diff --git a/CHANGELOG.md b/CHANGELOG.md
index 38e2d735..77d60f7a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,6 +18,18 @@ heading, and (b) bumping `project.yaml` `version:` in the same commit.
 
 ### Added
 
+- Editor subsystem in the daemon (`lib/src/editor/`). `EditorBuffer`
+  holds path + content + cursor/selection + dirty flag;
+  `EditorRegistry` owns the open-buffer set, active-buffer tracking,
+  and file I/O. IPC verbs land alongside (`editor.open | active |
+  activate | list | read | insert | replace-selection | set-selection
+  | set-content | save | close`) with matching events (`editor.opened
+  | active-changed | selection-changed | edited | saved | closed`).
+  Omitting `id` on mutating verbs targets the active buffer so the
+  tier-2 CLI shortcuts (`clide insert "…"`, `clide replace-selection
+  "…"`) read naturally. 16 new core tests cover the lifecycle +
+  dispatcher round-trips.
+
 - `builtin.claude` — Tier-1 stub upgraded to the real Claude pane per
   D-041. Contributes a primary `Claude` tab in the workspace slot that
   spawns `tmux new-session -A -s clide-claude-<hash> -- claude` via
diff --git a/bin/clide.dart b/bin/clide.dart
index 8ea75dcc..3a9f83e0 100644
--- a/bin/clide.dart
+++ b/bin/clide.dart
@@ -16,8 +16,10 @@ import 'package:clide/clide.dart';
 // Daemon-only deep imports — these pull in dart:ffi (PTY) and
 // daemon-subsystem wiring that the Flutter app doesn't need and
 // can't compile for web. See lib/clide.dart for the barrel split.
+import 'package:clide/src/daemon/editor_commands.dart';
 import 'package:clide/src/daemon/files_commands.dart';
 import 'package:clide/src/daemon/pane_commands.dart';
+import 'package:clide/src/editor/registry.dart' show EditorRegistry;
 import 'package:clide/src/panes/registry.dart';
 
 Future<void> main(List<String> argv) async {
@@ -85,6 +87,9 @@ Future<void> _runDaemon(List<String> args) async {
   final files = FilesService.atCwd(events: events);
   registerFilesCommands(dispatcher, files);
 
+  final editor = EditorRegistry(events: events, workspaceRoot: files.root);
+  registerEditorCommands(dispatcher, editor);
+
   final stopping = Completer<void>();
   void shutdown(ProcessSignal sig) {
     if (!stopping.isCompleted) {
@@ -99,6 +104,7 @@ Future<void> _runDaemon(List<String> args) async {
   await server.start();
   await stopping.future;
   await registry.shutdown();
+  await editor.shutdown();
   await files.shutdown();
   await server.stop();
   exit(0);
diff --git a/lib/clide.dart b/lib/clide.dart
index f1ba13ed..cfe5a20d 100644
--- a/lib/clide.dart
+++ b/lib/clide.dart
@@ -17,6 +17,7 @@ library;
 // they're pure data types that both the app and the daemon reference.
 
 export 'src/daemon/dispatcher.dart';
+export 'src/editor/buffer.dart';
 export 'src/files/ignore.dart';
 export 'src/files/listing.dart' show FileEntry, listDir;
 export 'src/ipc/envelope.dart';
diff --git a/lib/src/daemon/editor_commands.dart b/lib/src/daemon/editor_commands.dart
new file mode 100644
index 00000000..b953be8c
--- /dev/null
+++ b/lib/src/daemon/editor_commands.dart
@@ -0,0 +1,164 @@
+/// Registers `editor.*` command handlers on a [DaemonDispatcher].
+///
+/// Verb list matches CLAUDE.md's tier-2 surface:
+///   editor.open editor.active editor.activate editor.insert
+///   editor.replace-selection editor.save editor.close editor.list
+///   editor.read editor.set-selection editor.set-content
+///
+/// Single-word CLI shortcuts (`clide open`, `clide active`, …) map
+/// one-to-one onto these in `bin/clide.dart`.
