fix Claude pane hang: cap tail, parse off-isolate, coalesce notifies

The native Claude pane (T-137) picks the newest transcript JSONL by
mtime — which on a live session is this multi-MB active file. On attach
it read and parsed the whole file synchronously on the UI isolate and
fired notifyListeners per item, freezing the app.

Three fixes: cap the initial read to a recent tail (256KB, injectable
for tests); run JSON parsing in a background isolate via Isolate.run;
coalesce the controller's notifications with a zero-duration Timer so a
burst collapses into one rebuild (a microtask-scheduled notify wouldn't
— stream events deliver one per microtask and the notify interleaves).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.pql/changelog/ticket_history/2026-05.sql

@@ -2140,3 +2140,4 @@ Refinement (2026-05-22, refinement-agent review):
 ---
 Findings (2026-05-22): docs/spikes/cc-team-internals-2.1.148.md. All 5 checks answered from real artifacts + tmux manual + synthetic control-mode test (no live run/quota). Key risk for T-139: pane->teammate->transcript identity joins only on agentType (config <-> .meta.json), ambiguous for same-type teammates -> needs joinedAt/mtime or lead-transcript tiebreaker. tmux 3.6a has no %pane-died; poll list-panes -a. Teammate transcripts at <munged>/<sid>/subagents/agent-<hex>.jsonl + .meta.json{agentType,description}.', NULL, '2026-05-22 17:55:11', '2026-05-22 17:55:11', '2026-05-22 17:55:11', NULL, 'dbf4f56d1ec9d2234f255826c5f88f68', 1) ON CONFLICT(hash) DO NOTHING;
 INSERT INTO ticket_history (ticket_id, field, old_value, new_value, changed_by, changed_at, created_at, updated_at, deleted_at, hash, canonical_version) VALUES ('T-137', 'status', 'backlog', 'in_progress', NULL, '2026-05-22 17:58:07', '2026-05-22 17:58:07', '2026-05-22 17:58:07', NULL, '4e67262d4b04f8541e71fe8f03106fbf', 1) ON CONFLICT(hash) DO NOTHING;
+INSERT INTO ticket_history (ticket_id, field, old_value, new_value, changed_by, changed_at, created_at, updated_at, deleted_at, hash, canonical_version) VALUES ('T-137', 'status', 'in_progress', 'done', NULL, '2026-05-22 18:14:32', '2026-05-22 18:14:32', '2026-05-22 18:14:32', NULL, 'a284c7249dda3c63a52a202bad07f105', 1) ON CONFLICT(hash) DO NOTHING;

