.get(_keyChatPopups);
       chatPopups.value = ChatPopups.values.where((m) => m.name == popups).firstOrNull ?? ChatPopups.paused;
       final vol = box.get(_keySoundVolume);
@@ -203,6 +209,11 @@ class AppSettings {
     _put(_keyProgressLine, on);
   }
 
+  void setShareDiagnostics(bool on) {
+    shareDiagnostics.value = on;
+    _put(_keyShareDiagnostics, on);
+  }
+
   void setChatPopups(ChatPopups mode) {
     chatPopups.value = mode;
     _put(_keyChatPopups, mode.name);
diff --git a/apps/desktop/lib/src/widgets/app_keys.dart b/apps/desktop/lib/src/widgets/app_keys.dart
index 0ede7fd..0f3a379 100644
--- a/apps/desktop/lib/src/widgets/app_keys.dart
+++ b/apps/desktop/lib/src/widgets/app_keys.dart
@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 
+import '../shell/mark_problem.dart';
 import '../state/session_state.dart';
 import 'text_focus.dart';
 
@@ -15,11 +16,12 @@ Map<ShortcutActivator, Intent> get appShortcuts => {
       const SingleActivator(LogicalKeyboardKey.space): const DoNothingIntent(),
     };
 
-/// The window's two app-wide keys, ahead of whatever has focus: while in a
+/// The window's app-wide keys, ahead of whatever has focus: while in a
 /// server, Space is ready / not ready and Tab opens or closes the chat,
 /// on every tab and in every player mode. Not while a text field has
 /// focus (the chat field handles Tab and Esc itself) nor while a dialog or
-/// menu is in front of [child].
+/// menu is in front of [child]. Ctrl+Shift+B ([markProblemKey]) marks a
+/// problem anywhere, a text field focused or not.
 class AppKeys extends StatefulWidget {
   /// Who toggles the chat, latest first: the strip while it is shown, the
   /// full player in fullscreen (both hold the shell's toggle).
@@ -53,6 +55,11 @@ class _AppKeysState extends State<AppKeys> {
   bool _key(KeyEvent event) {
     if (event is! KeyDownEvent || !mounted) return false;
     final key = event.logicalKey;
+    if (markProblemKey.accepts(event, HardwareKeyboard.instance)) {
+      if (ModalRoute.of(context)?.isCurrent == false) return false;
+      showMarkProblem(context);
+      return true;
+    }
     if (key != LogicalKeyboardKey.space && key != LogicalKeyboardKey.tab) return false;
     if (SessionState.instance.route.peek() != AppRoute.session) return false;
     if (ModalRoute.of(context)?.isCurrent == false) return false;
diff --git a/apps/desktop/test/app_keys_test.dart b/apps/desktop/test/app_keys_test.dart
index 99c7434..4c3f660 100644
--- a/apps/desktop/test/app_keys_test.dart
+++ b/apps/desktop/test/app_keys_test.dart
@@ -65,4 +65,24 @@ void main() {
     expect(readies, 1);
     expect(panel, findsNothing, reason: 'Space pressed a focused chat button');
   });
+
+  testWidgets('Ctrl+Shift+B opens Mark a problem; Esc closes it unmarked', (tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData.dark(),
+      shortcuts: appShortcuts,
+      home: const AppKeys(child: Scaffold(body: SizedBox())),
+    ));
+    final card = find.byKey(const ValueKey('mark-problem-card'));
+    await tester.sendKeyDownEvent(LogicalKeyboardKey.controlLeft);
+    await tester.sendKeyDownEvent(LogicalKeyboardKey.shiftLeft);
+    await tester.sendKeyEvent(LogicalKeyboardKey.keyB);
+    await tester.sendKeyUpEvent(LogicalKeyboardKey.shiftLeft);
+    await tester.sendKeyUpEvent(LogicalKeyboardKey.controlLeft);
+    await tester.pumpAndSettle();
+    expect(card, findsOneWidget);
+    expect(textInputFocused(), isTrue, reason: 'the note field takes the keys');
+    await tester.sendKeyEvent(LogicalKeyboardKey.escape);
+    await tester.pumpAndSettle();
+    expect(card, findsNothing);
+  });
 }
diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
index c2b5852..3789bb8 100644
--- a/docs/ARCHITECTURE.md
+++ b/docs/ARCHITECTURE.md
@@ -1517,9 +1517,14 @@ dedicated server (`package:logging` loggers named by area: `relay`,
   `--config`, else `logs` next to the binary (systemd:
   `$STATE_DIRECTORY/logs`), else `~/.cache/watch-together/logs`; it
   prints the folder at start. Both also print every line to stdout.
+  The session log (below) sits next to them: the hosting app's server
+  writes it in the app's logs folder (not for a solo session), the
+  dedicated one in its log folder.
 - **Files** (`LogFiles`): one per day, the next `.1.log`, `.2.log` once a
   file passes 10 MiB; files older than 7 days, then the oldest beyond
-  100 MiB in all, are deleted whenever one is opened. A line is
+  100 MiB in all (the app's or server's files and the session log's
+  together, the oldest of either first, never the other writer's file of
+  today), are deleted whenever one is opened. A line is
   `2026-10-12 18:44:27.202 LEVEL logger: message`, the error and stack
   trace on the lines below. Writes are synchronous appends (nothing lost
   in a crash). Lines from before the folder is known (the app finds it
@@ -1527,7 +1532,7 @@ dedicated server (`package:logging` loggers named by area: `relay`,
 - **Redaction** (`redactLog`): every line, in one place
   (`formatLogLine`), before console or file: URL query strings cut to
   their parameter names (signed CDN links, Jellyfin `api_key`),
-  `user:pass@` in URLs, member tokens in `/r /m /j /a /i` paths, and the
+  `user:pass@` in URLs, member tokens in `/r /m /j /a /i /l` paths, and the
   values of passwords, secrets, tokens, API keys, cookies,
   `Authorization` / `X-Emby-*` headers (`name=value`, `"name":"value"`,
   `Name="value"`, `Name: value`) and `Bearer` credentials. Code logs
@@ -1554,12 +1559,51 @@ dedicated server (`package:logging` loggers named by area: `relay`,
   source (media opened and how long it took, channel up and down,
   reconnects, upstream refusals with the header names sent, re-resolves
   of expired format URLs).
+- **Never logged**: chat text (the server relays it without a log line,
+  the app does not log it), and no user-typed free text other than a
+  problem mark's note. Names, media titles and links (query cut) are.
+- **The session log** (`packages/server/lib/src/session_log.dart`,
+  `packages/core/lib/src/diag_share.dart`): one shared log per server so
+  a problem can be read afterwards from every side. While in a session
+  the app (`DiagUploader`, owned by the coordinator) keeps its own
+  records (not its hosted server's `server.*`, which the server logs
+  itself), redacted, and sends them every 10 s from the session's 1 s
+  ticker and when it leaves (before its token goes, at most 1.5 s), as
+  gzip batches to `POST /l/<sid>/<token>` (PROTOCOL
+  "Diagnostics"), timestamps moved to the server's clock with the ping
+  offset (`ClockSync`), the app version in every batch. Until it can
+  send (reconnecting, no clock sample yet) it keeps up to 2 MiB, the
+  oldest dropped and counted. The server (`SessionLog`) takes every
+  member's lines and its own records, holds them 15 s and writes them in
+  server-time order to `session-YYYY-MM-DD.log` (day and time on the
+  server's clock; a line later than that, a reconnect's backlog, goes in
+  with its own time, a little out of order). A line is
+  `HH:MM:SS.mmm [who] logger: message` (`LEVEL logger` from WARNING up),
+  `who` = `name (id)` or `server`, continuation lines indented; every
+  line is redacted again on the server. The server notes once per member
+  `Bob (m2) shares diagnostics (<version>)` or `Bob (m2) doesn't share
+  diagnostics`, and `N lines dropped` when the app's buffer overflowed.
+  Limits and rate caps: PROTOCOL "Diagnostics"; a refused batch stays in
+  the app's buffer for the next try.
+- **Opting out**: Settings → About → "Share diagnostics with the server"
+  (default on). Off: no lines are kept or sent; one batch per connection
+  says so (`share:false`). The local log is unchanged.
+- **Mark a problem**: Ctrl+Shift+B (`AppKeys`), the player's ⚙ card and
+  Settings → About open a small card with an optional note (200
+  characters, one line); the mark goes at once with the lines waiting,
+  opted out or not, and the server writes `[who] === PROBLEM marked by
+  <name>: <note> ===` at its time. The app's own log gets
+  `=== PROBLEM marked: <note> ===` (logger `problem`, never sent). Out
+  of a session only the app's log gets it. The toast says which.
 - **Getting it**: Settings → About → "Copy diagnostics" puts the app and
   OS versions, mpv/ffmpeg and yt-dlp versions, whether yt-dlp has
   cookies, and the last 6 hours of the log (up to 4 MiB) on the
   clipboard, already redacted, and saves the same text as
   `diagnostics-<time>.txt` in the logs folder (kept 7 days); "Open logs
-  folder" opens it.
