diff --git a/client/src/App.jsx b/client/src/App.jsx
index 3cacb15..ed3c6be 100644
--- a/client/src/App.jsx
+++ b/client/src/App.jsx
@@ -5,6 +5,7 @@ import MaintenanceGate from './components/MaintenanceGate.jsx'
 import RequireAuth from './components/RequireAuth.jsx'
 import RequirePlayer from './components/RequirePlayer.jsx'
 import RoleGate from './components/RoleGate.jsx'
+import { routesFor } from './modules/registry.js'
 
 // Public
 import Portal from './routes/public/Portal.jsx'
@@ -115,6 +116,16 @@ export default function App() {
             <Route path="/site/market/vendors/:serial" element={<MarketVendor />} />
             <Route path="/wiki" element={<Wiki />} />
             <Route path="/wiki/:slug" element={<WikiArticle />} />
+            {/* Installed modules' public pages, namespaced `/<id>/…` — the
+                registry prefixes the segment, so a module cannot spell its way
+                out of it (docs/website/MODULE_API.md §3.3). Declared before the
+                CMS catch-all below: React Router ranks a static segment over a
+                dynamic one, so the order is not what saves us, but keeping the
+                two adjacent makes the relationship visible to whoever adds the
+                next route here. */}
+            {routesFor('public').map((r) => (
+              <Route key={r.path} path={`/${r.path}`} element={r.element} />
+            ))}
             {/* CMS pages: top-level /:slug, matched only after the named routes
                 above (React Router ranks static routes over this dynamic one). */}
             <Route path="/:slug" element={<CmsPage />} />
@@ -205,6 +216,19 @@ export default function App() {
             <Route path="users/:id" element={<UserDetail />} />
             <Route path="invites" element={<InvitesAdmin />} />
             <Route path="account" element={<AccountAdmin />} />
+            {/* Installed modules' admin pages, at /admin/<id>/…, already inside
+                RequireAuth + AdminLayout. A module cannot supply its own auth
+                wrapper — only an optional { roles }, which core applies as the
+                same RoleGate its own routes above use, so the sidebar and the
+                route table cannot disagree about who may see what. Before the
+                `*` redirect, which would otherwise swallow every one of them. */}
+            {routesFor('admin').map((r) => (
+              <Route
+                key={r.path}
+                path={r.path}
+                element={r.gate ? <RoleGate roles={r.gate.roles}>{r.element}</RoleGate> : r.element}
+              />
+            ))}
             <Route path="*" element={<Navigate to="/admin" replace />} />
           </Route>
 
@@ -225,6 +249,17 @@ export default function App() {
             <Route path="/player/char/:serial" element={<PlayerCharacter />} />
             <Route path="/account" element={<PlayerAccount />} />
             <Route path="/account/appeals" element={<PlayerAppeals />} />
+            {/* Installed modules' player-portal pages, at /player/<id>/…. This
+                group's own routes are absolute (its layout route has no path),
+                so the prefix is written here rather than inherited — the one
+                place the three areas do not read alike. */}
+            {routesFor('player').map((r) => (
+              <Route
+                key={r.path}
+                path={`/player/${r.path}`}
+                element={r.gate ? <RoleGate roles={r.gate.roles}>{r.element}</RoleGate> : r.element}
+              />
+            ))}
           </Route>
 
           <Route path="*" element={<Navigate to="/" replace />} />
diff --git a/client/src/api/client.js b/client/src/api/client.js
index 4312872..068e99b 100644
--- a/client/src/api/client.js
+++ b/client/src/api/client.js
@@ -42,6 +42,17 @@ function safeParse(text) {
   }
 }
 
+// The request PRIMITIVE, exported for installed modules and handed to them on
+// `window.__rg.api` (docs/website/MODULE_API.md §3.5). Core owns the fetch
+// semantics — same-origin /api/v1, cookies included, JSON in and out, ApiError
+// on a non-2xx — and nothing above them: a module owns the paths it calls,
+// because it owns the routes at the other end.
+//
+// The `api` object below stays core's own binding surface. Its `atlas` and
+// `shard` namespaces are module bindings that only still live here because
+// Phase 3 has not moved them yet.
+export { req as request }
+
 export const api = {
   // ----- auth -----
   me: () => req('/auth/me'),
diff --git a/client/src/main.jsx b/client/src/main.jsx
index dc409af..dd70f9a 100644
--- a/client/src/main.jsx
+++ b/client/src/main.jsx
@@ -2,12 +2,54 @@ import React from 'react'
 import { createRoot } from 'react-dom/client'
 import { BrowserRouter } from 'react-router-dom'
 import App from './App.jsx'
+import { publishSharedDependencies } from './modules/shared.js'
 import './styles/theme.css'
 
