feat(tabs): 2-column tab grid with nestable subagent groups + tab menu (add/remove from group)

17 files changed, 1242 insertions, 185 deletions

diff --git a/src/adapters/ws/index.ts b/src/adapters/ws/index.ts
index 4dc0104..76d9e75 100644
--- a/src/adapters/ws/index.ts
+++ b/src/adapters/ws/index.ts
@@ -4,6 +4,7 @@ import type {
   ConversationCompactedMessage,
   ConversationOpenMessage,
   ConversationStatusChangedMessage,
+  ConversationTabGroupChangedMessage,
   ExtensionsChangedMessage,
   LayoutChangedMessage,
   TileLayoutChangedMessage,
@@ -30,6 +31,8 @@ export interface SurfaceSocketOptions {
   onConversationStatusChanged?: (msg: ConversationStatusChangedMessage) => void;
   /** Broadcast when a conversation's history has been compacted (reload needed). */
   onConversationCompacted?: (msg: ConversationCompactedMessage) => void;
+  /** Broadcast when a conversation's (visual) tab group parent changes. */
+  onConversationTabGroupChanged?: (msg: ConversationTabGroupChangedMessage) => void;
   /**
    * Broadcast after every plugin-manager mutation (plan 05 §11): every
    * connected frontend refetches its extension list (the server re-pushes the
@@ -93,6 +96,8 @@ export function createSurfaceSocket(opts: SurfaceSocketOptions): SurfaceSocketHa
           opts.onConversationOpen?.(msg as ConversationOpenMessage);
         } else if (msg.type === "conversation.statusChanged") {
           opts.onConversationStatusChanged?.(msg as ConversationStatusChangedMessage);
+        } else if (msg.type === "conversation.tabGroupChanged") {
+          opts.onConversationTabGroupChanged?.(msg as ConversationTabGroupChangedMessage);
         } else if (msg.type === "conversation.compacted") {
           opts.onConversationCompacted?.(msg as ConversationCompactedMessage);
         } else if (msg.type === "extensions-changed") {
diff --git a/src/adapters/ws/logic.test.ts b/src/adapters/ws/logic.test.ts
index 8fe2b5f..2179ad0 100644
--- a/src/adapters/ws/logic.test.ts
+++ b/src/adapters/ws/logic.test.ts
@@ -502,3 +502,63 @@ describe("parseServerMessage — layout (plan 02)", () => {
     expect(parseServerMessage(JSON.stringify({ type: "layout.unknown" }))).toBeNull();
   });
 });
+
+describe("parseServerMessage — subagent tab groups", () => {
+  it("forwards the effective tabGroupParentId on conversation.open", () => {
+    const result = parseServerMessage(
+      JSON.stringify({
+        type: "conversation.open",
+        conversationId: "child",
+        workspaceId: "w1",
+        tabGroupParentId: "parent",
+      }),
+    );
+    expect(result).toEqual({
+      type: "conversation.open",
+      conversationId: "child",
+      workspaceId: "w1",
+      tabGroupParentId: "parent",
+    });
+  });
+
+  it("omits a non-string/empty tabGroupParentId on conversation.open", () => {
+    for (const tabGroupParentId of ["", 42, null]) {
+      const result = parseServerMessage(
+        JSON.stringify({
+          type: "conversation.open",
+          conversationId: "c1",
+          workspaceId: "w1",
+          tabGroupParentId,
+        }),
+      );
+      expect(result).toEqual({
+        type: "conversation.open",
+        conversationId: "c1",
+        workspaceId: "w1",
+      });
+    }
+  });
+
+  it("parses conversation.tabGroupChanged with a parent id and with null (ungrouped)", () => {
+    for (const tabGroupParentId of ["parent", null]) {
+      const msg = {
+        type: "conversation.tabGroupChanged",
+        conversationId: "c1",
+        workspaceId: "w1",
+        tabGroupParentId,
+      };
+      expect(parseServerMessage(JSON.stringify(msg))).toEqual(msg);
+    }
+  });
+
+  it("rejects a malformed conversation.tabGroupChanged", () => {
+    const base = { type: "conversation.tabGroupChanged", conversationId: "c1", workspaceId: "w1" };
+    expect(parseServerMessage(JSON.stringify(base))).toBeNull(); // missing (undefined) parent
+    expect(parseServerMessage(JSON.stringify({ ...base, tabGroupParentId: "" }))).toBeNull();
+    expect(parseServerMessage(JSON.stringify({ ...base, tabGroupParentId: 1 }))).toBeNull();
+    const { workspaceId: _w, ...noWorkspace } = base;
+    expect(
+      parseServerMessage(JSON.stringify({ ...noWorkspace, tabGroupParentId: "p" })),
+    ).toBeNull();
+  });
+});
diff --git a/src/adapters/ws/logic.ts b/src/adapters/ws/logic.ts
index 4dbd173..cecc66c 100644
--- a/src/adapters/ws/logic.ts
+++ b/src/adapters/ws/logic.ts
@@ -4,6 +4,7 @@ import type {
   ConversationCompactedMessage,
   ConversationOpenMessage,
   ConversationStatusChangedMessage,
+  ConversationTabGroupChangedMessage,
   ExtensionsChangedMessage,
   LayoutChangedMessage,
   TileLayoutChangedMessage,
@@ -27,6 +28,7 @@ const VALID_SERVER_TYPES = new Set([
   "conversation.open",
   "conversation.statusChanged",
   "conversation.compacted",
+  "conversation.tabGroupChanged",
   "extensions-changed",
   "layout",
   "tile-layout",
@@ -126,11 +128,30 @@ export function parseServerMessage(data: string): WsServerMessage | null {
       // it, so a client can stamp the opened tab with the conversation's ACTUAL
       // model (fix-toolcall-duplicate-keys plan Part 4).
       const model = parsed.model;
+      // The EFFECTIVE tab group parent, so a summoned subagent's tab lands in
+      // its summoner's group immediately.
+      const tabGroupParentId = parsed.tabGroupParentId;
       const msg: ConversationOpenMessage = {
         type: "conversation.open",
         conversationId: parsed.conversationId,
         workspaceId: parsed.workspaceId,
         ...(typeof model === "string" && model.length > 0 ? { model } : {}),
+        ...(typeof tabGroupParentId === "string" && tabGroupParentId.length > 0
+          ? { tabGroupParentId }
+          : {}),
+      };
+      return msg;
+    }
+    case "conversation.tabGroupChanged": {
+      if (typeof parsed.conversationId !== "string") return null;
+      if (typeof parsed.workspaceId !== "string") return null;
+      const parent = parsed.tabGroupParentId;
+      if (parent !== null && (typeof parent !== "string" || parent.length === 0)) return null;
+      const msg: ConversationTabGroupChangedMessage = {
+        type: "conversation.tabGroupChanged",
+        conversationId: parsed.conversationId,
+        workspaceId: parsed.workspaceId,
+        tabGroupParentId: parent,
       };
       return msg;
     }
diff --git a/src/app/store.svelte.ts b/src/app/store.svelte.ts
index 0a9cbd1..a3bc226 100644
--- a/src/app/store.svelte.ts
+++ b/src/app/store.svelte.ts
@@ -12,6 +12,7 @@ import type {
   ConversationMetricsResponse,
   ConversationOpenMessage,
   ConversationStatusChangedMessage,
+  ConversationTabGroupChangedMessage,
   CwdResponse,
   ExtensionsChangedMessage,
   LayoutChangedMessage,
@@ -28,11 +29,13 @@ import type {
   SetModelRequest,
   SetReasoningEffortRequest,
   SetSystemPromptTemplateRequest,
+  SetTabGroupRequest,
   SetTitleRequest,
   SetVisionSettingsRequest,
   SystemPromptTemplateResponse,
   SystemPromptVariable,
   SystemPromptVariablesResponse,
+  TabGroupResponse,
   TestComputerResponse,
   TileLayoutChangedMessage,
   WarmRequest,
@@ -269,6 +272,13 @@ export interface AppStore {
   selectTab(conversationId: string): void;
   closeTab(conversationId: string): void;
   renameTab(conversationId: string, title: string): void;
+  /**
+   * Move a tab into a subagent group (the tab group parent's id) or out of
+   * any group (null). Purely presentational (NOT lineage): applied
+   * optimistically, persisted via `PUT /conversations/:id/tab-group` (synced
+   * to other clients by `conversation.tabGroupChanged`), reverted on failure.
+   */
+  setTabGroup(conversationId: string, parentId: string | null): void;
   invoke(surfaceId: string, actionId: string, payload?: unknown): void;
   /**
    * Explicitly (re-)subscribe one surface, scoped to `conversationId` (or the
@@ -1351,7 +1361,12 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
    * stamped with the conversation's actual `workspaceId`, NOT the viewer's
    * currently active workspace.
    */
-  function openConversation(conversationId: string, workspaceId: string, model?: string): void {
+  function openConversation(
+    conversationId: string,
+    workspaceId: string,
+    model?: string,
+    tabGroupParentId?: string,
+  ): void {
     if (chatStores.has(conversationId)) return;
     // Stamp the tab with the conversation's PERSISTED model when the open
     // broadcast carries it (falling back to the current global selection) —
@@ -1367,6 +1382,7 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
       model: tabModel,
       title: "Conversation",
       workspaceId,
+      ...(tabGroupParentId !== undefined ? { tabGroupParentId } : {}),
     });
   }
 