+library;
+
+import '../editor/registry.dart';
+import '../ipc/envelope.dart';
+import '../ipc/schema_v1.dart';
+import 'dispatcher.dart';
+
+export '../editor/buffer.dart' show Selection;
+
+void registerEditorCommands(DaemonDispatcher d, EditorRegistry registry) {
+  d.register('editor.open', (req) => _open(req, registry));
+  d.register('editor.active', (req) => _active(req, registry));
+  d.register('editor.activate', (req) => _activate(req, registry));
+  d.register('editor.list', (req) => _list(req, registry));
+  d.register('editor.read', (req) => _read(req, registry));
+  d.register('editor.insert', (req) => _insert(req, registry));
+  d.register('editor.replace-selection', (req) => _replace(req, registry));
+  d.register('editor.set-selection', (req) => _setSelection(req, registry));
+  d.register('editor.set-content', (req) => _setContent(req, registry));
+  d.register('editor.save', (req) => _save(req, registry));
+  d.register('editor.close', (req) => _close(req, registry));
+}
+
+IpcResponse _userErr(String id, String msg, {String? hint}) => IpcResponse.err(
+      id: id,
+      error: IpcError(
+        code: IpcExitCode.userError,
+        kind: IpcErrorKind.userError,
+        message: msg,
+        hint: hint,
+      ),
+    );
+
+IpcResponse _notFound(String id, String msg) => IpcResponse.err(
+      id: id,
+      error: IpcError(
+        code: IpcExitCode.notFound,
+        kind: IpcErrorKind.notFound,
+        message: msg,
+      ),
+    );
+
+String? _resolveId(IpcRequest req, EditorRegistry r) {
+  final id = req.args['id'] as String?;
+  if (id != null) return id;
+  // CLI shortcut: omitting `id` means the active buffer.
+  return r.active?.id;
+}
+
+Future<IpcResponse> _open(IpcRequest req, EditorRegistry r) async {
+  final path = req.args['path'] as String?;
+  if (path == null || path.isEmpty) {
+    return _userErr(req.id, 'path is required');
+  }
+  try {
+    final buf = await r.open(path);
+    return IpcResponse.ok(id: req.id, data: buf.toJson());
+  } catch (e) {
+    return IpcResponse.err(
+      id: req.id,
+      error: IpcError(
+        code: IpcExitCode.toolError,
+        kind: IpcErrorKind.toolError,
+        message: 'editor.open failed: $e',
+      ),
+    );
+  }
+}
+
+Future<IpcResponse> _active(IpcRequest req, EditorRegistry r) async {
+  final buf = r.active;
+  if (buf == null) {
+    return IpcResponse.ok(id: req.id, data: const {'active': null});
+  }
+  return IpcResponse.ok(id: req.id, data: {'active': buf.toJson()});
+}
+
+Future<IpcResponse> _activate(IpcRequest req, EditorRegistry r) async {
+  final id = req.args['id'] as String?;
+  if (id == null) return _userErr(req.id, 'id is required');
+  if (r.get(id) == null) return _notFound(req.id, 'no such buffer: $id');
+  r.activate(id);
+  return IpcResponse.ok(id: req.id, data: {'active': id});
+}
+
+Future<IpcResponse> _list(IpcRequest req, EditorRegistry r) async {
+  return IpcResponse.ok(
+    id: req.id,
+    data: {'buffers': [for (final b in r.buffers) b.toJson()]},
+  );
+}
+
+Future<IpcResponse> _read(IpcRequest req, EditorRegistry r) async {
+  final id = _resolveId(req, r);
+  if (id == null) return _notFound(req.id, 'no active buffer');
+  final buf = r.get(id);
+  if (buf == null) return _notFound(req.id, 'no such buffer: $id');
+  return IpcResponse.ok(id: req.id, data: buf.toFullJson());
+}
+
+Future<IpcResponse> _insert(IpcRequest req, EditorRegistry r) async {
+  final id = _resolveId(req, r);
+  if (id == null) return _notFound(req.id, 'no active buffer');
+  if (r.get(id) == null) return _notFound(req.id, 'no such buffer: $id');
+  final text = EditorRegistry.contentFromArgs(req.args);
+  r.insert(id, text);
+  return IpcResponse.ok(id: req.id, data: {'id': id, 'inserted': text.length});
+}
+
+Future<IpcResponse> _replace(IpcRequest req, EditorRegistry r) async {
+  final id = _resolveId(req, r);
+  if (id == null) return _notFound(req.id, 'no active buffer');
