fork a Claude conversation into a new pane

Adds fork-into-a-pane: /fork in the composer, a roster Fork button, and a
clide.agent.fork command all branch a session via
--resume <source> --fork-session, so the branch gets its own claude
session id and diverges without touching the original. SpawnSpec/
ManagedSession gain forkSourceSessionId; the orchestrator selects the
fork argv via a new forkSessionArgs helper; the session host opens the
fork as a new secondary pane.

The branch's real claude session-id (assigned by --fork-session, arriving
in the init event) is not yet captured back — tracked as T-185.

T-172.

Co-Authored-By: Claude <noreply@anthropic.com>

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

@@ -2290,3 +2290,5 @@ INSERT INTO ticket_history (ticket_id, field, old_value, new_value, changed_by,
 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-171', 'status', 'in_progress', 'done', NULL, '2026-05-30 21:11:26', '2026-05-30 21:11:26', '2026-05-30 21:11:26', NULL, '26cb27e63ad95ae94017ada95daef9d2', 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-174', 'status', 'in_progress', 'done', NULL, '2026-05-30 21:11:26', '2026-05-30 21:11:26', '2026-05-30 21:11:26', NULL, '4df20d29d49e7dd95dd9caafbe956cb2', 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-181', 'status', 'backlog', 'in_progress', NULL, '2026-05-30 21:14:56', '2026-05-30 21:14:56', '2026-05-30 21:14:56', NULL, '6426dc0355b68f0fd572e52d53101fc4', 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-181', 'status', 'in_progress', 'done', NULL, '2026-05-30 21:25:11', '2026-05-30 21:25:11', '2026-05-30 21:25:11', NULL, '81d8d388c7e4f53a4514f05cf5dbf37b', 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-172', 'status', 'backlog', 'in_progress', NULL, '2026-05-30 21:25:12', '2026-05-30 21:25:12', '2026-05-30 21:25:12', NULL, 'e42556ec9d67731533b85e6da2a1bc9c', 1) ON CONFLICT(hash) DO NOTHING;