@@ -1465,6 +1481,9 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
             model,
             title: conv.title,
             workspaceId: conv.workspaceId,
+            ...(conv.tabGroupParentId !== undefined
+              ? { tabGroupParentId: conv.tabGroupParentId }
+              : {}),
           });
         } else {
           // Already open — reconcile title + workspace from the authoritative
@@ -1473,12 +1492,14 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
           // authoritative conversation record is the source of truth, so a
           // differing tab workspace is corrected here.
           tabsStore.setTitle(conv.id, conv.title);
-          if (
-            tabsStore.tabs.find((t) => t.conversationId === conv.id)?.workspaceId !==
-            conv.workspaceId
-          ) {
+          const tab = tabsStore.tabs.find((t) => t.conversationId === conv.id);
+          if (tab?.workspaceId !== conv.workspaceId) {
             tabsStore.setWorkspaceId(conv.id, conv.workspaceId);
           }
+          // The backend's EFFECTIVE tab group parent is authoritative too.
+          if (tab?.tabGroupParentId !== conv.tabGroupParentId) {
+            tabsStore.setTabGroupParent(conv.id, conv.tabGroupParentId ?? null);
+          }
         }
       }
 
@@ -1533,7 +1554,12 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
     onMessage: handleServerMessage,
     onChat: handleChatMessage,
     onConversationOpen(msg: ConversationOpenMessage): void {
-      openConversation(msg.conversationId, msg.workspaceId, msg.model);
+      openConversation(msg.conversationId, msg.workspaceId, msg.model, msg.tabGroupParentId);
+    },
+    onConversationTabGroupChanged(msg: ConversationTabGroupChangedMessage): void {
+      // Another client (or this one, echoed) regrouped a tab — apply the
+      // authoritative EFFECTIVE value. Unknown tabs are a reducer no-op.
+      tabsStore.setTabGroupParent(msg.conversationId, msg.tabGroupParentId);
     },
     onConversationStatusChanged(msg: ConversationStatusChangedMessage): void {
       const { conversationId, status, workspaceId } = msg;
@@ -2003,6 +2029,34 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
       });
     },
 