+  if (r.get(id) == null) return _notFound(req.id, 'no such buffer: $id');
+  final text = EditorRegistry.contentFromArgs(req.args);
+  r.replaceSelection(id, text);
+  return IpcResponse.ok(id: req.id, data: {'id': id, 'length': text.length});
+}
+
+Future<IpcResponse> _setSelection(IpcRequest req, EditorRegistry r) async {
+  final id = _resolveId(req, r);
+  if (id == null) return _notFound(req.id, 'no active buffer');
+  if (r.get(id) == null) return _notFound(req.id, 'no such buffer: $id');
+  final sel = EditorRegistry.selectionFromArgs(req.args['selection']);
+  r.setSelection(id, sel);
+  return IpcResponse.ok(id: req.id, data: {'id': id});
+}
+
+Future<IpcResponse> _setContent(IpcRequest req, EditorRegistry r) async {
+  final id = _resolveId(req, r);
+  if (id == null) return _notFound(req.id, 'no active buffer');
+  if (r.get(id) == null) return _notFound(req.id, 'no such buffer: $id');
+  final content = EditorRegistry.contentFromArgs(req.args);
+  final sel = req.args['selection'] == null
+      ? null
+      : EditorRegistry.selectionFromArgs(req.args['selection']);
+  r.setContent(id, content, selection: sel);
+  return IpcResponse.ok(id: req.id, data: {'id': id, 'length': content.length});
+}
+
+Future<IpcResponse> _save(IpcRequest req, EditorRegistry r) async {
+  final id = _resolveId(req, r);
+  if (id == null) return _notFound(req.id, 'no active buffer');
+  final ok = await r.save(id);
+  if (!ok) return _notFound(req.id, 'no such buffer: $id');
+  return IpcResponse.ok(id: req.id, data: {'id': id, 'saved': true});
+}
+
+Future<IpcResponse> _close(IpcRequest req, EditorRegistry r) async {
+  final id = req.args['id'] as String?;
+  if (id == null) return _userErr(req.id, 'id is required');
+  if (r.get(id) == null) return _notFound(req.id, 'no such buffer: $id');
+  r.close(id);
+  return IpcResponse.ok(id: req.id, data: {'id': id});
+}
+
diff --git a/lib/src/editor/buffer.dart b/lib/src/editor/buffer.dart
new file mode 100644
index 00000000..546ef34d
--- /dev/null
+++ b/lib/src/editor/buffer.dart
@@ -0,0 +1,85 @@
+/// A buffer in the daemon's editor model.
+///
+/// Represents an open file: its on-disk path, the authoritative text
+/// content, the UI's current cursor/selection, and whether it has
+/// unsaved changes. Thin data class — the [EditorRegistry] owns the
+/// transitions.
+library;
+
+class Selection {
+  const Selection({required this.start, required this.end});
+
+  const Selection.collapsed(int offset)
+      : start = offset,
+        end = offset;
+
+  final int start;
+  final int end;
+
+  bool get isCollapsed => start == end;
+  int get length => end - start;
+
+  Map<String, Object?> toJson() => {'start': start, 'end': end};
+
+  factory Selection.fromJson(Map<String, Object?> j) => Selection(
+        start: (j['start'] as num).toInt(),
+        end: (j['end'] as num).toInt(),
+      );
+
+  @override
+  bool operator ==(Object other) =>
+      other is Selection && other.start == start && other.end == end;
+
+  @override
+  int get hashCode => Object.hash(start, end);
+
+  @override
+  String toString() => 'Selection($start-$end)';
+}
+
+class EditorBuffer {
+  EditorBuffer({
+    required this.id,
+    required this.path,
+    required this.content,
+    Selection? selection,
+    this.dirty = false,
+  }) : selection = selection ?? const Selection.collapsed(0);
+
+  /// Stable daemon-local id (`b_1`, `b_2`, …).
+  final String id;
+
+  /// Repo-relative path. Path is the identity key for "reopening the
+  /// same file" — opening an already-open path returns the existing
+  /// buffer.
+  final String path;
+
+  /// Authoritative text content. UI edits mutate this via IPC
+  /// (`editor.insert`, `editor.replace-selection`); the UI's local
+  /// copy reconciles to match.
+  String content;
+
+  /// Cursor / selection. Offsets are byte offsets into [content] —
+  /// utf-8 characters that span multiple bytes count as multiple
+  /// offsets, same convention Flutter's `TextEditingValue` uses.