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

@@ -2861,3 +2861,9 @@ Make the actions always reachable per T-173''s acceptance: keep them in the tree
 Acceptance: copy + custom actions are reachable and activatable by keyboard alone (Tab to focus, Enter/Space to invoke) with no mouse; Semantics button labels present; widget test drives a copy via focus+activation (no hover); a11y gate green.', 'done', 'medium', NULL, NULL, 'D-77', '2026-05-24 17:11:17', '2026-05-30 21:11:26', NULL, '2eb96628ffebd214dca72ffcd5f5da69', 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-171', 'task', 'T-164', 'P2: sidebar cockpit — task list + messages + roster controls', 'Evolve the Claude meta sidebar (T-141/T-157) from a read-only roster into the CONTROL surface for clide-managed agents. Because clide owns the broker, the task list and inter-agent messages are local data — so the sidebar both shows them live and lets the user ACT: spawn / show / hide / mute an agent, reassign a task, inject or redirect a message. Roster rows keep the live per-member status (T-157) but gain controls. Acceptance: from the sidebar the user can spawn/show/hide an agent, watch the shared task list update, see the message flow, and intervene (reassign/inject); widget tests for the controls wired to the orchestrator + broker. Blocked by T-169 (orchestrator) and T-170 (broker).', 'done', 'medium', NULL, NULL, 'D-77', '2026-05-24 16:27:09', '2026-05-30 21:11:26', NULL, '7d186872f15ef055161e51f2ae3f7479', 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-181', 'task', 'T-164', 'Per-agent permission-mode cycle badge in the cockpit roster', 'Per-agent permission-mode control in the cockpit roster (extends T-171). Each roster row shows a mode badge (D default / A acceptEdits / P plan). Click cycles the SAFE trio default -> acceptEdits -> plan and sends set_permission_mode to that agent session (a confirmed stream-json control subtype). bypassPermissions is a footgun, so it is reachable only on SHIFT-click (behind a confirm), and the tooltip documents both behaviours. Acceptance: clicking the badge cycles the safe modes and the session receives set_permission_mode; shift-click can reach bypass behind a confirm; the badge reflects the live mode (T-157 status); tooltip explains click vs shift-click; widget + transport tests. Wireframe: docs/design/wireframes/claude-prompts/05-team-cockpit-sidebar.png. Blocked by T-169 (orchestrator).', 'in_progress', 'low', NULL, NULL, 'D-77', '2026-05-25 09:16:05', '2026-05-30 21:14:56', NULL, '3e4c5116d77bfc451e05eb54d208e83d', 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-181', 'task', 'T-164', 'Per-agent permission-mode cycle badge in the cockpit roster', 'Per-agent permission-mode control in the cockpit roster (extends T-171). Each roster row shows a mode badge (D default / A acceptEdits / P plan). Click cycles the SAFE trio default -> acceptEdits -> plan and sends set_permission_mode to that agent session (a confirmed stream-json control subtype). bypassPermissions is a footgun, so it is reachable only on SHIFT-click (behind a confirm), and the tooltip documents both behaviours. Acceptance: clicking the badge cycles the safe modes and the session receives set_permission_mode; shift-click can reach bypass behind a confirm; the badge reflects the live mode (T-157 status); tooltip explains click vs shift-click; widget + transport tests. Wireframe: docs/design/wireframes/claude-prompts/05-team-cockpit-sidebar.png. Blocked by T-169 (orchestrator).', 'done', 'low', NULL, NULL, 'D-77', '2026-05-25 09:16:05', '2026-05-30 21:25:11', NULL, '5742b99a6d5b4315dcc85bc5f9e0ed39', 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-172', 'task', 'T-164', 'P2: fork-into-a-pane (--fork-session)', 'Branch a conversation into a new managed session/pane, reusing the orchestrator''s spawn plumbing. ''Fork'' = spawn a managed session seeded from another session''s context via --resume + --fork-session (resume the source, but create a new session-id so the branch diverges without touching the original). Surface it as a slash action and a sidebar/roster button on a session. Acceptance: forking an active session opens a new pane continuing from its context, independent thereafter; the original is unaffected; tests for the fork argv + the new managed session appearing in the roster. Blocked by T-169 (orchestrator).
+
+---
+Refinement (2026-05-30, /whats-next): --fork-session CONFIRMED against the live binary (claude --help: "When resuming, create a new session ID instead of reusing the original (use with --resume or --continue)"). Fork argv = [''--resume'', <sourceSessionId>, ''--fork-session''] (no --session-id; the new branch gets its own id from the init event). Add forkSourceSessionId to SpawnSpec (session_orchestrator.dart); build argv via a forkSessionArgs helper in session_naming.dart; add ''fork'' to kClideOwnedCommands + a case in claude_pane._send; add a fork button to _memberRow in claude_meta_sidebar.dart. SEQUENCE AFTER T-171 (shares _memberRow + orchestrator).', 'in_progress', 'low', NULL, NULL, 'D-77', '2026-05-24 16:27:16', '2026-05-30 21:25:12', NULL, '1aaeba013d95c3d2baa5488f20183775', 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-185', 'task', 'T-164', 'Capture the forked session''s real claude session-id from its init event', 'T-172 follow-up. A forked session is spawned with --resume <source> --fork-session and no --session-id, so claude assigns the branch a NEW session-id that only arrives in the stream-json init event. Today ManagedSession.sessionId for a fork holds a placeholder UUID (the orchestrator key), and the real claude session-id is never captured back. Consequence: the fork can''t itself be resumed/forked by id later, and any UI showing the session-id is wrong for forks. Do: capture session_id from the init/system event (StreamJsonSession already parses init for status — surface the session_id) and update the ManagedSession (make sessionId mutable or store a separate claudeSessionId that the orchestrator fills on init). Acceptance: after a fork''s first init event, the ManagedSession exposes the branch''s real claude session-id; a test drives an init event through a fake process and asserts the captured id. Surfaced by T-172; the fork argv/spawn/roster/original-unaffected behaviour already works without it.', 'backlog', 'low', NULL, NULL, 'D-77', '2026-05-30 21:34:26', '2026-05-30 21:34:26', NULL, 'f041094e4dea938f3e00997226e21edf', 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

@@ -72,6 +72,9 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
   safe trio default → acceptEdits → plan and sends `set_permission_mode` to
   that session; Shift-click reaches `bypassPermissions` behind a confirm.
   The badge reflects the live mode. (T-181, D-77)
+- Fork a Claude conversation into a new pane — `/fork` (or a roster Fork
+  button / `clide.agent.fork`) branches a session via `--resume … --fork-session`,
+  opening an independent continuation that leaves the original untouched. (T-172, D-77)
 - Team coordination broker (T-170, D-77) — clide hosts an in-process MCP
   server (`clide-team`) for managed sessions over the stream-json control
   channel, giving agents tools to message each other, broadcast, see the

lib/builtin/claude/src/claude_meta_sidebar.dart

@@ -203,6 +203,27 @@ class _ClaudeMetaSidebarState extends State<ClaudeMetaSidebar> {
     if (mounted) setState(() {});
   }
 