+    setTabGroup(conversationId: string, parentId: string | null): void {
+      const previous =
+        tabsStore.tabs.find((t) => t.conversationId === conversationId)?.tabGroupParentId ?? null;
+      tabsStore.setTabGroupParent(conversationId, parentId);
+      void (async () => {
+        try {
+          const res = await fetchImpl(
+            `${httpBase}/conversations/${encodeURIComponent(conversationId)}/tab-group`,
+            {
+              method: "PUT",
+              headers: { "Content-Type": "application/json" },
+              body: JSON.stringify({ parentId } satisfies SetTabGroupRequest),
+            },
+          );
+          if (!res.ok) {
+            const err = (await res.json().catch(() => null)) as { error?: string } | null;
+            throw new Error(err?.error ?? `HTTP ${res.status}`);
+          }
+          const body = (await res.json()) as TabGroupResponse;
+          tabsStore.setTabGroupParent(conversationId, body.tabGroupParentId);
+        } catch (err) {
+          // Revert the optimistic move, then surface the failure.
+          tabsStore.setTabGroupParent(conversationId, previous);
+          reportError("Failed to update the tab's subagent group", err);
+        }
+      })();
+    },
+
     invoke(surfaceId: string, actionId: string, payload?: unknown): void {
       invokeSurfaceOp(surfaceId, actionId, payload);
     },
diff --git a/src/app/views/TabsPane.svelte b/src/app/views/TabsPane.svelte
index f87b086..4547e06 100644
--- a/src/app/views/TabsPane.svelte
+++ b/src/app/views/TabsPane.svelte
@@ -20,5 +20,6 @@
     onClose={(id) => store.closeTab(id)}
     onNewDraft={() => store.newDraft()}
     onRename={(id, title) => store.renameTab(id, title)}
+    onSetTabGroup={(id, parentId) => store.setTabGroup(id, parentId)}
   />
 {/key}
diff --git a/src/core/wire/conformance.ts b/src/core/wire/conformance.ts
index b358641..855f7ea 100644
--- a/src/core/wire/conformance.ts
+++ b/src/core/wire/conformance.ts
@@ -91,6 +91,8 @@ export function assertWsServerMessageExhaustive(msg: WsServerMessage): string {
       return "conversation.statusChanged";
     case "conversation.compacted":
       return "conversation.compacted";
+    case "conversation.tabGroupChanged":
+      return "conversation.tabGroupChanged";
     case "extensions-changed":
       return "extensions-changed";
     case "layout":
diff --git a/src/features/tabs/index.ts b/src/features/tabs/index.ts
index 7520215..a0c53f8 100644
--- a/src/features/tabs/index.ts
+++ b/src/features/tabs/index.ts
@@ -1,3 +1,13 @@
+export type { TabGroupForest, TabGroupNode } from "./tab-groups";
+export {
+  buildTabGroups,
+  directGroupChildren,
+  groupBleedPx,
+  groupDescendants,
+  groupGutterPx,
+  groupParentCandidates,
+  visibleGroupParents,
+} from "./tab-groups";
 export type { Tab, TabsState } from "./tabs";
 export {
   activeTab,
@@ -9,6 +19,7 @@ export {
   newDraft,
   selectTab,
   setModel,
+  setTabGroupParent,
   setTitle,
   shortHandle,
 } from "./tabs";
diff --git a/src/features/tabs/tab-groups.test.ts b/src/features/tabs/tab-groups.test.ts
new file mode 100644
index 0000000..ca2492c
--- /dev/null
+++ b/src/features/tabs/tab-groups.test.ts
@@ -0,0 +1,124 @@
+import { describe, expect, it } from "vitest";
+import {
+  buildTabGroups,
+  directGroupChildren,
+  groupBleedPx,
+  groupDescendants,
+  groupGutterPx,
+  groupParentCandidates,
+  visibleGroupParents,
+} from "./tab-groups";
+import type { Tab } from "./tabs";
+
+function tab(id: string, parent?: string): Tab {
+  return {
+    conversationId: id,
+    model: "m",
+    title: id.toUpperCase(),
+    workspaceId: "w",
+    ...(parent !== undefined ? { tabGroupParentId: parent } : {}),
+  };
+}
+
+const ids = (tabs: readonly Tab[]): string[] => tabs.map((t) => t.conversationId);
+
+describe("visibleGroupParents", () => {
+  it("keeps a parent that is another open tab", () => {
+    const parents = visibleGroupParents([tab("a"), tab("b", "a")]);
+    expect(parents.get("a")).toBeNull();
+    expect(parents.get("b")).toBe("a");
+  });
+
+  it("drops a parent that is not open (closed / other workspace) or is the tab itself", () => {
+    const parents = visibleGroupParents([tab("a", "gone"), tab("b", "b")]);
+    expect(parents.get("a")).toBeNull();
+    expect(parents.get("b")).toBeNull();
+  });
+
+  it("treats every member of a cycle as a root, but keeps hangers-on below them", () => {
+    const parents = visibleGroupParents([tab("a", "b"), tab("b", "a"), tab("c", "a")]);
+    expect(parents.get("a")).toBeNull();
+    expect(parents.get("b")).toBeNull();
+    expect(parents.get("c")).toBe("a");
+  });
+});
+
+describe("buildTabGroups", () => {
+  it("returns flat roots and depth 0 when nothing is grouped", () => {
+    const forest = buildTabGroups([tab("a"), tab("b"), tab("c")]);
+    expect(forest.maxDepth).toBe(0);
+    expect(forest.roots.map((n) => n.tab.conversationId)).toEqual(["a", "b", "c"]);
+    expect(forest.roots.every((n) => n.children.length === 0)).toBe(true);
+  });
+
+  it("nests children under their parent in tab order, preserving root order", () => {
+    // Child listed BEFORE its parent still nests under it.
+    const forest = buildTabGroups([tab("x", "p"), tab("a"), tab("p"), tab("y", "p")]);
+    expect(forest.maxDepth).toBe(1);
+    expect(forest.roots.map((n) => n.tab.conversationId)).toEqual(["a", "p"]);
+    const p = forest.roots[1];
+    expect(p?.children.map((n) => n.tab.conversationId)).toEqual(["x", "y"]);
+  });
+
+  it("supports nested groups (subagents with subagents) and reports the depth", () => {
+    const forest = buildTabGroups([tab("p"), tab("c", "p"), tab("g", "c"), tab("gg", "g")]);
+    expect(forest.maxDepth).toBe(3);
+    const c = forest.roots[0]?.children[0];
+    expect(c?.tab.conversationId).toBe("c");
+    expect(c?.children[0]?.children[0]?.tab.conversationId).toBe("gg");
+  });
+
+  it("renders every tab exactly once", () => {
+    const tabs = [tab("a", "b"), tab("b", "a"), tab("c", "a"), tab("d", "nope"), tab("e", "c")];
+    const seen: string[] = [];
+    const walk = (nodes: ReturnType<typeof buildTabGroups>["roots"]): void => {
+      for (const n of nodes) {
+        seen.push(n.tab.conversationId);
+        walk(n.children);
+      }
+    };
+    walk(buildTabGroups(tabs).roots);
+    expect(seen.sort()).toEqual(["a", "b", "c", "d", "e"]);
+  });
+});
+
+describe("groupDescendants / directGroupChildren / groupParentCandidates", () => {
+  const tabs = [tab("p"), tab("c1", "p"), tab("c2", "p"), tab("g", "c1"), tab("solo")];
+
+  it("collects transitive descendants", () => {
+    expect([...groupDescendants(tabs, "p")].sort()).toEqual(["c1", "c2", "g"]);
+    expect([...groupDescendants(tabs, "c1")]).toEqual(["g"]);
+    expect(groupDescendants(tabs, "solo").size).toBe(0);
+  });
+
+  it("lists direct children only", () => {
+    expect(directGroupChildren(tabs, "p")).toEqual(["c1", "c2"]);
+  });
+
+  it("offers every tab except self, descendants (cycle) and the current parent", () => {
+    expect(ids(groupParentCandidates(tabs, "c1"))).toEqual(["c2", "solo"]);
+    expect(ids(groupParentCandidates(tabs, "p"))).toEqual(["solo"]);
+    expect(ids(groupParentCandidates(tabs, "solo"))).toEqual(["p", "c1", "c2", "g"]);
+  });
+});
+
+describe("groupBleedPx / groupGutterPx", () => {
+  it("is zero without groups", () => {
+    expect(groupGutterPx(0)).toBe(0);
+    expect(groupBleedPx(1, 0)).toBe(0);
+  });
+
+  it("bleeds outer boxes further than inner ones, the outermost filling the gutter", () => {
+    const max = 3;
+    const bleeds = [1, 2, 3].map((l) => groupBleedPx(l, max));
+    expect(bleeds[0]).toBe(groupGutterPx(max));
+    expect(bleeds[0]).toBeGreaterThan(bleeds[1] ?? 0);
+    expect(bleeds[1]).toBeGreaterThan(bleeds[2] ?? 0);
+    expect(bleeds[2]).toBeGreaterThan(0);
+  });
+
+  it("keeps the gutter bounded for deep nesting", () => {
+    expect(groupGutterPx(12)).toBeLessThanOrEqual(24);
+    expect(groupBleedPx(12, 12)).toBeGreaterThan(0);
+  });
+});
diff --git a/src/features/tabs/tab-groups.ts b/src/features/tabs/tab-groups.ts
new file mode 100644
index 0000000..ef16451
--- /dev/null
+++ b/src/features/tabs/tab-groups.ts
@@ -0,0 +1,155 @@
+import type { Tab } from "./tabs";
+
+/**
+ * Subagent tab groups — the PURE model behind the tab grid.
+ *
+ * Each tab may name a tab group parent (`Tab.tabGroupParentId`, the effective
+ * value from the backend). The tab grid renders the tabs as a forest: a tab
+ * with ≥1 visible child is a GROUP (double-width card + a darker box wrapping
+ * it and its children); everything else is a single half-width card. Groups
+ * nest (subagents can summon subagents). Purely presentational.
+ */
+
+export interface TabGroupNode {
+  readonly tab: Tab;
+  /** Visible children, in tab order. Non-empty ⇒ this node renders as a group. */
+  readonly children: readonly TabGroupNode[];
+}
+
+export interface TabGroupForest {
+  /** Top-level nodes, in tab order. */
+  readonly roots: readonly TabGroupNode[];
+  /** Deepest group-box nesting level (0 = no groups, 1 = only top-level groups, …). */
+  readonly maxDepth: number;
+}
+
+/**
+ * The VISIBLE tab group parent of every tab in `tabs`: its `tabGroupParentId`
+ * when that names ANOTHER tab in the same set, else null. A tab whose
+ * ancestor chain loops back to itself (a cycle — never produced by the
+ * backend, but tolerated) is treated as a root, so rendering never loops.
+ */
+export function visibleGroupParents(tabs: readonly Tab[]): ReadonlyMap<string, string | null> {
+  const byId = new Map(tabs.map((t) => [t.conversationId, t]));
+  const direct = (id: string): string | null => {
+    const p = byId.get(id)?.tabGroupParentId;
+    return p !== undefined && p !== id && byId.has(p) ? p : null;
+  };
+  const out = new Map<string, string | null>();
+  for (const tab of tabs) {
+    const id = tab.conversationId;
+    const parent = direct(id);
+    let cyclic = false;
+    const seen = new Set<string>([id]);
+    let cursor = parent;
+    while (cursor !== null) {
+      if (cursor === id) {
+        cyclic = true;
+        break;
+      }
+      if (seen.has(cursor)) break; // a loop above us that doesn't include us
+      seen.add(cursor);
+      cursor = direct(cursor);
+    }
+    out.set(id, cyclic ? null : parent);
+  }
+  return out;
+}
+
+/** Build the group forest for one workspace's tab set (input order preserved). */
+export function buildTabGroups(tabs: readonly Tab[]): TabGroupForest {
+  const parents = visibleGroupParents(tabs);
+  const childIds = new Map<string, string[]>();
+  for (const tab of tabs) {
+    const p = parents.get(tab.conversationId) ?? null;
+    if (p === null) continue;
+    const list = childIds.get(p);
+    if (list === undefined) childIds.set(p, [tab.conversationId]);
+    else list.push(tab.conversationId);
+  }
+  const byId = new Map(tabs.map((t) => [t.conversationId, t]));
+  let maxDepth = 0;
+  const build = (tab: Tab, depth: number): TabGroupNode => {
+    const ids = childIds.get(tab.conversationId) ?? [];
+    if (ids.length > 0) maxDepth = Math.max(maxDepth, depth + 1);
+    const children: TabGroupNode[] = [];
+    for (const cid of ids) {
+      const child = byId.get(cid);
+      if (child !== undefined) children.push(build(child, depth + 1));
+    }
+    return { tab, children };
+  };
+  const roots = tabs
+    .filter((t) => (parents.get(t.conversationId) ?? null) === null)
+    .map((t) => build(t, 0));
+  return { roots, maxDepth };
+}
+
+/** Every (transitive) descendant id of `conversationId` in the VISIBLE grouping. */
+export function groupDescendants(tabs: readonly Tab[], conversationId: string): Set<string> {
+  const parents = visibleGroupParents(tabs);
+  const out = new Set<string>();
+  let grew = true;
+  while (grew) {
+    grew = false;
+    for (const [id, p] of parents) {
+      if (out.has(id) || id === conversationId || p === null) continue;
+      if (p === conversationId || out.has(p)) {
+        out.add(id);
+        grew = true;
+      }
+    }
+  }
+  return out;
+}
+
+/**
+ * The tabs `conversationId` may be moved under (the "Add to group" menu):
+ * every other tab EXCEPT itself, its descendants (that would form a cycle),
+ * and its current visible group parent (already there). Tab order preserved.
+ */
+export function groupParentCandidates(tabs: readonly Tab[], conversationId: string): Tab[] {
+  const parents = visibleGroupParents(tabs);
+  const current = parents.get(conversationId) ?? null;
+  const descendants = groupDescendants(tabs, conversationId);
+  return tabs.filter(
+    (t) =>
+      t.conversationId !== conversationId &&
+      t.conversationId !== current &&
+      !descendants.has(t.conversationId),
+  );
+}
+
+/** Ids of the tabs DIRECTLY grouped under `conversationId` (visible grouping). */
+export function directGroupChildren(tabs: readonly Tab[], conversationId: string): string[] {
+  const parents = visibleGroupParents(tabs);
+  return tabs
+    .filter((t) => parents.get(t.conversationId) === conversationId)
+    .map((t) => t.conversationId);
+}
+
+/**
+ * Horizontal bleed (px) of a group box at nesting `level` (1 = outermost)
+ * when the deepest box is `maxDepth`. A box bleeds OUTWARD by this much (a
+ * negative margin + equal padding), so the cards inside every box sit on
+ * exactly the same two grid columns as top-level cards — all cards keep
+ * IDENTICAL dimensions at every depth. Outer boxes bleed more than inner
+ * ones so nested edges stay visible. The step shrinks for deep nesting so the
+ * total gutter (`groupGutterPx`) stays bounded.
+ */
+export function groupBleedPx(level: number, maxDepth: number): number {
+  if (maxDepth <= 0 || level < 1 || level > maxDepth) return 0;
+  return (maxDepth - level + 1) * groupStepPx(maxDepth);
+}
+
+/** The horizontal gutter the grid must reserve so the outermost box fits. */
+export function groupGutterPx(maxDepth: number): number {
+  return maxDepth <= 0 ? 0 : maxDepth * groupStepPx(maxDepth);
+}
+
+const MAX_STEP_PX = 6;
+const MAX_GUTTER_PX = 18;
+
+function groupStepPx(maxDepth: number): number {
+  return Math.max(2, Math.min(MAX_STEP_PX, Math.floor(MAX_GUTTER_PX / maxDepth)));
+}
diff --git a/src/features/tabs/tabs-store.svelte.ts b/src/features/tabs/tabs-store.svelte.ts
index 5ed08d4..8e63171 100644
--- a/src/features/tabs/tabs-store.svelte.ts
+++ b/src/features/tabs/tabs-store.svelte.ts
@@ -9,6 +9,7 @@ import {
   selectTab as reduceSelectTab,
   setModel as reduceSetModel,
   setSnapshot as reduceSetSnapshot,
+  setTabGroupParent as reduceSetTabGroupParent,
   setTitle as reduceSetTitle,
   setWorkspaceId as reduceSetWorkspaceId,
   activeTab as selectActiveTab,
@@ -37,6 +38,8 @@ export interface TabsStore {
   setSnapshot(conversationId: string, snapshot: NamedAgentSelectionSnapshot | null): void;
   setTitle(conversationId: string, title: string): void;
   setWorkspaceId(conversationId: string, workspaceId: string): void;
+  /** Set (string) or clear (null) a tab's tab group parent (visual subagent group). */
+  setTabGroupParent(conversationId: string, parentId: string | null): void;
 }
 
