feat(workspace): Phase A U2 — per-repo acquisition hardening

acquireWorkspaceRepoWorktree now hardens each sub-repo worktree at acquisition:
(1) installs the identity guard with the executor's settings args
(commitMsgHookEnabled/taskPrefix/taskAttributionTrailerName) for single-repo
parity — it was installing no guard before; (2) captures a per-repo
baseCommitSha local-first against the repo's resolved integration branch via
resolveIntegrationBranch(repoAbsPath, {...settings, integrationBranch: undefined})
— stripping the shared override so each sub-repo falls through to its own
origin/HEAD, not a project-wide branch; (3) persists baseCommitSha into the
workspaceWorktrees[repo] entry (Task type extended); (4) registers same-sub-repo
exclusivity on the sub-repo path via activeSessionRegistry under a distinct
"workspace-repo-acquire" kind (released in finally), so two concurrent workspace
tasks contending for the same sub-repo are serialized (throws
WorkspaceRepoAcquireBusyError). Idempotent re-acquire short-circuits.

resolveCapturedBaseCommitSha gains an optional trailing integrationBranch param
defaulting to "main", so existing single-repo callers + base-commit-capture
real-git tests stay green. New audit events worktree:workspace-repo-acquire-busy
/-failed. 6 new real-fixture tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.changeset/workspace-per-repo-acquisition-hardening.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": minor
+---
+
+Workspace mode (Phase A / U2): harden per-repo worktree acquisition. Each sub-repo worktree now gets the task identity guard installed (single-repo parity), a per-repo base commit SHA captured local-first against that sub-repo's resolved integration branch (shared `integrationBranch` override stripped so each repo falls through to its own `origin/HEAD`), and same-sub-repo acquisition exclusivity registered in the path-keyed active-session registry. Re-acquiring an already-acquired `(taskId, repo)` is idempotent, and acquisition failures surface an error plus an audit event instead of silently stalling.

packages/core/src/types.ts

@@ -2246,8 +2246,14 @@ export interface Task {
   /**
    * Workspace mode only. Keyed by repo path relative to workspace rootDir.
    * Each entry records the on-disk worktree path and git branch for one sub-repo.
+   *
+   * FNXC:Workspace 2026-06-21-20:10:
+   * `baseCommitSha` is the per-repo fork-point captured at acquisition (U2/KTD3)
+   * against that sub-repo's RESOLVED integration branch, local-first. It is the
+   * per-repo analogue of the single-repo base-commit capture and prevents
+   * cross-repo files-changed inflation when local integration is ahead of origin.
    */
-  workspaceWorktrees?: Record<string, { worktreePath: string; branch: string }>;
+  workspaceWorktrees?: Record<string, { worktreePath: string; branch: string; baseCommitSha?: string }>;
   steps: TaskStep[];
   currentStep: number;
   /**