+  /// Fork the team session identified by [memberName] (T-172, roster button).
+  ///
+  /// Resolves the managed session, then spawns a new fork session via the
+  /// orchestrator. The fork appears in the roster (it's a team session, visible
+  /// by default) and is independent from the source — the original is unaffected.
+  void _forkMember(String memberName) {
+    final orch = _orchestrator;
+    if (orch == null) return;
+    final managed = orch.byMemberName(memberName);
+    if (managed == null) return;
+    final forkId = 'fork:$memberName-${DateTime.now().millisecondsSinceEpoch}';
+    unawaited(orch.spawn(SpawnSpec(
+      id: forkId,
+      role: 'fork of $memberName',
+      // sessionId is a placeholder; real claude session id arrives via init.
+      sessionId: forkId,
+      cwd: managed.cwd,
+      forkSourceSessionId: managed.sessionId,
+    )));
+  }
+
   Future<void> _refreshStats() async {
     final stats = await _load();
     if (mounted) setState(() => _stats = stats);
@@ -318,6 +339,7 @@ class _ClaudeMetaSidebarState extends State<ClaudeMetaSidebar> {
             final managed = _orchestrator?.byMemberName(name);
             managed?.session.setPermissionMode(mode);
           },
+          onFork: (name) => _forkMember(name),
         ),
     ];
 
@@ -450,9 +472,8 @@ class _MetaRow {
 /// - eye / eye-slash — show / hide the session pane
 /// - speaker / speaker-slash — mute / unmute broker delivery
 /// - inject (chat icon) — expand the inline message input
+/// - fork (git-branch icon) — open a new pane branching from this session (T-172)
 /// - close (×) — kill the session
-///
-/// Seam for T-172: add a fork button to the _buildControls row.
 class _AgentRosterRow extends StatefulWidget {
   const _AgentRosterRow({
     super.key,
@@ -465,6 +486,7 @@ class _AgentRosterRow extends StatefulWidget {
     required this.onInjectSubmit,
     required this.onClose,
     required this.onSetPermissionMode,
+    required this.onFork,
   });
 
   final TeamMemberJoined member;
@@ -486,6 +508,10 @@ class _AgentRosterRow extends StatefulWidget {
   /// the mode to the session via [StreamJsonSession.setPermissionMode].
   final void Function(String memberName, String mode) onSetPermissionMode;
 
+  /// Called when the fork button is tapped (T-172). The session id of the
+  /// member's managed session is passed so the host can open a fork pane.
+  final void Function(String memberName) onFork;
+
   @override
   State<_AgentRosterRow> createState() => _AgentRosterRowState();
 }
@@ -660,6 +686,13 @@ class _AgentRosterRowState extends State<_AgentRosterRow> {
           color: isInjecting ? tokens.globalFocus : tokens.globalTextMuted,
           onTap: () => widget.onToggleInject(widget.member.name),
         ),
+        // Fork session (T-172): branch into a new pane without touching the original.
+        _IconButton(
+          painter: PhosphorIcons.gitBranch,
+          tooltip: 'Fork session',
+          color: tokens.globalTextMuted,
+          onTap: () => widget.onFork(widget.member.name),
+        ),
         // Close session
         _IconButton(
           painter: PhosphorIcons.xMark,

lib/builtin/claude/src/claude_pane.dart

@@ -31,6 +31,8 @@ class ClaudePane extends StatefulWidget {
     super.key,
     this.isPrimary = true,
     this.secondaryIndex,
+    this.forkSourceId,
+    this.onFork,
     this.showChrome = true,
     this.active = true,
     this.contributionId = 'claude.primary',
@@ -40,6 +42,16 @@ class ClaudePane extends StatefulWidget {
   final bool showChrome;
   final int? secondaryIndex;
 
+  /// When non-null, spawn this pane as a fork of the given claude session id
+  /// using `--resume <forkSourceId> --fork-session` (T-172). Takes precedence
+  /// over the normal fresh/resume logic for secondary panes.
+  final String? forkSourceId;
+
+  /// Called when the user issues `/fork` to branch this session into a new
+  /// pane. The argument is the current pane's claude session id, which the
+  /// host (ClaudeSessionHost) uses to open a fork tab (T-172).
+  final void Function(String sourceClaudeSessionId)? onFork;
+
   /// Whether this pane is the visible/focused sub-tab. Only the active
   /// pane publishes its status to the status-bar context slot (T-145).
   final bool active;
@@ -159,16 +171,6 @@ class _ClaudePaneState extends State<ClaudePane> {
     }
     _repoRoot = repoRoot;
 
