diff --git a/.gitea/workflows/check.yml b/.gitea/workflows/check.yml
index 3214e781..ee17d5ee 100644
--- a/.gitea/workflows/check.yml
+++ b/.gitea/workflows/check.yml
@@ -7,8 +7,12 @@
 # runs pytest NATIVELY against a `postgres` service (not the local docker-compose.test.yml, whose
 # bind-mount is unreliable under a mounted-socket runner); fidelity is kept by installing the SAME pinned
 # requirements, Postgres 16, and ffmpeg, and by setting SIFTLODE_TEST_REQUIRE_DB=1 so a DB/ffmpeg that
-# failed to come up is a hard error, never a silent skip. node 22 matches the prod build image
-# (Dockerfile `node:22-alpine`).
+# failed to come up is a hard error, never a silent skip.
+#
+# The frontend container's node major is NOT free: it must equal the prod build image's and the dev
+# machine's, because vitest/tsc read the engine's own Intl and web-API implementations and those split
+# between majors. `.nvmrc` is the single source of truth and
+# `frontend/src/lib/nodeEngine.contract.test.ts` fails this gate if any of them drift apart.
 name: gate
 
 on: [push, pull_request]
@@ -16,7 +20,7 @@ on: [push, pull_request]
 jobs:
   frontend:
     runs-on: ubuntu-latest
-    container: node:22-bookworm
+    container: node:24-bookworm
     defaults:
       run:
         working-directory: frontend
@@ -34,7 +38,11 @@ jobs:
     # A combined python+node image: python 3.12 (matches the prod Dockerfile) for the tests, and node
     # so the JS actions (checkout) run. actions/setup-python is NOT usable here — the runner can't reach
     # GitHub's python-versions download, so it must be baked into the image.
-    container: nikolaik/python-nodejs:python3.12-nodejs20
+    #
+    # This node runs the checkout action, never our code, so it is deliberately OUTSIDE the engine
+    # contract the frontend job is held to — it tracks `.nvmrc` as hygiene (node 20 went EOL in April
+    # 2026), but a missing upstream tag here must never be able to block the gate.
+    container: nikolaik/python-nodejs:python3.12-nodejs24
     services:
       postgres:
         image: postgres:16-alpine
diff --git a/.nvmrc b/.nvmrc
new file mode 100644
index 00000000..a45fd52c
--- /dev/null
+++ b/.nvmrc
@@ -0,0 +1 @@
+24
diff --git a/Dockerfile b/Dockerfile
index 67eb44ae..d6b362c5 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -10,7 +10,9 @@ ARG GIT_SHA=unknown
 ARG BUILD_DATE=
 
 # Stage 1: build the frontend SPA
-FROM node:22-alpine AS frontend
+# The major is pinned to `.nvmrc` and held there by frontend/src/lib/nodeEngine.contract.test.ts:
+# this image, the CI gate's image and the dev machine must be ONE engine (see that test for why).
+FROM node:24-alpine AS frontend
 WORKDIR /fe
 # Deps first, keyed on the lockfile, so editing app source never re-resolves node_modules.
 COPY frontend/package.json frontend/package-lock.json ./
diff --git a/docs/development.md b/docs/development.md
index c82a5d9a..0e7ae2f2 100644
--- a/docs/development.md
+++ b/docs/development.md
@@ -3,6 +3,17 @@
 How to run Siftlode locally and pass the gate. Conventions for the backend code itself live in
 [`backend/app/README.md`](../backend/app/README.md).
 
+## Node
+
+One major, everywhere. `.nvmrc` pins it; the CI container, the `Dockerfile`'s frontend build stage,
+`frontend/package.json`'s `engines` and `@types/node` must all match it — and so must the machine you
+run the gate on. `frontend/src/lib/nodeEngine.contract.test.ts` fails the gate the moment any of them
+drift apart, including when yours is the odd one out. `nvm use` in the repo root picks it up.
+
+This is a contract, not a preference: tsc and vitest read the engine's own `Intl` and web APIs, and
+those change between majors — the same currency format renders `$160M` on one and `$160.0M` on the
+next. A gate that is green on a major you do not ship is green about nothing.