 export function createTabsStore(storage: TabsStorage): TabsStore {
@@ -84,5 +87,8 @@ export function createTabsStore(storage: TabsStorage): TabsStore {
     setWorkspaceId(conversationId: string, workspaceId: string): void {
       apply(reduceSetWorkspaceId(state, conversationId, workspaceId));
     },
+    setTabGroupParent(conversationId: string, parentId: string | null): void {
+      apply(reduceSetTabGroupParent(state, conversationId, parentId));
+    },
   };
 }
diff --git a/src/features/tabs/tabs.test.ts b/src/features/tabs/tabs.test.ts
index ec93076..696e994 100644
--- a/src/features/tabs/tabs.test.ts
+++ b/src/features/tabs/tabs.test.ts
@@ -10,6 +10,7 @@ import {
   newDraft,
   selectTab,
   setModel,
+  setTabGroupParent,
   setTitle,
   shortHandle,
 } from "./tabs";
@@ -223,3 +224,31 @@ describe("shortHandle", () => {
     expect(shortHandle("abcd", ["abcd", "abcd1234"])).toBe("abcd");
   });
 });
+
+describe("setTabGroupParent", () => {
+  const base: TabsState = {
+    tabs: [
+      { conversationId: "a", model: "m", title: "A", workspaceId: "w" },
+      { conversationId: "b", model: "m", title: "B", workspaceId: "w" },
+    ],
+    activeConversationId: "a",
+  };
+
+  it("sets the tab group parent on the named tab only", () => {
+    const next = setTabGroupParent(base, "b", "a");
+    expect(next.tabs[1]?.tabGroupParentId).toBe("a");
+    expect(next.tabs[0]).toBe(base.tabs[0]);
+    expect(next.activeConversationId).toBe("a");
+  });
+
+  it("clears the key entirely with null", () => {
+    const grouped = setTabGroupParent(base, "b", "a");
+    const cleared = setTabGroupParent(grouped, "b", null);
+    expect(cleared.tabs[1]).toEqual(base.tabs[1]);
+    expect("tabGroupParentId" in (cleared.tabs[1] ?? {})).toBe(false);
+  });
+
+  it("is a no-op for an unknown tab", () => {
+    expect(setTabGroupParent(base, "zz", "a").tabs).toEqual(base.tabs);
+  });
+});
diff --git a/src/features/tabs/tabs.ts b/src/features/tabs/tabs.ts
index ff7bb4f..b2dad7d 100644
--- a/src/features/tabs/tabs.ts
+++ b/src/features/tabs/tabs.ts
@@ -13,6 +13,13 @@ export interface Tab {
    * renamed/removed. Absent for a raw model selection.
    */
   readonly snapshot?: NamedAgentSelectionSnapshot;
+  /**
+   * The EFFECTIVE tab group parent (backend `tabGroupParentId`): the
+   * conversation whose subagent group this tab is visually nested under in
+   * the tab grid. Purely presentational — distinct from the lineage
+   * `parentConversationId`. Absent = ungrouped (a top-level tab).
+   */
+  readonly tabGroupParentId?: string;
 }
 
 export interface TabsState {
@@ -120,6 +127,26 @@ export function setTitle(state: TabsState, conversationId: string, title: string
   return { tabs, activeConversationId: state.activeConversationId };
 }
 
+/**
+ * Set (string) or clear (null) a tab's tab group parent. Omits the key when
+ * cleared (`exactOptionalPropertyTypes`). No-op for an unknown tab.
+ */
+export function setTabGroupParent(
+  state: TabsState,
+  conversationId: string,
+  parentId: string | null,
+): TabsState {
+  const tabs = state.tabs.map((t) => {
+    if (t.conversationId !== conversationId) return t;
+    if (parentId !== null) return { ...t, tabGroupParentId: parentId };
+    if (!("tabGroupParentId" in t)) return t;
+    const { tabGroupParentId: _omit, ...rest } = t;
+    void _omit;
+    return rest;
+  });
+  return { tabs, activeConversationId: state.activeConversationId };
+}
+
 export function setWorkspaceId(
   state: TabsState,
   conversationId: string,
diff --git a/src/features/tabs/ui.test.ts b/src/features/tabs/ui.test.ts
index ae2a021..7c8b0db 100644
--- a/src/features/tabs/ui.test.ts
+++ b/src/features/tabs/ui.test.ts
@@ -165,7 +165,7 @@ describe("TabList", () => {
     expect(screen.getByText("7c2d")).toBeInTheDocument();
   });
 
-  it("renders each tab as a single vertical row (flex-col list, not a horizontal strip)", () => {
+  it("renders the tabs as a 2-column grid of same-size cards", () => {
     render(TabList, {
       props: {
         tabs: sampleTabs,
@@ -176,14 +176,17 @@ describe("TabList", () => {
       },
     });
 
-    // The scroll region containing the tab rows is a vertical flex column.
     const tabs = screen.getAllByRole("tab");
-    expect(tabs.length).toBeGreaterThan(0);
-    const region = tabs[0]?.parentElement;
-    expect(region).toHaveClass("flex-col");
+    const grid = tabs[0]?.parentElement;
+    expect(grid).toHaveClass("grid", "grid-cols-2");
+    // Every card has the same fixed height; none is double-width without a group.
+    for (const t of tabs) {
+      expect(t).toHaveClass("h-[4.5rem]");
+      expect(t).not.toHaveClass("col-span-2");
+    }
   });
 