+  folder" opens it. In a session it first asks whether to add the
+  server's session log of the same 6 hours (`GET /l/...`). Any member
+  may read the session log, not only the host: friends trust each other,
+  and it is redacted and holds no chat.
 
 ## Health reporting
 
diff --git a/docs/DEVELOPMENT.md b/docs/DEVELOPMENT.md
index 509845c..49997c1 100644
--- a/docs/DEVELOPMENT.md
+++ b/docs/DEVELOPMENT.md
@@ -145,7 +145,20 @@ on Linux (on Windows under `%APPDATA%`; Settings → About → "Open logs
 folder" finds it anywhere), with the hosted server's lines
 in the same file; a dedicated server prints its folder at start
 (`--log-dir`). Users send theirs with Settings → About → "Copy
-diagnostics". For a "can't play" report, grep both sides around the
+diagnostics".
+
+Start with the session log when there is one: `session-YYYY-MM-DD.log`
+in the server's folder (the hosting app's logs folder, or the dedicated
+server's `--log-dir`), every member's lines and the server's merged in
+server-clock order, each tagged `[name (id)]` or `[server]`. Any member
+gets it with "Copy diagnostics" in a session. Find
+`=== PROBLEM marked by` first (a member pressed Ctrl+Shift+B), then read
+the lines around it from every side; `grep '\[Bob (m2)\]'` follows one
+member. `[server] diag:` lines say who shares (with their app version)
+and who doesn't, so a missing side is explained. Times are the server's
+clock; a member's local log (`app-*.log`) is on its own clock.
+
+For a "can't play" report, grep both sides around the
 time: `swap by` (what went up and how), `relay channel` (when the
 source dialled in), `media:` (each viewer's requests: status and time to
 headers), `mpv:` (`cannot open the media` and ffmpeg's lines), `yt-dlp:`
diff --git a/docs/PROTOCOL.md b/docs/PROTOCOL.md
index 34a1caf..0813bf0 100644
--- a/docs/PROTOCOL.md
+++ b/docs/PROTOCOL.md
@@ -1,4 +1,4 @@
-# Protocol (proto = 14)
+# Protocol (proto = 15)
 
 Wire format only. Behavior (gate, clock, timeouts) is in
 [ARCHITECTURE.md](ARCHITECTURE.md). Source of truth:
@@ -26,12 +26,14 @@ with different `proto` refuse each other at `hello` (`proto mismatch`).
   "Jellyfin media" below). `/r/<sid>/<token>/...` is the relay of a
   member's media (see "Relay" below). `GET /a/<sid>/<token>/<hash>` serves
   a member's picture (see "Avatars" below), `GET /i/<sid>/<token>/<hash>`
-  the current media's art (see "Media art" below).
+  the current media's art (see "Media art" below). `POST` and `GET
+  /l/<sid>/<token>` send and read the session log (see "Diagnostics"
+  below).
 
 ## Client to server
 
 ```
-{"t":"hello","proto":14,"secret":"","user":{"name":"","avatar":""},
+{"t":"hello","proto":15,"secret":"","user":{"name":"","avatar":""},
  "media":{"name":"","size":0,"duration":0,"url":null},"hostKey":"...",
  "resume":"<token>"}
 {"t":"ready","ready":true,"source":"manual"}      source: manual | auto
@@ -467,3 +469,41 @@ state  "queue":[<item>,...], "queuePlay":"q3"
 - `state.queue` (omitted when empty) and `state.queuePlay` (omitted when
   none) ride in every state, `welcome.state` included. The first item's
   `media.relay` shows its warm relay (see "Relay").
+
+## Diagnostics (proto 15)
+
+Each app sends its log lines to the server, which merges them with its
+own into the session log (behavior, file and format: ARCHITECTURE.md,
+"Logging").
+
+```
+POST /l/<sid>/<token>   content-type application/gzip, body = gzip of:
+  {"v":"Starsync 1.4.0 on linux","share":false,
+   "lines":[{"t":1767225600123,"l":"WARNING","n":"mpv","m":"..."}],
+   "dropped":12,"marks":[{"t":1767225600500,"note":"..."}]}
+GET  /l/<sid>/<token>?hours=6
+```
+
+- `POST`: one batch. Every field is optional. `v`: the app and version,
+  noted once per member (and when it changes). `share:false` (omitted
+  when true): the member does not share; the server notes it once and
+  drops any `lines`. `lines[]`: `t` epoch ms on the server's clock (the
+  client converts with its `pong` offset), `l` level name, `n` logger
+  (`[A-Za-z0-9_.-]`, up to 40, else `app`), `m` the message, already
+  redacted, an error and stack trace on the lines below (cut to 8192
+  characters). `dropped`: lines the client dropped since its last batch
+  (its buffer was full). `marks[]`: "Mark a problem" at `t` with an
+  optional `note` (one line, cut to 200 characters); it goes in even
+  with `share:false`.
+- Limits: a body of at most 256 KiB, unpacking to at most 1 MiB (else
+  413); not gzip of a JSON object: 400. Per member and minute at most 30
+  posts and 4 MiB of lines, else 429 with `retry-after`: the client keeps
+  the batch and sends it later. Unknown token: 401. Success: 204, also
+  on a server that keeps no session log (the batch is dropped).
+- `GET`: the session log of the last `hours` (1 to 168, default 6; at
+  most 8 MiB, the newest kept), lines the server still holds for
+  ordering included, as gzip of UTF-8 text (`application/gzip`). Any
+  member may read it (it is redacted and holds no chat). 401 for an
+  unknown token, 404 on a server that keeps no session log.
+- Clients send every 10 s and when they leave; each batch carries at
+  most 256 KiB of lines.
diff --git a/packages/core/lib/core.dart b/packages/core/lib/core.dart
index 500041f..ce106ac 100644
--- a/packages/core/lib/core.dart
+++ b/packages/core/lib/core.dart
@@ -1,6 +1,7 @@
 export 'src/constants.dart';
 export 'src/errors.dart';
 export 'src/logging.dart';
+export 'src/diag_share.dart';
 export 'src/clock_sync.dart';
 export 'src/config.dart';
 export 'src/protocol.dart';
diff --git a/packages/core/lib/src/constants.dart b/packages/core/lib/src/constants.dart
index e000a85..7d272d5 100644
--- a/packages/core/lib/src/constants.dart
+++ b/packages/core/lib/src/constants.dart
@@ -1,5 +1,5 @@
 abstract class RelayConstants {
-  static const int protoVersion = 14;
+  static const int protoVersion = 15;
 
   // Relay (`/r/...`): a member's bytes through the server's buffer.
   static const int relaySliceBytes = 1 << 20;
@@ -70,4 +70,20 @@ abstract class RelayConstants {
   static const int maxQueueItems = 100;
   static const int maxQueueTextLength = 200;
   static const int queuePlayTimeoutSecs = 30;
+
+  /// Diagnostics sharing (`/l/...`, ARCHITECTURE "Logging"): a member's
+  /// batch carries at most [diagBatchBytes] of lines (gzip on the wire, at
+  /// most [diagBatchBytes] compressed and [diagDecodedBytes] unpacked); it
+  /// keeps up to [diagBufferBytes] while it cannot send (oldest dropped)
+  /// and sends every [diagIntervalSecs]. The server takes at most
+  /// [diagPostsPerMin] posts and [diagBytesPerMin] unpacked bytes per member
+  /// and minute (429 beyond). A "Mark a problem" note is cut to
+  /// [maxProblemNoteLength] characters.
+  static const int diagBatchBytes = 256 * 1024;
+  static const int diagDecodedBytes = 1 << 20;
+  static const int diagBufferBytes = 2 << 20;
+  static const int diagIntervalSecs = 10;
+  static const int diagPostsPerMin = 30;
+  static const int diagBytesPerMin = 4 << 20;
+  static const int maxProblemNoteLength = 200;
 }
diff --git a/packages/core/lib/src/diag_share.dart b/packages/core/lib/src/diag_share.dart
new file mode 100644
index 0000000..7a6d63d
--- /dev/null
+++ b/packages/core/lib/src/diag_share.dart
@@ -0,0 +1,389 @@
+import 'dart:async';
+import 'dart:collection';
+import 'dart:convert';
+import 'dart:io';
+import 'dart:typed_data';
+
+import 'package:logging/logging.dart';
+
+import 'constants.dart';
+import 'logging.dart';
+
+// Diagnostics sharing (ARCHITECTURE "Logging", PROTOCOL "Diagnostics"):
+// each app sends its log lines to the server, which merges them with its
+// own into one session log. The batch format both sides use, and the
+// app's sender.
+
+/// One log record of a member: [t] in epoch ms (the app's clock while it
+/// waits, the server's on the wire), the level name, the logger and the
+/// redacted message (an error and stack trace on the lines below).
+class DiagLine {
+  final int t;
+  final String level;
+  final String logger;
+  final String message;
+  const DiagLine({required this.t, required this.level, required this.logger, required this.message});
+
+  /// Roughly its bytes in a batch.
