diff --git a/.changeset/workspace-phase-a-u1-executor-session-scoping.md b/.changeset/workspace-phase-a-u1-executor-session-scoping.md
new file mode 100644
index 0000000000..6fc9911756
--- /dev/null
+++ b/.changeset/workspace-phase-a-u1-executor-session-scoping.md
@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": minor
+---
+
+Workspace mode Phase A (U1): executor session scoping. In workspace mode the executor now skips the root worktree acquisition and every rootDir git preflight (base-commit capture, contamination, worktree-liveness), runs the agent session rooted at the browse-only workspace root, and tracks acquired sub-repo worktrees as a per-task set. Single-repo tasks are unchanged (one-element set, byte-for-byte preflight parity).
diff --git a/packages/engine/src/__tests__/_workspace-fixture.ts b/packages/engine/src/__tests__/_workspace-fixture.ts
new file mode 100644
index 0000000000..5e94b78982
--- /dev/null
+++ b/packages/engine/src/__tests__/_workspace-fixture.ts
@@ -0,0 +1,66 @@
+/*
+FNXC:Workspace 2026-06-21-12:00:
+Shared REAL two-repo git fixture for workspace-mode engine tests (U1 + U2 + later phases). The foundation's executor-workspace test self-mocked the functions under test, which proves nothing; this harness instead builds genuine on-disk git repos under a NON-git workspace root so that any leaked rootDir git preflight actually fails. U2 and later units import `createWorkspaceFixture` directly — keep it dependency-light (only node:child_process + node:fs + saveWorkspaceConfig).
+
+A workspace root is a plain directory (NOT a git repo) containing N sub-repos. Each sub-repo is a real git repo with an initial commit on a default branch. `<root>/.fusion/workspace.json` lists the sub-repo relative paths so `loadWorkspaceConfig(root)` returns a populated config — the exact signal `this.workspaceConfig` keys off in the executor.
+*/
+import { execSync, spawnSync } from "node:child_process";
+import { mkdtempSync, mkdirSync, rmSync, writeFileSync } from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { saveWorkspaceConfig } from "@fusion/core";
+
+export const hasGit = spawnSync("git", ["--version"], { stdio: "pipe" }).status === 0;
+
+function git(repo: string, command: string): string {
+  return execSync(command, { cwd: repo, encoding: "utf-8", stdio: ["pipe", "pipe", "pipe"] }).trim();
+}
+
+/** Initialize a real git repo at `repoDir` with one commit on `defaultBranch`. */
+export function initRepoWithCommit(repoDir: string, defaultBranch = "main"): void {
+  mkdirSync(repoDir, { recursive: true });
+  git(repoDir, `git init -b ${defaultBranch}`);
+  git(repoDir, 'git config user.email "test@example.com"');
+  git(repoDir, 'git config user.name "Test"');
+  writeFileSync(path.join(repoDir, "README.md"), `# ${path.basename(repoDir)}\n`, "utf-8");
+  git(repoDir, "git add README.md");
+  git(repoDir, "git commit -m 'init'");
+}
+
+export interface WorkspaceFixture {
+  /** Absolute path to the non-git workspace root. */
+  rootDir: string;
+  /** Relative sub-repo paths (workspace.json `repos`). */
+  repos: string[];
+  /** Absolute path to a sub-repo by relative name. */
+  repoPath(rel: string): string;
+  /** Run a git command inside a sub-repo. */
+  git(rel: string, command: string): string;
+  /** Remove all on-disk fixture state. */
+  cleanup(): void;
+}
+
+/**
+ * Create a real two-repo (by default) workspace fixture on disk.
+ * - `rootDir` is a plain non-git directory.
+ * - Each `repos[i]` is a real git repo with an initial commit.
+ * - `<root>/.fusion/workspace.json` is written so loadWorkspaceConfig() resolves.
+ */
+export async function createWorkspaceFixture(
+  repos: string[] = ["repo-a", "repo-b"],
+  defaultBranch = "main",
+): Promise<WorkspaceFixture> {
+  const rootDir = mkdtempSync(path.join(os.tmpdir(), "fusion-workspace-"));
+  for (const rel of repos) {
+    initRepoWithCommit(path.join(rootDir, rel), defaultBranch);
+  }
+  await saveWorkspaceConfig(rootDir, { repos });
+
+  return {
+    rootDir,
+    repos,
+    repoPath: (rel: string) => path.join(rootDir, rel),
+    git: (rel: string, command: string) => git(path.join(rootDir, rel), command),
+    cleanup: () => rmSync(rootDir, { recursive: true, force: true }),
+  };
+}
diff --git a/packages/engine/src/__tests__/executor-paused-abort-todo-benign.test.ts b/packages/engine/src/__tests__/executor-paused-abort-todo-benign.test.ts
index 351ee343bf..a8237d0755 100644
--- a/packages/engine/src/__tests__/executor-paused-abort-todo-benign.test.ts
+++ b/packages/engine/src/__tests__/executor-paused-abort-todo-benign.test.ts
@@ -88,7 +88,7 @@ describe("pause-abort benign requeue-to-todo (FN-6782)", () => {
     // executor must retry the agent session in place rather than bouncing the
     // task through todo (and must not fire a failure notification).
     const { store, task, executor } = makeHarness({ column: "todo" });
-    (executor as any).activeWorktrees.set(task.id, task.worktree);
+    (executor as any).addActiveWorktree(task.id, task.worktree);
     const executeSpy = vi
       .spyOn(executor as any, "execute")
       .mockResolvedValue(undefined);
@@ -137,7 +137,7 @@ describe("pause-abort benign requeue-to-todo (FN-6782)", () => {
       // and a retry scheduled); the task then changes state before the timer
       // fires, and the fire-time re-fetch must abort the dispatch.
       const { store, task, executor } = makeHarness({ column: "todo" });
-      (executor as any).activeWorktrees.set(task.id, task.worktree);
+      (executor as any).addActiveWorktree(task.id, task.worktree);
       const executeSpy = vi.spyOn(executor as any, "execute").mockResolvedValue(undefined);
 
       await invokeGraphFailure(executor, task);
@@ -164,7 +164,7 @@ describe("pause-abort benign requeue-to-todo (FN-6782)", () => {
       status: "failed",
       error: "Workflow graph failure surfaced after paused engine abort during pause/resume",
     });
-    (executor as any).activeWorktrees.set(task.id, task.worktree);
+    (executor as any).addActiveWorktree(task.id, task.worktree);
     const executeSpy = vi.spyOn(executor as any, "execute").mockResolvedValue(undefined);
 
     await invokeGraphFailure(executor, task);
@@ -197,7 +197,7 @@ describe("pause-abort benign requeue-to-todo (FN-6782)", () => {
       // pause that ended up in todo must stay parked-benign and wait for
       // explicit resume — auto-resuming it would override the operator's intent.
       const { store, task, executor } = makeHarness(overrides, provenance);
-      (executor as any).activeWorktrees.set(task.id, task.worktree);
+      (executor as any).addActiveWorktree(task.id, task.worktree);
       const executeSpy = vi.spyOn(executor as any, "execute").mockResolvedValue(undefined);
 
       await invokeGraphFailure(executor, task);
@@ -217,7 +217,7 @@ describe("pause-abort benign requeue-to-todo (FN-6782)", () => {
       column: "todo",
       graphResumeRetryCount: 2,
     });
-    (executor as any).activeWorktrees.set(task.id, task.worktree);
+    (executor as any).addActiveWorktree(task.id, task.worktree);
     const executeSpy = vi
       .spyOn(executor as any, "execute")
       .mockResolvedValue(undefined);
@@ -246,7 +246,7 @@ describe("pause-abort benign requeue-to-todo (FN-6782)", () => {
       status: "failed",
       error: "Workflow graph failure surfaced after paused engine abort during pause/resume",
     });
-    (executor as any).activeWorktrees.set(task.id, task.worktree);
+    (executor as any).addActiveWorktree(task.id, task.worktree);
 
     await invokeGraphFailure(executor, task);
 
diff --git a/packages/engine/src/__tests__/executor-recovery.test.ts b/packages/engine/src/__tests__/executor-recovery.test.ts
index ae9e39343b..c6d0e8bf35 100644
--- a/packages/engine/src/__tests__/executor-recovery.test.ts
+++ b/packages/engine/src/__tests__/executor-recovery.test.ts
@@ -605,7 +605,7 @@ describe("TaskExecutor bounded recovery retries", () => {
 