+  Selection selection;
+
+  /// True after an edit landed that hasn't been persisted via
+  /// `editor.save` (or `files.save` in future).
+  bool dirty;
+
+  Map<String, Object?> toJson() => {
+        'id': id,
+        'path': path,
+        'length': content.length,
+        'selection': selection.toJson(),
+        'dirty': dirty,
+      };
+
+  /// Full snapshot including [content] — for `editor.read` / tests /
+  /// anything that needs the text explicitly.
+  Map<String, Object?> toFullJson() => {
+        ...toJson(),
+        'content': content,
+      };
+}
diff --git a/lib/src/editor/registry.dart b/lib/src/editor/registry.dart
new file mode 100644
index 00000000..d5078631
--- /dev/null
+++ b/lib/src/editor/registry.dart
@@ -0,0 +1,230 @@
+/// [EditorRegistry] — daemon-side state for open editor buffers.
+///
+/// Owns a set of [EditorBuffer]s keyed by id. At most one buffer is
+/// `active` at a time — the UI tells the daemon which one it's
+/// focused on via `editor.activate`. All state transitions emit
+/// events through the [DaemonEventSink].
+library;
+
+import 'dart:convert';
+import 'dart:io';
+
+import '../ipc/envelope.dart';
+import '../panes/event_sink.dart';
+import 'buffer.dart';
+
+class EditorRegistry {
+  EditorRegistry({
+    required this.events,
+    required this.workspaceRoot,
+  });
+
+  final DaemonEventSink events;
+
+  /// Workspace root used to resolve repo-relative paths to disk.
+  final Directory workspaceRoot;
+
+  final Map<String, EditorBuffer> _buffers = {};
+  final Map<String, String> _pathToId = {}; // repo-rel path → id
+  int _nextId = 1;
+  String? _activeId;
+
+  Iterable<EditorBuffer> get buffers => _buffers.values;
+  EditorBuffer? get(String id) => _buffers[id];
+  EditorBuffer? get active =>
+      _activeId == null ? null : _buffers[_activeId!];
+
+  /// Open a file. If [path] is already open, returns the existing
+  /// buffer (no re-read from disk — the in-memory content is the
+  /// source of truth between save points).
+  Future<EditorBuffer> open(String path) async {
+    final existing = _pathToId[path];
+    if (existing != null) {
+      final buf = _buffers[existing]!;
+      _setActive(buf.id);
+      return buf;
+    }
+
+    final absolute = _absolutePathOf(path);
+    final file = File(absolute);
+    String content = '';
+    if (await file.exists()) {
+      content = await file.readAsString();
+    }
+
+    final id = 'b_${_nextId++}';
+    final buf = EditorBuffer(id: id, path: path, content: content);
+    _buffers[id] = buf;
+    _pathToId[path] = id;
+
+    _emit('editor.opened', {
+      ...buf.toJson(),
+      'content': buf.content, // snapshot at open time
+    });
+    _setActive(id);
+    return buf;
+  }
+
+  /// Mark [id] as the active buffer. Idempotent.
+  void activate(String id) {
+    if (!_buffers.containsKey(id)) return;
+    _setActive(id);
+  }
+
+  /// Insert [text] at the buffer's cursor (or replace the selection
+  /// if one exists). Advances the cursor past the inserted text.
+  void insert(String id, String text) {
+    final buf = _buffers[id];
+    if (buf == null) return;
+    final sel = buf.selection;
+    final before = buf.content.substring(0, sel.start);
+    final after = buf.content.substring(sel.end);
+    buf.content = '$before$text$after';
+    final newCaret = sel.start + text.length;
+    buf.selection = Selection.collapsed(newCaret);
+    buf.dirty = true;
+    _emit('editor.edited', {
+      'id': id,
+      'kind': 'insert',
+      'inserted': text,
+      'at': sel.start,
+      'replaced': sel.length,
+      'length': buf.content.length,
+      'selection': buf.selection.toJson(),
+    });
+    _emitSelection(buf);
+  }
+
+  /// Replace the current selection (or insert at cursor if no
+  /// selection) with [text]. Same mechanic as [insert] — kept as a
+  /// named verb because the CLI surface exposes it separately per
+  /// CLAUDE.md's tier-2 list.
+  void replaceSelection(String id, String text) => insert(id, text);
+
+  /// Update the UI's cursor / selection for [id]. Broadcasts so other
+  /// subscribers can mirror it.