+  int get size => message.length + logger.length + level.length + 32;
+
+  Map<String, Object?> toJson() => {'t': t, 'l': level, 'n': logger, 'm': message};
+
+  static DiagLine? fromJson(Object? j) {
+    if (j is! Map) return null;
+    final t = j['t'], l = j['l'], n = j['n'], m = j['m'];
+    if (t is! num || !t.isFinite || m is! String) return null;
+    return DiagLine(t: t.toInt(), level: l is String ? l : 'INFO', logger: n is String ? n : 'app', message: m);
+  }
+}
+
+/// A "Mark a problem" from a member: when (epoch ms, server clock) and
+/// its optional note.
+class DiagMark {
+  final int t;
+  final String note;
+  const DiagMark({required this.t, this.note = ''});
+
+  Map<String, Object?> toJson() => {'t': t, if (note.isNotEmpty) 'note': note};
+
+  static DiagMark? fromJson(Object? j) {
+    if (j is! Map) return null;
+    final t = j['t'], note = j['note'];
+    if (t is! num || !t.isFinite) return null;
+    return DiagMark(t: t.toInt(), note: note is String ? note : '');
+  }
+}
+
+/// The body of `POST /l/<sid>/<token>` (gzip of its JSON): the app's
+/// [version], whether it [share]s its lines (false: none, the server notes
+/// it once), the [lines], how many it [dropped] since the last batch (its
+/// buffer was full) and any problem [marks].
+class DiagBatch {
+  final String version;
+  final bool share;
+  final List<DiagLine> lines;
+  final int dropped;
+  final List<DiagMark> marks;
+  const DiagBatch({this.version = '', this.share = true, this.lines = const [], this.dropped = 0, this.marks = const []});
+
+  Map<String, Object?> toJson() => {
+        if (version.isNotEmpty) 'v': version,
+        if (!share) 'share': false,
+        if (lines.isNotEmpty) 'lines': [for (final l in lines) l.toJson()],
+        if (dropped > 0) 'dropped': dropped,
+        if (marks.isNotEmpty) 'marks': [for (final m in marks) m.toJson()],
+      };
+
+  List<int> encode() => gzip.encode(utf8.encode(jsonEncode(toJson())));
+
+  factory DiagBatch.fromJson(Map<String, Object?> j) {
+    final lines = j['lines'], marks = j['marks'], dropped = j['dropped'], v = j['v'];
+    return DiagBatch(
+      version: v is String ? v : '',
+      share: j['share'] != false,
+      lines: lines is List ? lines.map(DiagLine.fromJson).whereType<DiagLine>().toList() : const [],
+      dropped: dropped is num && dropped.isFinite && dropped > 0 ? dropped.toInt() : 0,
+      marks: marks is List ? marks.map(DiagMark.fromJson).whereType<DiagMark>().toList() : const [],
+    );
+  }
+
+  /// Unpacks a body; throws [FormatException] when it is not gzip of a
+  /// batch or unpacks to more than [maxBytes].
+  static DiagBatch decode(List<int> body, {int maxBytes = RelayConstants.diagDecodedBytes}) {
+    final text = utf8.decode(gunzipCapped(body, maxBytes), allowMalformed: true);
+    final j = jsonDecode(text);
+    if (j is! Map) throw const FormatException('not a batch');
+    return DiagBatch.fromJson(Map<String, Object?>.from(j));
+  }
+}
+
+/// [body] gunzipped, or [FormatException] once it passes [maxBytes] (a
+/// small body must not unpack to gigabytes).
+List<int> gunzipCapped(List<int> body, int maxBytes) {
+  final out = BytesBuilder(copy: false);
+  final sink = _CappedSink(out, maxBytes);
+  try {
+    gzip.decoder.startChunkedConversion(sink)
+      ..add(body)
+      ..close();
+  } on FormatException {
+    rethrow;
+  } catch (e) {
+    throw FormatException('not gzip: $e');
+  }
+  return out.takeBytes();
+}
+
+class _CappedSink implements Sink<List<int>> {
+  final BytesBuilder out;
+  final int max;
+  _CappedSink(this.out, this.max);
+
+  @override
+  void add(List<int> data) {
+    if (out.length + data.length > max) throw const FormatException('too large');
+    out.add(data);
+  }
+
+  @override
+  void close() {}
+}
+
+/// Where the app's lines go: the server's base URL (`http://host:port`),
+/// this member's token and the server's clock minus the local one (ms).
+typedef DiagTarget = ({Uri base, String token, double offsetMs});
+
+/// The app's side: keeps its log records (not the hosted server's, which
+/// the server logs itself) while in a session, and sends them to the
+/// server every [RelayConstants.diagIntervalSecs] ([tick]) and on [stop].
+/// [target] is null while there is nowhere to send (reconnecting, no
+/// clock sample yet): lines wait, up to [bufferBytes], the oldest dropped
+/// and counted. With [share] off nothing is kept or sent but one batch
+/// saying so per connection; problem marks ([markProblem]) go either way.
+class DiagUploader {
+  final DiagTarget? Function() target;
+  /// The app and its version, e.g. `Watch Together 1.4.0 on linux`.
+  final String version;
+  final int bufferBytes;
+  final int batchBytes;
+  final Duration interval;
+  final DateTime Function() _now;
+  DiagUploader({
+    required this.target,
+    required this.version,
+    this.bufferBytes = RelayConstants.diagBufferBytes,
+    this.batchBytes = RelayConstants.diagBatchBytes,
+    this.interval = const Duration(seconds: RelayConstants.diagIntervalSecs),
+    DateTime Function()? now,
+  }) : _now = now ?? DateTime.now;
+
+  /// Loggers never sent: the hosted server's records (`server.*`, the
+  /// server logs them itself) and the local copy of a mark ([markLogger]).
+  static bool sends(String logger) => logger != 'server' && !logger.startsWith('server.') && logger != markLogger;
+
+  /// The local log's copy of a problem mark.
+  static const markLogger = 'problem';
+  static final _markLog = Logger(markLogger);
+
+  final _lines = Queue<DiagLine>();
+  var _size = 0;
+  var _dropped = 0;
+  final _marks = <DiagMark>[];
+  StreamSubscription<LogRecord>? _sub;
+  HttpClient? _http;
+  var _busy = false;
+  var _again = false;
+  DateTime? _last;
+  /// The token the server was told "doesn't share" for.
+  String? _toldOff;
+  var _share = true;
+
+  bool get running => _sub != null;
+
+  /// Lines waiting to go.
+  int get pending => _lines.length;
+
+  /// Share the log lines (Settings); off drops what waits.
+  bool get share => _share;
+  set share(bool on) {
+    if (on == _share) return;
+    _share = on;
+    if (!on) _clear();
+    _toldOff = null;
+  }
+
+  /// Starts keeping the log's records (a session starts).
+  void start() {
+    _sub ??= Logger.root.onRecord.listen(_onRecord);
+    _last = _now();
+  }
+
+  void _onRecord(LogRecord r) {
+    if (!_share || !sends(r.loggerName)) return;
+    final b = StringBuffer(r.message);
+    if (r.error != null) b.write('\n  error: ${r.error}');
+    final s = '${r.stackTrace ?? ''}'.trimRight();
+    if (s.isNotEmpty) b.write('\n$s');
+    add(DiagLine(t: r.time.millisecondsSinceEpoch, level: r.level.name, logger: r.loggerName, message: redactLog('$b')));
+  }
+
+  /// Keeps [line] (local clock) for the next batch.
+  void add(DiagLine line) {
+    _lines.add(line);
+    _size += line.size;
+    _trim();
+  }
+
+  void _trim() {
+    while (_size > bufferBytes && _lines.isNotEmpty) {
+      _size -= _lines.removeFirst().size;
+      _dropped++;
+    }
+  }
+
+  void _clear() {
+    _lines.clear();
+    _size = 0;
+    _dropped = 0;
+  }
+
+  /// "Mark a problem": a marker line in the server's session log with
+  /// [note] (cut to [RelayConstants.maxProblemNoteLength], one line), sent
+  /// now with the lines waiting. Also written to the local log. True once
+  /// the server has it; false when it waits (not connected).
+  Future<bool> markProblem(String note) {
+    var clean = note.replaceAll(RegExp(r'[\x00-\x1f\x7f]+'), ' ').trim();
+    if (clean.length > RelayConstants.maxProblemNoteLength) {
+      clean = clean.substring(0, RelayConstants.maxProblemNoteLength);
+    }
+    _markLog.warning('=== PROBLEM marked${clean.isEmpty ? '' : ': $clean'} ===');
+    _marks.add(DiagMark(t: _now().millisecondsSinceEpoch, note: redactLog(clean)));
+    return flush();
+  }
+
+  /// Once a second (the app's ticker): sends when [interval] has passed.
+  void tick() {
+    final last = _last;
+    if (last != null && _now().difference(last) < interval) return;
+    unawaited(flush());
+  }
+
+  /// Sends what waits: true when the server took all of it (or there was
+  /// nothing to send).
+  Future<bool> flush() async {
+    if (_busy) {
+      // Once the one under way is done (a mark goes now, not next tick).