       (executor as any).executing.add(taskId);
       executingTaskLock.tryClaim(taskId);
-      (executor as any).activeWorktrees.set(taskId, worktreePath);
+      (executor as any).addActiveWorktree(taskId, worktreePath);
       (executor as any).activeSessions.set(taskId, { session });
       (executor as any).activeStepExecutors.set(taskId, stepExecutor);
       (executor as any).activeWorkflowStepSessions.set(taskId, workflowSession);
@@ -686,7 +686,7 @@ describe("TaskExecutor bounded recovery retries", () => {
       });
       (executor as any).executing.add("FN-001");
       executingTaskLock.tryClaim("FN-001");
-      (executor as any).activeWorktrees.set("FN-001", "/tmp/test/.worktrees/FN-001");
+      (executor as any).addActiveWorktree("FN-001", "/tmp/test/.worktrees/FN-001");
       (executor as any).activeSessions.set("FN-001", { session });
 
       executor.markStuckAborted("FN-001", true);
@@ -750,7 +750,7 @@ describe("TaskExecutor bounded recovery retries", () => {
       vi.mocked(removeWorktree).mockRejectedValue(new Error("worktree busy"));
       (executor as any).executing.add("FN-001");
       executingTaskLock.tryClaim("FN-001");
-      (executor as any).activeWorktrees.set("FN-001", "/tmp/test/.worktrees/FN-001");
+      (executor as any).addActiveWorktree("FN-001", "/tmp/test/.worktrees/FN-001");
       (executor as any).activeSessions.set("FN-001", { session });
 
       executor.markStuckAborted("FN-001", true);
@@ -797,7 +797,7 @@ describe("TaskExecutor bounded recovery retries", () => {
       vi.mocked(removeWorktree).mockResolvedValue(undefined as any);
       (executor as any).executing.add("FN-001");
       executingTaskLock.tryClaim("FN-001");
-      (executor as any).activeWorktrees.set("FN-001", "/tmp/test/.worktrees/FN-001");
+      (executor as any).addActiveWorktree("FN-001", "/tmp/test/.worktrees/FN-001");
       (executor as any).activeSessions.set("FN-001", { session });
 
       executor.markStuckAborted("FN-001", true);
diff --git a/packages/engine/src/__tests__/executor-workspace-session-cwd.test.ts b/packages/engine/src/__tests__/executor-workspace-session-cwd.test.ts
new file mode 100644
index 0000000000..19d101f8dc
--- /dev/null
+++ b/packages/engine/src/__tests__/executor-workspace-session-cwd.test.ts
@@ -0,0 +1,120 @@
+/*
+FNXC:Workspace 2026-06-21-12:00:
+U1 session-cwd scenarios that require driving the real TaskExecutor.execute() to the agent-session boundary. Uses the shared executor-test-helpers harness — it mocks the AI/session/git/fs seams (NOT the workspace gating, NOT acquireTaskWorktree), so setting `(executor as any).workspaceConfig` exercises the genuine KTD1 gate: root acquisition is skipped, and every agent session (initial + retry) is created with `cwd === rootDir` (browse-only workspace root). The non-workspace path is the regression control (cwd === the acquired worktree path).
+*/
+import { afterEach, beforeEach, describe, it, expect, vi } from "vitest";
+import "./executor-test-helpers.js";
+import { TaskExecutor } from "../executor.js";
+import { acquireTaskWorktree } from "../worktree-acquisition.js";
+import type { WorkspaceConfig } from "@fusion/core";
+import {
+  createMockStore,
+  mockedCreateFnAgent,
+  mockedExecSync,
+  resetExecutorMocks,
+} from "./executor-test-helpers.js";
+
+vi.mock("../worktree-acquisition.js", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("../worktree-acquisition.js")>();
+  return { ...actual, acquireTaskWorktree: vi.fn(actual.acquireTaskWorktree) };
+});
+
+const mockedAcquireTaskWorktree = vi.mocked(acquireTaskWorktree);
+
+const ROOT = "/tmp/workspace-root";
+
+function inProgressTask(overrides: Record<string, unknown> = {}) {
+  return {
+    id: "FN-001",
+    title: "Test",
+    description: "Test",
+    column: "in-progress",
+    dependencies: [],
+    steps: [],
+    currentStep: 0,
+    log: [],
+    createdAt: new Date().toISOString(),
+    updatedAt: new Date().toISOString(),
+    ...overrides,
+  } as any;
+}
+
+describe("U1 KTD1 — session cwd is the browse-only workspace root", () => {
+  beforeEach(() => {
+    resetExecutorMocks();
+    // Make any accidental git invocation observable: empty stdout keeps real-git
+    // helpers from throwing, but acquireTaskWorktree assertions catch a leak.
+    mockedExecSync.mockReturnValue("");
+  });
+  afterEach(() => vi.restoreAllMocks());
+
+  it("skips root acquireTaskWorktree and creates every session (initial + retry) with cwd === rootDir", async () => {
+    const store = createMockStore();
+    const mockPrompt = vi.fn().mockResolvedValue(undefined); // no fn_task_done → drives retries too
+    mockedCreateFnAgent.mockResolvedValue({
+      session: { prompt: mockPrompt, dispose: vi.fn() },
+      sessionFile: "/tmp/sessions/ws.jsonl",
+    } as any);
+
+    const executor = new TaskExecutor(store, ROOT);
+    // Drive the genuine workspace gate (loadWorkspaceConfig is covered elsewhere).
+    (executor as any).workspaceConfig = { repos: ["repo-a", "repo-b"] } as WorkspaceConfig;
+
+    await executor.execute(inProgressTask({ worktree: null }));
+
+    // KTD1: the non-git root is never acquired as a worktree.
+    expect(mockedAcquireTaskWorktree).not.toHaveBeenCalled();
+
+    // Every agent session (initial + the retries fired because fn_task_done was
+    // never called) is rooted at the workspace root.
+    expect(mockedCreateFnAgent.mock.calls.length).toBeGreaterThanOrEqual(2);
+    for (const call of mockedCreateFnAgent.mock.calls) {
+      expect((call[0] as any).cwd).toBe(ROOT);
+    }
+
+    // task.worktree is never set in workspace mode.
+    const worktreeWrites = (store.updateTask as any).mock.calls.filter(
+      (c: any[]) => c[1] && Object.prototype.hasOwnProperty.call(c[1], "worktree") && c[1].worktree,
+    );
+    expect(worktreeWrites).toHaveLength(0);
+  });
+});
+
+describe("U1 regression — non-workspace task acquires a worktree and roots the session there", () => {
+  beforeEach(() => {
+    resetExecutorMocks();
+    mockedExecSync.mockReturnValue("");
+  });
+  afterEach(() => vi.restoreAllMocks());
+
+  it("calls acquireTaskWorktree and creates the session with cwd === the acquired worktree path", async () => {
+    const store = createMockStore();
+    const ACQUIRED = "/tmp/test/.worktrees/swift-falcon";
+    mockedAcquireTaskWorktree.mockResolvedValue({
+      worktreePath: ACQUIRED,
+      branch: "fusion/fn-001",
+      source: "fresh",
+      hydrated: false,
+      isResume: false,
+    });
+
+    const mockPrompt = vi.fn().mockResolvedValue(undefined);
+    mockedCreateFnAgent.mockResolvedValue({
+      session: { prompt: mockPrompt, dispose: vi.fn() },
+      sessionFile: "/tmp/sessions/ns.jsonl",
+    } as any);
+
+    const executor = new TaskExecutor(store, "/tmp/test");
+    // No workspaceConfig → single-repo path. Pin the lazy-load guard so the real
+    // loader is never consulted (it would return null for /tmp/test anyway).
+    (executor as any).workspaceConfig = null;
+
+    await executor.execute(inProgressTask({ worktree: null }));
+
+    expect(mockedAcquireTaskWorktree).toHaveBeenCalledTimes(1);
+    expect(mockedCreateFnAgent.mock.calls.length).toBeGreaterThanOrEqual(1);
+    for (const call of mockedCreateFnAgent.mock.calls) {
+      expect((call[0] as any).cwd).toBe(ACQUIRED);
+    }
+  });
+});
diff --git a/packages/engine/src/__tests__/executor-workspace.test.ts b/packages/engine/src/__tests__/executor-workspace.test.ts
index 1916b52367..330915e966 100644
--- a/packages/engine/src/__tests__/executor-workspace.test.ts
+++ b/packages/engine/src/__tests__/executor-workspace.test.ts
@@ -1,89 +1,220 @@
-// @ts-nocheck
-import { describe, it, expect, vi, beforeEach } from "vitest";
-import { loadWorkspaceConfig } from "@fusion/core";
-import { acquireWorkspaceRepoWorktree } from "../worktree-acquisition.js";
+/*
+FNXC:Workspace 2026-06-21-12:00:
+U1 executor session-scoping tests. REWRITTEN from the foundation's self-mocking version (which vi.mock'd the very functions under test and proved nothing). These tests use a REAL two-repo git fixture (`createWorkspaceFixture`) under a NON-git workspace root, so a leaked rootDir git preflight would actually fail. They drive the real TaskExecutor methods that U1 changed: the activeWorktrees Set conversion + every enumerated consumer (KTD2), the preflight gate + browse-only-root scoping (KTD1), and the synthetic-acquisition cwd.
 
