diff --git a/packages/engine/src/__tests__/worktree-primary-checkout-invariant.test.ts b/packages/engine/src/__tests__/worktree-primary-checkout-invariant.test.ts
new file mode 100644
index 0000000000..cc38ca959f
--- /dev/null
+++ b/packages/engine/src/__tests__/worktree-primary-checkout-invariant.test.ts
@@ -0,0 +1,133 @@
+import { execFileSync } from "node:child_process";
+import { mkdtemp, mkdir, rm, writeFile } from "node:fs/promises";
+import { readFileSync, realpathSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join, normalize } from "node:path";
+import { fileURLToPath } from "node:url";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { TaskExecutor } from "../executor.js";
+
+function git(cwd: string, args: string[]): string {
+  return execFileSync("git", ["-C", cwd, ...args], { encoding: "utf8" }).trim();
+}
+
+function canonicalPath(path: string): string {
+  return normalize(realpathSync(path));
+}
+
+function registeredWorktrees(rootDir: string): Array<{ path: string; branch?: string }> {
+  return git(rootDir, ["worktree", "list", "--porcelain"])
+    .split(/\r?\n(?=worktree )/)
+    .filter(Boolean)
+    .map((record) => {
+      const fields = new Map(record.split("\n").map((line) => {
+        const separator = line.indexOf(" ");
+        return [line.slice(0, separator), line.slice(separator + 1)];
+      }));
+      return { path: canonicalPath(fields.get("worktree")!), branch: fields.get("branch") };
+    });
+}
+
+function createExecutor(rootDir: string): TaskExecutor {
+  const store = {
+    on: vi.fn(),
+    getSettings: vi.fn().mockResolvedValue({
+      commitMsgHookEnabled: false,
+      worktreeRebaseBeforeMerge: false,
+    }),
+    updateTask: vi.fn().mockResolvedValue(undefined),
+    logEntry: vi.fn().mockResolvedValue(undefined),
+  };
+  return new TaskExecutor(store as any, rootDir);
+}
+
+const executorSource = readFileSync(fileURLToPath(new URL("../executor.ts", import.meta.url)), "utf8");
+const acquisitionSource = readFileSync(fileURLToPath(new URL("../worktree-acquisition.ts", import.meta.url)), "utf8");
+const mergerSource = readFileSync(fileURLToPath(new URL("../merger.ts", import.meta.url)), "utf8");
+
+function sourceRegion(source: string, start: string, end: string): string {
+  const startIndex = source.indexOf(start);
+  const endIndex = source.indexOf(end, startIndex + start.length);
+  expect(startIndex, `missing source-scan start marker: ${start}`).toBeGreaterThanOrEqual(0);
+  expect(endIndex, `missing source-scan end marker: ${end}`).toBeGreaterThan(startIndex);
+  return source.slice(startIndex, endIndex);
+}
+
+describe("TaskExecutor primary-checkout worktree invariant", () => {
+  const tempRoots: string[] = [];
+
+  afterEach(async () => {
+    await Promise.all(tempRoots.splice(0).map((rootDir) => rm(rootDir, { recursive: true, force: true })));
+  });
+
+  it("creates and reuses task branches in distinct worktrees while primary main remains unchanged", async () => {
+    const rootDir = await mkdtemp(join(tmpdir(), "fusion-worktree-invariant-"));
+    tempRoots.push(rootDir);
+    git(rootDir, ["init", "--initial-branch=main"]);
+    git(rootDir, ["config", "user.name", "Fusion Test"]);
+    git(rootDir, ["config", "user.email", "fusion-test@example.test"]);
+    await writeFile(join(rootDir, "README.md"), "fixture\n");
+    git(rootDir, ["add", "README.md"]);
+    git(rootDir, ["commit", "-m", "initial commit"]);
+
+    const executor = createExecutor(rootDir);
+    const createWorktree = (executor as any).createWorktree.bind(executor) as (
+      branch: string,
+      path: string,
+      taskId: string,
+      startPoint?: string,
+    ) => Promise<{ path: string; branch: string }>;
+
+    const cases = [
+      { name: "fresh", branch: "fusion/fn-8370-fresh", setup: async (_path: string) => {} },
+      { name: "unregistered", branch: "fusion/fn-8370-unregistered", setup: async (path: string) => mkdir(path) },
+      { name: "registered-reuse", branch: "fusion/fn-8370-reuse", setup: async (_path: string) => {} },
+    ];
+
+    for (const { name, branch, setup } of cases) {
+      const worktreePath = join(rootDir, ".worktrees", name);
+      await setup(worktreePath);