.pql/changelog/tickets/2026-05.sql

@@ -2485,3 +2485,4 @@ Refinement (2026-05-22, refinement-agent review):
 ---
 Findings (2026-05-22): docs/spikes/cc-team-internals-2.1.148.md. All 5 checks answered from real artifacts + tmux manual + synthetic control-mode test (no live run/quota). Key risk for T-139: pane->teammate->transcript identity joins only on agentType (config <-> .meta.json), ambiguous for same-type teammates -> needs joinedAt/mtime or lead-transcript tiebreaker. tmux 3.6a has no %pane-died; poll list-panes -a. Teammate transcripts at <munged>/<sid>/subagents/agent-<hex>.jsonl + .meta.json{agentType,description}.', 'done', 'high', NULL, NULL, NULL, '2026-05-22 15:59:34', '2026-05-22 17:55:11', NULL, '2f9a445df8c60beafe126aff339410b8', 1) ON CONFLICT(id) DO UPDATE SET type=excluded.type, parent_id=excluded.parent_id, title=excluded.title, description=excluded.description, status=excluded.status, priority=excluded.priority, assigned_to=excluded.assigned_to, team=excluded.team, decision_ref=excluded.decision_ref, updated_at=excluded.updated_at, deleted_at=excluded.deleted_at, hash=excluded.hash, canonical_version=excluded.canonical_version WHERE excluded.updated_at > tickets.updated_at OR (excluded.updated_at = tickets.updated_at AND excluded.hash > tickets.hash);
 INSERT INTO tickets (id, type, parent_id, title, description, status, priority, assigned_to, team, decision_ref, created_at, updated_at, deleted_at, hash, canonical_version) VALUES ('T-137', 'story', 'T-132', 'ConversationView + replace the terminal in the Claude pane', 'New lib/builtin/claude/src/conversation_view.dart rendering user / assistant(markdown via ClideMarkdown) / tool-call card / tool-result+diff card; whole tree under one SelectionArea. Swap ClidePtyView -> ConversationView in lib/builtin/claude/src/claude_pane.dart (~line 287). Claude still runs in tmux (detached / output ignored) but content comes from the transcript (TranscriptReader). Keep the terminal builtin + ClidePtyView for general use. Surface conversation as events too (D-6). Cite D-75. Acceptance: make run shows the native Claude conversation from the live transcript; select+copy across cards works; no terminal in the Claude pane; terminal builtin still works; widget tests. Blocked by T3, T4.', 'in_progress', 'high', NULL, NULL, NULL, '2026-05-22 15:59:34', '2026-05-22 17:58:07', NULL, 'e8244b90386314574545f0c1a7bd012c', 1) ON CONFLICT(id) DO UPDATE SET type=excluded.type, parent_id=excluded.parent_id, title=excluded.title, description=excluded.description, status=excluded.status, priority=excluded.priority, assigned_to=excluded.assigned_to, team=excluded.team, decision_ref=excluded.decision_ref, updated_at=excluded.updated_at, deleted_at=excluded.deleted_at, hash=excluded.hash, canonical_version=excluded.canonical_version WHERE excluded.updated_at > tickets.updated_at OR (excluded.updated_at = tickets.updated_at AND excluded.hash > tickets.hash);
+INSERT INTO tickets (id, type, parent_id, title, description, status, priority, assigned_to, team, decision_ref, created_at, updated_at, deleted_at, hash, canonical_version) VALUES ('T-137', 'story', 'T-132', 'ConversationView + replace the terminal in the Claude pane', 'New lib/builtin/claude/src/conversation_view.dart rendering user / assistant(markdown via ClideMarkdown) / tool-call card / tool-result+diff card; whole tree under one SelectionArea. Swap ClidePtyView -> ConversationView in lib/builtin/claude/src/claude_pane.dart (~line 287). Claude still runs in tmux (detached / output ignored) but content comes from the transcript (TranscriptReader). Keep the terminal builtin + ClidePtyView for general use. Surface conversation as events too (D-6). Cite D-75. Acceptance: make run shows the native Claude conversation from the live transcript; select+copy across cards works; no terminal in the Claude pane; terminal builtin still works; widget tests. Blocked by T3, T4.', 'done', 'high', NULL, NULL, NULL, '2026-05-22 15:59:34', '2026-05-22 18:14:32', NULL, '6651a2c3c9099f7d38553f6cf9e41e98', 1) ON CONFLICT(id) DO UPDATE SET type=excluded.type, parent_id=excluded.parent_id, title=excluded.title, description=excluded.description, status=excluded.status, priority=excluded.priority, assigned_to=excluded.assigned_to, team=excluded.team, decision_ref=excluded.decision_ref, updated_at=excluded.updated_at, deleted_at=excluded.deleted_at, hash=excluded.hash, canonical_version=excluded.canonical_version WHERE excluded.updated_at > tickets.updated_at OR (excluded.updated_at = tickets.updated_at AND excluded.hash > tickets.hash);

CHANGELOG.md

@@ -97,6 +97,9 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Fixed
 
+- Claude pane no longer freezes the app on open — the transcript reader
+  caps its initial read to the recent tail, parses off the UI isolate,
+  and coalesces view notifications into one rebuild per burst (T-137).
 - Daemon-not-connected on startup — panels and the Claude pane raced
   the socket loopback. Requests now wait briefly for an in-flight
   connection, the server isn't restarted for the same workspace, and

lib/builtin/claude/src/conversation_controller.dart

@@ -33,6 +33,8 @@ class ConversationController extends ChangeNotifier {
   final Future<void> Function()? _onDispose;
   late final StreamSubscription<ConversationItem> _sub;
   final List<ConversationItem> _items = [];
+  Timer? _notifyTimer;
+  bool _disposed = false;
 
   /// Items in arrival (transcript) order.
   List<ConversationItem> get items => List.unmodifiable(_items);
@@ -41,11 +43,22 @@ class ConversationController extends ChangeNotifier {
 
   void _onItem(ConversationItem item) {
     _items.add(item);
-    notifyListeners();
+    // Coalesce notifications: the reader emits a burst (the initial tail
+    // read), and a notify-per-item would thrash the view's rebuild +
+    // auto-scroll. A zero-duration Timer fires only after the microtask
+    // queue drains — the stream delivers one event per microtask, so a
+    // microtask-scheduled notify would interleave between deliveries and
+    // fire per item. The timer collapses a whole burst into one rebuild.
+    _notifyTimer ??= Timer(Duration.zero, () {
+      _notifyTimer = null;
+      if (!_disposed) notifyListeners();
+    });
   }
 