-vi.mock("@fusion/core", async (importOriginal) => {
-  const actual = await importOriginal<typeof import("@fusion/core")>();
+Seam choice (FN-5048): `(executor as any).workspaceConfig` is set directly to drive the gating with real git — loadWorkspaceConfig is covered by its own unit and is not the subject here. No mock-the-world child_process/fs shell.
+*/
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { EventEmitter } from "node:events";
+import { loadWorkspaceConfig, type Task, type TaskStore, type WorkspaceConfig } from "@fusion/core";
+import { TaskExecutor, buildExecutionPrompt } from "../executor.js";
+import { createWorkspaceFixture, hasGit, type WorkspaceFixture } from "./_workspace-fixture.js";
+
+const describeIfGit = hasGit ? describe : describe.skip;
+
+function createStore(overrides: Partial<Record<string, unknown>> = {}): TaskStore & EventEmitter {
+  const emitter = new EventEmitter();
+  return Object.assign(emitter, {
+    updateTask: vi.fn().mockResolvedValue(undefined),
+    logEntry: vi.fn().mockResolvedValue(undefined),
+    getSettings: vi.fn().mockResolvedValue({ autoMerge: false }),
+    on: emitter.on.bind(emitter),
+    ...overrides,
+  }) as unknown as TaskStore & EventEmitter;
+}
+
+function makeTask(id = "FN-WS-1", overrides: Partial<Task> = {}): Task {
   return {
-    ...actual,
-    loadWorkspaceConfig: vi.fn(),
-  };
-});
+    id,
+    title: "Workspace task",
+    description: "",
+    column: "in-progress",
+    dependencies: [],
+    steps: [],
+    currentStep: 0,
+    log: [],
+    createdAt: new Date().toISOString(),
+    updatedAt: new Date().toISOString(),
+    ...overrides,
+  } as Task;
+}
 
-vi.mock("../worktree-acquisition.js", async (importOriginal) => {
-  const actual = await importOriginal<typeof import("../worktree-acquisition.js")>();
-  return {
-    ...actual,
-    acquireWorkspaceRepoWorktree: vi.fn(),
-  };
-});
+const repoAPath = (fx: WorkspaceFixture) => `${fx.repoPath("repo-a")}/.worktrees/fn-ws-1`;
+const repoBPath = (fx: WorkspaceFixture) => `${fx.repoPath("repo-b")}/.worktrees/fn-ws-1`;
 
-const mockedLoadWorkspaceConfig = vi.mocked(loadWorkspaceConfig);
-const mockedAcquireWorkspaceRepoWorktree = vi.mocked(acquireWorkspaceRepoWorktree);
+describeIfGit("workspace fixture", () => {
+  let fx: WorkspaceFixture;
+  afterEach(() => fx?.cleanup());
 
-const MOCK_WORKSPACE_CONFIG = {
-  repos: ["wolf-server", "wolf-community-frontend-1"],
-};
-
-describe("acquireWorkspaceRepoWorktree", () => {
-  beforeEach(() => {
-    vi.clearAllMocks();
-  });
-
-  it("returns alreadyAcquired=false for a fresh repo", async () => {
-    mockedAcquireWorkspaceRepoWorktree.mockResolvedValueOnce({
-      worktreePath: "/workspace/wolf-server/.worktrees/fn-001-abc",
-      branch: "fusion/fn-001",
-      alreadyAcquired: false,
-    });
-
-    const result = await acquireWorkspaceRepoWorktree({
-      repoRelPath: "wolf-server",
-      workspaceRootDir: "/workspace",
-      task: { id: "FN-001", workspaceWorktrees: undefined } as never,
-      store: { getTask: vi.fn(), updateTask: vi.fn(), logEntry: vi.fn() } as never,
-      settings: {},
-    });
-
-    expect(result.alreadyAcquired).toBe(false);
-    expect(result.worktreePath).toContain("wolf-server");
-  });
-
-  it("returns alreadyAcquired=true when worktree already acquired", async () => {
-    mockedAcquireWorkspaceRepoWorktree.mockResolvedValueOnce({
-      worktreePath: "/workspace/wolf-server/.worktrees/fn-001-abc",
-      branch: "fusion/fn-001",
-      alreadyAcquired: true,
-    });
-
-    const result = await acquireWorkspaceRepoWorktree({
-      repoRelPath: "wolf-server",
-      workspaceRootDir: "/workspace",
-      task: {
-        id: "FN-001",
-        workspaceWorktrees: {
-          "wolf-server": { worktreePath: "/workspace/wolf-server/.worktrees/fn-001-abc", branch: "fusion/fn-001" },
-        },
-      } as never,
-      store: { getTask: vi.fn(), updateTask: vi.fn(), logEntry: vi.fn() } as never,
-      settings: {},
-    });
-
-    expect(result.alreadyAcquired).toBe(true);
+  it("builds a non-git root with two real git sub-repos and a resolvable workspace config", async () => {
+    fx = await createWorkspaceFixture();
+    // Root is NOT a git repo.
+    expect(() => fx.git("..", "git rev-parse --git-dir")).toThrow();
+    // Each sub-repo is a real git repo with a commit on main.
+    expect(fx.git("repo-a", "git rev-parse --abbrev-ref HEAD")).toBe("main");
+    expect(fx.git("repo-b", "git rev-list --count HEAD")).toBe("1");
+    // loadWorkspaceConfig resolves the on-disk config the executor keys off.
+    const config = await loadWorkspaceConfig(fx.rootDir);
+    expect(config?.repos).toEqual(["repo-a", "repo-b"]);
   });
 });
 