+      _again = true;
+      return false;
+    }
+    final tg = target();
+    if (tg == null) return false;
+    _busy = true;
+    _last = _now();
+    try {
+      final offset = tg.offsetMs.round();
+      List<DiagMark> takeMarks() {
+        final m = [for (final x in _marks) DiagMark(t: x.t + offset, note: x.note)];
+        _marks.clear();
+        return m;
+      }
+
+      if (!_share) {
+        if (_toldOff == tg.token && _marks.isEmpty) return true;
+        final marks = takeMarks();
+        final ok = await _post(tg, DiagBatch(version: version, share: false, marks: marks));
+        if (ok) {
+          _toldOff = tg.token;
+        } else {
+          _marks.insertAll(0, [for (final m in marks) DiagMark(t: m.t - offset, note: m.note)]);
+        }
+        return ok;
+      }
+      // A few batches at most per flush; the rest goes next time.
+      for (var i = 0; i < 8 && (_lines.isNotEmpty || _marks.isNotEmpty || _dropped > 0); i++) {
+        final taken = <DiagLine>[];
+        var bytes = 0;
+        while (_lines.isNotEmpty && (taken.isEmpty || bytes + _lines.first.size <= batchBytes)) {
+          final l = _lines.removeFirst();
+          _size -= l.size;
+          bytes += l.size;
+          taken.add(l);
+        }
+        final dropped = _dropped;
+        _dropped = 0;
+        final marks = takeMarks();
+        final batch = DiagBatch(
+          version: version,
+          lines: [for (final l in taken) DiagLine(t: l.t + offset, level: l.level, logger: l.logger, message: l.message)],
+          dropped: dropped,
+          marks: marks,
+        );
+        if (!await _post(tg, batch)) {
+          // Back in front, in order; the cap applies again.
+          for (final l in taken.reversed) {
+            _lines.addFirst(l);
+            _size += l.size;
+          }
+          _dropped += dropped;
+          _marks.insertAll(0, [for (final m in marks) DiagMark(t: m.t - offset, note: m.note)]);
+          _trim();
+          return false;
+        }
+      }
+      return _lines.isEmpty;
+    } finally {
+      _busy = false;
+      if (_again) {
+        _again = false;
+        if (_sub != null) unawaited(flush());
+      }
+    }
+  }
+
+  static Uri _httpUri(Uri base, String path, [Map<String, String>? query]) => base.replace(
+      scheme: base.scheme == 'wss' || base.scheme == 'https' ? 'https' : 'http',
+      path: path,
+      queryParameters: query,
+      fragment: null);
+
+  /// The server's `http://host:port` for a `/sync` socket URL.
+  static Uri baseOf(Uri wsUri) => _httpUri(wsUri, '/');
+
+  Future<bool> _post(DiagTarget tg, DiagBatch batch) async {
+    try {
+      final http = _http ??= HttpClient()..connectionTimeout = const Duration(seconds: 5);
+      final req = await http.postUrl(_httpUri(tg.base, '/l/default/${tg.token}')).timeout(const Duration(seconds: 5));
+      final body = batch.encode();
+      req.headers.contentType = ContentType('application', 'gzip');
+      req.contentLength = body.length;
+      req.add(body);
+      final res = await req.close().timeout(const Duration(seconds: 10));
+      await res.drain<void>();
+      // 413: this batch can never go (should not happen); drop it.
+      return res.statusCode == 204 || res.statusCode == 200 || res.statusCode == 413;
+    } catch (_) {
+      return false;
+    }
+  }
+
+  /// The server's session log of the last [hours] (every member and the
+  /// server, merged), or null when it cannot be had (not connected, a
+  /// server without one, an older server).
+  Future<String?> fetchSessionLog({int hours = 6}) async {
+    final tg = target();
+    if (tg == null) return null;
+    try {
+      final http = _http ??= HttpClient()..connectionTimeout = const Duration(seconds: 5);
+      final req = await http
+          .getUrl(_httpUri(tg.base, '/l/default/${tg.token}', {'hours': '$hours'}))
+          .timeout(const Duration(seconds: 5));
+      final res = await req.close().timeout(const Duration(seconds: 10));
+      final bytes = <int>[];
+      await for (final c in res.timeout(const Duration(seconds: 30))) {
+        bytes.addAll(c);
+      }
+      if (res.statusCode != 200) return null;
+      return utf8.decode(gunzipCapped(bytes, 64 << 20), allowMalformed: true);
+    } catch (_) {
+      return null;
+    }
+  }
+
+  /// The session ends: what waits goes now (best effort, [timeout]), then
+  /// nothing is kept until the next [start].
+  Future<void> stop({Duration timeout = const Duration(milliseconds: 1500)}) async {
+    await _sub?.cancel();
+    _sub = null;
+    if ((_lines.isNotEmpty || _marks.isNotEmpty || _dropped > 0) && target() != null) {
+      try {
+        await flush().timeout(timeout);
+      } catch (_) {}
+    }
+    _clear();
+    _marks.clear();
+    _toldOff = null;
+    _last = null;
+    _http?.close(force: true);
+    _http = null;
+  }
+}
diff --git a/packages/core/lib/src/logging.dart b/packages/core/lib/src/logging.dart
index d6e7959..b7bf07b 100644
--- a/packages/core/lib/src/logging.dart
+++ b/packages/core/lib/src/logging.dart
@@ -10,6 +10,10 @@ import 'package:logging/logging.dart';
 
 Logger createLogger(String name) => Logger(name);
 
+/// The server's combined log of every member and itself
+/// (`session-YYYY-MM-DD.log`, ARCHITECTURE "Logging").
+const sessionLogPrefix = 'session';
+
 var _listening = false;
 var _console = true;
 LogFiles? _files;
@@ -35,12 +39,13 @@ void initLogging({Level level = Level.INFO, bool console = true}) {
 
 /// Writes the files in [dir] from now on: `<prefix>-YYYY-MM-DD.log`,
 /// rotated by day and [LogFiles.maxFileBytes], pruned by age and total
-/// size. Lines logged before go in first. A folder that cannot be written
-/// leaves logging on the console only (and says so there).
+/// size (shared with the session log, [sessionLogPrefix], in the same
+/// folder). Lines logged before go in first. A folder that cannot be
+/// written leaves logging on the console only (and says so there).
 LogFiles? startLogFiles(String dir, {String prefix = 'app'}) {
   if (_files != null) return _files;
   try {
-    final f = LogFiles(dir, prefix: prefix)..open();
+    final f = LogFiles(dir, prefix: prefix, shareWith: const [sessionLogPrefix])..open();
     _files = f;
     for (final l in _early) {
       f.write(l);
@@ -123,13 +128,13 @@ final _headerSecret = RegExp(
 final _pairSecret = RegExp('\\b($_secretNames)\\s*=\\s*[^&\\s"\',;)}\\]]+', caseSensitive: false);
 final _bearer = RegExp(r'\b(Bearer|Basic)\s+[A-Za-z0-9._~+/=-]{8,}', caseSensitive: false);
 final _url = RegExp(r'''\b(?:https?|wss?)://[^\s"'<>]+''', caseSensitive: false);
-// A member token in a server path: /r/<sid>/<token>/..., /m, /j, /a, /i.
-final _pathToken = RegExp(r'(/[rmjai]/[^/\s?#]+/)[0-9a-fA-F]{32}\b');
+// A member token in a server path: /r/<sid>/<token>/..., /m, /j, /a, /i, /l.
+final _pathToken = RegExp(r'(/[rmjail]/[^/\s?#]+/)[0-9a-fA-F]{32}\b');
 final _userInfo = RegExp(r'^([a-z]+://)[^/@\s]+@', caseSensitive: false);
 
 /// [s] with nothing secret left: every URL's query string down to its
 /// parameter names (signed CDN links, `api_key`), user:password@ in URLs,
-/// member tokens in `/r/ /m/ /j/ /a/ /i/` paths, and the values of
+/// member tokens in `/r/ /m/ /j/ /a/ /i/ /l/` paths, and the values of
 /// passwords, tokens, API keys, cookies and auth headers. Applied to
 /// every log line ([formatLogLine]).
 String redactLog(String s) {
@@ -203,16 +208,21 @@ class LogThrottle {
 /// Rotating log files in [dir]: `<prefix>-YYYY-MM-DD.log` (then `.1.log`,
 /// `.2.log` ... once one passes [maxFileBytes]); files older than
 /// [keepDays] and the oldest beyond [maxTotalBytes] are deleted whenever a
-/// file is opened.