+      const created = await createWorktree(branch, worktreePath, `FN-8370-${name}`, "main");
+
+      expect(canonicalPath(created.path)).toBe(canonicalPath(worktreePath));
+      expect(created.branch).toBe(branch);
+      expect(git(rootDir, ["rev-parse", "--abbrev-ref", "HEAD"])).toBe("main");
+
+      const registration = registeredWorktrees(rootDir).find(({ path }) => path === canonicalPath(worktreePath));
+      expect(registration).toEqual({ path: canonicalPath(worktreePath), branch: `refs/heads/${branch}` });
+      expect(canonicalPath(worktreePath)).not.toBe(canonicalPath(rootDir));
+
+      if (name === "registered-reuse") {
+        const reused = await createWorktree(branch, worktreePath, "FN-8370-reuse", "main");
+        expect(canonicalPath(reused.path)).toBe(canonicalPath(worktreePath));
+        expect(git(rootDir, ["rev-parse", "--abbrev-ref", "HEAD"])).toBe("main");
+        expect(registeredWorktrees(rootDir).filter(({ path }) => path === canonicalPath(worktreePath))).toHaveLength(1);
+      }
+    }
+  });
+
+  it("guards every task-worktree creation or acquisition surface against primary-checkout branch switches", () => {
+    /*
+    FNXC:Worktrees 2026-07-19-15:47:
+    This source guard complements the real-git test above. It must fail if a task-worktree creation or
+    acquisition surface reintroduces `git checkout`/`git switch` against the project root to select a
+    task branch. Merger's later integration-target checkout is deliberately outside the reacquire slice.
+    */
+    const executorCreation = sourceRegion(executorSource, "private async createWorktree(", "private async cleanupConflictingWorktree(");
+    const acquisition = sourceRegion(acquisitionSource, "const createWorktreeImpl = createWorktree", "const logConfiguredCopyFileResults");
+    const mergerReacquire = sourceRegion(mergerSource, "const reacquireReuseIntegrationWorktree = async", "// 3b. Ensure rootDir is based on the resolved integration target before merging.");
+
+    expect(executorCreation).toContain("git worktree add");
+    expect(acquisition).toContain("backend.create(");
+    expect(mergerReacquire).toContain("git worktree add -f");
+
+    const rootCheckoutSwitch = /execAsync\(\s*`git\s+(?:checkout|switch)(?:\s+(?:-b|-c))?\b/;
+    for (const [surface, source] of [
+      ["TaskExecutor.createWorktree", executorCreation],
+      ["acquireTaskWorktree createWorktreeImpl", acquisition],
+      ["merger reacquire callback", mergerReacquire],
+    ] as const) {
+      expect(source, `${surface} must not select a task branch in the primary checkout`).not.toMatch(rootCheckoutSwitch);
+    }
+  });
+});
diff --git a/packages/engine/src/executor.ts b/packages/engine/src/executor.ts
index 21c98e074d..71a5178d82 100644
--- a/packages/engine/src/executor.ts
+++ b/packages/engine/src/executor.ts
@@ -17045,6 +17045,13 @@ You have access to the file system to review changes.${inlineFixBlock}${verdictB
     return "retry";
   }
 
+  /*
+  FNXC:Worktrees 2026-07-19-15:47:
+  Branch-needing task work must be created with `git worktree add` in an isolated checkout. Per the
+  AGENTS.md “Prefer main For Direct Work; Use Worktrees For Branches” standing rule, this.rootDir is
+  never switched with `git checkout` or `git switch` to select a task branch; see the primary-checkout
+  invariant regression test for the executable guard.
+  */
   private async createWorktree(
     branch: string,
     path: string,
diff --git a/packages/engine/src/worktree-acquisition.ts b/packages/engine/src/worktree-acquisition.ts
index 660e81de62..6a34d7ed0d 100644
--- a/packages/engine/src/worktree-acquisition.ts
+++ b/packages/engine/src/worktree-acquisition.ts
@@ -326,6 +326,11 @@ export async function acquireTaskWorktree(opts: AcquireTaskWorktreeOptions): Pro
     }
   };
 
+  /*
+  FNXC:Worktrees 2026-07-19-15:47:
+  Acquisition delegates branch creation to the isolated-worktree primitive. The project root remains
+  on its current branch; task branch selection must never use a root-checkout `git checkout` or `git switch`.
+  */
   const createWorktreeImpl = createWorktree
     ? createWorktree
     : async (createBranch: string, createPath: string, createTaskId: string, startPoint?: string, allowRename?: boolean) => {