-describe("workspace config", () => {
-  it("loadWorkspaceConfig returns null for non-workspace", async () => {
-    mockedLoadWorkspaceConfig.mockResolvedValueOnce(null);
-    const config = await loadWorkspaceConfig("/some/single-repo");
-    expect(config).toBeNull();
+describeIfGit("U1 KTD2 — activeWorktrees Set + every enumerated consumer", () => {
+  let fx: WorkspaceFixture;
+  afterEach(() => fx?.cleanup());
+
+  function workspaceExecutor() {
+    fx ??= undefined as never;
+    const store = createStore();
+    const executor = new TaskExecutor(store, fx.rootDir);
+    (executor as any).workspaceConfig = { repos: fx.repos } as WorkspaceConfig;
+    return executor;
+  }
+
+  it("a workspace task holding TWO sub-repo paths is found by membership, not equality", async () => {
+    fx = await createWorkspaceFixture();
+    const executor = workspaceExecutor();
+    const pA = repoAPath(fx);
+    const pB = repoBPath(fx);
+    (executor as any).addActiveWorktree("FN-WS-1", pA);
+    (executor as any).addActiveWorktree("FN-WS-1", pB);
+
+    // hasActiveWorktreeBinding: both held paths match; an unheld path does not.
+    expect((executor as any).hasActiveWorktreeBinding("FN-WS-1", pA)).toBe(true);
+    expect((executor as any).hasActiveWorktreeBinding("FN-WS-1", pB)).toBe(true);
+    expect((executor as any).hasActiveWorktreeBinding("FN-WS-1", "/nope")).toBe(false);
+
+    // findActiveWorktreeOwner: another task asking about either held path finds FN-WS-1.
+    await expect((executor as any).findActiveWorktreeOwner(pA, "FN-OTHER")).resolves.toBe("FN-WS-1");
+    await expect((executor as any).findActiveWorktreeOwner(pB, "FN-OTHER")).resolves.toBe("FN-WS-1");
+    // The owner itself is excluded.
+    await expect((executor as any).findActiveWorktreeOwner(pA, "FN-WS-1")).resolves.toBeNull();
   });
 
-  it("loadWorkspaceConfig returns config for workspace", async () => {
-    mockedLoadWorkspaceConfig.mockResolvedValueOnce(MOCK_WORKSPACE_CONFIG);
-    const config = await loadWorkspaceConfig("/some/workspace");
-    expect(config?.repos).toEqual(["wolf-server", "wolf-community-frontend-1"]);
+  it("listWorktreeHolders flat-maps the Set into N holder rows for one task", async () => {
+    fx = await createWorkspaceFixture();
+    const executor = workspaceExecutor();
+    const pA = repoAPath(fx);
+    const pB = repoBPath(fx);
+    (executor as any).addActiveWorktree("FN-WS-1", pA);
+    (executor as any).addActiveWorktree("FN-WS-1", pB);
+
+    const holders = executor.listWorktreeHolders();
+    expect(holders).toHaveLength(2);
+    expect(holders).toContainEqual({ taskId: "FN-WS-1", worktreePath: pA });
+    expect(holders).toContainEqual({ taskId: "FN-WS-1", worktreePath: pB });
+  });
+
+  it("shouldGenerateNewWorktreeName iterates the Set (conflict membership)", async () => {
+    fx = await createWorkspaceFixture();
+    const store = createStore({ listTasks: vi.fn().mockResolvedValue([]) });
+    const executor = new TaskExecutor(store, fx.rootDir);
+    (executor as any).workspaceConfig = { repos: fx.repos } as WorkspaceConfig;
+    const pA = repoAPath(fx);
+    (executor as any).addActiveWorktree("FN-HOLDER", pA);
+
+    // A different task contending for FN-HOLDER's path must be told to generate a new name.
+    await expect((executor as any).shouldGenerateNewWorktreeName(pA, "FN-WS-1")).resolves.toBe(true);
+    // The holder asking about its own path is not a conflict (excluded), and the
+    // DB liveness fallback returns no other user.
+    await expect((executor as any).shouldGenerateNewWorktreeName(pA, "FN-HOLDER")).resolves.toBe(false);
+  });
+
+  it("getWorktreePath returns undefined for a multi-worktree workspace task (Set-collapse contract)", async () => {
+    fx = await createWorkspaceFixture();
+    const executor = workspaceExecutor();
+    (executor as any).addActiveWorktree("FN-WS-1", repoAPath(fx));
+    (executor as any).addActiveWorktree("FN-WS-1", repoBPath(fx));
+    expect(executor.getWorktreePath("FN-WS-1")).toBeUndefined();
+  });
+
+  it("cleanup drops in-memory tracking in workspace mode but never removes the root", async () => {
+    fx = await createWorkspaceFixture();
+    const removeSpy = vi.fn();
+    const executor = workspaceExecutor();
+    (executor as any).removeOwnWorktreeWithReconcile = removeSpy;
+    (executor as any).addActiveWorktree("FN-WS-1", repoAPath(fx));
+    (executor as any).addActiveWorktree("FN-WS-1", repoBPath(fx));
+
+    await executor.cleanup("FN-WS-1");
+
+    expect(executor.getWorktreePath("FN-WS-1")).toBeUndefined();
+    expect((executor as any).activeWorktrees.has("FN-WS-1")).toBe(false);
+    // The browse-only root must never be torn down as if it were a worktree.
+    expect(removeSpy).not.toHaveBeenCalled();
+  });
+
+  it("clearPhantomExecutorBinding (FN-6736) unregisters every held path, not one", async () => {
+    fx = await createWorkspaceFixture();
+    const executor = workspaceExecutor();
+    const pA = repoAPath(fx);
+    const pB = repoBPath(fx);
+    (executor as any).addActiveWorktree("FN-WS-1", pA);
+    (executor as any).addActiveWorktree("FN-WS-1", pB);
+
+    const ok = (executor as any).clearPhantomExecutorBinding("FN-WS-1");
+    expect(ok).toBe(true);
+    expect((executor as any).activeWorktrees.has("FN-WS-1")).toBe(false);
+  });
+});
+
+describeIfGit("U1 KTD2 — non-workspace task is a one-element Set (regression: unchanged)", () => {
+  let fx: WorkspaceFixture;
+  afterEach(() => fx?.cleanup());
+
+  it("getWorktreePath returns the sole path; listWorktreeHolders emits exactly one row", async () => {
+    fx = await createWorkspaceFixture();
+    const store = createStore();
+    const executor = new TaskExecutor(store, fx.repoPath("repo-a")); // single-repo root
+    // No workspaceConfig set → single-repo mode.
+    const wt = `${fx.repoPath("repo-a")}/.worktrees/fn-001`;
+    (executor as any).addActiveWorktree("FN-001", wt);
+
+    expect(executor.getWorktreePath("FN-001")).toBe(wt);
+    expect(executor.listWorktreeHolders()).toEqual([{ taskId: "FN-001", worktreePath: wt }]);
+    expect((executor as any).hasActiveWorktreeBinding("FN-001", wt)).toBe(true);
+  });
+});
+
+describeIfGit("U1 KTD1 — verifyWorktreeInvariants gated off in workspace mode", () => {
+  let fx: WorkspaceFixture;
+  afterEach(() => fx?.cleanup());
+
+  it("returns ok for a zero-acquire workspace task (no task.worktree) so fn_task_done does not requeue", async () => {
+    fx = await createWorkspaceFixture();
+    const store = createStore();
+    const executor = new TaskExecutor(store, fx.rootDir);
+    (executor as any).workspaceConfig = { repos: fx.repos } as WorkspaceConfig;
+
+    // A workspace task that acquired ZERO sub-repos has no task.worktree and no
+    // tracked paths. The singular invariant would otherwise refuse on
+    // "missing task.worktree"; in workspace mode it is gated OFF.
+    const result = await (executor as any).verifyWorktreeInvariants(makeTask("FN-WS-1", { worktree: undefined }));
+    expect(result).toEqual({ ok: true });
+  });
+
+  it("non-workspace task with no worktree still fails the invariant (regression: gate is workspace-only)", async () => {
+    fx = await createWorkspaceFixture();
+    const store = createStore();
+    const executor = new TaskExecutor(store, fx.repoPath("repo-a"));
+    // No workspaceConfig.
+    const result = await (executor as any).verifyWorktreeInvariants(makeTask("FN-001", { worktree: undefined }));
+    expect(result.ok).toBe(false);
+  });
+});
+
+describeIfGit("U1 KTD1 — scopePromptToWorktree / buildExecutionPrompt no-op in workspace mode", () => {
+  let fx: WorkspaceFixture;
+  afterEach(() => fx?.cleanup());
+
+  it("does not rewrite root-anchored paths when a workspace config is present", async () => {
+    fx = await createWorkspaceFixture();
+    const task = makeTask("FN-WS-1", { prompt: `Edit ${fx.rootDir}/repo-a/src/index.ts and commit.` });
+    const config: WorkspaceConfig = { repos: fx.repos };
+    // worktreePath === rootDir in workspace mode; the prompt must be returned verbatim.
+    const prompt = buildExecutionPrompt(task as any, fx.rootDir, { autoMerge: false } as any, fx.rootDir, undefined, undefined, config);
+    expect(prompt).toContain(`${fx.rootDir}/repo-a/src/index.ts`);
+    // The workspace repo list is appended (foundation behavior).
+    expect(prompt).toContain("repo-a");
   });
 });
diff --git a/packages/engine/src/__tests__/executor-worktree-conflict.test.ts b/packages/engine/src/__tests__/executor-worktree-conflict.test.ts
index c2714fda0b..adbc691c86 100644
--- a/packages/engine/src/__tests__/executor-worktree-conflict.test.ts
+++ b/packages/engine/src/__tests__/executor-worktree-conflict.test.ts
@@ -39,7 +39,7 @@ describe("FN-4973: executor worktree conflict cleanup", () => {
     const store = createMockStore();
     const executor = new TaskExecutor(store, "/tmp/test");
     store.listTasks.mockResolvedValue([]);
-    (executor as any).activeWorktrees.set("FN-4973", CONFLICT_PATH);
+    (executor as any).addActiveWorktree("FN-4973", CONFLICT_PATH);
     activeSessionRegistry.registerPath(CONFLICT_PATH, { taskId: "FN-4973", kind: "executor", ownerKey: "FN-4973" });
 