-  it("fixes the tab list region at 40vh so a long set scrolls internally", () => {
+  it("fixes the tab grid region at 40vh so a long set scrolls internally", () => {
     render(TabList, {
       props: {
         tabs: sampleTabs,
@@ -195,7 +198,7 @@ describe("TabList", () => {
     });
 
     const tabs = screen.getAllByRole("tab");
-    const region = tabs[0]?.parentElement;
+    const region = tabs[0]?.parentElement?.parentElement;
     expect(region).toHaveClass("h-[40vh]");
     expect(region).toHaveClass("overflow-y-auto");
   });
@@ -295,4 +298,167 @@ describe("TabList", () => {
     // c3 has no status → no spinner at all.
     expect(idleTab.querySelector(".loading")).toBeNull();
   });
+
+  describe("subagent groups", () => {
+    const grouped: readonly Tab[] = [
+      { conversationId: "p", model: "m", title: "Parent", workspaceId: "w" },
+      {
+        conversationId: "c1",
+        model: "m",
+        title: "Child 1",
+        workspaceId: "w",
+        tabGroupParentId: "p",
+      },
+      {
+        conversationId: "c2",
+        model: "m",
+        title: "Child 2",
+        workspaceId: "w",
+        tabGroupParentId: "p",
+      },
+      {
+        conversationId: "g",
+        model: "m",
+        title: "Grandchild",
+        workspaceId: "w",
+        tabGroupParentId: "c1",
+      },
+      { conversationId: "solo", model: "m", title: "Solo", workspaceId: "w" },
+    ];
+
+    function renderGrouped(onSetTabGroup = vi.fn()) {
+      render(TabList, {
+        props: {
+          tabs: grouped,
+          activeConversationId: "p",
+          onSelect: vi.fn(),
+          onClose: vi.fn(),
+          onNewDraft: vi.fn(),
+          onSetTabGroup,
+        },
+      });
+      return onSetTabGroup;
+    }
+
+    const card = (id: string): HTMLElement => {
+      const el = document.querySelector<HTMLElement>(`[role="tab"][data-conversation-id="${id}"]`);
+      if (el === null) throw new Error(`card ${id} not found`);
+      return el;
+    };
+
+    it("renders a group parent double-width inside a darker group box with its subagents", () => {
+      renderGrouped();
+      const box = document.querySelector<HTMLElement>('[data-tab-group="p"]');
+      expect(box).not.toBeNull();
+      expect(box).toHaveClass("col-span-2");
+      expect(card("p")).toHaveClass("col-span-2");
+      expect(card("p")).toHaveTextContent("2 subagents");
+      expect(box?.contains(card("c1"))).toBe(true);
+      expect(box?.contains(card("c2"))).toBe(true);
+      expect(box?.contains(card("solo"))).toBe(false);
+      // Subagents are regular half-width cards.
+      expect(card("c2")).not.toHaveClass("col-span-2");
+    });
+
+    it("nests a subagent that has its own subagents in an inner box", () => {
+      renderGrouped();
+      const outer = document.querySelector<HTMLElement>('[data-tab-group="p"]');
+      const inner = document.querySelector<HTMLElement>('[data-tab-group="c1"]');
+      expect(inner).not.toBeNull();
+      expect(outer?.contains(inner)).toBe(true);
+      expect(inner?.dataset.level).toBe("2");
+      expect(card("c1")).toHaveClass("col-span-2");
+      expect(inner?.contains(card("g"))).toBe(true);
+      // Inner boxes bleed less than outer ones, so nested edges stay visible
+      // while the cards keep identical widths.
+      const pad = (el: HTMLElement | null) => el?.style.getPropertyValue("padding-inline") ?? "";
+      const bleed = (el: HTMLElement | null) => Number.parseFloat(pad(el));
+      expect(bleed(inner)).toBeGreaterThan(0);
+      expect(bleed(outer)).toBeGreaterThan(bleed(inner));
+      expect(outer?.style.getPropertyValue("margin-inline")).toBe(`-${pad(outer)}`);
+    });
+
+    it("right-click opens the menu; 'Remove from group' ungroups the tab", async () => {
+      const onSetTabGroup = renderGrouped();
+      await fireEvent.contextMenu(card("c2"), { clientX: 10, clientY: 10 });
+      const menu = screen.getByRole("menu");
+      expect(menu).toBeInTheDocument();
+      await fireEvent.click(screen.getByRole("menuitem", { name: /Remove from group/ }));
+      expect(onSetTabGroup).toHaveBeenCalledWith("c2", null);
+      expect(screen.queryByRole("menu")).toBeNull();
+    });
+
+    it("'Add to group' lists valid parents (no self/descendants/current) and moves the tab", async () => {
+      const onSetTabGroup = renderGrouped();
+      await fireEvent.contextMenu(card("c1"));
+      const candidates = [...document.querySelectorAll<HTMLElement>("[data-group-candidate]")].map(
+        (el) => el.dataset.groupCandidate,
+      );
+      expect(candidates).toEqual(["c2", "solo"]);
+      const solo = document.querySelector<HTMLElement>('[data-group-candidate="solo"]');
+      if (solo === null) throw new Error("solo candidate missing");
+      await fireEvent.click(solo);
+      expect(onSetTabGroup).toHaveBeenCalledWith("c1", "solo");
+    });
+
+    it("a group parent's menu can ungroup all of its direct subagents", async () => {
+      const onSetTabGroup = renderGrouped();
+      await fireEvent.contextMenu(card("p"));
+      expect(screen.queryByRole("menuitem", { name: /Remove from group/ })).toBeNull();
+      await fireEvent.click(screen.getByRole("menuitem", { name: /Ungroup 2 subagents/ }));
+      expect(onSetTabGroup).toHaveBeenCalledTimes(2);
+      expect(onSetTabGroup).toHaveBeenCalledWith("c1", null);
+      expect(onSetTabGroup).toHaveBeenCalledWith("c2", null);
+    });
+
+    it("Escape closes the menu without changing anything", async () => {
+      const onSetTabGroup = renderGrouped();
+      await fireEvent.contextMenu(card("solo"));
+      expect(screen.getByRole("menu")).toBeInTheDocument();
+      await fireEvent.keyDown(window, { key: "Escape" });
+      expect(screen.queryByRole("menu")).toBeNull();
+      expect(onSetTabGroup).not.toHaveBeenCalled();
+    });
+
+    it("touch-and-hold opens the menu and swallows the follow-up click", async () => {
+      vi.useFakeTimers();
+      try {
+        const onSelect = vi.fn();
+        render(TabList, {
+          props: {
+            tabs: grouped,
+            activeConversationId: "p",
+            onSelect,
+            onClose: vi.fn(),
+            onNewDraft: vi.fn(),
+            onSetTabGroup: vi.fn(),
+          },
+        });
+        await fireEvent.pointerDown(card("solo"), { pointerType: "touch", clientX: 5, clientY: 5 });
+        await vi.advanceTimersByTimeAsync(600);
+        expect(screen.getByRole("menu")).toBeInTheDocument();
+        await fireEvent.click(card("solo"));
+        expect(onSelect).not.toHaveBeenCalled();
+      } finally {
+        vi.useRealTimers();
+      }
+    });
+
+    it("a touch that moves away cancels the long-press", async () => {
+      vi.useFakeTimers();
+      try {
+        renderGrouped();
+        await fireEvent.pointerDown(card("solo"), { pointerType: "touch", clientX: 5, clientY: 5 });
+        await fireEvent.pointerMove(card("solo"), {
+          pointerType: "touch",
+          clientX: 5,
+          clientY: 60,
+        });
+        await vi.advanceTimersByTimeAsync(600);
+        expect(screen.queryByRole("menu")).toBeNull();
+      } finally {
+        vi.useRealTimers();
+      }
+    });
+  });
 });
diff --git a/src/features/tabs/ui/TabCard.svelte b/src/features/tabs/ui/TabCard.svelte
new file mode 100644
index 0000000..a95f8e6
--- /dev/null
+++ b/src/features/tabs/ui/TabCard.svelte
@@ -0,0 +1,232 @@
+<script lang="ts">
+  import type { Tab } from "../tabs";
+
+  /**
+   * One tab in the tab grid — a fixed-height card. Every card has IDENTICAL
+   * dimensions; a group parent (`wide`) simply spans both grid columns.
+   * Right-click (or touch-and-hold) asks the owner to open the tab menu.
+   */
+  let {
+    tab,
+    handle,
+    active,
+    status,
+    wide = false,
+    subagentCount = 0,
+    onSelect,
+    onClose,
+    onRename,
+    onMenu,
+  }: {
+    tab: Tab;
+    /** Short-handle (git-style unique prefix) shown in the id badge. */
+    handle: string;
+    active: boolean;
+    /** Lifecycle status (drives the spinner), when known. */
+    status: string | undefined;
+    /** Group parent: spans both grid columns. */
+    wide?: boolean;
+    /** Number of direct subagents (shown on a group parent). */
+    subagentCount?: number;
+    onSelect: (conversationId: string) => void;
+    onClose: (conversationId: string) => void;
+    onRename?: ((conversationId: string, title: string) => void) | undefined;
+    /** Open the tab menu at viewport coordinates. */
+    onMenu: (conversationId: string, x: number, y: number) => void;
+  } = $props();
+
+  // Inline rename: double-click the title to edit, Enter/blur to save.
+  let editing = $state(false);
+  let editValue = $state("");
+  let editEl = $state<HTMLInputElement>();
+
+  function startRename(): void {