-    // Bind this pane to a specific session id (T-146). Primary: deterministic
-    // → resumes across restarts. Secondary: fresh → a clean session.
-    _sessionId ??= widget.isPrimary ? primarySessionId(repoRoot) : freshSessionId();
-
-    // A transcript already on disk means the session existed before, so resume
-    // it; `claude --session-id <id>` refuses an existing id (T-161/D-77).
-    final home = Platform.environment['HOME'] ?? '';
-    final transcriptFile = '$home/.claude/projects/${repoRoot.replaceAll('/', '-')}/$_sessionId.jsonl';
-    final resume = await File(transcriptFile).exists();
-
     // The orchestrator owns the session (T-169): spawn-or-bind by our pane key,
     // so the session (and its accumulating conversation) outlives this pane.
     final orch = activeSessionOrchestrator;
@@ -176,21 +178,55 @@ class _ClaudePaneState extends State<ClaudePane> {
       setState(() => _error = 'Session orchestrator unavailable.');
       return;
     }
+
     final ManagedSession managed;
-    try {
-      managed = await orch.spawn(SpawnSpec(
-        id: _orchId,
-        role: widget.isPrimary ? 'primary' : 'session ${widget.secondaryIndex}',
-        sessionId: _sessionId!,
-        cwd: repoRoot,
-        resume: resume,
-        transcriptPath: resume ? transcriptFile : null,
-      ));
-    } catch (e) {
-      if (mounted) setState(() => _error = 'Could not start claude: $e');
-      return;
+    final forkSource = widget.forkSourceId;
+    if (forkSource != null) {
+      // Fork pane: branch source session into a new clide-managed session.
+      // The clide-internal id is a fresh UUID; the real claude session id is
+      // assigned by `--fork-session` and arrives in the init event (T-172).
+      _sessionId ??= freshSessionId();
+      try {
+        managed = await orch.spawn(SpawnSpec(
+          id: _orchId,
+          role: 'fork ${widget.secondaryIndex}',
+          sessionId: _sessionId!,
+          cwd: repoRoot,
+          forkSourceSessionId: forkSource,
+        ));
+      } catch (e) {
+        if (mounted) setState(() => _error = 'Could not start fork: $e');
+        return;
+      }
+      if (!mounted) return;
+      setState(() => _statusLine = 'fork of $forkSource');
+    } else {
+      // Bind this pane to a specific session id (T-146). Primary: deterministic
+      // → resumes across restarts. Secondary: fresh → a clean session.
+      _sessionId ??= widget.isPrimary ? primarySessionId(repoRoot) : freshSessionId();
+
+      // A transcript already on disk means the session existed before, so resume
+      // it; `claude --session-id <id>` refuses an existing id (T-161/D-77).
+      final home = Platform.environment['HOME'] ?? '';
+      final transcriptFile = '$home/.claude/projects/${repoRoot.replaceAll('/', '-')}/$_sessionId.jsonl';
+      final resume = await File(transcriptFile).exists();
+
+      try {
+        managed = await orch.spawn(SpawnSpec(
+          id: _orchId,
+          role: widget.isPrimary ? 'primary' : 'session ${widget.secondaryIndex}',
+          sessionId: _sessionId!,
+          cwd: repoRoot,
+          resume: resume,
+          transcriptPath: resume ? transcriptFile : null,
+        ));
+      } catch (e) {
+        if (mounted) setState(() => _error = 'Could not start claude: $e');
+        return;
+      }
+      if (!mounted) return;
+      setState(() => _statusLine = resume ? 'resumed · $_sessionId' : 'new session · $_sessionId');
     }
-    if (!mounted) return;
 
     _session = managed.session;
     _conversation = managed.conversation;
@@ -198,13 +234,13 @@ class _ClaudePaneState extends State<ClaudePane> {
       if (!mounted) return;
       setState(() => _status = s);
     });
-    setState(() => _statusLine = resume ? 'resumed · $_sessionId' : 'new session · $_sessionId');
   }
 
   // Send composed text to Claude over the stream-json channel. Commands clide
   // owns (T-156) are handled here, never forwarded — /clear and /resume fork
   // the session to a new id, so clide drives them: /clear starts fresh,
-  // /resume picks a past session and re-binds to it.
+  // /resume picks a past session and re-binds to it. /fork branches the
+  // conversation into a new pane (T-172).
   void _send(String text) {
     switch (clideOwnedCommand(text)) {
       case 'clear':
@@ -213,10 +249,24 @@ class _ClaudePaneState extends State<ClaudePane> {
       case 'resume':
         unawaited(_resumeFlow());
         return;
+      case 'fork':
+        _forkSession();
+        return;
     }
     _session?.send(text);
   }
 