     vi.spyOn(worktreePoolModule, "removeWorktree").mockRejectedValue(
diff --git a/packages/engine/src/__tests__/reliability-interactions/active-worktree-removal-liveness.test.ts b/packages/engine/src/__tests__/reliability-interactions/active-worktree-removal-liveness.test.ts
index 4889abc25d..d35c55ff7f 100644
--- a/packages/engine/src/__tests__/reliability-interactions/active-worktree-removal-liveness.test.ts
+++ b/packages/engine/src/__tests__/reliability-interactions/active-worktree-removal-liveness.test.ts
@@ -58,7 +58,7 @@ describe("FN-4811: active worktree removal liveness gate", () => {
     it("returns the owner taskId when activeWorktrees has another task using the path", async () => {
       const store = createMockStore();
       const executor = new TaskExecutor(store, "/tmp/test");
-      (executor as any).activeWorktrees.set("FN-OTHER", ACTIVE_PATH);
+      (executor as any).addActiveWorktree("FN-OTHER", ACTIVE_PATH);
 
       const owner = await (executor as any).findActiveWorktreeOwner(ACTIVE_PATH, "FN-4811");
       expect(owner).toBe("FN-OTHER");
@@ -67,7 +67,7 @@ describe("FN-4811: active worktree removal liveness gate", () => {
     it("returns null when activeWorktrees only has the requesting task at the path", async () => {
       const store = createMockStore();
       const executor = new TaskExecutor(store, "/tmp/test");
-      (executor as any).activeWorktrees.set("FN-4811", ACTIVE_PATH);
+      (executor as any).addActiveWorktree("FN-4811", ACTIVE_PATH);
       store.listTasks.mockResolvedValue([]);
 
       const owner = await (executor as any).findActiveWorktreeOwner(ACTIVE_PATH, "FN-4811");
@@ -125,7 +125,7 @@ describe("FN-4811: active worktree removal liveness gate", () => {
     it("refuses removal when worktree is in activeWorktrees for another task", async () => {
       const store = createMockStore();
       const executor = new TaskExecutor(store, "/tmp/test");
-      (executor as any).activeWorktrees.set("FN-OTHER", ACTIVE_PATH);
+      (executor as any).addActiveWorktree("FN-OTHER", ACTIVE_PATH);
       store.listTasks.mockResolvedValue([]);
 
       const result = await (executor as any).cleanupConflictingWorktree(
@@ -226,7 +226,7 @@ describe("FN-4811: active worktree removal liveness gate", () => {
     it("returns 'sticky' without invoking inspection when conflict path is actively owned", async () => {
       const store = createMockStore();
       const executor = new TaskExecutor(store, "/tmp/test");
-      (executor as any).activeWorktrees.set("FN-OWNER", ACTIVE_PATH);
+      (executor as any).addActiveWorktree("FN-OWNER", ACTIVE_PATH);
       store.listTasks.mockResolvedValue([]);
 