+/// file is opened. [shareWith]: other prefixes in [dir] that count toward
+/// the same [maxTotalBytes] (the app's or server's log and the session
+/// log); the oldest of all of them go first, never another prefix's file
+/// of today (its writer may have it open).
 class LogFiles {
   final String dir;
   final String prefix;
+  final List<String> shareWith;
   final int maxFileBytes;
   final int maxTotalBytes;
   final int keepDays;
   final DateTime Function() _now;
   LogFiles(this.dir,
       {this.prefix = 'app',
+      this.shareWith = const [],
       this.maxFileBytes = 10 * 1024 * 1024,
       this.maxTotalBytes = 100 * 1024 * 1024,
       this.keepDays = 7,
@@ -229,7 +239,10 @@ class LogFiles {
 
   String _dayOf(DateTime t) => '${t.year}-${_two(t.month)}-${_two(t.day)}';
 
-  String _dayOfFile(File f) => f.uri.pathSegments.last.substring(prefix.length + 1, prefix.length + 11);
+  String _dayOfFile(File f) => _fileDay(prefix, f);
+
+  static String _fileDay(String prefix, File f) =>
+      f.uri.pathSegments.last.substring(prefix.length + 1, prefix.length + 11);
 
   /// Opens today's file (creating [dir]); throws when it cannot.
   void open() {
@@ -256,9 +269,10 @@ class LogFiles {
   }
 
   /// Appends [line]; a new day or a full file moves on to the next file.
-  void write(String line) {
+  /// [at]: the line's own time picks the day's file (else now).
+  void write(String line, {DateTime? at}) {
     try {
-      final day = _dayOf(_now());
+      final day = _dayOf(at ?? _now());
       if (_out == null || day != _day || _size >= maxFileBytes) _roll(day);
       final bytes = utf8.encode('$line\n');
       _out!.writeFromSync(bytes);
@@ -269,7 +283,10 @@ class LogFiles {
   }
 
   /// This prefix's files, oldest first.
-  List<File> files() {
+  List<File> files() => filesOf(prefix);
+
+  /// The files of [prefix] in [dir], oldest first.
+  List<File> filesOf(String prefix) {
     final d = Directory(dir);
     if (!d.existsSync()) return const [];
     final name = RegExp('^${RegExp.escape(prefix)}-(\\d{4}-\\d{2}-\\d{2})(?:\\.(\\d+))?\\.log\$');
@@ -295,10 +312,18 @@ class LogFiles {
         kept.add(f);
       }
     }
-    var total = kept.fold<int>(0, (a, f) => a + _len(f));
-    for (final f in kept) {
+    final today = _dayOf(_now());
+    // Every file under the budget, oldest day first: (day, prefix, file).
+    final all = [
+      for (final f in kept) (_dayOfFile(f), true, f),
+      for (final p in shareWith)
+        if (p != prefix)
+          for (final f in filesOf(p)) (_fileDay(p, f), false, f),
+    ]..sort((a, b) => a.$1.compareTo(b.$1));
+    var total = all.fold<int>(0, (a, f) => a + _len(f.$3));
+    for (final (day, own, f) in all) {
       if (total <= maxTotalBytes) break;
-      if (f.path == _path) continue;
+      if (f.path == _path || (!own && day == today)) continue;
       total -= _len(f);
       _delete(f);
     }
diff --git a/packages/core/test/diag_share_test.dart b/packages/core/test/diag_share_test.dart
new file mode 100644
index 0000000..d66ed4c
--- /dev/null
+++ b/packages/core/test/diag_share_test.dart
@@ -0,0 +1,87 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:logging/logging.dart';
+import 'package:sync_relay_core/core.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('a batch survives gzip both ways; a bomb or junk is refused', () {
+    const b = DiagBatch(
+        version: 'WT 1.0',
+        lines: [DiagLine(t: 5, level: 'WARNING', logger: 'mpv', message: 'x\n  error: y')],
+        dropped: 2,
+        marks: [DiagMark(t: 7, note: 'lag')]);
+    final back = DiagBatch.decode(b.encode());
+    expect(back.toJson(), b.toJson());
+    expect(DiagBatch.decode(const DiagBatch(share: false).encode()).share, isFalse);
+    expect(() => DiagBatch.decode(gzip.encode(List.filled(2 << 20, 32)), maxBytes: 1 << 20), throwsFormatException);
+    expect(() => DiagBatch.decode(utf8.encode('plain')), throwsFormatException);
+  });
+
+  group('DiagUploader', () {
+    late HttpServer http;
+    final got = <DiagBatch>[];
+    var accept = true;
+    setUp(() async {
+      got.clear();
+      accept = true;
+      http = await HttpServer.bind(InternetAddress.loopbackIPv4, 0);
+      http.listen((req) async {
+        final body = await req.fold<List<int>>([], (a, c) => a..addAll(c));
+        if (accept) got.add(DiagBatch.decode(body));
+        req.response.statusCode = accept ? 204 : 503;
+        await req.response.close();
+      });
+    });
+    tearDown(() => http.close(force: true));
+
+    DiagUploader make({bool connected = true, int buffer = 1 << 20}) => DiagUploader(
+        target: () => connected ? (base: Uri.parse('http://127.0.0.1:${http.port}'), token: 't', offsetMs: 1000) : null,
+        version: 'WT 1.0',
+        bufferBytes: buffer);
+
+    test('sends its records on the server clock, not the hosted server\'s; keeps them while it cannot', () async {
+      final u = make()..start();
+      Logger('relay').info('hello api_key=abc');
+      Logger('server.relay').info('the server logs this itself');
+      await Future<void>.delayed(Duration.zero);
+      accept = false;
+      expect(await u.flush(), isFalse);
+      expect(u.pending, 1);
+      accept = true;
+      expect(await u.flush(), isTrue);
+      final line = got.single.lines.single;
+      expect(line.message, 'hello api_key=***');
+      expect(line.logger, 'relay');
+      expect(got.single.version, 'WT 1.0');
+      await u.stop();
+    });
+
+    test('a full buffer drops the oldest and says how many', () async {
+      final u = make(buffer: 400);
+      for (var i = 0; i < 10; i++) {
+        u.add(DiagLine(t: i, level: 'INFO', logger: 'a', message: 'line $i ${'x' * 40}'));
+      }
+      expect(await u.flush(), isTrue);
+      expect(got.single.dropped, greaterThan(0));
+      expect(got.single.lines.last.message, startsWith('line 9'));
+      expect(got.single.lines.first.t, greaterThanOrEqualTo(1000), reason: 'offset to the server clock');
+    });
+
+    test('opted out: no lines, one "does not share" per connection; marks still go', () async {
+      final u = make()
+        ..share = false
+        ..start();
+      Logger('relay').info('private');
+      expect(await u.flush(), isTrue);
+      expect(await u.flush(), isTrue);
+      expect(got.length, 1);
+      expect(got.single.share, isFalse);
+      expect(got.single.lines, isEmpty);
+      expect(await u.markProblem('it\nfroze'), isTrue);
+      expect(got.last.marks.single.note, 'it froze');
+      await u.stop();
+    });
+  });
+}
diff --git a/packages/core/test/logging_test.dart b/packages/core/test/logging_test.dart
index 1ed7dda..0f5f343 100644
--- a/packages/core/test/logging_test.dart
+++ b/packages/core/test/logging_test.dart
@@ -19,6 +19,7 @@ void main() {
       expect(redactLog('open http://1.2.3.4:8080/r/default/$tok/2/0/name'), 'open http://1.2.3.4:8080/r/default/***/2/0/name');
       expect(redactLog('ws://h:1/r/default/$tok/source?q=7'), 'ws://h:1/r/default/***/source?q=…');
       expect(redactLog('/m/default/$tok/file.mkv range=bytes=0-'), '/m/default/***/file.mkv range=bytes=0-');
+      expect(redactLog('POST /l/default/$tok'), 'POST /l/default/***');
       expect(redactLog('edl://!no_clip;%160%http://h/r/default/$tok/2/0/n;!new_stream;!no_clip;%160%http://h/r/default/$tok/2/1/n'),
           isNot(contains(tok)));
     });
@@ -85,5 +86,16 @@ void main() {
       expect(small.files().map((x) => x.uri.pathSegments.last), ['app-2026-10-12.1.log', 'app-2026-10-13.log']);
       small.close();
     });
+
+    test('the session log shares the total budget, never touching another writer\'s today', () {
+      final now = DateTime(2026, 10, 12, 10);
+      File('${dir.path}/session-2026-10-10.log').writeAsStringSync('s' * 100);
+      File('${dir.path}/session-2026-10-12.log').writeAsStringSync('s' * 100);
+      File('${dir.path}/app-2026-10-11.log').writeAsStringSync('a' * 100);
+      final f = LogFiles(dir.path, maxTotalBytes: 150, shareWith: const ['session'], now: () => now)..open();
+      final left = dir.listSync().map((e) => e.uri.pathSegments.last).toSet();
+      expect(left, {'session-2026-10-12.log', 'app-2026-10-12.log'});
+      f.close();
+    });
   });
 }
diff --git a/packages/server/bin/server.dart b/packages/server/bin/server.dart
index d9bc2d1..6a519fa 100644
--- a/packages/server/bin/server.dart
+++ b/packages/server/bin/server.dart
@@ -31,7 +31,8 @@ Future<void> main(List<String> args) async {
             'An account login: keep it private. Copied into --yt-dlp-dir at start.')
     ..addOption('config', help: 'YAML or JSON config file (keys as in AppConfig); overrides the other flags.')