   @override
   void dispose() {
+    _disposed = true;
+    _notifyTimer?.cancel();
     unawaited(_sub.cancel());
     unawaited(_onDispose?.call());
     super.dispose();

lib/builtin/claude/src/transcript_reader.dart

@@ -31,6 +31,7 @@ library;
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
+import 'dart:isolate';
 
 // ---------------------------------------------------------------------------
 // Data model
@@ -145,6 +146,12 @@ String _shortId(String uuid) => uuid.length >= 8 ? uuid.substring(0, 8) : uuid;
 // Reader
 // ---------------------------------------------------------------------------
 
+/// On first attach to a session file, read at most this many recent
+/// bytes (not the whole file) — an active transcript can be many MB and
+/// parsing it all synchronously would freeze the UI. Appends after that
+/// stream incrementally.
+const _defaultInitialTailBytes = 256 * 1024;
+
 /// Known major transcript versions.
 const _knownMajorVersions = {1, 2};
 
@@ -174,14 +181,20 @@ class TranscriptReader {
     Duration pollInterval = const Duration(milliseconds: 500),
     void Function(String)? onWarn,
     String? projectsBase,
+    int? initialTailBytes,
   })  : _pollInterval = pollInterval,
         _onWarn = onWarn ?? _defaultWarn,
-        _projectsBase = projectsBase ?? _defaultProjectsBase();
+        _projectsBase = projectsBase ?? _defaultProjectsBase(),
+        _initialTailBytes = initialTailBytes ?? _defaultInitialTailBytes;
 
   final String workspacePath;
   final Duration _pollInterval;
   final void Function(String) _onWarn;
 
+  /// Max bytes of recent history to read when first attaching to a
+  /// session file (overridable for tests).
+  final int _initialTailBytes;
+
   /// Base dir holding the per-workspace transcript dirs. Defaults to
   /// `~/.claude/projects`; overridable so tests point the real reader at a
   /// temp directory instead of the user's home.
@@ -276,11 +289,16 @@ class TranscriptReader {
     if (newest == null) return;
 
     if (newest != _currentPath) {
-      // Session switch — reset cursor so we replay from the beginning of the
-      // new file. We intentionally re-emit items from the new file start;
-      // a future UI layer can de-dup by uuid if required.
+      // New session file. Start from the recent tail rather than byte 0:
+      // an active transcript can be many MB (thousands of records), and
+      // parsing the whole thing synchronously on first attach freezes the
+      // UI. Read at most [_initialTailBytes] of recent history, then stream
+      // appends. A partial first line (from landing mid-record) simply
+      // fails to JSON-parse and is skipped. Older scrollback is a future
+      // load-more concern.
       _currentPath = newest;
-      _cursor = 0;
+      final length = await File(newest).length();
+      _cursor = length > _initialTailBytes ? length - _initialTailBytes : 0;
     }
 
     await _tail(controller, newest);
@@ -291,199 +309,186 @@ class TranscriptReader {
     final length = await file.length();
     if (length <= _cursor) return; // no new bytes
 
+    final String chunk;
     final raf = await file.open();
     try {
       await raf.setPosition(_cursor);
       final newBytes = await raf.read(length - _cursor);
       _cursor = length;
-
-      final chunk = utf8.decode(newBytes, allowMalformed: true);
-      final lines = chunk.split('\n');
-
-      for (final raw in lines) {
-        final line = raw.trim();
-        if (line.isEmpty) continue;
-        for (final item in parseLine(line)) {
-          controller.add(item);
-        }
-      }
+      chunk = utf8.decode(newBytes, allowMalformed: true);
     } finally {
       await raf.close();
     }
+    if (controller.isClosed) return;
+
+    // Parse off the UI isolate — the initial chunk can be sizeable and
+    // JSON-decoding it on the main thread would jank the frame.
+    final parsed = await Isolate.run(() => parseTranscriptChunk(chunk));
+    if (controller.isClosed) return;
+    for (final w in parsed.warnings) {
+      _onWarn(w);
+    }
+    for (final item in parsed.items) {
+      if (controller.isClosed) break;
+      controller.add(item);
+    }
   }
 