       const inspectSpy = vi.spyOn(branchConflictModule, "inspectBranchConflict");
diff --git a/packages/engine/src/__tests__/reliability-interactions/post-completion-stale-self-owned-binding.test.ts b/packages/engine/src/__tests__/reliability-interactions/post-completion-stale-self-owned-binding.test.ts
index abe29b5b53..3717a9e355 100644
--- a/packages/engine/src/__tests__/reliability-interactions/post-completion-stale-self-owned-binding.test.ts
+++ b/packages/engine/src/__tests__/reliability-interactions/post-completion-stale-self-owned-binding.test.ts
@@ -23,7 +23,7 @@ describe("FN-5346 reliability interactions: post-completion stale self-owned bin
   it("reconciles stale same-task registry entry during cleanup()", async () => {
     const store = createMockStore();
     const executor = new TaskExecutor(store, ROOT);
-    (executor as any).activeWorktrees.set(TASK_ID, PATH);
+    (executor as any).addActiveWorktree(TASK_ID, PATH);
     activeSessionRegistry.registerPath(PATH, { taskId: TASK_ID, kind: "executor", ownerKey: TASK_ID });
     (activeSessionRegistry.lookupByPath(PATH) as any).registeredAt = 0;
     const removeSpy = vi.spyOn(worktreePoolModule, "removeWorktree").mockResolvedValue(undefined);
@@ -56,7 +56,7 @@ describe("FN-5346 reliability interactions: post-completion stale self-owned bin
   it("preserves refusal for truly-live same-task bindings", async () => {
     const store = createMockStore();
     const executor = new TaskExecutor(store, ROOT);
-    (executor as any).activeWorktrees.set(TASK_ID, PATH);
+    (executor as any).addActiveWorktree(TASK_ID, PATH);
     activeSessionRegistry.registerPath(PATH, { taskId: TASK_ID, kind: "executor", ownerKey: TASK_ID });
     vi.spyOn(worktreePoolModule, "removeWorktree").mockRejectedValue(
       new ActiveSessionWorktreeRemovalError({
@@ -82,7 +82,7 @@ describe("FN-5346 reliability interactions: post-completion stale self-owned bin
     const store = createMockStore();
     store.listTasks.mockResolvedValue([]);
     const executor = new TaskExecutor(store, ROOT);
-    (executor as any).activeWorktrees.set("FN-FOREIGN", PATH);
+    (executor as any).addActiveWorktree("FN-FOREIGN", PATH);
     activeSessionRegistry.registerPath(PATH, { taskId: "FN-FOREIGN", kind: "executor", ownerKey: "FN-FOREIGN" });
     const removeSpy = vi.spyOn(worktreePoolModule, "removeWorktree").mockResolvedValue(undefined);
 
@@ -96,13 +96,13 @@ describe("FN-5346 reliability interactions: post-completion stale self-owned bin
   it("is idempotent across repeated cleanup sweeps", async () => {
     const store = createMockStore();
     const executor = new TaskExecutor(store, ROOT);
-    (executor as any).activeWorktrees.set(TASK_ID, PATH);
+    (executor as any).addActiveWorktree(TASK_ID, PATH);
     activeSessionRegistry.registerPath(PATH, { taskId: TASK_ID, kind: "executor", ownerKey: TASK_ID });
     (activeSessionRegistry.lookupByPath(PATH) as any).registeredAt = 0;
     const removeSpy = vi.spyOn(worktreePoolModule, "removeWorktree").mockResolvedValue(undefined);
 
     await executor.cleanup(TASK_ID);
-    (executor as any).activeWorktrees.set(TASK_ID, PATH);
+    (executor as any).addActiveWorktree(TASK_ID, PATH);
     await executor.cleanup(TASK_ID);
 
     const clearedCalls = (store.logEntry as any).mock.calls.filter(
@@ -120,7 +120,7 @@ describe("FN-5346 reliability interactions: post-completion stale self-owned bin
 
     const store = createMockStore();
     const executor = new TaskExecutor(store, ROOT);
-    (executor as any).activeWorktrees.set(TASK_ID, PATH);
+    (executor as any).addActiveWorktree(TASK_ID, PATH);
     activeSessionRegistry.registerPath(PATH, { taskId: TASK_ID, kind: "executor", ownerKey: TASK_ID });
     vi.spyOn(worktreePoolModule, "removeWorktree").mockResolvedValue(undefined);
 
diff --git a/packages/engine/src/__tests__/reliability-interactions/stale-self-owned-active-session-recovery.test.ts b/packages/engine/src/__tests__/reliability-interactions/stale-self-owned-active-session-recovery.test.ts
index f555c81d5f..9ee54c545f 100644
--- a/packages/engine/src/__tests__/reliability-interactions/stale-self-owned-active-session-recovery.test.ts
+++ b/packages/engine/src/__tests__/reliability-interactions/stale-self-owned-active-session-recovery.test.ts
@@ -118,7 +118,7 @@ describe("FN-4973 reliability interactions: stale self-owned active-session reco
     const store = createMockStore();
     store.listTasks.mockResolvedValue([]);
     const executor = new TaskExecutor(store, "/tmp/test");
-    (executor as any).activeWorktrees.set(TASK_ID, CONFLICT_PATH);
+    (executor as any).addActiveWorktree(TASK_ID, CONFLICT_PATH);
     activeSessionRegistry.registerPath(CONFLICT_PATH, { taskId: TASK_ID, kind: "executor", ownerKey: TASK_ID });
 
     vi.spyOn(worktreePoolModule, "removeWorktree").mockRejectedValue(
diff --git a/packages/engine/src/__tests__/reliability-interactions/stale-self-owned-session-registry.test.ts b/packages/engine/src/__tests__/reliability-interactions/stale-self-owned-session-registry.test.ts
index 91e77b0ac4..869d6303bf 100644
--- a/packages/engine/src/__tests__/reliability-interactions/stale-self-owned-session-registry.test.ts
+++ b/packages/engine/src/__tests__/reliability-interactions/stale-self-owned-session-registry.test.ts
@@ -45,7 +45,7 @@ describe("FN-4976: stale self-owned activeSessionRegistry deadlock backstop", ()
   it("FN-4976 does not clear foreign-owned activeSessionRegistry entry and FN-4811 refusal still fires", async () => {
     const store = createMockStore();
     const executor = new TaskExecutor(store, ROOT);
-    (executor as any).activeWorktrees.set("FN-OTHER", PATH);
+    (executor as any).addActiveWorktree("FN-OTHER", PATH);
     store.listTasks.mockResolvedValue([]);
     activeSessionRegistry.registerPath(PATH, { taskId: "FN-OTHER", kind: "executor", ownerKey: "FN-OTHER" });
 
diff --git a/packages/engine/src/executor.ts b/packages/engine/src/executor.ts
index de96d138d0..1f7c91769b 100644
--- a/packages/engine/src/executor.ts
+++ b/packages/engine/src/executor.ts
@@ -143,7 +143,7 @@ import { isContextLimitError } from "./context-limit-detector.js";
 import { StepSessionExecutor } from "./step-session-executor.js";
 import { makeAncestryBlastRadiusGuard, resetStepToBaseline, runTaskStep } from "./step-runner.js";
 // FNXC:MergerUnification 2026-06-21-19:05: the foundation branch imported `acquireWorkspaceRepoWorktree` here but never used it in executor.ts (the agent tool wraps it via agent-tools.ts), which fails lint on the inherited base. Removed until master-plan U1 re-adds it together with its per-repo acquisition usage.
-import { acquireTaskWorktree } from "./worktree-acquisition.js";
+import { acquireTaskWorktree, type AcquireTaskWorktreeResult } from "./worktree-acquisition.js";
 import { resolveCapturedBaseCommitSha } from "./base-commit-capture.js";
 import { installTaskWorktreeIdentityGuard } from "./worktree-hooks.js";
 import {
@@ -1465,7 +1465,28 @@ interface ActiveExecutorSessionState {
 }
 