     ..addOption('log-dir',
-        help: 'Folder for the rotating log files (server-YYYY-MM-DD.log, kept 7 days). '
+        help: 'Folder for the rotating log files (server-YYYY-MM-DD.log, and session-YYYY-MM-DD.log with '
+            'every member\'s shared diagnostics merged in; kept 7 days). '
             'Default: logs next to --config, else logs next to this binary (systemd: \$STATE_DIRECTORY/logs).')
     ..addFlag('upgrade',
         negatable: false,
@@ -117,7 +118,9 @@ Future<void> main(List<String> args) async {
   }
 
   final events = ReceivePort('server events');
-  final done = serve(config.toMap(), events: events.sendPort);
+  // The session log (every member's diagnostics and the server's records
+  // merged) goes next to the server's own.
+  final done = serve({...config.toMap(), if (files != null) 'logDir': files.dir}, events: events.sendPort);
   final signals = <StreamSubscription<ProcessSignal>>[];
   await for (final msg in events) {
     if (msg is! Map) continue;
diff --git a/packages/server/lib/server.dart b/packages/server/lib/server.dart
index 40b476b..f71a4a7 100644
--- a/packages/server/lib/server.dart
+++ b/packages/server/lib/server.dart
@@ -7,3 +7,4 @@ export 'src/jellyfin_relay.dart';
 export 'src/relay_buffer.dart';
 export 'src/relay_routes.dart';
 export 'src/relay_stats.dart';
+export 'src/session_log.dart';
diff --git a/packages/server/lib/src/serve.dart b/packages/server/lib/src/serve.dart
index a2a1dc5..06df86b 100644
--- a/packages/server/lib/src/serve.dart
+++ b/packages/server/lib/src/serve.dart
@@ -19,7 +19,8 @@ import 'sync_server.dart';
 /// swap; a following `swap` with its name and size puts it up) and answers
 /// `{'ok':true,'name':n,'size':s}` or `{'ok':false}`. Without [events] a
 /// start failure throws. `'loopbackOnly': true` in [config] (next to the
-/// [AppConfig] keys) binds 127.0.0.1 only: the app's solo session.
+/// [AppConfig] keys) binds 127.0.0.1 only: the app's solo session;
+/// `'logDir'` is the folder of the session log ([SyncServer.logDir]).
 Future<void> serve(Map<String, Object?> config, {SendPort? events}) async {
   StreamSubscription<pkg_logging.LogRecord>? logs;
   if (events != null) {
@@ -34,7 +35,9 @@ Future<void> serve(Map<String, Object?> config, {SendPort? events}) async {
   }
   final SyncServer server;
   try {
-    server = SyncServer(AppConfig.fromMap(config), loopbackOnly: config['loopbackOnly'] == true);
+    final logDir = config['logDir'];
+    server = SyncServer(AppConfig.fromMap(config),
+        loopbackOnly: config['loopbackOnly'] == true, logDir: logDir is String && logDir.isNotEmpty ? logDir : null);
     await server.start();
   } catch (e) {
     await logs?.cancel();
diff --git a/packages/server/lib/src/session_log.dart b/packages/server/lib/src/session_log.dart
new file mode 100644
index 0000000..5fd515f
--- /dev/null
+++ b/packages/server/lib/src/session_log.dart
@@ -0,0 +1,288 @@
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+import 'dart:typed_data';
+
+import 'package:logging/logging.dart' as pkg_logging;
+import 'package:relic/relic.dart';
+import 'package:sync_relay_core/core.dart';
+
+import 'server_hub.dart';
+import 'session.dart';
+
+// The session log (ARCHITECTURE "Logging"): every member's diagnostics
+// (`POST /l/...`) and the server's own records, merged in server-clock
+// order into one file per day, `session-YYYY-MM-DD.log`, in the server's
+// log folder.
+
+class _Entry {
+  final int t;
+  final int seq;
+  final String text;
+  const _Entry(this.t, this.seq, this.text);
+}
+
+/// The merged log. Lines wait [window] (members send every 10 s) and are
+/// written in time order; a line later than that (a reconnect's backlog)
+/// still goes in with its own time, a little out of order.
+class SessionLog {
+  final LogFiles files;
+  final Duration window;
+  final DateTime Function() _now;
+  final _pending = <_Entry>[];
+  var _seq = 0;
+  StreamSubscription<pkg_logging.LogRecord>? _own;
+  /// Per member token: the version and share state already noted.
+  final _noted = <String, ({String version, bool share})>{};
+
+  SessionLog(String dir,
+      {this.window = const Duration(seconds: 15),
+      List<String> shareWith = const ['app', 'server'],
+      DateTime Function()? now,
+      int maxFileBytes = 10 * 1024 * 1024,
+      int maxTotalBytes = 100 * 1024 * 1024})
+      : _now = now ?? DateTime.now,
+        files = LogFiles(dir,
+            prefix: sessionLogPrefix,
+            shareWith: shareWith,
+            maxFileBytes: maxFileBytes,
+            maxTotalBytes: maxTotalBytes,
+            now: now) {
+    files.open();
+  }
+
+  /// Takes this isolate's own log records as the server's lines.
+  void captureOwn() {
+    _own ??= pkg_logging.Logger.root.onRecord.listen((r) {
+      final b = StringBuffer(r.message);
+      if (r.error != null) b.write('\n  error: ${r.error}');
+      final s = '${r.stackTrace ?? ''}'.trimRight();
+      if (s.isNotEmpty) b.write('\n$s');
+      add(r.time.millisecondsSinceEpoch, 'server', r.level.name, r.loggerName, '$b');
+    });
+  }
+
+  static final _control = RegExp(r'[\x00-\x08\x0b-\x1f\x7f]');
+  static final _loggerName = RegExp(r'^[A-Za-z0-9_.\-]{1,40}$');
+  static const _levels = {'FINEST', 'FINER', 'FINE', 'CONFIG', 'INFO', 'WARNING', 'SEVERE', 'SHOUT'};
+  static const _quiet = {'FINEST', 'FINER', 'FINE', 'CONFIG', 'INFO'};
+
+  static String _clean(String s, int max) {
+    var out = s.replaceAll('\r', '').replaceAll(_control, ' ');
+    if (out.length > max) out = '${out.substring(0, max)}…';
+    return out;
+  }
+
+  static String _two(int n) => n.toString().padLeft(2, '0');
+
+  static String _clock(DateTime t) =>
+      '${_two(t.hour)}:${_two(t.minute)}:${_two(t.second)}.${t.millisecond.toString().padLeft(3, '0')}';
+
+  /// One record at [t] (epoch ms, server clock) from [who] (`name (id)`
+  /// or `server`): `HH:MM:SS.mmm [who] logger: message`, the level before
+  /// the logger from WARNING up, continuation lines indented. Redacted
+  /// again here.
+  void add(int t, String who, String level, String logger, String message) {
+    final lvl = _levels.contains(level) ? level : 'INFO';
+    final name = _loggerName.hasMatch(logger) ? logger : 'app';
+    final body = redactLog(_clean(message, 8192)).split('\n');
+    final head = '[${_clean(who, 80).replaceAll('\n', ' ')}] ${_quiet.contains(lvl) ? '' : '$lvl '}$name: ${body.first}';
+    _push(t, [head, for (final l in body.skip(1)) '  $l'].join('\n'));
+  }
+
+  /// A problem mark by [name] ([who]) at [t]:
+  /// `=== PROBLEM marked by <name>: <note> ===`.
+  void mark(int t, String who, String name, String note) {
+    final n = redactLog(_clean(note, RelayConstants.maxProblemNoteLength).replaceAll('\n', ' ')).trim();
+    _push(t, '[${_clean(who, 80).replaceAll('\n', ' ')}] === PROBLEM marked by $name${n.isEmpty ? '' : ': $n'} ===');
+  }
+
+  /// A line of the server's about a member, now.
+  void note(String message) => add(_now().millisecondsSinceEpoch, 'server', 'INFO', 'diag', message);
+
+  void _push(int t, String text) {
+    final now = _now().millisecondsSinceEpoch;
+    // A clock far ahead lands now; older than the files keep is dropped.
+    if (t > now + 60000) t = now;
+    if (t < now - files.keepDays * 86400000) return;
+    _pending.add(_Entry(t, _seq++, text));
+    // A flood must not grow the wait without bound: write early.
+    if (_pending.length > 20000) flush(all: true);
+  }
+
+  /// Writes the lines older than [window] ([all]: every one), in order.
+  /// Called every second.
+  void flush({bool all = false}) {
+    if (_pending.isEmpty) return;
+    final until = all ? null : _now().millisecondsSinceEpoch - window.inMilliseconds;
+    _pending.sort(_order);
+    var n = 0;
+    for (final e in _pending) {
+      if (until != null && e.t > until) break;
+      final at = DateTime.fromMillisecondsSinceEpoch(e.t);
+      files.write('${_clock(at)} ${e.text}', at: at);
+      n++;
+    }
+    _pending.removeRange(0, n);
+  }
+
+  static int _order(_Entry a, _Entry b) => a.t != b.t ? a.t.compareTo(b.t) : a.seq.compareTo(b.seq);
+
+  /// A batch from the member [token] ([who] for the log, [name] for a
+  /// mark): its lines, a note the first time (or when its version or
+  /// sharing changes), how many it dropped, its marks.