+
 ## Run it locally
 
 The whole stack (Postgres + API + worker) comes up from the localdev compose file:
diff --git a/frontend/package-lock.json b/frontend/package-lock.json
index 6eb41273..8d6a49e1 100644
--- a/frontend/package-lock.json
+++ b/frontend/package-lock.json
@@ -5,6 +5,7 @@
   "packages": {
     "": {
       "name": "siftlode-frontend",
+      "license": "AGPL-3.0-only",
       "dependencies": {
         "@dnd-kit/core": "^6.3.1",
         "@dnd-kit/sortable": "^10.0.0",
@@ -26,7 +27,7 @@
         "@playwright/test": "^1.61.1",
         "@tailwindcss/postcss": "^4.3.3",
         "@testing-library/react": "^16.3.2",
-        "@types/node": "^26.1.1",
+        "@types/node": "^24.13.6",
         "@types/react": "^18.3.3",
         "@types/react-dom": "^18.3.0",
         "@vitejs/plugin-react": "^4.3.1",
@@ -44,6 +45,9 @@
         "typescript-eslint": "8.46.0",
         "vite": "^5.3.4",
         "vitest": "^3.2.7"
+      },
+      "engines": {
+        "node": "24.x"
       }
     },
     "node_modules/@alloc/quick-lru": {
@@ -2909,13 +2913,13 @@
       "license": "MIT"
     },
     "node_modules/@types/node": {
-      "version": "26.1.1",
-      "resolved": "https://registry.npmjs.org/@types/node/-/node-26.1.1.tgz",
-      "integrity": "sha512-nxAkRSVkN1Y0JC1W8ky/fTfkGsMmcrRsbx+3XoZE+rMOX71kLYTV7fLXpqud1GpbpP5TuffXFqfX7fH2GgZREw==",
+      "version": "24.13.6",
+      "resolved": "https://registry.npmjs.org/@types/node/-/node-24.13.6.tgz",
+      "integrity": "sha512-SGrw/h3KPFshy3OE6ZL53LMBG5vGQQ8/gIpiqz/kRZhPJ7HgwCEs8LBuNtWLa8dvGZVpSF7+Bf+c11HUrCb/yg==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
-        "undici-types": "~8.3.0"
+        "undici-types": "~7.18.0"
       }
     },
     "node_modules/@types/prop-types": {
@@ -6409,9 +6413,9 @@
       }
     },
     "node_modules/undici-types": {
-      "version": "8.3.0",
-      "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-8.3.0.tgz",
-      "integrity": "sha512-j375ScV60dom+YkPFIfTLcOiPxkN/buHz5GobjLhixFuANaNs3C9l4GmrWqejgXWJ7BbJcFYpTEUkS1Ge8bpZQ==",
+      "version": "7.18.2",
+      "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-7.18.2.tgz",
+      "integrity": "sha512-AsuCzffGHJybSaRrmr5eHr81mwJU3kjw6M+uprWvCXiNeN9SOGwQ3Jn8jb8m3Z6izVgknn1R0FTCEAP2QrLY/w==",
       "dev": true,
       "license": "MIT"
     },
diff --git a/frontend/package.json b/frontend/package.json
index bb308fbf..ab3e43d6 100644
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -3,6 +3,9 @@
   "private": true,
   "license": "AGPL-3.0-only",
   "type": "module",
+  "engines": {
+    "node": "24.x"
+  },
   "scripts": {
     "dev": "vite",
     "typecheck": "npm run typecheck:app && npm run typecheck:node && npm run typecheck:e2e",
@@ -40,7 +43,7 @@
     "@playwright/test": "^1.61.1",
     "@tailwindcss/postcss": "^4.3.3",
     "@testing-library/react": "^16.3.2",
-    "@types/node": "^26.1.1",
+    "@types/node": "^24.13.6",
     "@types/react": "^18.3.3",
     "@types/react-dom": "^18.3.0",
     "@vitejs/plugin-react": "^4.3.1",
diff --git a/frontend/src/lib/nodeEngine.contract.test.ts b/frontend/src/lib/nodeEngine.contract.test.ts
new file mode 100644
index 00000000..90ae85bf
--- /dev/null
+++ b/frontend/src/lib/nodeEngine.contract.test.ts
@@ -0,0 +1,80 @@
+import { readFileSync } from "node:fs";
+import { expect, it } from "vitest";
+
+// Four machines build or check this app, and for months they did not agree on which Node they were.