 export class TaskExecutor {
-  private activeWorktrees = new Map<string, string>();
+  /*
+  FNXC:Workspace 2026-06-21-12:00:
+  activeWorktrees tracks the worktree paths a task currently holds for liveness/owner checks. In workspace mode a single task acquires N sub-repo worktrees (foundation `task.workspaceWorktrees`), so the value is a SET of paths, not one path. A non-workspace (single-repo) task holds a one-element set — every consumer is converted to membership semantics so the single-repo path is byte-for-byte unchanged (KTD2). Helpers below add/remove/iterate the set.
+  */
+  private activeWorktrees = new Map<string, Set<string>>();
+
+  /**
+   * FNXC:Workspace 2026-06-21-12:00: Register a worktree path under a task's active set, creating the set on first add (KTD2). Single-repo tasks call this once → one-element set.
+   */
+  private addActiveWorktree(taskId: string, worktreePath: string): void {
+    const set = this.activeWorktrees.get(taskId) ?? new Set<string>();
+    set.add(worktreePath);
+    this.activeWorktrees.set(taskId, set);
+  }
+
+  /**
+   * FNXC:Workspace 2026-06-21-12:00: Read-only snapshot of every worktree path a task currently holds (KTD2). Empty when the task holds none.
+   */
+  private getActiveWorktreePaths(taskId: string): string[] {
+    const set = this.activeWorktrees.get(taskId);
+    return set ? Array.from(set) : [];
+  }
   private executing = new Set<string>();
   /** Tasks currently being prepared for unpause resume, before execute() has registered them. */
   private resumingUnpaused = new Set<string>();
@@ -1583,9 +1604,10 @@ export class TaskExecutor {
     this.activeSessions.delete(taskId);
     // U5: drop the effective column-agent principal for this task's session.
     this.effectiveColumnAgentByTask.delete(taskId);
-    const resolvedWorktreePath = worktreePath ?? this.activeWorktrees.get(taskId);
-    if (resolvedWorktreePath) {
-      activeSessionRegistry.unregisterPath(resolvedWorktreePath);
+    // FNXC:Workspace 2026-06-21-12:00: KTD2 — when no explicit path is given, unregister EVERY worktree path the task holds (a workspace task holds N sub-repo paths); single-repo tasks resolve a one-element set.
+    const resolvedWorktreePaths = worktreePath ? [worktreePath] : this.getActiveWorktreePaths(taskId);
+    for (const path of resolvedWorktreePaths) {
+      activeSessionRegistry.unregisterPath(path);
     }
   }
 
@@ -1600,9 +1622,10 @@ export class TaskExecutor {
     this.activeStepExecutorSeenSteeringIds.delete(taskId);
     // U5: drop the effective column-agent principal for this task's step session.
     this.effectiveColumnAgentByTask.delete(taskId);
-    const resolvedWorktreePath = worktreePath ?? this.activeWorktrees.get(taskId);
-    if (resolvedWorktreePath) {
-      activeSessionRegistry.unregisterPath(resolvedWorktreePath);
+    // FNXC:Workspace 2026-06-21-12:00: KTD2 — unregister every held worktree path (Set), not one.
+    const resolvedWorktreePaths = worktreePath ? [worktreePath] : this.getActiveWorktreePaths(taskId);
+    for (const path of resolvedWorktreePaths) {
+      activeSessionRegistry.unregisterPath(path);
     }
   }
 
@@ -1615,9 +1638,10 @@ export class TaskExecutor {
   private deleteActiveWorkflowStepSession(taskId: string, worktreePath?: string): void {
     this.activeWorkflowStepSessions.delete(taskId);
     this.activeWorkflowStepSessionSeenSteeringIds.delete(taskId);
-    const resolvedWorktreePath = worktreePath ?? this.activeWorktrees.get(taskId);
-    if (resolvedWorktreePath) {
-      activeSessionRegistry.unregisterPath(resolvedWorktreePath);
+    // FNXC:Workspace 2026-06-21-12:00: KTD2 — unregister every held worktree path (Set), not one.
+    const resolvedWorktreePaths = worktreePath ? [worktreePath] : this.getActiveWorktreePaths(taskId);
+    for (const path of resolvedWorktreePaths) {
+      activeSessionRegistry.unregisterPath(path);
     }
   }
 
@@ -2053,7 +2077,8 @@ export class TaskExecutor {
       return false;
     }
 
-    const worktreePath = this.activeWorktrees.get(taskId);
+    // FNXC:Workspace 2026-06-21-12:00: KTD2 — collect every worktree path the task holds (a workspace task holds N) before clearing the binding, so the registry sweep below unregisters all of them, not just one.
+    const heldWorktreePaths = this.getActiveWorktreePaths(taskId);
     this.activeWorktrees.delete(taskId);
     this.executing.delete(taskId);
     this.recoveringCompleted.delete(taskId);
@@ -2063,8 +2088,8 @@ export class TaskExecutor {
     this.effectiveColumnAgentByTask.delete(taskId);
 
     const registeredPaths = new Set(activeSessionRegistry.pathsForTask(taskId));
-    if (worktreePath) {
-      registeredPaths.add(worktreePath);
+    for (const path of heldWorktreePaths) {
+      registeredPaths.add(path);
     }
     for (const path of registeredPaths) {
       activeSessionRegistry.unregisterPath(path);
@@ -7430,7 +7455,19 @@ export class TaskExecutor {
       const hadAssignedWorktree = Boolean(task.worktree);
       const taskCommandAbortController = new AbortController();
       this.registerConfiguredCommandController(task.id, taskCommandAbortController);
-      const acquisition = await (async () => {
+      /*
+      FNXC:Workspace 2026-06-21-12:00:
+      KTD1 — in workspace mode `this.rootDir` is a NON-git parent. Acquiring a root worktree there fails. Skip root acquisition entirely and run the agent session rooted at the browse-only workspace root; the agent acquires per-sub-repo worktrees on demand via fn_acquire_repo_worktree. `task.worktree` stays unset. We synthesize a non-fresh, non-resume acquisition with an empty branch so the downstream env-injection/onStart bookkeeping runs unchanged while every rootDir git preflight (base capture, contamination, liveness) is gated off below. The non-workspace branch is byte-for-byte the original acquisition path.
+      */
+      const acquisition: AcquireTaskWorktreeResult = this.workspaceConfig
+        ? {
+            worktreePath: this.rootDir,
+            branch: "",
+            source: "existing",
+            hydrated: true,
+            isResume: Boolean(task.sessionFile),
+          }
+        : await (async () => {
         try {
           return await acquireTaskWorktree({
             task,
@@ -7520,6 +7557,11 @@ export class TaskExecutor {
         }
       }
 
+      /*
+      FNXC:Workspace 2026-06-21-12:00:
+      KTD1 — every preflight below (base-commit capture, contamination check, worktree-liveness gate) runs git against `worktreePath`, which equals the non-git workspace root in workspace mode. They would all fail. Gate the whole block off in workspace mode; the per-repo equivalents return in Phase B (master U3) against each acquired sub-repo worktree. The non-workspace branch is unchanged.
+      */
+      if (!this.workspaceConfig) {
       // Capture the base commit SHA for diff computation whenever a task
       // starts with a newly assigned worktree.
       if (!acquisition.isResume) {
@@ -7664,8 +7706,10 @@ export class TaskExecutor {
         this.options.onError?.(task, new Error(failureMessage));
         return;
       }
+      } // end !this.workspaceConfig preflight gate (FNXC:Workspace KTD1)
 