-  // ---------------------------------------------------------------------------
-  // Parsing — pure: takes a JSONL line, returns the items it yields.
-  //
-  // Public so tests exercise the real parser directly (no duplicate). The tail
-  // loop above feeds the returned items into the stream. Malformed JSON and
-  // skip/unknown types yield an empty list; the version drift-guard warns via
-  // [onWarn] but still parses what it can.
-  // ---------------------------------------------------------------------------
-
+  /// Parse a single JSONL line into its items (forwarding any version
+  /// warnings to [onWarn]). Public so tests exercise the real parser.
   List<ConversationItem> parseLine(String line) {
-    Map<String, dynamic> envelope;
-    try {
-      envelope = (jsonDecode(line) as Map).cast<String, dynamic>();
-    } catch (_) {
-      return const []; // malformed JSON — skip silently
+    final parsed = parseTranscriptChunk(line);
+    for (final w in parsed.warnings) {
+      _onWarn(w);
     }
+    return parsed.items;
+  }
+}
 
-    // Version drift-guard.
-    final rawVersion = envelope['version'] as String?;
-    if (rawVersion != null) {
-      final dotIdx = rawVersion.indexOf('.');
-      final majorStr = dotIdx > 0 ? rawVersion.substring(0, dotIdx) : rawVersion;
-      final major = int.tryParse(majorStr);
-      if (major != null && !_knownMajorVersions.contains(major)) {
-        _onWarn('unfamiliar transcript version "$rawVersion" (major=$major); '
-            'parsing will degrade gracefully');
-      }
-    }
+/// Result of [parseTranscriptChunk]: parsed items + version-drift warnings.
+typedef ParsedChunk = ({List<ConversationItem> items, List<String> warnings});
 
-    final type = envelope['type'] as String?;
-    if (type == null || _skipTypes.contains(type)) return const [];
+// ---------------------------------------------------------------------------
+// Parsing — pure + isolate-safe. Top-level (no instance state) so it can run
+// via Isolate.run. Malformed JSON and skip/unknown types are dropped; an
+// unfamiliar major `version` adds a warning but parsing still proceeds.
+// ---------------------------------------------------------------------------
 
-    final uuid = envelope['uuid'] as String? ?? '';
-    final isSidechain = envelope['isSidechain'] as bool? ?? false;
+ParsedChunk parseTranscriptChunk(String chunk) {
+  final items = <ConversationItem>[];
+  final warnings = <String>[];
+  for (final raw in chunk.split('\n')) {
+    final line = raw.trim();
+    if (line.isEmpty) continue;
+    _parseLineInto(line, items, warnings);
+  }
+  return (items: items, warnings: warnings);
+}
 
-    DateTime timestamp;
-    try {
-      timestamp = DateTime.parse(envelope['timestamp'] as String? ?? '');
-    } catch (_) {
-      timestamp = DateTime.fromMillisecondsSinceEpoch(0, isUtc: true);
-    }
-
-    final out = <ConversationItem>[];
-    switch (type) {
-      case 'user':
-        _parseUser(envelope, uuid, timestamp, isSidechain, out);
-      case 'assistant':
-        _parseAssistant(envelope, uuid, timestamp, isSidechain, out);
-      default:
-        // Unknown type — degrade gracefully (don't emit, don't crash).
-        break;
-    }
-    return out;
+void _parseLineInto(String line, List<ConversationItem> out, List<String> warnings) {
+  Map<String, dynamic> envelope;
+  try {
+    envelope = (jsonDecode(line) as Map).cast<String, dynamic>();
+  } catch (_) {
+    return; // malformed JSON — skip silently
   }
 
-  void _parseUser(
-    Map<String, dynamic> envelope,
-    String uuid,
-    DateTime timestamp,
-    bool isSidechain,
-    List<ConversationItem> out,
-  ) {
-    final message = envelope['message'] as Map?;
-    if (message == null) return;
+  final rawVersion = envelope['version'] as String?;
+  if (rawVersion != null) {
+    final dotIdx = rawVersion.indexOf('.');
+    final majorStr = dotIdx > 0 ? rawVersion.substring(0, dotIdx) : rawVersion;
+    final major = int.tryParse(majorStr);
+    if (major != null && !_knownMajorVersions.contains(major)) {
+      warnings.add('unfamiliar transcript version "$rawVersion" (major=$major); '
+          'parsing will degrade gracefully');
+    }
+  }
 
-    final content = message['content'];
+  final type = envelope['type'] as String?;
+  if (type == null || _skipTypes.contains(type)) return;
 