packages/engine/src/__tests__/worktree-acquisition-workspace.test.ts

@@ -0,0 +1,273 @@
+/*
+FNXC:Workspace 2026-06-21-20:10:
+U2 per-repo acquisition hardening tests. A REAL two-repo git fixture is required
+because the invariants under test are git-shaped: local-ahead-of-origin base
+capture, a resolved-per-repo (non-shared) integration branch, and a working
+identity-guard hook that actually rejects a commit. The shared harness from
+./_workspace-fixture.ts builds genuine on-disk repos under a NON-git workspace
+root. The TaskStore is an in-memory fake (no DB / no network) per FN-5048 — real
+git only where the invariant needs it; everything else is a narrow seam.
+*/
+import { execSync, spawnSync } from "node:child_process";
+import { existsSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+import { afterEach, describe, expect, it } from "vitest";
+import type { Settings, Task, TaskStore } from "@fusion/core";
+import {
+  acquireWorkspaceRepoWorktree,
+  WorkspaceRepoAcquireBusyError,
+} from "../worktree-acquisition.js";
+import { ActiveSessionRegistry } from "../active-session-registry.js";
+import { createWorkspaceFixture, hasGit, type WorkspaceFixture } from "./_workspace-fixture.js";
+
+const describeIfGit = hasGit ? describe : describe.skip;
+
+function git(repo: string, command: string): string {
+  return execSync(command, { cwd: repo, encoding: "utf-8", stdio: ["pipe", "pipe", "pipe"] }).trim();
+}
+
+/**
+ * Minimal in-memory TaskStore covering exactly what acquireWorkspaceRepoWorktree
+ * and its acquireTaskWorktree callee touch: updateTask (merge-in-place so the
+ * idempotency re-read sees persisted workspaceWorktrees), logEntry, getTask.
+ */
+function makeFakeStore(task: Task): { store: TaskStore; current: () => Task; logs: string[] } {
+  let current = task;
+  const logs: string[] = [];
+  const store = {
+    async updateTask(id: string, patch: Partial<Task>): Promise<void> {
+      if (id === current.id) current = { ...current, ...patch };
+    },
+    async logEntry(_id: string, message: string): Promise<void> {
+      logs.push(message);
+    },
+    async getTask(id: string): Promise<Task | null> {
+      return id === current.id ? current : null;
+    },
+  } as unknown as TaskStore;
+  return { store, current: () => current, logs };
+}
+
+function makeTask(id: string): Task {
+  return {
+    id,
+    title: `task ${id}`,
+    description: "workspace task",
+    status: "in-progress",
+  } as unknown as Task;
+}
+
+const SETTINGS: Partial<Settings> = {
+  worktreeNaming: "task-id",
+  commitMsgHookEnabled: true,
+  taskPrefix: "FN",
+  taskAttributionTrailerNames: ["Fusion-Task-Id"],
+};
+
+describeIfGit("acquireWorkspaceRepoWorktree (U2 per-repo hardening)", { timeout: 60_000 }, () => {
+  let fixture: WorkspaceFixture;
+
+  afterEach(() => {
+    fixture?.cleanup();
+  });
+
+  it("captures the LOCAL integration tip as baseCommitSha even when origin is behind (inflation invariant)", async () => {
+    // Give repo-a a real origin so origin/main can lag behind local main.
+    fixture = await createWorkspaceFixture(["repo-a"]);
+    const repoA = fixture.repoPath("repo-a");
+    const origin = `${repoA}-origin`;
+    git(repoA, "git init --bare " + JSON.stringify(origin));
+    git(repoA, `git remote add origin ${JSON.stringify(origin)}`);
+    git(repoA, "git push -u origin main");
+
+    // Local main advances by an unpushed predecessor commit (FN-5937 shape).
+    git(repoA, "git commit --allow-empty -m 'FN-9000: unpushed predecessor'");
+    const localTip = git(repoA, "git rev-parse HEAD");
+    const originTip = git(repoA, "git rev-parse origin/main");
+    expect(localTip).not.toBe(originTip);
+
+    const { store, current } = makeFakeStore(makeTask("FN-1"));
+    const registry = new ActiveSessionRegistry();
+    const result = await acquireWorkspaceRepoWorktree({
+      repoRelPath: "repo-a",
+      workspaceRootDir: fixture.rootDir,
+      task: current(),
+      store,
+      settings: SETTINGS,
+      registry,
+    });
+
+    // Base must be the LOCAL tip, never the behind origin tip.
+    expect(result.baseCommitSha).toBe(localTip);
+    expect(current().workspaceWorktrees?.["repo-a"]?.baseCommitSha).toBe(localTip);
+  });
+
+  it("captures against a NON-main integration branch and does not inherit a shared settings.integrationBranch (KTD3)", async () => {
+    // repo-a's default branch is 'develop'; origin/HEAD points at it. A shared
+    // settings.integrationBranch override must be STRIPPED so per-repo resolution
+    // falls through to this repo's own origin/HEAD.
+    fixture = await createWorkspaceFixture(["repo-a"], "develop");
+    const repoA = fixture.repoPath("repo-a");
+    const origin = `${repoA}-origin`;
+    git(repoA, "git init --bare " + JSON.stringify(origin));
+    git(repoA, `git remote add origin ${JSON.stringify(origin)}`);
+    git(repoA, "git push -u origin develop");
+    // Point origin/HEAD at develop so resolveIntegrationBranch resolves it.
+    git(repoA, "git remote set-head origin develop");
+    const developTip = git(repoA, "git rev-parse develop");
+
+    const { store, current } = makeFakeStore(makeTask("FN-2"));
+    const registry = new ActiveSessionRegistry();
+    const result = await acquireWorkspaceRepoWorktree({
+      repoRelPath: "repo-a",
+      workspaceRootDir: fixture.rootDir,
+      task: current(),
+      store,
+      // A SHARED integration branch that does NOT exist in this sub-repo. If it
+      // leaked through, base capture would resolve against 'shared-trunk' and
+      // (absent that branch) fall back to HEAD — not develop's tip.