+//
+// On 2026-09-20 the server gate had been red for 73 of its last 90 frontend runs while the local gate
+// was green, and nothing told anyone: the dev machine ran Node 24, the CI container `node:22-bookworm`,
+// the prod build image `node:22-alpine`. The three failures were all engine splits, not code bugs —
+// `Intl.NumberFormat({notation:"compact", maximumFractionDigits:1})` renders "$160.0M" on 22 and
+// "$160M" on 24 (the minimum is the engine's to choose when you only give it a maximum), and
+// `Response.blob()` under jsdom hands back node's own Blob class on 22 but jsdom's on 24, so
+// `toBeInstanceOf(Blob)` asserts a realm rather than a type. Both are the engine showing through the
+// test — and the first one the engine showing through to the USER, since the browser that renders the
+// money string splits along the same line the build image does.
+//
+// So the version is a contract, not a coincidence, and this test is where it is held. `.nvmrc` is the
+// single source of truth; everything below must agree with it, including the engine actually running
+// this test. That last assertion is the one that pays: it turns "the local gate and the shipping image
+// have drifted apart" from something discovered months later in a CI log nobody reads into a red lane
+// on the very first run after the drift.
+//
+// To move the version: edit `.nvmrc`, then follow this test's failures. Every place it names is a place
+// that would otherwise have been forgotten.
+const readRoot = (rel: string) => readFileSync(new URL(`../../../${rel}`, import.meta.url), "utf8");
+
+function majorIn(source: string, pattern: RegExp, where: string): number {
+  const m = pattern.exec(source);
+  // A rename or a moved line lands here. Re-point the test — the contract did not go away with it.
+  expect(m, `${pattern} not found in ${where}`).not.toBeNull();
+  return Number(m![1]);
+}
+
+const pinned = majorIn(readRoot(".nvmrc"), /^v?(\d+)\b/, ".nvmrc");
+const pkg = JSON.parse(readFileSync(new URL("../../package.json", import.meta.url), "utf8")) as {
+  engines?: { node?: string };
+  devDependencies: Record<string, string>;
+};
+
+it("pins a plausible Node major", () => {
+  // Guards against a typo'd `.nvmrc` silently agreeing with nothing (`Number("v24.x") === NaN`).
+  expect(pinned).toBeGreaterThanOrEqual(24);
+});
+
+it("builds the shipped bundle on the pinned engine", () => {
+  // The image that compiles the SPA our users download.
+  expect(
+    majorIn(readRoot("Dockerfile"), /^FROM node:(\d+)-alpine AS frontend$/m, "Dockerfile"),
+  ).toBe(pinned);
+});
+
+it("runs the server gate on the pinned engine", () => {
+  // The frontend job only — the backend job's node runs the checkout action, never our code, so it is
+  // deliberately not part of this contract (see the note in check.yml).
+  expect(
+    majorIn(
+      readRoot(".gitea/workflows/check.yml"),
+      /^\s*container: node:(\d+)-bookworm$/m,
+      "check.yml",
+    ),
+  ).toBe(pinned);
+});
+
+it("declares the pinned engine to npm and to tsc", () => {
+  expect(pkg.engines?.node).toBe(`${pinned}.x`);
+  // Types a major ahead of the runtime make tsc green over APIs the shipped engine does not have —
+  // the same defect as the gate split, just moved to compile time.
+  const types = pkg.devDependencies["@types/node"];
+  expect(types, "@types/node is gone from devDependencies").toBeTruthy();
+  expect(majorIn(types!, /^\D*(\d+)\./, "@types/node")).toBe(pinned);
+});
+
+it("is itself running on the pinned engine", () => {
+  const running = Number(process.versions.node.split(".")[0]);
+  expect(
+    running,
+    `this machine runs Node ${process.versions.node}, but the gate and the prod image ship Node ` +
+      `${pinned}.x. Install it (\`nvm use\` reads .nvmrc) — a green gate on another major does not ` +
+      `mean the build is green.`,
+  ).toBe(pinned);
+});