-    if (content is String) {
-      // Plain string content.
-      if (content.isNotEmpty) {
-        out.add(UserMessage(
+  final uuid = envelope['uuid'] as String? ?? '';
+  final isSidechain = envelope['isSidechain'] as bool? ?? false;
+
+  DateTime timestamp;
+  try {
+    timestamp = DateTime.parse(envelope['timestamp'] as String? ?? '');
+  } catch (_) {
+    timestamp = DateTime.fromMillisecondsSinceEpoch(0, isUtc: true);
+  }
+
+  switch (type) {
+    case 'user':
+      _parseUserInto(envelope, uuid, timestamp, isSidechain, out);
+    case 'assistant':
+      _parseAssistantInto(envelope, uuid, timestamp, isSidechain, out);
+    default:
+      break; // unknown type — degrade gracefully
+  }
+}
+
+void _parseUserInto(
+  Map<String, dynamic> envelope,
+  String uuid,
+  DateTime timestamp,
+  bool isSidechain,
+  List<ConversationItem> out,
+) {
+  final message = envelope['message'] as Map?;
+  if (message == null) return;
+  final content = message['content'];
+
+  if (content is String) {
+    if (content.isNotEmpty) {
+      out.add(UserMessage(uuid: uuid, timestamp: timestamp, isSidechain: isSidechain, text: content));
+    }
+    return;
+  }
+  if (content is! List) return;
+
+  final textParts = <String>[];
+  for (final item in content) {
+    if (item is! Map) continue;
+    switch (item['type'] as String?) {
+      case 'text':
+        final text = item['text'] as String? ?? '';
+        if (text.isNotEmpty) textParts.add(text);
+      case 'tool_result':
+        final rawContent = item['content'];
+        out.add(ToolResultMessage(
           uuid: uuid,
           timestamp: timestamp,
           isSidechain: isSidechain,
-          text: content,
+          toolUseId: item['tool_use_id'] as String? ?? '',
+          content: rawContent is String ? rawContent : jsonEncode(rawContent),
+          isError: item['is_error'] as bool? ?? false,
         ));
-      }
-      return;
-    }
-
-    if (content is! List) return;
-
-    // Array content — may contain text parts and/or tool_result parts.
-    final textParts = <String>[];
-    for (final item in content) {
-      if (item is! Map) continue;
-      final itemType = item['type'] as String?;
-      switch (itemType) {
-        case 'text':
-          final text = item['text'] as String? ?? '';
-          if (text.isNotEmpty) textParts.add(text);
-        case 'tool_result':
-          final toolUseId = item['tool_use_id'] as String? ?? '';
-          final rawContent = item['content'];
-          final resultContent = rawContent is String ? rawContent : jsonEncode(rawContent);
-          final isError = item['is_error'] as bool? ?? false;
-          out.add(ToolResultMessage(
-            uuid: uuid,
-            timestamp: timestamp,
-            isSidechain: isSidechain,
-            toolUseId: toolUseId,
-            content: resultContent,
-            isError: isError,
-          ));
-        default:
-          break;
-      }
-    }
-
-    if (textParts.isNotEmpty) {
-      out.add(UserMessage(
-        uuid: uuid,
-        timestamp: timestamp,
-        isSidechain: isSidechain,
-        text: textParts.join('\n'),
-      ));
+      default:
+        break;
     }
   }
+  if (textParts.isNotEmpty) {
+    out.add(UserMessage(uuid: uuid, timestamp: timestamp, isSidechain: isSidechain, text: textParts.join('\n')));
+  }
+}
 
-  void _parseAssistant(
-    Map<String, dynamic> envelope,
-    String uuid,
-    DateTime timestamp,
-    bool isSidechain,
-    List<ConversationItem> out,
-  ) {
-    final message = envelope['message'] as Map?;
-    if (message == null) return;
+void _parseAssistantInto(
+  Map<String, dynamic> envelope,
+  String uuid,
+  DateTime timestamp,
+  bool isSidechain,
+  List<ConversationItem> out,
+) {
+  final message = envelope['message'] as Map?;
+  if (message == null) return;
+  final content = message['content'];
+  if (content is! List) return;
 