+      settings: { ...SETTINGS, integrationBranch: "shared-trunk" },
+      registry,
+    });
+
+    expect(result.baseCommitSha).toBe(developTip);
+  });
+
+  it("installs the identity-guard hook so a commit on a non-fusion branch is rejected", async () => {
+    fixture = await createWorkspaceFixture(["repo-a"]);
+    const { store, current } = makeFakeStore(makeTask("FN-3"));
+    const registry = new ActiveSessionRegistry();
+    const result = await acquireWorkspaceRepoWorktree({
+      repoRelPath: "repo-a",
+      workspaceRootDir: fixture.rootDir,
+      task: current(),
+      settings: SETTINGS,
+      store,
+      registry,
+    });
+
+    const wt = result.worktreePath;
+    expect(existsSync(join(wt, ".git"))).toBe(true);
+    git(wt, 'git config user.email "test@example.com"');
+    git(wt, 'git config user.name "Test"');
+
+    // On the fusion/<id> branch the guard permits a commit (real staged change,
+    // so the FN-5345 empty-commit guard also installed by the identity guard
+    // does not refuse it).
+    git(wt, "git checkout fusion/fn-3");
+    writeFileSync(join(wt, "own.txt"), "own work\n", "utf-8");
+    git(wt, "git add own.txt");
+    git(wt, "git commit -m 'FN-3: ok on own branch'");
+
+    // Switch to a foreign branch; the pre-commit identity guard must refuse.
+    git(wt, "git checkout -B rogue-branch");
+    writeFileSync(join(wt, "rogue.txt"), "rogue work\n", "utf-8");
+    git(wt, "git add rogue.txt");
+    const attempt = spawnSync("git", ["commit", "-m", "rogue"], {
+      cwd: wt,
+      encoding: "utf-8",
+    });
+    expect(attempt.status).not.toBe(0);
+    expect(`${attempt.stderr}`).toMatch(/refusing commit/i);
+  });
+
+  it("serializes two concurrent acquisitions of the SAME sub-repo via the exclusivity registry (KTD4)", async () => {
+    fixture = await createWorkspaceFixture(["repo-a"]);
+    const repoAbs = fixture.repoPath("repo-a");
+    const registry = new ActiveSessionRegistry();
+
+    // Pre-register the sub-repo path as if task FN-A is mid-acquisition, then
+    // prove a second task is rejected while it is held.
+    registry.registerPath(repoAbs, { taskId: "FN-A", kind: "workspace-repo-acquire", ownerKey: "workspace-repo-acquire" });
+
+    const { store, current } = makeFakeStore(makeTask("FN-B"));
+    await expect(
+      acquireWorkspaceRepoWorktree({
+        repoRelPath: "repo-a",
+        workspaceRootDir: fixture.rootDir,
+        task: current(),
+        store,
+        settings: SETTINGS,
+        registry,
+      }),
+    ).rejects.toBeInstanceOf(WorkspaceRepoAcquireBusyError);
+
+    // The holder's entry is untouched by the rejected loser.
+    expect(registry.lookupByPath(repoAbs)?.taskId).toBe("FN-A");
+
+    // Once released, the same task acquires cleanly and the registry is freed.
+    registry.unregisterPath(repoAbs);
+    const result = await acquireWorkspaceRepoWorktree({
+      repoRelPath: "repo-a",
+      workspaceRootDir: fixture.rootDir,
+      task: current(),
+      store,
+      settings: SETTINGS,
+      registry,
+    });
+    expect(result.alreadyAcquired).toBe(false);
+    // Acquisition releases its own exclusivity entry on completion.
+    expect(registry.isPathActive(repoAbs)).toBe(false);
+  });
+
+  it("is idempotent across (taskId, repo): re-acquire returns the existing entry without re-capture", async () => {
+    fixture = await createWorkspaceFixture(["repo-a"]);
+    const { store, current } = makeFakeStore(makeTask("FN-4"));
+    const registry = new ActiveSessionRegistry();
+
+    const first = await acquireWorkspaceRepoWorktree({
+      repoRelPath: "repo-a",
+      workspaceRootDir: fixture.rootDir,
+      task: current(),
+      store,
+      settings: SETTINGS,
+      registry,
+    });
+    expect(first.alreadyAcquired).toBe(false);
+
+    // Re-acquire with the now-populated task: returns the persisted entry,
+    // does not re-register exclusivity, does not re-create a worktree.
+    const second = await acquireWorkspaceRepoWorktree({
+      repoRelPath: "repo-a",
+      workspaceRootDir: fixture.rootDir,
+      task: current(),
+      store,
+      settings: SETTINGS,
+      registry,
+    });
+    expect(second.alreadyAcquired).toBe(true);
+    expect(second.worktreePath).toBe(first.worktreePath);
+    expect(second.baseCommitSha).toBe(first.baseCommitSha);
+    expect(registry.isPathActive(fixture.repoPath("repo-a"))).toBe(false);
+  });
+
+  it("surfaces an error and persists an audit event when acquisition fails (no swallowed stall)", async () => {
+    fixture = await createWorkspaceFixture(["repo-a"]);
+    const { store, current, logs } = makeFakeStore(makeTask("FN-5"));
+    const registry = new ActiveSessionRegistry();
+    const auditEvents: Array<{ type: string }> = [];
+    const audit = {
+      async git(e: { type: string }): Promise<void> {
+        auditEvents.push(e);
+      },
+      async filesystem(): Promise<void> {},
+    };
+
+    await expect(
+      acquireWorkspaceRepoWorktree({
+        repoRelPath: "does-not-exist",
+        workspaceRootDir: fixture.rootDir,
+        task: current(),
+        store,
+        settings: SETTINGS,
+        registry,
+        audit: audit as never,
+      }),
+    ).rejects.toThrow();
+
+    expect(auditEvents.some((e) => e.type === "worktree:workspace-repo-acquire-failed")).toBe(true);
+    expect(logs.some((m) => /acquisition failed/i.test(m))).toBe(true);
+    // The exclusivity entry is released even on the failure path.
+    expect(registry.isPathActive(join(fixture.rootDir, "does-not-exist"))).toBe(false);
+  });
+});