+    if (onRename === undefined) return;
+    editing = true;
+    editValue = tab.title;
+    queueMicrotask(() => editEl?.focus());
+  }
+
+  function commitRename(): void {
+    if (!editing) return;
+    const trimmed = editValue.trim();
+    if (trimmed.length > 0 && onRename !== undefined) onRename(tab.conversationId, trimmed);
+    editing = false;
+  }
+
+  // Click-to-copy the FULL conversation id; the badge text is selected as the
+  // feedback (and as a manual-copy fallback when the clipboard is unavailable).
+  async function copyId(el: HTMLElement): Promise<void> {
+    const range = document.createRange();
+    range.selectNodeContents(el);
+    const selection = window.getSelection();
+    selection?.removeAllRanges();
+    selection?.addRange(range);
+    const clipboard = navigator.clipboard;
+    if (clipboard === undefined) return;
+    try {
+      await clipboard.writeText(tab.conversationId);
+    } catch {
+      // Selection already lets the user copy manually.
+    }
+  }
+
+  // Touch-and-hold → menu (iOS Safari never fires `contextmenu`; Android does,
+  // and opening twice at the same spot is idempotent).
+  const LONG_PRESS_MS = 500;
+  const MOVE_TOLERANCE_PX = 10;
+  let pressTimer: ReturnType<typeof setTimeout> | null = null;
+  let pressX = 0;
+  let pressY = 0;
+  let suppressClick = false;
+
+  function cancelPress(): void {
+    if (pressTimer !== null) clearTimeout(pressTimer);
+    pressTimer = null;
+  }
+
+  function onPointerDown(e: PointerEvent): void {
+    if (e.pointerType !== "touch") return;
+    cancelPress();
+    pressX = e.clientX;
+    pressY = e.clientY;
+    pressTimer = setTimeout(() => {
+      pressTimer = null;
+      suppressClick = true;
+      onMenu(tab.conversationId, pressX, pressY);
+    }, LONG_PRESS_MS);
+  }
+
+  function onPointerMove(e: PointerEvent): void {
+    if (pressTimer === null) return;
+    if (Math.hypot(e.clientX - pressX, e.clientY - pressY) > MOVE_TOLERANCE_PX) cancelPress();
+  }
+
+  $effect(() => cancelPress);
+</script>
+
+<div
+  class="tab-card group/card relative flex h-[4.5rem] min-w-0 cursor-pointer flex-col justify-between rounded-md border px-2 py-1.5 text-sm transition-colors {active
+    ? 'border-primary bg-primary/15'
+    : 'border-base-content/10 bg-base-300 hover:border-base-content/25'}"
+  class:col-span-2={wide}
+  role="tab"
+  tabindex="0"
+  aria-selected={active}
+  title={tab.title}
+  data-conversation-id={tab.conversationId}
+  data-wide={wide ? "true" : undefined}
+  onclick={() => {
+    if (suppressClick) {
+      suppressClick = false;
+      return;
+    }
+    onSelect(tab.conversationId);
+  }}
+  onkeydown={(e) => {
+    if (e.target !== e.currentTarget) return;
+    if (e.key === "Enter") onSelect(tab.conversationId);
+    else if (e.key === "ContextMenu" || (e.shiftKey && e.key === "F10")) {
+      e.preventDefault();
+      const r = e.currentTarget.getBoundingClientRect();
+      onMenu(tab.conversationId, r.left + 8, r.bottom);
+    }
+  }}
+  oncontextmenu={(e) => {
+    e.preventDefault();
+    cancelPress();
+    onMenu(tab.conversationId, e.clientX, e.clientY);
+  }}
+  onpointerdown={onPointerDown}
+  onpointermove={onPointerMove}
+  onpointerup={cancelPress}
+  onpointercancel={cancelPress}
+  onpointerleave={cancelPress}
+>
+  <div class="flex min-w-0 items-center gap-1">
+    <button
+      type="button"
+      class="shrink-0 select-text rounded bg-base-100 px-1 py-0.5 font-mono text-[10px] leading-none text-base-content/60 transition-colors hover:bg-primary hover:text-primary-content"
+      data-copy-id={tab.conversationId}
+      title="Click to copy conversation id"
+      aria-label={`Copy conversation id ${tab.conversationId}`}
+      onclick={(e) => {
+        e.stopPropagation();
+        void copyId(e.currentTarget);
+      }}
+    >
+      {handle}
+    </button>
+    {#if wide && subagentCount > 0}
+      <span
+        class="shrink-0 truncate rounded bg-base-100 px-1 py-0.5 text-[10px] leading-none text-base-content/60"
+        data-subagent-count={subagentCount}
+      >
+        {subagentCount}
+        {subagentCount === 1 ? "subagent" : "subagents"}
+      </span>
+    {/if}
+    <span class="flex-1"></span>
+    {#if status === "queued"}
+      <!-- Waiting for a concurrency slot — a ring (vs the dots of `active`). -->
+      <span
+        class="loading loading-ring loading-xs shrink-0 text-primary"
+        aria-label="Queued"
+        title="Waiting for a concurrency slot"
+      ></span>
+    {:else if status === "active"}
+      <span class="loading loading-dots loading-xs shrink-0 text-primary"></span>
+    {/if}
+    <button
+      type="button"
+      class="btn btn-ghost btn-xs -mr-1 h-5 min-h-5 w-5 shrink-0 p-0 text-base-content/60"
+      aria-label="Close tab"
+      onclick={(e) => {
+        e.stopPropagation();
+        onClose(tab.conversationId);
+      }}
+    >
+      &times;
+    </button>
+  </div>
+  {#if editing}
+    <input
+      bind:this={editEl}
+      bind:value={editValue}
+      class="w-full min-w-0 rounded bg-base-100 px-1 py-0.5 text-left text-xs outline outline-1 outline-primary"
+      onclick={(e) => e.stopPropagation()}
+      onkeydown={(e) => {
+        if (e.key === "Enter") {
+          e.preventDefault();
+          commitRename();
+        } else if (e.key === "Escape") {
+          e.preventDefault();
+          editing = false;
+        }
+      }}
+      onblur={commitRename}
+    />
+  {:else}
+    <span
+      class="line-clamp-2 min-w-0 break-words text-left text-xs leading-tight"
+      role="button"
+      tabindex="-1"
+      title={tab.title}
+      ondblclick={(e) => {
+        e.stopPropagation();
+        startRename();
+      }}
+    >
+      {tab.title}
+    </span>
+  {/if}
+</div>
+
+<style>
+  /* Touch-and-hold opens the tab menu — suppress the native callout/selection. */
+  .tab-card {
+    -webkit-touch-callout: none;
+    -webkit-user-select: none;
+    user-select: none;
+  }
+</style>
diff --git a/src/features/tabs/ui/TabGroupBox.svelte b/src/features/tabs/ui/TabGroupBox.svelte
new file mode 100644
index 0000000..5cbfb27
--- /dev/null
+++ b/src/features/tabs/ui/TabGroupBox.svelte
@@ -0,0 +1,53 @@
+<script lang="ts">
+  import type { Snippet } from "svelte";
+  import type { TabGroupNode } from "../tab-groups";
+  import { groupBleedPx } from "../tab-groups";
+  import type { Tab } from "../tabs";
+  import TabGroupBox from "./TabGroupBox.svelte";
+
+  /**
+   * A subagent group: a darker box spanning both grid columns that wraps the
+   * group parent's (double-width) card + its subagents' cards. Recursive —
+   * a subagent with its own subagents renders a nested box. The box BLEEDS
+   * outward (negative margin + equal padding, `groupBleedPx`) so the cards
+   * inside stay on the same two columns as top-level cards: every card keeps
+   * identical dimensions at every depth. The translucent black background
+   * compounds, so each nesting level reads darker than the one around it.
+   */
+  let {
+    node,
+    level,
+    maxDepth,
+    card,
+  }: {
+    node: TabGroupNode;
+    /** Nesting level of THIS box (1 = outermost). */
+    level: number;
+    maxDepth: number;
+    /** Renders one card: (tab, wide, subagentCount). */
+    card: Snippet<[Tab, boolean, number]>;
+  } = $props();
+
+  const bleed = $derived(groupBleedPx(level, maxDepth));
+</script>
+
+<div
+  class="col-span-2 rounded-lg border border-black/50 bg-black/40 py-1.5 shadow-inner"
+  style:margin-inline="-{bleed}px"
+  style:padding-inline="{bleed}px"
+  role="group"
+  aria-label={`Subagents of ${node.tab.title}`}
+  data-tab-group={node.tab.conversationId}
+  data-level={level}
+>
+  <div class="grid grid-flow-row-dense grid-cols-2 gap-1.5">
+    {@render card(node.tab, true, node.children.length)}
+    {#each node.children as child (child.tab.conversationId)}
+      {#if child.children.length > 0}
+        <TabGroupBox node={child} level={level + 1} {maxDepth} {card} />
+      {:else}
+        {@render card(child.tab, false, 0)}
+      {/if}
+    {/each}
+  </div>
+</div>
diff --git a/src/features/tabs/ui/TabList.svelte b/src/features/tabs/ui/TabList.svelte
index 6d85065..06d5aa6 100644
--- a/src/features/tabs/ui/TabList.svelte
+++ b/src/features/tabs/ui/TabList.svelte
@@ -1,183 +1,149 @@
 <script lang="ts">
-	import type { Tab } from "../tabs";
-	import { shortHandle } from "../tabs";
+  import {
+    buildTabGroups,
+    directGroupChildren,
+    groupGutterPx,
+    groupParentCandidates,
+    visibleGroupParents,
+  } from "../tab-groups";
+  import type { Tab } from "../tabs";
+  import { shortHandle } from "../tabs";
+  import TabCard from "./TabCard.svelte";
+  import TabGroupBox from "./TabGroupBox.svelte";
+  import TabMenu from "./TabMenu.svelte";
 