-    final content = message['content'];
-    if (content is! List) return;
-
-    for (final item in content) {
-      if (item is! Map) continue;
-      final itemType = item['type'] as String?;
-      switch (itemType) {
-        case 'text':
-          final text = item['text'] as String? ?? '';
-          if (text.isNotEmpty) {
-            out.add(AssistantTextMessage(
-              uuid: uuid,
-              timestamp: timestamp,
-              isSidechain: isSidechain,
-              text: text,
-            ));
-          }
-        case 'thinking':
-          final thinking = item['thinking'] as String? ?? '';
-          if (thinking.isNotEmpty) {
-            out.add(AssistantThinkingMessage(
-              uuid: uuid,
-              timestamp: timestamp,
-              isSidechain: isSidechain,
-              thinking: thinking,
-            ));
-          }
-        case 'tool_use':
-          final toolUseId = item['id'] as String? ?? '';
-          final name = item['name'] as String? ?? '';
-          final rawInput = item['input'];
-          final input = rawInput is Map ? rawInput.cast<String, dynamic>() : <String, dynamic>{};
-          out.add(AssistantToolUse(
-            uuid: uuid,
-            timestamp: timestamp,
-            isSidechain: isSidechain,
-            toolUseId: toolUseId,
-            name: name,
-            input: input,
-          ));
-        default:
-          break;
-      }
+  for (final item in content) {
+    if (item is! Map) continue;
+    switch (item['type'] as String?) {
+      case 'text':
+        final text = item['text'] as String? ?? '';
+        if (text.isNotEmpty) {
+          out.add(AssistantTextMessage(uuid: uuid, timestamp: timestamp, isSidechain: isSidechain, text: text));
+        }
+      case 'thinking':
+        final thinking = item['thinking'] as String? ?? '';
+        if (thinking.isNotEmpty) {
+          out.add(AssistantThinkingMessage(uuid: uuid, timestamp: timestamp, isSidechain: isSidechain, thinking: thinking));
+        }
+      case 'tool_use':
+        final rawInput = item['input'];
+        out.add(AssistantToolUse(
+          uuid: uuid,
+          timestamp: timestamp,
+          isSidechain: isSidechain,
+          toolUseId: item['id'] as String? ?? '',
+          name: item['name'] as String? ?? '',
+          input: rawInput is Map ? rawInput.cast<String, dynamic>() : <String, dynamic>{},
+        ));
+      default:
+        break;
     }
   }
 }

test/builtin/claude/conversation_view_test.dart

@@ -56,11 +56,15 @@ void main() {
       expect(c.isEmpty, isTrue);
       ctrl.add(_user('hi'));
       ctrl.add(_asst('hello'));
-      await Future<void>.delayed(Duration.zero);
+      // Wait past the coalescing timer (zero-duration, fires after the
+      // microtask queue drains).
+      await Future<void>.delayed(const Duration(milliseconds: 20));
 
       expect(c.items, hasLength(2));
       expect(c.items.first, isA<UserMessage>());
-      expect(notifications, 2);
+      // Notifications are coalesced: a burst of items collapses to a
+      // single notify so the view rebuilds once, not per item.
+      expect(notifications, 1);
       await ctrl.close();
     });
 

test/builtin/claude/transcript_reader_test.dart

@@ -669,6 +669,43 @@ void main() {
       expect(collected, hasLength(1));
       expect((collected.first as UserMessage).text, 'real message');
     });
+
+    test('initial attach reads only the recent tail (initialTailBytes cap)', () async {
+      final projectDir = mungedDir(tempBase, workspace);
+      await projectDir.create(recursive: true);
+      final sessionFile = File('${projectDir.path}/session-abc.jsonl');
+
+      // A long pre-existing transcript — the kind that froze the UI when
+      // parsed in full on attach.
+      writeLines(sessionFile, [
+        for (var i = 0; i < 200; i++) assistantText('a$i', 'reply number $i'),
+      ]);
+
+      // Cap the initial read well below the file size so only the last
+      // records are within the tail window.
+      final reader = TranscriptReader(
+        workspace,
+        projectsBase: tempBase.path,
+        pollInterval: const Duration(milliseconds: 20),
+        initialTailBytes: 256,
+      );
+      final collected = <ConversationItem>[];
+      final sub = reader.stream.listen(collected.add);
+
+      await Future<void>.delayed(const Duration(milliseconds: 200));
+
+      await sub.cancel();
+      await reader.dispose();
+
+      // Far fewer than the 200 records — only the recent tail was read.
+      expect(collected, isNotEmpty);
+      expect(collected.length, lessThan(200));
+      // The most recent record is always intact at the end of the file.
+      expect(
+        collected.whereType<AssistantTextMessage>().last.text,
+        'reply number 199',
+      );
+    });
   });
 }