packages/engine/src/active-session-registry.ts

@@ -1,4 +1,13 @@
-export type ActiveSessionKind = "executor" | "step-session" | "workflow-step" | "step-session-parallel" | "ai-merge";
+/*
+FNXC:Workspace 2026-06-21-20:10:
+"workspace-repo-acquire" is a DISTINCT registry kind reserved for the
+acquisition-time same-sub-repo exclusivity entry (U2/KTD4). It is keyed by the
+sub-repo absolute path (NOT the worktree path) so two concurrent workspace tasks
+contending for the SAME sub-repo are serialized. Keeping it distinct from
+"executor"/"step-session" means it does not collide with the executor's later
+session registration on the produced worktree path.
+*/
+export type ActiveSessionKind = "executor" | "step-session" | "workflow-step" | "step-session-parallel" | "ai-merge" | "workspace-repo-acquire";
 
 export interface ActiveSessionRegistration {
   taskId: string;

packages/engine/src/base-commit-capture.ts

@@ -22,15 +22,31 @@ const execAsync = promisify(exec);
  *
  * Returns `undefined` only when every git invocation fails (caller treats a
  * missing base as non-fatal).
+ *
+ * FNXC:Workspace 2026-06-21-20:10:
+ * `integrationBranch` is an OPTIONAL TRAILING param defaulting to the historic
+ * "main" literal so the single-repo executor caller and the real-git tests stay
+ * green without change. Workspace mode (U2/KTD3) passes each sub-repo's RESOLVED
+ * integration branch so per-repo base capture forks against the right branch
+ * instead of a hardcoded "main". The local-first ordering (merge-base HEAD
+ * <local> then origin/<branch>) is preserved per-branch to keep the
+ * inflation-prevention invariant (FN-5937) intact for non-main integration
+ * branches too.
  */
 export async function resolveCapturedBaseCommitSha(
   worktreePath: string,
   logger?: { warn: (msg: string) => void },
+  integrationBranch: string = "main",
 ): Promise<string | undefined> {
+  const branch = integrationBranch.trim() || "main";
+  // Shell-quote defensively; integration branch names are normalized upstream
+  // but may carry slashes (e.g. "release/2026-06") that are valid in refs.
+  const localRef = JSON.stringify(branch);
+  const originRef = JSON.stringify(`origin/${branch}`);
   let baseCommitSha: string | undefined;
   try {
     const { stdout } = await execAsync(
-      "git merge-base HEAD main 2>/dev/null || git merge-base HEAD origin/main",
+      `git merge-base HEAD ${localRef} 2>/dev/null || git merge-base HEAD ${originRef}`,
       { cwd: worktreePath, encoding: "utf-8" },
     );
     baseCommitSha = stdout.trim() || undefined;