+  /// clide-owned `/fork` (T-172): branch this conversation into a new pane.
+  ///
+  /// Delegates to the [onFork] callback supplied by [ClaudeSessionHost] with
+  /// the current pane's claude session id. If the session hasn't started yet
+  /// or no callback was supplied, the command is silently ignored.
+  void _forkSession() {
+    final sourceId = _sessionId;
+    if (sourceId == null) return;
+    widget.onFork?.call(sourceId);
+  }
+
   /// clide-owned `/clear` (T-156): respawn on a brand-new, empty session.
   Future<void> _clearSession() async {
     if (mounted) setState(() => _statusLine = 'clearing…');

lib/builtin/claude/src/claude_session_host.dart

@@ -55,6 +55,20 @@ class ClaudeSessionHostState extends State<ClaudeSessionHost> {
     ));
   }
 
+  /// Open a new pane as a fork of [sourceClaudeSessionId] (T-172).
+  ///
+  /// The fork pane is a secondary tab seeded with `--resume <source>
+  /// --fork-session` so the branch diverges into its own claude session
+  /// without touching the original.
+  void addFork(String sourceClaudeSessionId) {
+    final index = _nextSecondary++;
+    _controller.add(MultitabEntry<_Session>(
+      id: 'secondary-$index',
+      title: 'fork $index',
+      payload: _Session(isPrimary: false, secondaryIndex: index, forkSourceId: sourceClaudeSessionId),
+    ));
+  }
+
   @override
   Widget build(BuildContext context) {
     return MultitabPane<_Session>(
@@ -66,6 +80,8 @@ class ClaudeSessionHostState extends State<ClaudeSessionHost> {
         return ClaudePane(
           isPrimary: s.isPrimary,
           secondaryIndex: s.secondaryIndex,
+          forkSourceId: s.forkSourceId,
+          onFork: addFork,
           // The MultitabPane already provides the tab strip header;
           // suppressing the ClaudePane's own chrome avoids a double row.
           showChrome: false,
@@ -78,7 +94,11 @@ class ClaudeSessionHostState extends State<ClaudeSessionHost> {
 }
 
 class _Session {
-  const _Session({required this.isPrimary, this.secondaryIndex});
+  const _Session({required this.isPrimary, this.secondaryIndex, this.forkSourceId});
   final bool isPrimary;
   final int? secondaryIndex;
+
+  /// When non-null, spawn this pane as a fork of the given claude session id
+  /// (T-172). Forwarded to [ClaudePane.forkSourceId].
+  final String? forkSourceId;
 }

lib/builtin/claude/src/extension.dart

@@ -174,17 +174,39 @@ class ClaudeExtension extends ClideExtension {
             return IpcResponse.ok(id: '', data: {'taskId': taskId, 'toId': toId, 'ok': ok});
           },
         ),
-        // claude.agent.spawn: spawning a new agent session programmatically.
-        // Full implementation deferred — requires the caller to supply
-        // SpawnSpec fields (sessionId, cwd, role, team flag, etc.) which are
-        // non-trivial to serialize over a flat CLI arg list.  The UI affordance
-        // (TeamPanelHost spawn) is the primary surface for now; this stub
-        // satisfies D-6 parity and will be fleshed out in T-172.
+        // claude.agent.fork: branch a managed session into a new fork session
+        // (T-172, D-6 CLI/UI parity for the roster fork button).
+        // Usage: clide claude.agent.fork <sourceSessionId> [<cwd>]
+        // <sourceSessionId>: the clide-internal id of the session to fork.
+        // <cwd>: optional working directory; defaults to the source session's cwd.
         CommandContribution(
-          id: 'claude.agent.spawn',
-          command: 'claude.agent.spawn',
-          title: 'Claude: spawn a new agent session (stub — T-172)',
-          run: (_) async => IpcResponse.ok(id: '', data: const {'status': 'not-implemented', 'ticket': 'T-172'}),
+          id: 'claude.agent.fork',
+          command: 'claude.agent.fork',
+          title: 'Claude: fork a managed session into a new branch session',
+          run: (args) async {
+            final sourceId = args.firstOrNull;
+            if (sourceId == null) {
+              return IpcResponse.ok(id: '', data: const {'error': 'usage: claude.agent.fork <sourceSessionId> [<cwd>]'});
+            }
+            final orch = _orchestrator;
+            if (orch == null) {
+              return IpcResponse.ok(id: '', data: const {'error': 'orchestrator unavailable'});
+            }
+            final source = orch.byId(sourceId);
+            if (source == null) {
+              return IpcResponse.ok(id: '', data: {'error': 'unknown session "$sourceId"'});
+            }
+            final cwd = args.length >= 2 ? args[1] : source.cwd;
+            final forkId = 'fork:$sourceId-${DateTime.now().millisecondsSinceEpoch}';
+            await orch.spawn(SpawnSpec(
+              id: forkId,
+              role: 'fork of $sourceId',
+              sessionId: forkId,
+              cwd: cwd,
+              forkSourceSessionId: source.sessionId,
+            ));
+            return IpcResponse.ok(id: '', data: {'forkId': forkId, 'sourceId': sourceId, 'status': 'spawned'});
+          },
         ),
         // Always-pickable left-panel tab: Claude activity (from
         // stats-cache.json) + the team roster when a team is running (T-141).

