diff --git a/package.json b/package.json
index 74a15da08b..310762f59b 100644
--- a/package.json
+++ b/package.json
@@ -61,6 +61,7 @@
     "ui:preview": "npm run ui:build && wrangler dev --config apps/loopover-ui/dist/server/wrangler.json --ip 127.0.0.1 --port 4173 --local",
     "ui:lint": "npm run ui:kit:build && npm --workspace @loopover/ui run lint && npm --workspace @loopover/ui-miner run lint",
     "ui:typecheck": "npm run ui:kit:build && npm --workspace @loopover/ui run typecheck && npm --workspace @loopover/ui-miner run typecheck",
+    "preui:test": "npm run check-node-version",
     "ui:test": "npm run ui:kit:build && npm --workspace @loopover/ui run test && npm --workspace @loopover/ui-miner run test && npm --workspace @loopover/miner-extension run test",
     "ui:openapi": "tsx scripts/write-ui-openapi.ts",
     "ui:openapi:check": "tsx scripts/write-ui-openapi.ts --check",
@@ -88,6 +89,8 @@
     "mcp:release-due": "node scripts/check-mcp-release-due.mjs --json",
     "mcp:release-candidate": "node scripts/check-mcp-release-candidate.mjs",
     "typecheck": "tsc --noEmit",
+    "check-node-version": "node scripts/check-node-version.mjs",
+    "pretest": "npm run check-node-version",
     "test": "vitest run",
     "test:unit": "vitest run test/unit",
     "test:integration": "vitest run test/integration",
@@ -96,13 +99,16 @@
     "test:live-gate-parity": "vitest run test/contract/live-gate-parity.test.ts",
     "test:driver-parity": "vitest run test/contract/coding-agent-driver-parity.test.ts",
     "test:changed": "vitest run --changed=origin/main",
+    "pretest:workers": "npm run check-node-version",
     "test:workers": "vitest run --config vitest.workers.config.ts",
+    "pretest:coverage": "npm run check-node-version",
     "test:coverage": "vitest run --coverage --pool=forks",
     "test:smoke:production": "node scripts/smoke-production.mjs",
     "test:smoke:observability": "node scripts/smoke-observability-traces.mjs",
     "test:smoke:observability:metrics": "node scripts/smoke-observability-metrics.mjs",
     "test:smoke:browser:install": "playwright install chromium",
     "test:smoke:browser": "node scripts/smoke-ui-browser.mjs",
+    "pretest:ci": "npm run check-node-version",
     "test:ci": "git diff --check && npm run actionlint && npm run lint:composite-actions && npm run db:migrations:check && npm run db:schema-drift:check && npm run selfhost:env-reference:check && npm run miner:env-reference:check && npm run selfhost:validate-observability && npm run cf-typegen:check && npm run build --workspace @loopover/engine && npm run build --workspace @loopover/discovery-index && npm run typecheck && npm run test:coverage && npm run test:engine-parity && npm run test:live-gate-parity && npm run test:driver-parity && npm run test --workspace @loopover/engine && npm run test:workers && npm run build:mcp && npm run test:mcp-pack && npm run build:miner && npm run test:miner-pack && npm run test:miner-deployment-docs-audit && npm run rees:test && npm run ui:openapi:check && npm run ui:openapi:settings-parity && npm run ui:version-audit && npm run docs:drift-check && npm run branding-drift:check && npm run manifest:drift-check && npm run engine-parity:drift-check && npm run engines-nvmrc:check && npm run release-manifest:sync:check && npm run command-reference:check && npm run ui:lint && npm run ui:typecheck && npm run ui:test && npm run ui:build",
     "test:release": "npm run test:ci && npm run changelog:check",
     "test:release:mcp": "npm run test:ci",
diff --git a/scripts/check-node-version.d.mts b/scripts/check-node-version.d.mts
new file mode 100644
index 0000000000..c836d2f3bb
--- /dev/null
+++ b/scripts/check-node-version.d.mts
@@ -0,0 +1,8 @@
+export function checkNodeVersion(options?: {
+  nodeVersion?: string;
+  readFile?: () => string;
+}): {
+  ok: boolean;
+  requiredRange: string | undefined;
+  nodeVersion?: string;
+};
diff --git a/scripts/check-node-version.mjs b/scripts/check-node-version.mjs
new file mode 100644
index 0000000000..7d8606889a
--- /dev/null
+++ b/scripts/check-node-version.mjs
@@ -0,0 +1,36 @@
+#!/usr/bin/env node
+// Fails fast, with a clear message, when the running Node doesn't satisfy root package.json's
+// engines.node -- even when node_modules is already installed. The root .npmrc's engine-strict=true only
+// fires during npm install/ci (dependency resolution); a node_modules installed while on the pinned Node,
+// followed by simply switching the active `node` (nvm/homebrew default change) with no reinstall, sails
+// straight past engine-strict on every later `npm run`. That's exactly the shape of gap that let the
+// Node 26 jsdom/localStorage bug (#7592/#7597/#7612) go unnoticed the first two times: a pile of
+// confusing downstream test failures instead of one clear "wrong Node version" message up front. Wired as
+// a `pretest*` hook (see package.json) on the commands people actually run vitest through.
