diff --git a/scripts/verify-dep-lockstep.main.test.mjs b/scripts/verify-dep-lockstep.main.test.mjs
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+// End-to-end tests for the dep-lockstep guard's executable path (Copilot,
+// #1962). The sibling tests cover the pure helpers; nothing exercised `main()`,
+// so a regression in source enumeration, install discovery, lockfile loading,
+// the sibling-guard vouch, or the nonzero exit on real skew would have left the
+// whole suite green.
+//
+// Each case builds a throwaway repo — the guard derives its root from its own
+// file location, so the script is copied into the fixture rather than pointed
+// at one — `git add`s it (the enumeration is `git ls-files`, which reads the
+// index), and runs the guard as a subprocess to assert the exit status and
+// message. Run via `npm run test:scripts`.
+
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { execFileSync, spawnSync } from "node:child_process";
+import {
+  cpSync,
+  mkdirSync,
+  mkdtempSync,
+  realpathSync,
+  rmSync,
+  symlinkSync,
+  writeFileSync,
+} from "node:fs";
+import { createRequire } from "node:module";
+import { tmpdir } from "node:os";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+const scriptsDir = path.dirname(fileURLToPath(import.meta.url));
+const repoRoot = path.join(scriptsDir, "..");
+
+/** The real typescript install, symlinked into each fixture so the guard resolves it. */
+const typescriptDir = path.dirname(
+  createRequire(path.join(repoRoot, "clients", "web", "package.json")).resolve(
+    "typescript/package.json",
+  ),
+);
+
+const lock = (deps) => ({
+  lockfileVersion: 3,
+  packages: Object.fromEntries(
+    Object.entries(deps).map(([name, version]) => [
+      `node_modules/${name}`,
+      { version },
+    ]),
+  ),
+});
+
+/**
+ * Build a fixture repo: shared sources importing `zod` and `express`, a root
+ * install and one client install, and the guard itself. `rootDeps`/`webDeps`
+ * decide whether the two installs agree.
+ */
+function makeFixture({ rootDeps, webDeps, scripts }) {
+  // realpath matters: on macOS `tmpdir()` is `/var/...`, a symlink to
+  // `/private/var/...`. The guard only runs `main()` when `import.meta.url`
+  // (always the resolved path) matches `process.argv[1]`, so launching it via
+  // the unresolved path would load the module and silently do nothing —
+  // exiting 0 with no output, which every assertion here would misread.
+  const dir = realpathSync(mkdtempSync(path.join(tmpdir(), "dep-lockstep-")));
+  const write = (rel, contents) => {
+    mkdirSync(path.join(dir, path.dirname(rel)), { recursive: true });
+    writeFileSync(
+      path.join(dir, rel),
+      typeof contents === "string"
+        ? contents
+        : JSON.stringify(contents, null, 2),
+    );
+  };
+
+  write("package.json", {
+    name: "fixture",
+    scripts: scripts ?? {
+      validate: "npm run verify:format-coverage && npm run verify:dep-lockstep",
+      "verify:format-coverage": "node scripts/verify-format-coverage.mjs",
+      "verify:dep-lockstep": "node scripts/verify-dep-lockstep.mjs",
+    },
+  });
+  write("core/client.ts", 'import { z } from "zod";\nexport const s = z;\n');
+  write(
+    "test-servers/src/server.ts",
+    'import express from "express";\nexport const app = express;\n',
+  );
+  write(
+    "vitest.shared.mts",
+    'import path from "node:path";\nexport default path;\n',
+  );
+  write("package-lock.json", lock(rootDeps));
+  write("clients/web/package.json", { name: "web" });
+  write("clients/web/package-lock.json", lock(webDeps));
+
+  // The guard resolves its repo root from its own location, so it has to live
+  // inside the fixture; `lib/npm-scripts.mjs` comes along as its import.