packages/engine/src/run-audit.ts

@@ -99,6 +99,11 @@ export type GitMutationType =
   | "worktree:incomplete-detected"
   | "worktree:reanchored"
   | "worktree:auto-recovered"
+  // FNXC:Workspace 2026-06-21-20:10: workspace per-repo acquisition audit events (U2).
+  // -busy: another task holds the same sub-repo's acquisition exclusivity lock (KTD4).
+  // -failed: a sub-repo worktree acquisition threw; surfaced + audited, never swallowed.
+  | "worktree:workspace-repo-acquire-busy"
+  | "worktree:workspace-repo-acquire-failed"
   /**
    * worktrunk run-audit metadata shape:
    *

packages/engine/src/worktree-acquisition.ts

@@ -34,6 +34,10 @@ import {
 import type { RunAuditor } from "./run-audit.js";
 import { writeSecretsEnvFile } from "./secrets-env-writer.js";
 import { removeDesktopBuildArtifacts } from "./worktree-desktop-artifacts.js";
+import { installTaskWorktreeIdentityGuard } from "./worktree-hooks.js";
+import { resolveCapturedBaseCommitSha } from "./base-commit-capture.js";
+import { resolveIntegrationBranch } from "./integration-branch.js";
+import { activeSessionRegistry, type ActiveSessionRegistry } from "./active-session-registry.js";
 
 const execAsync = promisify(exec);
 
@@ -604,47 +608,184 @@ export interface AcquireWorkspaceRepoWorktreeOptions {
   settings: Partial<Settings>;
   logger?: { log: (m: string) => void; warn: (m: string) => void; error?: (m: string) => void };
   secretsStore?: Pick<SecretsStore, "listEnvExportable">;
+  audit?: Pick<RunAuditor, "git" | "filesystem">;
+  runContext?: RunMutationContext;
+  /** Test seam: inject the path-keyed exclusivity registry (defaults to the process singleton). */
+  registry?: ActiveSessionRegistry;
 }
 
+/*
+FNXC:Workspace 2026-06-21-20:10:
+Acquisition-time exclusivity owner key for the same-sub-repo lock (U2/KTD4). The
+registry record is keyed by the sub-repo ABSOLUTE path and carries this distinct
+ownerKey so it never collides with the executor's later "executor"/"step-session"
+registration on the produced WORKTREE path.
+*/
+const WORKSPACE_REPO_ACQUIRE_OWNER_KEY = "workspace-repo-acquire";
+
 export async function acquireWorkspaceRepoWorktree(
   opts: AcquireWorkspaceRepoWorktreeOptions,
-): Promise<{ worktreePath: string; branch: string; alreadyAcquired: boolean }> {
-  const { repoRelPath, workspaceRootDir, task, store, settings, logger, secretsStore } = opts;
+): Promise<{ worktreePath: string; branch: string; baseCommitSha?: string; alreadyAcquired: boolean }> {
+  const { repoRelPath, workspaceRootDir, task, store, settings, logger, secretsStore, audit, runContext } = opts;
+  const registry = opts.registry ?? activeSessionRegistry;
   const { join } = await import("node:path");
 
   const existing = task.workspaceWorktrees?.[repoRelPath];
   if (existing) {
+    /*
+    FNXC:Workspace 2026-06-21-20:10:
+    Idempotency across (taskId, repo): a re-acquire of an already-acquired sub-repo
+    returns the persisted entry verbatim — no second identity-guard install, no
+    re-capture of the base SHA, no second exclusivity registration.
+    */
     return { ...existing, alreadyAcquired: true };
   }
 
   const repoAbsPath = join(workspaceRootDir, repoRelPath);
 