+  void takeBatch(String token, String who, String name, DiagBatch batch) {
+    final version = _clean(batch.version, 100).replaceAll('\n', ' ');
+    final before = _noted[token];
+    if (before == null || before.version != version || before.share != batch.share) {
+      _noted[token] = (version: version, share: batch.share);
+      final what = version.isEmpty ? '' : ' ($version)';
+      if (!batch.share) {
+        note("$who doesn't share diagnostics$what");
+      } else if (before == null || !before.share) {
+        note('$who shares diagnostics$what');
+      } else {
+        note('$who now runs $version');
+      }
+    }
+    if (batch.share) {
+      for (final l in batch.lines) {
+        add(l.t, who, l.level, l.logger, l.message);
+      }
+      if (batch.dropped > 0) {
+        final t = batch.lines.isEmpty ? _now().millisecondsSinceEpoch : batch.lines.first.t;
+        add(t, who, 'WARNING', 'diag', '${batch.dropped} lines dropped (the app could not send them in time)');
+      }
+    }
+    for (final m in batch.marks) {
+      mark(m.t, who, name, m.note);
+    }
+  }
+
+  /// The lines of the last [since], written and waiting, oldest first; at
+  /// most about [maxBytes], the newest kept.
+  String recent(Duration since, {int maxBytes = 8 * 1024 * 1024}) {
+    final from = _now().subtract(since);
+    final fromDay = _day(from);
+    final out = <String>[];
+    final head = RegExp(r'^(\d{2}):(\d{2}):(\d{2})\.(\d{3}) ');
+    for (final f in files.files()) {
+      final name = f.uri.pathSegments.last;
+      final day = name.substring(sessionLogPrefix.length + 1, sessionLogPrefix.length + 11);
+      if (day.compareTo(fromDay) < 0) continue;
+      final date = DateTime.tryParse(day);
+      if (date == null) continue;
+      List<String> lines;
+      try {
+        lines = const LineSplitter().convert(utf8.decode(f.readAsBytesSync(), allowMalformed: true));
+      } catch (_) {
+        continue;
+      }
+      var keep = false;
+      for (final l in lines) {
+        final m = head.firstMatch(l);
+        if (m != null) {
+          final t = DateTime(date.year, date.month, date.day, int.parse(m[1]!), int.parse(m[2]!), int.parse(m[3]!),
+              int.parse(m[4]!));
+          keep = !t.isBefore(from);
+        }
+        if (keep) out.add(l);
+      }
+    }
+    final fromMs = from.millisecondsSinceEpoch;
+    final waiting = [..._pending]..sort(_order);
+    for (final e in waiting) {
+      if (e.t >= fromMs) out.add('${_clock(DateTime.fromMillisecondsSinceEpoch(e.t))} ${e.text}');
+    }
+    var size = 0, start = out.length;
+    while (start > 0 && size + out[start - 1].length + 1 <= maxBytes) {
+      size += out[--start].length + 1;
+    }
+    return out.sublist(start).join('\n');
+  }
+
+  static String _day(DateTime t) => '${t.year}-${_two(t.month)}-${_two(t.day)}';
+
+  Future<void> close() async {
+    await _own?.cancel();
+    _own = null;
+    flush(all: true);
+    files.close();
+  }
+}
+
+class _Rate {
+  var start = DateTime.now();
+  var posts = 0;
+  var bytes = 0;
+}
+
+/// `POST /l/<sid>/<token>`: a member's diagnostics batch ([DiagBatch],
+/// gzip) into the [sessionLog]; `GET /l/<sid>/<token>?hours=N`: the session log of
+/// the last N hours (gzip text), for any member. Without one (a
+/// server with no log folder, the solo session) batches are taken and
+/// dropped, and the GET is 404.
+void mountSessionLogRoutes(RelicApp app, ServerHub hub, SessionLog? Function() sessionLog) {
+  Response status(int code, [String? why]) => Response(code, body: Body.fromString(why ?? 'status $code'));
+  final rates = <String, _Rate>{};
+
+  ViewerToken? member(Request req) {
+    final session = hub.sessions.get(req.pathParameters.raw[#sid] ?? '');
+    final token = req.pathParameters.raw[#token] ?? '';
+    if (session == null || session.gate(token) != MediaGate.ok) return null;
+    return session.tokens[token];
+  }
+
+  app.post('/l/:sid/:token', (final req) async {
+    final viewer = member(req);
+    if (viewer == null) return status(401);
+    final body = BytesBuilder(copy: false);
+    try {
+      await for (final c in req.read(maxLength: RelayConstants.diagBatchBytes)) {
+        body.add(c);
+      }
+    } catch (_) {
+      return status(413);
+    }
+    final DiagBatch batch;
+    try {
+      batch = DiagBatch.decode(body.takeBytes());
+    } on FormatException catch (e) {
+      return status(e.message == 'too large' ? 413 : 400, 'bad batch');
+    }
+    final now = DateTime.now();
+    if (rates.length > 500) rates.removeWhere((k, _) => !hub.session.tokens.containsKey(k));
+    final rate = rates.putIfAbsent(viewer.token, _Rate.new);
+    if (now.difference(rate.start) >= const Duration(minutes: 1)) {
+      rate
+        ..start = now
+        ..posts = 0
+        ..bytes = 0;
+    }
+    final size = batch.lines.fold<int>(0, (a, l) => a + l.size);
+    if (rate.posts >= RelayConstants.diagPostsPerMin || rate.bytes + size > RelayConstants.diagBytesPerMin) {
+      return Response(429,
+          body: Body.fromString('too many'), headers: Headers.build((h) => h['retry-after'] = ['30']));
+    }
+    rate
+      ..posts += 1
+      ..bytes += size;
+    final log = sessionLog();
+    if (log == null) return Response.noContent();
+    log.takeBatch(viewer.token, hub.who(viewer.token), viewer.name, batch);
+    return Response.noContent();
+  });
+
+  app.get('/l/:sid/:token', (final req) {
+    if (member(req) == null) return status(401);
+    final log = sessionLog();
+    if (log == null) return status(404, 'this server keeps no session log');
+    final hours = (int.tryParse(req.url.queryParameters['hours'] ?? '') ?? 6).clamp(1, 24 * 7);
+    final text = log.recent(Duration(hours: hours));
+    return Response.ok(
+      body: Body.fromData(Uint8List.fromList(gzip.encode(utf8.encode(text))),
+          mimeType: MimeType('application', 'gzip')),
+      headers: Headers.build((h) => h['cache-control'] = ['no-store']),
+    );
+  });
+}
diff --git a/packages/server/lib/src/sync_server.dart b/packages/server/lib/src/sync_server.dart
index c3ce794..cfd7424 100644
--- a/packages/server/lib/src/sync_server.dart
+++ b/packages/server/lib/src/sync_server.dart
@@ -11,6 +11,7 @@ import 'media_endpoint.dart';
 import 'relay_routes.dart';
 import 'server_hub.dart';
 import 'session.dart';
+import 'session_log.dart';
 
 /// [configured], else systemd's `$STATE_DIRECTORY/yt-dlp`, else `yt-dlp`
 /// next to the server binary when that is writable (not under `dart run`),
@@ -56,8 +57,16 @@ class SyncServer {
   /// IPv4 interface.
   final bool loopbackOnly;
 
+  /// The folder of the session log (`session-YYYY-MM-DD.log`, every
+  /// member's diagnostics and the server's records merged); null: none.
+  final String? logDir;
+  SessionLog? _sessionLog;
+
+  /// The session log while running with a [logDir].
+  SessionLog? get sessionLog => _sessionLog;
+
   /// [ytDlp] replaces the server's own (tests).