+  mkdirSync(path.join(dir, "scripts", "lib"), { recursive: true });
+  for (const rel of [
+    "verify-dep-lockstep.mjs",
+    path.join("lib", "npm-scripts.mjs"),
+  ])
+    cpSync(path.join(scriptsDir, rel), path.join(dir, "scripts", rel));
+
+  mkdirSync(path.join(dir, "node_modules"), { recursive: true });
+  symlinkSync(
+    typescriptDir,
+    path.join(dir, "node_modules", "typescript"),
+    "dir",
+  );
+
+  execFileSync("git", ["init", "-q"], { cwd: dir });
+  execFileSync("git", ["add", "-A"], { cwd: dir });
+  return dir;
+}
+
+function runGuard(dir) {
+  const r = spawnSync(
+    process.execPath,
+    [path.join(dir, "scripts", "verify-dep-lockstep.mjs")],
+    { cwd: dir, encoding: "utf8" },
+  );
+  return { status: r.status, out: `${r.stdout}${r.stderr}` };
+}
+
+/** Run `fn` against a fresh fixture, always cleaning the temp dir up. */
+function withFixture(options, fn) {
+  const dir = makeFixture(options);
+  try {
+    return fn(dir);
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+}
+
+const ALIGNED = { zod: "4.4.3", express: "5.2.1" };
+
+test("main: exits 0 when every install agrees", () => {
+  withFixture({ rootDeps: ALIGNED, webDeps: ALIGNED }, (dir) => {
+    const { status, out } = runGuard(dir);
+    assert.equal(status, 0, out);
+    assert.match(out, /verify:dep-lockstep — OK/);
+    // Both shared trees and the named shared file contributed their imports.
+    assert.match(out, /2 install-crossing dependencies/);
+  });
+});
+
+test("main: exits 1 and names the skewed package and every holder", () => {
+  withFixture(
+    { rootDeps: { ...ALIGNED, zod: "4.3.6" }, webDeps: ALIGNED },
+    (dir) => {
+      const { status, out } = runGuard(dir);
+      assert.equal(status, 1, out);
+      assert.match(out, /\bzod\b/);
+      assert.match(out, /4\.3\.6\s+\(\.\)/);
+      assert.match(out, /4\.4\.3\s+\(clients\/web\)/);
+      // `express` agrees, so it must not be reported.
+      assert.doesNotMatch(out, /^\s+express$/m);
+    },
+  );
+});
+
+test("main: a package imported only by test-servers/src is still checked", () => {
+  // Guards the enumeration of the *second* shared tree: if only `core/` were
+  // scanned, this skew would pass.
+  withFixture(
+    { rootDeps: { ...ALIGNED, express: "5.0.0" }, webDeps: ALIGNED },
+    (dir) => {
+      const { status, out } = runGuard(dir);
+      assert.equal(status, 1, out);
+      assert.match(out, /\bexpress\b/);
+    },
+  );
+});
+
+test("main: exits 1 when a configured shared source matches no file", () => {
+  withFixture({ rootDeps: ALIGNED, webDeps: ALIGNED }, (dir) => {
+    // Drop `core/` from the index — the other sources keep the file count
+    // nonzero, which is exactly what the old aggregate check missed.
+    execFileSync("git", ["rm", "-r", "-q", "--cached", "core"], { cwd: dir });
+    const { status, out } = runGuard(dir);
+    assert.equal(status, 1, out);
+    assert.match(out, /matched no tracked file/);
+    assert.match(out, /^\s+core$/m);
+  });
+});
+
+test("main: exits 1 when the root validate no longer runs the sibling guard", () => {
+  withFixture(
+    {
+      rootDeps: ALIGNED,
+      webDeps: ALIGNED,
+      // `verify:format-coverage` dropped from the chain: the vouch must fail
+      // even though the dependency versions themselves are fine.
+      scripts: {
+        validate: "npm run verify:dep-lockstep",
+        "verify:dep-lockstep": "node scripts/verify-dep-lockstep.mjs",
+      },
+    },
+    (dir) => {
+      const { status, out } = runGuard(dir);
+      assert.equal(status, 1, out);
+      assert.match(out, /no longer runs `verify:format-coverage`/);
+    },
+  );
+});