-	let {
-		tabs,
-		activeConversationId,
-		statusFor,
-		onSelect,
-		onClose,
-		onNewDraft,
-		onRename,
-	}: {
-		tabs: readonly Tab[];
-		activeConversationId: string | null;
-		/** Returns the conversation's lifecycle status, or undefined when unknown. */
-		statusFor?: (conversationId: string) => string | undefined;
-		onSelect: (conversationId: string) => void;
-		onClose: (conversationId: string) => void;
-		onNewDraft: () => void;
-		onRename?: (conversationId: string, title: string) => void;
-	} = $props();
+  let {
+    tabs,
+    activeConversationId,
+    statusFor,
+    onSelect,
+    onClose,
+    onNewDraft,
+    onRename,
+    onSetTabGroup,
+  }: {
+    tabs: readonly Tab[];
+    activeConversationId: string | null;
+    /** Returns the conversation's lifecycle status, or undefined when unknown. */
+    statusFor?: (conversationId: string) => string | undefined;
+    onSelect: (conversationId: string) => void;
+    onClose: (conversationId: string) => void;
+    onNewDraft: () => void;
+    onRename?: (conversationId: string, title: string) => void;
+    /**
+     * Move a tab into a subagent group (parent id) or out of it (null). Purely
+     * presentational grouping. When absent, the tab menu is disabled.
+     */
+    onSetTabGroup?: (conversationId: string, parentId: string | null) => void;
+  } = $props();
 
-	// Git-style short handle (shortest unique prefix) per open tab — the visible
-	// "tab ID". Derived from the set of open conversation ids; pure helper.
-	const handles = $derived.by(() => {
-		const ids = tabs.map((t) => t.conversationId);
-		const map = new Map<string, string>();
-		for (const id of ids) map.set(id, shortHandle(id, ids));
-		return map;
-	});
+  // Git-style short handle (shortest unique prefix) per open tab — the visible
+  // "tab ID". Derived from the set of open conversation ids; pure helper.
+  const handles = $derived.by(() => {
+    const ids = tabs.map((t) => t.conversationId);
+    const map = new Map<string, string>();
+    for (const id of ids) map.set(id, shortHandle(id, ids));
+    return map;
+  });
+  const handleFor = (id: string): string => handles.get(id) ?? id;
 
-	// Inline rename: double-click a tab's title to edit, Enter/blur to save.
-	let editingId = $state<string | null>(null);
-	let editValue = $state("");
-	let editEl = $state<HTMLInputElement>();
+  // The subagent-group forest (pure) + the gutter the outermost box bleeds into.
+  const forest = $derived(buildTabGroups(tabs));
+  const gutter = $derived(groupGutterPx(forest.maxDepth));
 
-	function startRename(tab: Tab): void {
-		if (onRename === undefined) return;
-		editingId = tab.conversationId;
-		editValue = tab.title;
-		// Focus the input after it renders.
-		queueMicrotask(() => editEl?.focus());
-	}
+  // Tab menu (right-click / touch-and-hold). Local UI state owned here.
+  let menu = $state<{ conversationId: string; x: number; y: number } | null>(null);
+  const menuTab = $derived(
+    menu === null ? null : (tabs.find((t) => t.conversationId === menu?.conversationId) ?? null),
+  );
+  const menuGroupParent = $derived.by(() => {
+    if (menuTab === null) return null;
+    const pid = visibleGroupParents(tabs).get(menuTab.conversationId) ?? null;
+    return pid === null ? null : (tabs.find((t) => t.conversationId === pid) ?? null);
+  });
+  const menuChildren = $derived(
+    menuTab === null ? [] : directGroupChildren(tabs, menuTab.conversationId),
+  );
+  const menuCandidates = $derived(
+    menuTab === null ? [] : groupParentCandidates(tabs, menuTab.conversationId),
+  );
 
-	function commitRename(): void {
-		const id = editingId;
-		if (id !== null && onRename !== undefined) {
-			const trimmed = editValue.trim();
-			if (trimmed.length > 0) onRename(id, trimmed);
-		}
-		editingId = null;
-	}
-
-	function cancelRename(): void {
-		editingId = null;
-	}
-
-	// Click-to-copy the agent (conversation) id: clicking the ID badge copies the
-	// FULL conversationId to the clipboard (the stable, useful id — the badge
-	// only shows a short prefix) and highlights the badge text as feedback. The
-	// highlight (text selection) is the indicator — no text is swapped, so the
-	// badge keeps a stable width. If the clipboard write fails, the selection is
-	// already in place so the user can Ctrl+C the text manually.
-	async function copyId(conversationId: string, el: HTMLElement): Promise<void> {
-		selectText(el);
-		const clipboard = navigator.clipboard;
-		if (clipboard === undefined) return;
-		try {
-			await clipboard.writeText(conversationId);
-		} catch {
-			// Selection already lets the user copy manually.
-		}
-	}
-
-	function selectText(el: HTMLElement): void {
-		const range = document.createRange();
-		range.selectNodeContents(el);
-		const selection = window.getSelection();
-		selection?.removeAllRanges();
-		selection?.addRange(range);
-	}
+  function openMenu(conversationId: string, x: number, y: number): void {
+    if (onSetTabGroup === undefined) return;
+    menu = { conversationId, x, y };
+  }
 </script>
 
+{#snippet card(tab: Tab, wide: boolean, subagentCount: number)}
+  <TabCard
+    {tab}
+    handle={handleFor(tab.conversationId)}
+    active={tab.conversationId === activeConversationId}
+    status={statusFor?.(tab.conversationId)}
+    {wide}
+    {subagentCount}
+    {onSelect}
+    {onClose}
+    {onRename}
+    onMenu={openMenu}
+  />
+{/snippet}
+
 <div class="flex flex-col gap-2">
-	<!-- Single-column vertical tab list. Fixed at 40% of the viewport height so a
-	     long tab set scrolls inside this region instead of growing the whole sidebar. -->
-	<div class="flex h-[40vh] flex-col gap-1 overflow-y-auto pr-1">
-		{#each tabs as tab (tab.conversationId)}
-			<div
-				class="flex items-center gap-1.5 rounded px-2 py-1.5 text-sm hover:bg-base-300"
-				class:bg-base-300={tab.conversationId === activeConversationId}
-				role="tab"
-				tabindex="0"
-				aria-selected={tab.conversationId === activeConversationId}
-				title={tab.title}
-				onclick={() => onSelect(tab.conversationId)}
-				onkeydown={(e) => {
-					if (e.key === "Enter") onSelect(tab.conversationId);
-				}}
-			>
-				<button
-					type="button"
-					class="shrink-0 rounded bg-base-300 px-1 py-0.5 font-mono text-[10px] leading-none text-base-content/60 transition-colors hover:bg-primary hover:text-primary-content"
-					data-copy-id={tab.conversationId}
-					title="Click to copy conversation id"
-					aria-label={`Copy conversation id ${tab.conversationId}`}
-					onclick={(e) => {
-						e.stopPropagation();
-						void copyId(tab.conversationId, e.currentTarget);
-					}}
-				>
-					{handles.get(tab.conversationId) ?? tab.conversationId}
-				</button>
-				{#if editingId === tab.conversationId}
-					<input
-						bind:this={editEl}
-						bind:value={editValue}
-						class="min-w-0 flex-1 rounded bg-base-100 px-1 py-0.5 text-left text-sm outline outline-1 outline-primary"
-						onclick={(e) => e.stopPropagation()}
-						onkeydown={(e) => {
-							if (e.key === "Enter") {
-								e.preventDefault();
-								commitRename();
-							} else if (e.key === "Escape") {
-								e.preventDefault();
-								cancelRename();
-							}
-						}}
-						onblur={commitRename}
-					/>
-				{:else}
-					<span
-						class="min-w-0 flex-1 cursor-pointer truncate text-left"
-						role="button"
-						tabindex="-1"
-						title={tab.title}
-						ondblclick={(e) => {
-							e.stopPropagation();
-							startRename(tab);
-						}}
-					>
-						{tab.title}
-					</span>
-				{/if}
-				{#if statusFor?.(tab.conversationId) === "queued"}
-					<!-- Waiting for a concurrency slot — a ring (vs the dots of `active`). -->
-					<span
-						class="loading loading-ring loading-xs shrink-0 text-primary"
-						aria-label="Queued"
-						title="Waiting for a concurrency slot"
-					></span>
-				{:else if statusFor?.(tab.conversationId) === "active"}
-					<span class="loading loading-dots loading-xs shrink-0 text-primary"></span>
-				{/if}
-				<button
-					class="btn btn-ghost btn-xs shrink-0"
-					aria-label="Close tab"
-					onclick={(e) => {
-						e.stopPropagation();
-						onClose(tab.conversationId);
-					}}
-				>
-					&times;
-				</button>
-			</div>
-		{/each}
-	</div>
+  <!-- 2-column grid of same-size tab cards, fixed at 40% of the viewport height
+       so a long tab set scrolls inside this region instead of growing the
+       sidebar. Subagent groups span both columns; `dense` lets a later card
+       fill the half-row a group would otherwise leave empty. -->
+  <div
+    class="h-[40vh] overflow-y-auto py-0.5"
+    style:padding-inline="{gutter}px"
+    data-tab-grid
+    onscroll={() => (menu = null)}
+  >
+    <div class="grid grid-flow-row-dense grid-cols-2 gap-1.5" role="tablist">
+      {#each forest.roots as node (node.tab.conversationId)}
+        {#if node.children.length > 0}
+          <TabGroupBox {node} level={1} maxDepth={forest.maxDepth} {card} />
+        {:else}
+          {@render card(node.tab, false, 0)}
+        {/if}
+      {/each}
+    </div>
+  </div>
 