-      this.activeWorktrees.set(task.id, worktreePath);
+      // FNXC:Workspace 2026-06-21-12:00: KTD2 — register the worktree path under the task's Set. In workspace mode `worktreePath` is the browse-only root; per-repo paths are added as the agent acquires them. Non-workspace tasks add exactly one path → a one-element set (unchanged liveness/owner semantics).
+      this.addActiveWorktree(task.id, worktreePath);
       executorLog.log(`${task.id}: worktree ready at ${worktreePath}`);
 
       const injected = await this.buildInjectedRuntimeEnv(task.id, worktreePath, acquisition.branch ?? undefined);
@@ -10457,8 +10501,13 @@ export class TaskExecutor {
     options?: { noOpCompletion?: boolean; noOpCompletionReason?: string },
   ): Promise<{ ok: true } | { ok: false; reason: "wrong_toplevel" | "wrong_branch" | "no_commits"; observed: string; expected: string }> {
     const settings = await this.store.getSettings();
+    // FNXC:Workspace 2026-06-21-12:00: KTD1/KTD2 — workspace tasks have no root worktree and no single `task.worktree`; the singular per-task invariant is meaningless against the non-git root. Phase B (master U3) iterates this check per sub-repo worktree. Until then it is gated OFF in workspace mode so fn_task_done (its only caller path) does not requeue a zero-acquire workspace task for "missing task.worktree".
+    if (this.workspaceConfig) {
+      return { ok: true };
+    }
     const branchName = resolveTaskWorkingBranch(task);
-    const worktreePath = worktreePathOverride ?? task.worktree ?? this.activeWorktrees.get(task.id) ?? null;
+    // Non-workspace tasks hold a one-element set; fall back to its sole member to preserve the original singular resolution.
+    const worktreePath = worktreePathOverride ?? task.worktree ?? this.getActiveWorktreePaths(task.id)[0] ?? null;
 
     if (!worktreePath) {
       return {
@@ -14440,9 +14489,9 @@ You have access to the file system to review changes.${verdictBlock}`;
     conflictPath: string,
     currentTaskId: string,
   ): Promise<boolean> {
-    // Check if conflicting worktree is in our active set
-    for (const [taskId, worktreePath] of this.activeWorktrees) {
-      if (taskId !== currentTaskId && worktreePath === conflictPath) {
+    // FNXC:Workspace 2026-06-21-12:00: KTD2 — a task may hold N worktree paths; the conflict check is membership across the set, not equality on a single path.
+    for (const [taskId, worktreePaths] of this.activeWorktrees) {
+      if (taskId !== currentTaskId && worktreePaths.has(conflictPath)) {
         return true;
       }
     }
@@ -14479,8 +14528,11 @@ You have access to the file system to review changes.${verdictBlock}`;
    */
   listWorktreeHolders(): Array<{ taskId: string; worktreePath: string }> {
     const holders: Array<{ taskId: string; worktreePath: string }> = [];
-    for (const [taskId, worktreePath] of this.activeWorktrees) {
-      holders.push({ taskId, worktreePath });
+    // FNXC:Workspace 2026-06-21-12:00: KTD2 — flat-map each task's Set into one holder row per worktree path. A workspace task emits N rows; the FN-6782 reaper (self-healing.ts) and in-process-runtime adapter key purely off taskId (verified) and are idempotent across duplicate-task rows, so multi-row holders do not mis-count maxWorktrees slots.
+    for (const [taskId, worktreePaths] of this.activeWorktrees) {
+      for (const worktreePath of worktreePaths) {
+        holders.push({ taskId, worktreePath });
+      }
     }
     return holders;
   }
@@ -14489,8 +14541,9 @@ You have access to the file system to review changes.${verdictBlock}`;
     worktreePath: string,
     requestingTaskId: string,
   ): Promise<string | null> {
-    for (const [taskId, path] of this.activeWorktrees) {
-      if (taskId !== requestingTaskId && path === worktreePath) {
+    // FNXC:Workspace 2026-06-21-12:00: KTD2 — membership across the task's worktree set (a workspace task holds N).
+    for (const [taskId, paths] of this.activeWorktrees) {
+      if (taskId !== requestingTaskId && paths.has(worktreePath)) {
         return taskId;
       }
     }
@@ -14516,12 +14569,9 @@ You have access to the file system to review changes.${verdictBlock}`;
    * Returns true if cleanup succeeded.
    */
   private hasActiveWorktreeBinding(taskId: string, worktreePath: string): boolean {
-    for (const [activeTaskId, activePath] of this.activeWorktrees) {
-      if (activeTaskId === taskId && activePath === worktreePath) {
-        return true;
-      }
-    }
-    return false;
+    // FNXC:Workspace 2026-06-21-12:00: KTD2 — membership across the task's worktree set.
+    const paths = this.activeWorktrees.get(taskId);
+    return paths ? paths.has(worktreePath) : false;
   }
 
   private async reconcileSelfOwnedBeforeRemove(worktreePath: string, taskId: string): Promise<void> {
@@ -14919,11 +14969,18 @@ You have access to the file system to review changes.${verdictBlock}`;
    * always cleaned up by the merger on a per-task basis.
    */
   async cleanup(taskId: string): Promise<void> {
-    const worktreePath = this.activeWorktrees.get(taskId);
-    if (!worktreePath) return;
+    const worktreePaths = this.getActiveWorktreePaths(taskId);
+    if (worktreePaths.length === 0) return;
 
     this.activeWorktrees.delete(taskId);
 
+    // FNXC:Workspace 2026-06-21-12:00: KTD1 — in workspace mode the tracked path is the non-git workspace root (browse-only), never a removable worktree. Drop the in-memory tracking above but never remove the root. Per-repo worktree teardown returns in Phase B.
+    if (this.workspaceConfig) {
+      return;
+    }
+    // Non-workspace tasks hold a one-element set — preserve the original single-path removal semantics.
+    const worktreePath = worktreePaths[0];
+
     // Check if another task still needs this worktree
     const otherUser = await findWorktreeUser(this.store, worktreePath, taskId);
     if (otherUser) {
@@ -15420,8 +15477,14 @@ You have access to the file system to review changes.${verdictBlock}`;
     return true;
   }
 
+  /**
+   * FNXC:Workspace 2026-06-21-12:00: KTD2 single-path-getter contract. Returns the task's sole worktree path for single-repo tasks (one-element set). For a multi-worktree workspace task there is no single answer — callers must read the per-repo `task.workspaceWorktrees` entry instead — so this returns undefined. A workspace task tracked only at the browse-only root also returns undefined, matching the "no removable single worktree" semantics.
+   */
   getWorktreePath(taskId: string): string | undefined {
-    return this.activeWorktrees.get(taskId);
+    if (this.workspaceConfig) {
+      return undefined;
+    }
+    return this.getActiveWorktreePaths(taskId)[0];
   }
 
   // ── Agent Spawning ─────────────────────────────────────────────────────
@@ -15721,7 +15784,11 @@ function formatTimestamp(iso: string): string {
 // Project commands are injected here (for reliability) and also in the PROMPT.md (by triage).
 // This ensures the executor agent always sees the authoritative commands from settings,
 // even if the PROMPT.md was written manually or before commands were configured.
-function scopePromptToWorktree(prompt: string, rootDir?: string, worktreePath?: string): string {
+function scopePromptToWorktree(prompt: string, rootDir?: string, worktreePath?: string, workspaceConfig?: WorkspaceConfig | null): string {
+  // FNXC:Workspace 2026-06-21-12:00: KTD1 — in workspace mode the session is rooted at the workspace root itself (worktreePath === rootDir) and path rewriting to a per-task root worktree is meaningless: edits happen in per-sub-repo worktrees the agent acquires, not at the root. No-op the rewrite. (The rootDir === worktreePath guard below already covers this, but gate explicitly so intent survives future refactors.)
+  if (workspaceConfig) {
+    return prompt;
+  }
   if (!rootDir || !worktreePath || rootDir === worktreePath || !prompt.includes(rootDir)) {
     return prompt;
   }
@@ -15755,7 +15822,7 @@ export function buildExecutionPrompt(
   customFieldDefs?: WorkflowFieldDefinition[],
   workspaceConfig?: WorkspaceConfig | null,
 ): string {
-  const prompt = scopePromptToWorktree(task.prompt, rootDir, worktreePath);
+  const prompt = scopePromptToWorktree(task.prompt, rootDir, worktreePath, workspaceConfig);
   const reviewLevel = parseReviewLevelFromPrompt(prompt);
 
   // Build co-author trailer arg for git commits based on settings. The user's