   /*
-  FNXC:WorkspaceWorktree 2026-06-21-19:05:
-  Workspace mode acquires one worktree per sub-repo for a single task. `acquireTaskWorktree`
-  is single-repo: it reads `task.worktree`/`task.branch` to decide resume-vs-fresh and rewrites
-  those singular fields on the task row after each acquisition. Passing the live task straight
-  through means the second repo's acquisition sees the first repo's `task.worktree` (which exists
-  on disk), classifies it as a resume, and reuses repo A's worktree inside repo B — cross-repo
-  contamination. Clear the singular worktree/branch fields on the copy handed to the single-repo
-  helper so each sub-repo always gets a fresh worktree; per-repo state is tracked in
-  `task.workspaceWorktrees`, not the singular column.
+  FNXC:Workspace 2026-06-21-20:10:
+  Same-sub-repo exclusivity (KTD4): register the sub-repo absolute path in the
+  path-keyed activeSessionRegistry BEFORE acquiring so two concurrent workspace
+  tasks contending for the SAME sub-repo are serialized. WorktreePool is a recycle
+  cache, not a cross-task lock, and disjoint-scope contention on one sub-repo is
+  otherwise unprotected (file-scope leases don't catch it). The entry is keyed by
+  the sub-repo path with a distinct ownerKey so it does not collide with the
+  executor's later session registration on the produced worktree path. We release
+  it once acquisition completes (success or failure) — it guards the acquisition
+  critical section, not the whole task lifetime.
   */
-  const result = await acquireTaskWorktree({
-    task: { ...task, worktree: undefined, branch: undefined },
-    rootDir: repoAbsPath,
-    store,
-    settings,
-    logger,
-    secretsStore,
-    runInitCommand: true,
+  const exclusivityHolder = registry.lookupByPath(repoAbsPath);
+  if (exclusivityHolder && exclusivityHolder.ownerKey === WORKSPACE_REPO_ACQUIRE_OWNER_KEY && exclusivityHolder.taskId !== task.id) {
+    const message = `sub-repo ${repoRelPath} is being acquired by ${exclusivityHolder.taskId}; serializing concurrent workspace acquisition`;
+    logger?.warn(`${task.id}: ${message}`);
+    await store.logEntry(task.id, message, undefined, runContext);
+    const err = new WorkspaceRepoAcquireBusyError(repoRelPath, exclusivityHolder.taskId, task.id);
+    await audit?.git({
+      type: "worktree:workspace-repo-acquire-busy",
+      target: repoAbsPath,
+      metadata: { repoRelPath, holderTaskId: exclusivityHolder.taskId, requestingTaskId: task.id },
+    });
+    throw err;
+  }
+  registry.registerPath(repoAbsPath, {
+    taskId: task.id,
+    kind: "workspace-repo-acquire",
+    ownerKey: WORKSPACE_REPO_ACQUIRE_OWNER_KEY,
   });
 
-  const updated: Record<string, { worktreePath: string; branch: string }> = {
-    ...(task.workspaceWorktrees ?? {}),
-    [repoRelPath]: { worktreePath: result.worktreePath, branch: result.branch },
-  };
-  await store.updateTask(task.id, { workspaceWorktrees: updated });
+  try {
+    /*
+    FNXC:WorkspaceWorktree 2026-06-21-19:05:
+    Workspace mode acquires one worktree per sub-repo for a single task. `acquireTaskWorktree`
+    is single-repo: it reads `task.worktree`/`task.branch` to decide resume-vs-fresh and rewrites
+    those singular fields on the task row after each acquisition. Passing the live task straight
+    through means the second repo's acquisition sees the first repo's `task.worktree` (which exists
+    on disk), classifies it as a resume, and reuses repo A's worktree inside repo B — cross-repo
+    contamination. Clear the singular worktree/branch fields on the copy handed to the single-repo
+    helper so each sub-repo always gets a fresh worktree; per-repo state is tracked in
+    `task.workspaceWorktrees`, not the singular column.
+    */
+    const result = await acquireTaskWorktree({
+      task: { ...task, worktree: undefined, branch: undefined },
+      rootDir: repoAbsPath,
+      store,
+      settings,
+      logger,
+      secretsStore,
+      audit,
+      runContext,
+      runInitCommand: true,
+    });
 