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import semver from "semver";
+
+const PACKAGE_JSON_URL = new URL("../package.json", import.meta.url);
+
+export function checkNodeVersion({ nodeVersion = process.version, readFile = () => readFileSync(PACKAGE_JSON_URL, "utf8") } = {}) {
+  const pkg = JSON.parse(readFile());
+  const requiredRange = pkg.engines?.node;
+  if (!requiredRange) return { ok: true, requiredRange: undefined };
+
+  const ok = semver.satisfies(nodeVersion, requiredRange);
+  return { ok, requiredRange, nodeVersion };
+}
+
+function main() {
+  const { ok, requiredRange, nodeVersion } = checkNodeVersion();
+  if (!ok) {
+    console.error(
+      `\nRunning Node ${nodeVersion}, but this repo requires ${requiredRange} (see .nvmrc / package.json engines).\n` +
+        `Switch to the pinned Node version (e.g. \`nvm use\`) before running this command.\n`,
+    );
+    process.exit(1);
+  }
+}
+
+if (process.argv[1] === fileURLToPath(import.meta.url)) main();
diff --git a/test/unit/check-node-version-script.test.ts b/test/unit/check-node-version-script.test.ts
new file mode 100644
index 0000000000..a691d3a6b6
--- /dev/null
+++ b/test/unit/check-node-version-script.test.ts
@@ -0,0 +1,47 @@
+import { execFileSync } from "node:child_process";
+import { describe, expect, it } from "vitest";
+import { checkNodeVersion } from "../../scripts/check-node-version.mjs";
+
+describe("check-node-version script", () => {
+  it("passes when the running Node satisfies the declared engines.node range", () => {
+    const result = checkNodeVersion({
+      nodeVersion: "v22.23.1",
+      readFile: () => JSON.stringify({ engines: { node: ">=22.0.0 <23.0.0" } }),
+    });
+
+    expect(result).toEqual({ ok: true, requiredRange: ">=22.0.0 <23.0.0", nodeVersion: "v22.23.1" });
+  });
+
+  it("fails when the running Node is outside the declared engines.node range (the Node 26 case)", () => {
+    const result = checkNodeVersion({
+      nodeVersion: "v26.5.0",
+      readFile: () => JSON.stringify({ engines: { node: ">=22.0.0 <23.0.0" } }),
+    });
+
+    expect(result).toEqual({ ok: false, requiredRange: ">=22.0.0 <23.0.0", nodeVersion: "v26.5.0" });
+  });
+
+  it("passes trivially when package.json declares no engines.node at all", () => {
+    const result = checkNodeVersion({
+      nodeVersion: "v26.5.0",
+      readFile: () => JSON.stringify({}),
+    });
+
+    expect(result).toEqual({ ok: true, requiredRange: undefined });
+  });
+
+  // Regression guard: proves the process actually running this test suite -- CI's own Node, or whatever a
+  // contributor is running locally -- genuinely satisfies the real repo's engines.node. If this fails, the
+  // repo is being tested on the wrong Node right now.
+  it("the real repo's engines.node is satisfied by whatever Node is running this suite", () => {
+    const result = checkNodeVersion();
+
+    expect(result.ok).toBe(true);
+  });
+
+  it("prints nothing and exits 0 as a subprocess on a satisfying Node", () => {
+    const output = execFileSync(process.execPath, ["scripts/check-node-version.mjs"], { encoding: "utf8" });
+
+    expect(output).toBe("");
+  });
+});