+  void setSelection(String id, Selection sel) {
+    final buf = _buffers[id];
+    if (buf == null) return;
+    final clamped = Selection(
+      start: sel.start.clamp(0, buf.content.length),
+      end: sel.end.clamp(0, buf.content.length),
+    );
+    if (clamped.start == buf.selection.start && clamped.end == buf.selection.end) {
+      return;
+    }
+    buf.selection = clamped;
+    _emitSelection(buf);
+  }
+
+  /// Overwrite [id]'s content (used when the UI owns authoritative
+  /// text — diff-style editor, paste, etc.) and reconcile the
+  /// registry's view. Emits a single `editor.edited` event with
+  /// kind='replace' so subscribers don't need to diff.
+  void setContent(String id, String content, {Selection? selection}) {
+    final buf = _buffers[id];
+    if (buf == null) return;
+    buf.content = content;
+    if (selection != null) {
+      buf.selection = Selection(
+        start: selection.start.clamp(0, content.length),
+        end: selection.end.clamp(0, content.length),
+      );
+    } else {
+      buf.selection = Selection(
+        start: buf.selection.start.clamp(0, content.length),
+        end: buf.selection.end.clamp(0, content.length),
+      );
+    }
+    buf.dirty = true;
+    _emit('editor.edited', {
+      'id': id,
+      'kind': 'replace',
+      'length': content.length,
+      'selection': buf.selection.toJson(),
+    });
+  }
+
+  /// Persist [id] to disk. Clears the dirty flag on success.
+  Future<bool> save(String id) async {
+    final buf = _buffers[id];
+    if (buf == null) return false;
+    final absolute = _absolutePathOf(buf.path);
+    await File(absolute).writeAsString(buf.content);
+    buf.dirty = false;
+    _emit('editor.saved', {'id': id, 'path': buf.path});
+    return true;
+  }
+
+  /// Close a buffer. Idempotent.
+  void close(String id) {
+    final buf = _buffers.remove(id);
+    if (buf == null) return;
+    _pathToId.remove(buf.path);
+    if (_activeId == id) {
+      _activeId = _buffers.values.isEmpty ? null : _buffers.values.first.id;
+      if (_activeId != null) _emitActive();
+    }
+    _emit('editor.closed', {'id': id, 'path': buf.path});
+  }
+
+  Future<void> shutdown() async {
+    _buffers.clear();
+    _pathToId.clear();
+    _activeId = null;
+  }
+
+  // -----------------------------------------------------------------
+
+  void _setActive(String id) {
+    if (_activeId == id) return;
+    _activeId = id;
+    _emitActive();
+  }
+
+  void _emitActive() {
+    final buf = active;
+    _emit('editor.active-changed', {
+      'id': buf?.id,
+      'path': buf?.path,
+    });
+  }
+
+  void _emitSelection(EditorBuffer buf) {
+    _emit('editor.selection-changed', {
+      'id': buf.id,
+      'selection': buf.selection.toJson(),
+    });
+  }
+
+  void _emit(String kind, Map<String, Object?> data) {
+    events.emit(IpcEvent(
+      subsystem: 'editor',
+      kind: kind,
+      timestamp: DateTime.now().toUtc(),
+      data: data,
+    ));
+  }
+
+  String _absolutePathOf(String repoRelative) {
+    if (repoRelative.startsWith('/')) return repoRelative;
+    final sep = Platform.pathSeparator;
+    return '${workspaceRoot.absolute.path}$sep${repoRelative.replaceAll('/', sep)}';
+  }
+
+  // Support JSON decode of Selection from IPC args.
+  static Selection selectionFromArgs(Object? raw) {
+    if (raw is! Map) return const Selection.collapsed(0);
+    return Selection.fromJson(raw.cast<String, Object?>());
+  }
+
+  // Support JSON decode of content payloads (base64 for binary safety
+  // or plain text).