-  SyncServer(this.config, {YtDlp? ytDlp, this.loopbackOnly = false}) {
+  SyncServer(this.config, {YtDlp? ytDlp, this.loopbackOnly = false, this.logDir}) {
     hub = ServerHub(sessions, config.secret,
         hostKey: config.hostKey,
         jellyfin: config.jellyfin,
@@ -103,6 +112,7 @@ class SyncServer {
     mountRelayRoutes(_app, sessions, relayAt: hub.relayAt, sourceOf: hub.sourceOf, onLog: logRequest);
     mountAvatarRoute(_app, hub);
     mountArtRoute(_app, hub);
+    mountSessionLogRoutes(_app, hub, () => _sessionLog);
     mountJellyfinRelayRoute(
         _app, sessions, () => hub.jellyfin, () => _relayClient ??= HttpClient(),
         next: () => hub.nextJellyfinRef, onLog: logRequest);
@@ -139,6 +149,14 @@ class SyncServer {
     if (config.secret.isEmpty) {
       _log.warning('no password set: open server, anyone who can reach it can join');
     }
+    final dir = logDir;
+    if (dir != null && _sessionLog == null) {
+      try {
+        _sessionLog = SessionLog(dir)..captureOwn();
+      } catch (e) {
+        _log.warning('no session log: cannot write $dir ($e)');
+      }
+    }
     await _resolveJellyfinServerId();
     _mountRoutes();
     _server = await _app.serve(
@@ -158,6 +176,7 @@ class SyncServer {
       hub.sampleRates();
       hub.broadcastState();
       hub.broadcastFeed();
+      _sessionLog?.flush();
     });
     _log.info('server "${config.name}" listening on $boundPort');
   }
@@ -193,5 +212,7 @@ class SyncServer {
     await _app.close();
     _server = null;
     _log.info('server "${config.name}" stopped');
+    await _sessionLog?.close();
+    _sessionLog = null;
   }
 }
diff --git a/packages/server/test/session_log_test.dart b/packages/server/test/session_log_test.dart
new file mode 100644
index 0000000..69732f6
--- /dev/null
+++ b/packages/server/test/session_log_test.dart
@@ -0,0 +1,130 @@
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:logging/logging.dart';
+import 'package:sync_relay_core/core.dart';
+import 'package:sync_relay_server/server.dart';
+import 'package:test/test.dart';
+import 'package:web_socket_channel/web_socket_channel.dart';
+
+void main() {
+  late Directory dir;
+  setUp(() async => dir = await Directory.systemTemp.createTemp('session_log'));
+  tearDown(() async => dir.delete(recursive: true));
+
+  String read() => File('${dir.path}/session-2026-10-12.log').readAsStringSync();
+
+  test('merges lines of every side in server-clock order after the window', () async {
+    var now = DateTime(2026, 10, 12, 18, 0, 30);
+    final log = SessionLog(dir.path, window: const Duration(seconds: 15), now: () => now);
+    int at(int s, [int ms = 0]) => DateTime(2026, 10, 12, 18, 0, s, ms).millisecondsSinceEpoch;
+    log.add(at(12), 'Bob (m2)', 'INFO', 'relay', 'second');
+    log.add(at(10, 5), 'server', 'WARNING', 'server_hub', 'first\n  error: boom');
+    log.add(at(20), 'Alice (m1)', 'INFO', 'mpv', 'too new');
+    log.add(at(14), 'Alice (m1)', 'INFO', 'media', 'open http://h:1/r/default/50989dda73c45a4710751f8aff4a1982/2/0/x?sig=abc');
+    log.mark(at(13), 'Alice (m1)', 'Alice', 'stutter\nhere');
+    log.flush();
+    expect(read().trimRight().split('\n'), [
+      '18:00:10.005 [server] WARNING server_hub: first',
+      '    error: boom',
+      '18:00:12.000 [Bob (m2)] relay: second',
+      '18:00:13.000 [Alice (m1)] === PROBLEM marked by Alice: stutter here ===',
+      '18:00:14.000 [Alice (m1)] media: open http://h:1/r/default/***/2/0/x?sig=…',
+    ]);
+    expect(log.recent(const Duration(hours: 1)), endsWith('[Alice (m1)] mpv: too new'), reason: 'waiting lines too');
+    now = DateTime(2026, 10, 12, 18, 1);
+    await log.close();
+    expect(read(), contains('18:00:20.000 [Alice (m1)] mpv: too new'));
+  });
+
+  test('takeBatch: notes the version once, opt-out once, dropped lines', () async {
+    final now = DateTime(2026, 10, 12, 18);
+    final log = SessionLog(dir.path, now: () => now);
+    final t = now.millisecondsSinceEpoch;
+    log.takeBatch('tok', 'Bob (m2)', 'Bob',
+        DiagBatch(version: 'WT 1.0', lines: [DiagLine(t: t, level: 'INFO', logger: 'app', message: 'hi')], dropped: 3));
+    log.takeBatch('tok', 'Bob (m2)', 'Bob', DiagBatch(version: 'WT 1.0', lines: [DiagLine(t: t, level: 'INFO', logger: 'app', message: 'again')]));
+    log.takeBatch('tok2', 'Cleo (m3)', 'Cleo', const DiagBatch(version: 'WT 1.0', share: false, lines: [DiagLine(t: 0, level: 'INFO', logger: 'app', message: 'never')]));
+    log.takeBatch('tok2', 'Cleo (m3)', 'Cleo', const DiagBatch(version: 'WT 1.0', share: false));
+    await log.close();
+    final text = read();
+    expect('shares diagnostics'.allMatches(text).length, 1);
+    expect(text, contains('[server] diag: Bob (m2) shares diagnostics (WT 1.0)'));
+    expect(text, contains("[server] diag: Cleo (m3) doesn't share diagnostics (WT 1.0)"));
+    expect("doesn't share".allMatches(text).length, 1);
+    expect(text, contains('[Bob (m2)] WARNING diag: 3 lines dropped'));
+    expect(text, isNot(contains('never')));
+  });
+
+  group('live', () {
+    late SyncServer server;
+    setUp(() async {
+      server = SyncServer(const AppConfig(port: 0, secret: 's', name: 't'), logDir: dir.path);
+      await server.start();
+    });
+    tearDown(() async => server.stop());
+
+    Future<(WebSocketChannel, String)> join(String name) async {
+      final ch = WebSocketChannel.connect(Uri.parse('ws://127.0.0.1:${server.boundPort}/sync'));
+      final msgs = StreamIterator(ch.stream);
+      ch.sink.add(jsonEncode(HelloMessage(
+              proto: RelayConstants.protoVersion,
+              secret: 's',
+              user: HelloUser(name: name),
+              media: const HelloMedia(name: '', size: 0, duration: 0))
+          .toJson()));
+      while (await msgs.moveNext()) {
+        final m = jsonDecode('${msgs.current}') as Map;
+        if (m['t'] == 'welcome') {
+          unawaited(() async {
+            while (await msgs.moveNext()) {}
+          }());
+          return (ch, m['token'] as String);
+        }
+      }
+      throw StateError('no welcome');
+    }
+
+    DiagUploader uploader(String token, {String version = 'WT 1.0'}) => DiagUploader(
+        target: () => (base: Uri.parse('http://127.0.0.1:${server.boundPort}'), token: token, offsetMs: 0),
+        version: version);
+
+    test('members send lines and marks; chat never lands; any member reads it back', () async {
+      final (alice, aliceTok) = await join('Alice');
+      final (bob, bobTok) = await join('Bob');
+      final a = uploader(aliceTok)..start();
+      final b = uploader(bobTok)
+        ..share = false
+        ..start();
+      Logger('relay').info('alice line password=hunter2');
+      alice.sink.add(jsonEncode(const ChatMessage(text: 'secret chat words').toJson()));
+      await Future<void>.delayed(const Duration(milliseconds: 200));
+      expect(await a.flush(), isTrue);
+      expect(await b.flush(), isTrue);
+      expect(await a.markProblem('audio drifts'), isTrue);
+      final text = await a.fetchSessionLog(hours: 1);
+      expect(text, isNotNull);
+      expect(text, contains('[Alice (m1)] relay: alice line password=***'));
+      expect(text, contains('=== PROBLEM marked by Alice: audio drifts ==='));
+      expect(text, contains("Bob (m2) doesn't share diagnostics"));
+      expect(text, contains('[server] server_hub: join Alice'));
+      expect(text, isNot(contains('secret chat words')));
+      // Bob reads it too (friends); a stranger does not.
+      expect(await b.fetchSessionLog(), contains('PROBLEM'));
+      expect(await uploader('0' * 32).fetchSessionLog(), isNull);
+      final bad = await HttpClient().postUrl(Uri.parse('http://127.0.0.1:${server.boundPort}/l/default/$aliceTok'));
+      bad.add(utf8.encode('not gzip'));
+      expect((await bad.close()).statusCode, 400);
+      await a.stop();
+      await b.stop();
+      await alice.sink.close();
+      await bob.sink.close();
+      await server.stop();
+      final files = dir.listSync().map((e) => e.uri.pathSegments.last).where((n) => n.startsWith('session-'));
+      final all = [for (final n in files) File('${dir.path}/$n').readAsStringSync()].join();
+      expect(all, contains('PROBLEM'));
+      expect(all, isNot(contains('secret chat words')));
+    });
+  });
+}
diff --git a/packages/server/test/session_test.dart b/packages/server/test/session_test.dart
index 24e65e5..06e015b 100644
--- a/packages/server/test/session_test.dart
+++ b/packages/server/test/session_test.dart
@@ -64,8 +64,8 @@ void main() {
   });
 
   group('core welcome + proto', () {
-    test('proto 14: avatars by hash, profile intent', () {
-      expect(RelayConstants.protoVersion, 14);
+    test('proto 15: avatars by hash, profile intent', () {
+      expect(RelayConstants.protoVersion, 15);
       const v = MemberView(id: 'm1', name: 'a', ready: false, inSet: false, buffered: 0, stale: false);
       expect(v.toJson().containsKey('avatarHash'), isFalse);
       expect(v.toJson().containsKey('avatar'), isFalse);