-  return { worktreePath: result.worktreePath, branch: result.branch, alreadyAcquired: false };
+    /*
+    FNXC:Workspace 2026-06-21-20:10:
+    Identity guard (single-repo parity): acquireTaskWorktree above runs WITHOUT a
+    createWorktree override, so the default native backend installs NO identity
+    hooks for a sub-repo worktree. Install the same guard the executor installs for
+    single-repo tasks (executor.ts identity-guard call), passing the SAME settings
+    args (commitMsgHookEnabled / taskPrefix / first taskAttributionTrailerName) so a
+    commit on a non-fusion/<id> branch is refused inside every sub-repo worktree too.
+    */
+    await installTaskWorktreeIdentityGuard({
+      worktreePath: result.worktreePath,
+      taskId: task.id,
+      commitMsgHookEnabled: settings.commitMsgHookEnabled,
+      taskPrefix: settings.taskPrefix,
+      taskAttributionTrailerName: settings.taskAttributionTrailerNames?.[0],
+    });
+
+    /*
+    FNXC:Workspace 2026-06-21-20:10:
+    Per-repo base SHA (KTD3): resolve THIS sub-repo's integration branch with the
+    shared settings.integrationBranch override STRIPPED. resolveIntegrationBranch
+    checks settings.integrationBranch FIRST, so without stripping it every sub-repo
+    would resolve to the shared workspace branch — defeating per-repo resolution.
+    With it undefined, each sub-repo falls through to its own origin/HEAD. Capture
+    the base local-first against that branch so local-ahead-of-origin integration
+    tips don't inflate the per-repo diff (FN-5937 invariant, per sub-repo).
+    */
+    const integrationBranch = await resolveIntegrationBranch(
+      repoAbsPath,
+      { ...settings, integrationBranch: undefined },
+      { logger },
+    );
+    const baseCommitSha = await resolveCapturedBaseCommitSha(result.worktreePath, logger, integrationBranch);
+
+    const updated: Record<string, { worktreePath: string; branch: string; baseCommitSha?: string }> = {
+      ...(task.workspaceWorktrees ?? {}),
+      [repoRelPath]: { worktreePath: result.worktreePath, branch: result.branch, baseCommitSha },
+    };
+    await store.updateTask(task.id, { workspaceWorktrees: updated });
+
+    return { worktreePath: result.worktreePath, branch: result.branch, baseCommitSha, alreadyAcquired: false };
+  } catch (err) {
+    /*
+    FNXC:Workspace 2026-06-21-20:10:
+    Acquisition failure must surface an error and leave an audit trail (no swallowed
+    stall): persist the failure as an audit event + task log, then re-throw so the
+    caller observes the failure rather than silently proceeding with an unacquired
+    sub-repo.
+    */
+    if (!(err instanceof WorkspaceRepoAcquireBusyError)) {
+      const message = err instanceof Error ? err.message : String(err);
+      logger?.error?.(`${task.id}: workspace sub-repo acquisition failed for ${repoRelPath}: ${message}`);
+      await store.logEntry(task.id, `Workspace sub-repo acquisition failed for ${repoRelPath}: ${message}`, undefined, runContext);
+      await audit?.git({
+        type: "worktree:workspace-repo-acquire-failed",
+        target: repoAbsPath,
+        metadata: { repoRelPath, taskId: task.id, error: message },
+      });
+    }
+    throw err;
+  } finally {
+    /*
+    FNXC:Workspace 2026-06-21-20:10:
+    Release the acquisition-time exclusivity entry only when WE hold it. The busy-path
+    throw above does NOT enter this try (it short-circuits before registerPath), so a
+    serialized loser never unregisters the winner's entry.
+    */
+    const held = registry.lookupByPath(repoAbsPath);
+    if (held && held.taskId === task.id && held.ownerKey === WORKSPACE_REPO_ACQUIRE_OWNER_KEY) {
+      registry.unregisterPath(repoAbsPath);
+    }
+  }
+}
+
+/*
+FNXC:Workspace 2026-06-21-20:10:
+Thrown when a second workspace task tries to acquire a sub-repo already inside
+another task's acquisition critical section (KTD4). Distinct from generic
+acquisition failures so the caller (and tests) can tell "serialized, retry later"
+apart from "this sub-repo is broken".
+*/
+export class WorkspaceRepoAcquireBusyError extends Error {
+  constructor(
+    public readonly repoRelPath: string,
+    public readonly holderTaskId: string,
+    public readonly requestingTaskId: string,
+  ) {
+    super(`workspace sub-repo ${repoRelPath} acquisition is in progress for task ${holderTaskId}`);
+    this.name = "WorkspaceRepoAcquireBusyError";
+  }
 }