lib/builtin/claude/src/session_naming.dart

@@ -74,6 +74,14 @@ String primarySessionId(String repoRoot) => _deterministicUuid(_primarySessionSe
 /// (T-161). Appended after the stream-json flags by [ClaudeStreamJsonProcess].
 List<String> claudeLaunchArgs(String sessionId, {required bool resume}) => resume ? ['--resume', sessionId] : ['--session-id', sessionId];
 
+/// The session-selection args for forking a session (T-172, D-77).
+///
+/// `--resume <sourceSessionId> --fork-session` resumes [sourceSessionId] but
+/// creates a NEW claude session id so the branch diverges without touching the
+/// original. No `--session-id` is passed — the fork gets its own id from the
+/// `init` event.
+List<String> forkSessionArgs(String sourceSessionId) => ['--resume', sourceSessionId, '--fork-session'];
+
 /// A fresh random session id for a secondary pane — secondaries are
 /// always clean sessions, never resumed.
 String freshSessionId() {

lib/builtin/claude/src/session_orchestrator.dart

@@ -50,6 +50,7 @@ class SpawnSpec {
     this.visible = true,
     this.team = false,
     this.memberName,
+    this.forkSourceSessionId,
   });
 
   final String id;
@@ -75,6 +76,15 @@ class SpawnSpec {
   /// Name teammates address this session by (`send_message(to: …)`); defaults
   /// to [role] when omitted. Only meaningful when [team] is true.
   final String? memberName;
+
+  /// When non-null, spawn a forked branch of this claude session id (T-172).
+  /// Uses `--resume <forkSourceSessionId> --fork-session` so the branch
+  /// diverges into a NEW claude session without touching the original.
+  /// Takes precedence over [resume]/[sessionId] for arg selection.
+  final String? forkSourceSessionId;
+
+  /// Whether this spec spawns a forked session.
+  bool get isFork => forkSourceSessionId != null;
 }
 
 /// One clide-managed session: the process wrapper plus the conversation it
@@ -85,16 +95,27 @@ class ManagedSession {
     required this.id,
     required this.role,
     required this.sessionId,
+    required this.cwd,
     required this.session,
     required this.conversation,
     this.memberName,
     this.visible = true,
     this.muted = false,
+    this.forkSourceSessionId,
   });
 
   final String id;
   final String role;
+
+  /// The clide-internal session id. For fork sessions this is the placeholder
+  /// UUID passed via [SpawnSpec.sessionId]; the real claude-assigned session id
+  /// arrives in the `init` event and is not yet captured here (T-172 follow-up).
   final String sessionId;
+
+  /// The working directory this session was spawned in. Retained so forks and
+  /// the UI can reference the source context (T-172).
+  final String cwd;
+
   final StreamJsonSession session;
   final ConversationController conversation;
 
@@ -110,6 +131,13 @@ class ManagedSession {
   /// The session process still runs; teammates' messages accumulate in its
   /// inbox but are not injected into stdin until unmuted (T-171).
   bool muted;
+
+  /// The source claude session id this was forked from (T-172), or null for
+  /// non-fork sessions. For display / provenance only.
+  final String? forkSourceSessionId;
+
+  /// Whether this is a forked session.
+  bool get isFork => forkSourceSessionId != null;
 }
 