+  static String contentFromArgs(Map<String, Object?> args) {
+    final text = args['text'];
+    if (text is String) return text;
+    final b64 = args['content_b64'];
+    if (b64 is String) return utf8.decode(base64Decode(b64));
+    return '';
+  }
+}
diff --git a/test/daemon/editor_commands_test.dart b/test/daemon/editor_commands_test.dart
new file mode 100644
index 00000000..f3660ced
--- /dev/null
+++ b/test/daemon/editor_commands_test.dart
@@ -0,0 +1,110 @@
+import 'dart:io';
+
+import 'package:clide/clide.dart';
+import 'package:clide/src/daemon/editor_commands.dart';
+import 'package:clide/src/editor/registry.dart';
+import 'package:test/test.dart';
+
+void main() {
+  late Directory sandbox;
+  late DaemonDispatcher dispatcher;
+  late EditorRegistry reg;
+
+  setUp(() async {
+    sandbox = await Directory.systemTemp.createTemp('clide-ed-cmd-test-');
+    await File('${sandbox.path}/doc.md').writeAsString('alpha beta');
+    final sink = RecordingEventSink();
+    reg = EditorRegistry(events: sink, workspaceRoot: sandbox);
+    dispatcher = DaemonDispatcher();
+    registerEditorCommands(dispatcher, reg);
+  });
+
+  tearDown(() async {
+    await reg.shutdown();
+    if (sandbox.existsSync()) sandbox.deleteSync(recursive: true);
+  });
+
+  Future<IpcResponse> call(String cmd, [Map<String, Object?> args = const {}]) {
+    return dispatcher.dispatch(IpcRequest(id: '1', cmd: cmd, args: args));
+  }
+
+  test('editor.open requires a path', () async {
+    final r = await call('editor.open');
+    expect(r.ok, isFalse);
+    expect(r.error!.kind, 'user_error');
+  });
+
+  test('editor.open + editor.active round-trip', () async {
+    final open = await call('editor.open', {'path': 'doc.md'});
+    expect(open.ok, isTrue);
+    final id = open.data['id']! as String;
+    expect(id, startsWith('b_'));
+
+    final active = await call('editor.active');
+    expect(active.ok, isTrue);
+    final act = active.data['active']! as Map;
+    expect(act['id'], id);
+    expect(act['path'], 'doc.md');
+  });
+
+  test('editor.insert without id targets the active buffer', () async {
+    await call('editor.open', {'path': 'doc.md'});
+    final r = await call('editor.insert', {'text': 'X '});
+    expect(r.ok, isTrue);
+    expect(r.data['inserted'], 2);
+
+    final read = await call('editor.read');
+    expect((read.data['content'] as String).startsWith('X '), isTrue);
+  });
+
+  test('editor.replace-selection swaps selected range', () async {
+    final open = await call('editor.open', {'path': 'doc.md'});
+    final id = open.data['id'] as String?;
+
+    await call('editor.set-selection', {
+      'id': id,
+      'selection': {'start': 0, 'end': 5}, // 'alpha'
+    });
+    final r = await call('editor.replace-selection', {'text': 'gamma'});
+    expect(r.ok, isTrue);
+
+    final read = await call('editor.read');
+    expect(read.data['content'], 'gamma beta');
+  });
+
+  test('editor.save persists to disk', () async {
+    await call('editor.open', {'path': 'doc.md'});
+    await call('editor.insert', {'text': 'Z '});
+    final save = await call('editor.save');
+    expect(save.ok, isTrue);
+    final disk = await File('${sandbox.path}/doc.md').readAsString();
+    expect(disk.startsWith('Z '), isTrue);
+  });
+
+  test('editor.close removes the buffer', () async {
+    final open = await call('editor.open', {'path': 'doc.md'});
+    final id = open.data['id'] as String?;
+    final r = await call('editor.close', {'id': id});
+    expect(r.ok, isTrue);
+    final list = await call('editor.list');
+    expect((list.data['buffers'] as List), isEmpty);
+  });
+
+  test('editor.list includes all open buffers', () async {
+    await File('${sandbox.path}/a.md').writeAsString('a');
+    await File('${sandbox.path}/b.md').writeAsString('b');
+    await call('editor.open', {'path': 'a.md'});
+    await call('editor.open', {'path': 'b.md'});
+    final r = await call('editor.list');
+    final names = [
+      for (final b in (r.data['buffers'] as List).cast<Map>()) b['path'],
+    ];