-	<button
-		type="button"
-		class="btn btn-ghost btn-sm w-full border border-base-300"
-		class:btn-primary={activeConversationId === null}
-		aria-label="New chat"
-		onclick={() => onNewDraft()}
-	>
-		{#if activeConversationId === null}
-			New Chat
-		{:else}
-			+ New chat
-		{/if}
-	</button>
+  <button
+    type="button"
+    class="btn btn-ghost btn-sm w-full border border-base-300"
+    class:btn-primary={activeConversationId === null}
+    aria-label="New chat"
+    onclick={() => onNewDraft()}
+  >
+    {#if activeConversationId === null}
+      New Chat
+    {:else}
+      + New chat
+    {/if}
+  </button>
 </div>
+
+{#if menu !== null && menuTab !== null && onSetTabGroup !== undefined}
+  {@const setGroup = onSetTabGroup}
+  {@const target = menuTab.conversationId}
+  {@const children = [...menuChildren]}
+  <TabMenu
+    tab={menuTab}
+    handle={handleFor(target)}
+    x={menu.x}
+    y={menu.y}
+    groupParent={menuGroupParent}
+    childCount={children.length}
+    candidates={menuCandidates}
+    {handleFor}
+    onAddToGroup={(parentId) => setGroup(target, parentId)}
+    onRemoveFromGroup={() => setGroup(target, null)}
+    onUngroupChildren={() => {
+      for (const id of children) setGroup(id, null);
+    }}
+    onDismiss={() => (menu = null)}
+  />
+{/if}
diff --git a/src/features/tabs/ui/TabMenu.svelte b/src/features/tabs/ui/TabMenu.svelte
new file mode 100644
index 0000000..513463f
--- /dev/null
+++ b/src/features/tabs/ui/TabMenu.svelte
@@ -0,0 +1,145 @@
+<script lang="ts">
+  import type { Tab } from "../tabs";
+
+  /**
+   * The tab context menu (right-click / touch-and-hold on a tab card): add the
+   * tab to a subagent group, remove it from its group, or release its own
+   * subagents. Purely presentational grouping. Fixed-positioned at the
+   * pointer, clamped into the viewport; closes on Escape / outside press /
+   * scroll / resize.
+   */
+  let {
+    tab,
+    handle,
+    x,
+    y,
+    groupParent,
+    childCount,
+    candidates,
+    handleFor,
+    onAddToGroup,
+    onRemoveFromGroup,
+    onUngroupChildren,
+    onDismiss,
+  }: {
+    tab: Tab;
+    handle: string;
+    /** Requested viewport position (pointer). */
+    x: number;
+    y: number;
+    /** The tab's current visible group parent, or null when top-level. */
+    groupParent: Tab | null;
+    /** Number of tabs directly grouped under this tab. */
+    childCount: number;
+    /** Tabs this tab may be moved under. */
+    candidates: readonly Tab[];
+    handleFor: (conversationId: string) => string;
+    onAddToGroup: (parentId: string) => void;
+    onRemoveFromGroup: () => void;
+    onUngroupChildren: () => void;
+    onDismiss: () => void;
+  } = $props();
+
+  let menuEl = $state<HTMLDivElement>();
+  let width = $state(0);
+  let height = $state(0);
+  const MARGIN = 8;
+
+  const left = $derived(
+    Math.max(MARGIN, Math.min(x, (globalThis.innerWidth ?? 0) - width - MARGIN)),
+  );
+  const top = $derived(
+    Math.max(MARGIN, Math.min(y, (globalThis.innerHeight ?? 0) - height - MARGIN)),
+  );
+
+  $effect(() => {
+    menuEl?.focus();
+  });
+
+  function act(fn: () => void): void {
+    fn();
+    onDismiss();
+  }
+</script>
+
+<svelte:window
+  onkeydown={(e) => {
+    if (e.key === "Escape") onDismiss();
+  }}
+  onpointerdown={(e) => {
+    if (menuEl !== undefined && !menuEl.contains(e.target as Node)) onDismiss();
+  }}
+  onresize={onDismiss}
+/>
+
+<div
+  bind:this={menuEl}
+  bind:clientWidth={width}
+  bind:clientHeight={height}
+  class="fixed z-50 flex w-64 max-w-[calc(100vw-1rem)] flex-col rounded-lg border border-base-content/15 bg-base-100 p-1 text-sm shadow-xl outline-none"
+  style:left="{left}px"
+  style:top="{top}px"
+  role="menu"
+  tabindex="-1"
+  aria-label={`Tab menu for ${tab.title}`}
+  oncontextmenu={(e) => e.preventDefault()}
+>
+  <div class="flex min-w-0 items-center gap-1.5 px-2 py-1 text-xs text-base-content/60">
+    <span class="shrink-0 rounded bg-base-300 px-1 py-0.5 font-mono text-[10px] leading-none">
+      {handle}
+    </span>
+    <span class="truncate">{tab.title}</span>
+  </div>
+
+  {#if groupParent !== null}
+    <button
+      type="button"
+      role="menuitem"
+      class="rounded px-2 py-1.5 text-left hover:bg-base-300"
+      onclick={() => act(onRemoveFromGroup)}
+    >
+      Remove from group
+      <span class="block truncate text-xs text-base-content/50">
+        Currently under {handleFor(groupParent.conversationId)} · {groupParent.title}
+      </span>
+    </button>
+  {/if}
+
+  {#if childCount > 0}
+    <button
+      type="button"
+      role="menuitem"
+      class="rounded px-2 py-1.5 text-left hover:bg-base-300"
+      onclick={() => act(onUngroupChildren)}
+    >
+      Ungroup {childCount}
+      {childCount === 1 ? "subagent" : "subagents"}
+    </button>
+  {/if}
+
+  <div class="mt-1 border-t border-base-content/10 px-2 pt-1.5 pb-1 text-xs text-base-content/60">
+    Add to group under…
+  </div>
+  {#if candidates.length === 0}
+    <div class="px-2 py-1.5 text-xs text-base-content/40">No other tabs</div>
+  {:else}
+    <div class="flex max-h-56 flex-col overflow-y-auto">
+      {#each candidates as candidate (candidate.conversationId)}
+        <button
+          type="button"
+          role="menuitem"
+          class="flex min-w-0 items-center gap-1.5 rounded px-2 py-1.5 text-left hover:bg-base-300"
+          data-group-candidate={candidate.conversationId}
+          onclick={() => act(() => onAddToGroup(candidate.conversationId))}
+        >
+          <span
+            class="shrink-0 rounded bg-base-300 px-1 py-0.5 font-mono text-[10px] leading-none text-base-content/60"
+          >
+            {handleFor(candidate.conversationId)}
+          </span>
+          <span class="truncate">{candidate.title}</span>
+        </button>
+      {/each}
+    </div>
+  {/if}
+</div>