 /// App-wide orchestrator, set by the Claude extension on activate (like
@@ -154,7 +182,10 @@ class ClaudeSessionOrchestrator extends ChangeNotifier {
     // injected into their system prompt (T-170). Register the member before
     // spawning so a peer that messages it immediately resolves.
     final mcpServers = <McpServer>[];
-    var sessionArgs = claudeLaunchArgs(spec.sessionId, resume: spec.resume);
+    // Fork sessions use --resume <source> --fork-session so the branch gets its
+    // own claude session id from the init event (T-172). All other sessions use
+    // the normal --resume / --session-id selection.
+    var sessionArgs = spec.isFork ? forkSessionArgs(spec.forkSourceSessionId!) : claudeLaunchArgs(spec.sessionId, resume: spec.resume);
     if (spec.team) {
       final name = spec.memberName ?? spec.role;
       broker.addMember(TeamMemberRef(id: spec.id, name: name, role: spec.role));
@@ -174,10 +205,12 @@ class ClaudeSessionOrchestrator extends ChangeNotifier {
       id: spec.id,
       role: spec.role,
       sessionId: spec.sessionId,
+      cwd: spec.cwd,
       session: session,
       conversation: conversation,
       memberName: spec.memberName,
       visible: spec.visible,
+      forkSourceSessionId: spec.forkSourceSessionId,
     );
     _sessions[spec.id] = managed;
     notifyListeners();

lib/builtin/claude/src/slash_commands.dart

@@ -30,7 +30,8 @@ bool isKnownSlashCommand(String text, Iterable<String> known) {
 /// Slash commands clide handles itself instead of forwarding to Claude:
 /// Claude Code's own handling forks the session to a new id that clide's
 /// transcript reader can't follow, so clide owns the semantics (T-156).
-const Set<String> kClideOwnedCommands = {'clear', 'resume'};
+/// `/fork` branches the current session into a new pane (T-172).
+const Set<String> kClideOwnedCommands = {'clear', 'resume', 'fork'};
 
 /// The clide-owned command in [text] (a single-line leading-slash token in
 /// [kClideOwnedCommands]), or null.

test/builtin/claude/claude_meta_sidebar_test.dart

@@ -624,4 +624,82 @@ void main() {
       orch.dispose();
     });
   });
+
+  // T-172: fork button in the roster -------------------------------------------
+
+  group('T-172 fork session button', () {
+    Future<ClaudeSessionOrchestrator> orchWithMember(WidgetTester tester, {String name = 'Forker', String agentId = 'f1'}) async {
+      final orch = _fakeOrchestrator();
+      await orch.spawn(SpawnSpec(
+        id: 'teammate:$name',
+        role: 'teammate',
+        sessionId: '$name-uuid',
+        cwd: '/repo',
+        team: true,
+        memberName: name,
+      ));
+
+      await tester.pumpWidget(harness(f, sidebar(orchestrator: orch, initialTab: SidebarTab.team)));
+
+      f.services.events.emit(TeamMemberJoined(
+        team: 't',
+        agentId: agentId,
+        name: name,
+        agentType: 'coder',
+        paneId: '%1',
+        color: 'teal',
+      ));
+      await tester.pump();
+      await tester.pump();
+      return orch;
+    }
+
+    testWidgets('fork button appears in the roster controls', (tester) async {
+      final semantics = tester.ensureSemantics();
+      final orch = await orchWithMember(tester);
+
+      // The fork button is an _IconButton with tooltip 'Fork session'.
+      expect(find.bySemanticsLabel('Fork session'), findsOneWidget);
+
+      semantics.dispose();
+      orch.dispose();
+    });
+
+    testWidgets('tapping fork button spawns a new fork session in the orchestrator', (tester) async {
+      final semantics = tester.ensureSemantics();
+      final orch = await orchWithMember(tester);
+
+      expect(orch.sessions, hasLength(1)); // just the source
+
+      await tester.tap(find.bySemanticsLabel('Fork session').first);
+      await tester.pump();
+      await tester.pump();
+
+      // A second (fork) session is now registered.
+      expect(orch.sessions, hasLength(2));
+      final forkSession = orch.sessions.last;
+      expect(forkSession.isFork, isTrue);
+      expect(forkSession.forkSourceSessionId, 'Forker-uuid');
+
+      semantics.dispose();
+      orch.dispose();
+    });
+
+    testWidgets('fork leaves the source session untouched', (tester) async {
+      final semantics = tester.ensureSemantics();
+      final orch = await orchWithMember(tester);
+      final source = orch.byId('teammate:Forker')!;
+
+      await tester.tap(find.bySemanticsLabel('Fork session').first);
+      await tester.pump();
+      await tester.pump();
+
+      // Source session is still present and unchanged.
+      expect(orch.byId('teammate:Forker'), same(source));
+      expect(source.visible, isTrue);
+
+      semantics.dispose();
+      orch.dispose();
+    });
+  });
 }