+    expect(names, containsAll(['a.md', 'b.md']));
+  });
+
+  test('insert on unknown id returns not-found', () async {
+    final r = await call('editor.insert', {'id': 'b_404', 'text': 'x'});
+    expect(r.ok, isFalse);
+    expect(r.error!.code, IpcExitCode.notFound);
+  });
+}
diff --git a/test/editor/registry_test.dart b/test/editor/registry_test.dart
new file mode 100644
index 00000000..1861ab6f
--- /dev/null
+++ b/test/editor/registry_test.dart
@@ -0,0 +1,97 @@
+import 'dart:io';
+
+import 'package:clide/clide.dart';
+import 'package:clide/src/editor/registry.dart';
+import 'package:test/test.dart';
+
+void main() {
+  late Directory sandbox;
+  late RecordingEventSink sink;
+  late EditorRegistry reg;
+
+  setUp(() async {
+    sandbox = await Directory.systemTemp.createTemp('clide-editor-test-');
+    await File('${sandbox.path}/README.md').writeAsString('# Hello\n\nbody\n');
+    sink = RecordingEventSink();
+    reg = EditorRegistry(events: sink, workspaceRoot: sandbox);
+  });
+
+  tearDown(() async {
+    await reg.shutdown();
+    if (sandbox.existsSync()) sandbox.deleteSync(recursive: true);
+  });
+
+  test('open loads file content + emits editor.opened + active-changed', () async {
+    final buf = await reg.open('README.md');
+    expect(buf.content, contains('Hello'));
+    expect(buf.dirty, isFalse);
+    expect(reg.active, same(buf));
+    expect(sink.ofKind('editor.opened'), hasLength(1));
+    expect(sink.ofKind('editor.active-changed'), hasLength(1));
+  });
+
+  test('opening the same path returns the existing buffer', () async {
+    final a = await reg.open('README.md');
+    final b = await reg.open('README.md');
+    expect(b.id, a.id);
+    // Still only one open event — re-open is an activate, not a reload.
+    expect(sink.ofKind('editor.opened'), hasLength(1));
+  });
+
+  test('insert at caret appends + advances cursor', () async {
+    final buf = await reg.open('README.md');
+    // caret at 0
+    reg.insert(buf.id, 'PREFIX ');
+    expect(buf.content.startsWith('PREFIX '), isTrue);
+    expect(buf.selection.isCollapsed, isTrue);
+    expect(buf.selection.start, 'PREFIX '.length);
+    expect(buf.dirty, isTrue);
+    expect(sink.ofKind('editor.edited'), hasLength(1));
+  });
+
+  test('replace-selection swaps selected text + resets cursor', () async {
+    final buf = await reg.open('README.md');
+    reg.setSelection(buf.id, const Selection(start: 2, end: 7)); // 'Hello'
+    reg.replaceSelection(buf.id, 'WORLD');
+    expect(buf.content.substring(2, 7), 'WORLD');
+    expect(buf.selection, const Selection(start: 7, end: 7));
+  });
+
+  test('set-selection clamps out-of-range offsets', () async {
+    final buf = await reg.open('README.md');
+    reg.setSelection(buf.id, const Selection(start: -5, end: 99999));
+    expect(buf.selection.start, 0);
+    expect(buf.selection.end, buf.content.length);
+  });
+
+  test('save writes the content back + clears dirty', () async {
+    final buf = await reg.open('README.md');
+    reg.insert(buf.id, 'X');
+    expect(buf.dirty, isTrue);
+    final ok = await reg.save(buf.id);
+    expect(ok, isTrue);
+    expect(buf.dirty, isFalse);
+    final onDisk = await File('${sandbox.path}/README.md').readAsString();
+    expect(onDisk.startsWith('X'), isTrue);
+    expect(sink.ofKind('editor.saved'), hasLength(1));
+  });
+
+  test('close picks a new active buffer when the active one closes',
+      () async {
+    final a = await reg.open('README.md');
+    await File('${sandbox.path}/b.txt').writeAsString('two');
+    final b = await reg.open('b.txt');
+    expect(reg.active, same(b));
+    reg.close(b.id);
+    expect(reg.active, same(a));
+    expect(sink.ofKind('editor.closed'), hasLength(1));
+    // Active changed at least twice: a→b (on open), b→a (after close)
+    expect(sink.ofKind('editor.active-changed').length, greaterThanOrEqualTo(2));
+  });
+
+  test('opening a non-existent path creates an empty buffer', () async {
+    final buf = await reg.open('NEW.md');
+    expect(buf.content, isEmpty);
+    expect(buf.dirty, isFalse);
+  });
+}