test/builtin/claude/session_naming_test.dart

@@ -52,4 +52,29 @@ void main() {
       expect(resumed.last, id);
     });
   });
+
+  group('forkSessionArgs (T-172)', () {
+    const sourceId = 'aaaa1111-1111-4111-8111-111111111111';
+
+    test('fork args lead with --resume then the source id', () {
+      final args = forkSessionArgs(sourceId);
+      expect(args[0], '--resume');
+      expect(args[1], sourceId);
+    });
+
+    test('fork args include --fork-session to diverge without touching the original', () {
+      final args = forkSessionArgs(sourceId);
+      expect(args, contains('--fork-session'));
+    });
+
+    test('fork args contain exactly three elements', () {
+      // [--resume, <sourceId>, --fork-session] — no --session-id so claude
+      // assigns its own new session id (the branch).
+      expect(forkSessionArgs(sourceId), hasLength(3));
+    });
+
+    test('fork args do not contain --session-id', () {
+      expect(forkSessionArgs(sourceId), isNot(contains('--session-id')));
+    });
+  });
 }

test/builtin/claude/session_orchestrator_test.dart

@@ -175,4 +175,95 @@ void main() {
       expect(managed.conversation.items, isEmpty);
     });
   });
+
+  // T-172: fork-into-a-pane --------------------------------------------------
+
+  group('fork session (T-172)', () {
+    List<String>? capturedArgs;
+
+    setUp(() {
+      capturedArgs = null;
+      orch = ClaudeSessionOrchestrator(
+        processFactory: ({required sessionArgs, required cwd, env}) async {
+          capturedArgs = sessionArgs;
+          return _FakeProc();
+        },
+      );
+    });
+
+    SpawnSpec forkSpec(String id, String sourceSessionId) => SpawnSpec(
+          id: id,
+          role: 'fork',
+          sessionId: '$id-placeholder',
+          cwd: '/repo',
+          forkSourceSessionId: sourceSessionId,
+        );
+
+    test('fork spawn passes --resume <source> --fork-session instead of --session-id', () async {
+      const sourceId = 'bbbb2222-2222-4222-8222-222222222222';
+      await orch.spawn(forkSpec('fork-1', sourceId));
+      expect(capturedArgs, isNotNull);
+      expect(capturedArgs![0], '--resume');
+      expect(capturedArgs![1], sourceId);
+      expect(capturedArgs!, contains('--fork-session'));
+      expect(capturedArgs!, isNot(contains('--session-id')));
+    });
+
+    test('fork SpawnSpec.isFork is true when forkSourceSessionId is set', () {
+      const sourceId = 'cccc3333-3333-4333-8333-333333333333';
+      final s = forkSpec('fork-1', sourceId);
+      expect(s.isFork, isTrue);
+    });
+
+    test('non-fork SpawnSpec.isFork is false', () {
+      final s = spec('primary');
+      expect(s.isFork, isFalse);
+    });
+
+    test('fork registers in the orchestrator under its own id', () async {
+      const sourceId = 'dddd4444-4444-4444-8444-444444444444';
+      final managed = await orch.spawn(forkSpec('fork-1', sourceId));
+      expect(orch.byId('fork-1'), same(managed));
+      expect(managed.isFork, isTrue);
+      expect(managed.forkSourceSessionId, sourceId);
+    });
+
+    test('fork is independent from the source — source session is unaffected', () async {
+      // Spawn the source session first.
+      final source = await orch.spawn(spec('primary'));
+      expect(orch.sessions, hasLength(1));
+
+      // Fork it.
+      await orch.spawn(forkSpec('fork-1', source.sessionId));
+      // Both sessions exist; source is unchanged.
+      expect(orch.sessions, hasLength(2));
+      expect(orch.byId('primary'), same(source));
+    });
+
+    test('fork session appears in sessions list and is visible by default', () async {
+      const sourceId = 'eeee5555-5555-4555-8555-555555555555';
+      final managed = await orch.spawn(forkSpec('fork-1', sourceId));
+      expect(orch.sessions.contains(managed), isTrue);
+      expect(managed.visible, isTrue);
+      expect(orch.visibleSessions.contains(managed), isTrue);
+    });
+
+    test('fork notifies listeners when spawned', () async {
+      const sourceId = 'ffff6666-6666-4666-8666-666666666666';
+      var notified = false;
+      orch.addListener(() => notified = true);
+      await orch.spawn(forkSpec('fork-1', sourceId));
+      expect(notified, isTrue);
+    });
+
+    test('ManagedSession.cwd reflects the spec cwd', () async {
+      final managed = await orch.spawn(SpawnSpec(
+        id: 'primary',
+        role: 'primary',
+        sessionId: 'primary-uuid',
+        cwd: '/my/project',
+      ));
+      expect(managed.cwd, '/my/project');
+    });
+  });
 }