-createRoot(document.getElementById('root')).render(
-  <React.StrictMode>
-    <BrowserRouter>
-      <App />
-    </BrowserRouter>
-  </React.StrictMode>,
-)
+// Publish window.__rg BEFORE rendering and before any module chunk evaluates.
+// Installed modules arrive as `<script type="module" src="/modules/<id>/…">`
+// tags the server injects into the shell (server/src/utils/htmlShell.js), placed
+// after this bundle's own tag; module scripts execute in document order, so they
+// resolve their externals against the global this call sets up
+// (docs/website/MODULE_API.md §3.2).
+publishSharedDependencies()
+
+// Render on DOMContentLoaded rather than immediately, and that is the one line
+// of core's boot the module system changes.
+//
+// Deferred scripts — which every `type="module"` script is — execute in document
+// order and ALL of them finish before DOMContentLoaded fires. Waiting for that
+// event is therefore the guarantee that every installed module has registered
+// its routes before React reads the registry: no loading state, no re-render,
+// and no ordering race between core's bundle and a module's. A module chunk that
+// 404s or throws does not hold the event back, so a broken module costs its own
+// pages and not the site.
+//
+// The readyState check below is `'complete'`, and it is not the obvious
+// `'loading'`. A DEFERRED script — which every `type="module"` script is — runs
+// after the document has been parsed, so by the time this line executes
+// readyState is already `'interactive'`; DOMContentLoaded has NOT fired yet and
+// still comes after every deferred script. Testing for `'loading'` therefore
+// mounts immediately, before any module chunk has evaluated, and a module's
+// routes are missing from the very first render — which looks exactly like a
+// module that failed to load: its URL falls through to core's catch-all and
+// redirects home. Found by loading a real chunk in a browser; no unit test in
+// this repo can see it.
+//
+// `'complete'` is only reached after `load`, which is strictly later than any
+// static deferred script, so this branch is the genuine "the event has already
+// been and gone" case and not a wrong guess about our own timing.
+function mount() {
+  createRoot(document.getElementById('root')).render(
+    <React.StrictMode>
+      <BrowserRouter>
+        <App />
+      </BrowserRouter>
+    </React.StrictMode>,
+  )
+}
+
+if (document.readyState === 'complete') {
+  mount()
+} else {
+  document.addEventListener('DOMContentLoaded', mount, { once: true })
+}
diff --git a/client/src/modules/registry.js b/client/src/modules/registry.js
new file mode 100644
index 0000000..fb5d9d4
--- /dev/null
+++ b/client/src/modules/registry.js
@@ -0,0 +1,137 @@
+// ── The client-side module registry ────────────────────────────────────────
+//
+// Phase 2, PR 7 of docs/website/MODULE_SYSTEM.md §2.7. The normative contract is
+// docs/website/MODULE_API.md §3.3; where the two disagree, the contract wins.
+//
+// A module's prebuilt chunk registers its routes, its nav entries and its feature
+// provider here, and core reads them back. This is the client twin of the
+// server's modules/loader.js — with one structural difference worth stating,
+// because it is what makes the file this short: core *hands* the registry to the
+// module (on `window.__rg`, see shared.js) rather than discovering it. There is
+// nothing to scan, nothing to validate a manifest against, and no failure mode
+// where half a module is registered.
+//
+// **Timing is the whole design.** Module chunks are `<script type="module" src>`
+// tags the server injects before `</body>` (server/src/utils/htmlShell.js), after
+// core's own bundle. Module scripts are deferred, so they evaluate after that
+// bundle has run — which is where `window.__rg` is published — and all of them
+// finish before DOMContentLoaded. main.jsx waits for that same event before
+// calling render(), so registration is complete before React reads any of this.
+//
+// That is what buys the simplicity here: registration is a plain synchronous
+// write with no subscribers, not an observable store, because nothing can
+// register after the first render. If that ever stops being true it changes in
+// this file and in main.jsx, not in a dozen consumers.
+//
+// What PR 7 wires up is `routesFor` (App.jsx). `navFor` and `featureProviderFor`
+// are stored and returned faithfully but core does not read them yet — PR 8 adds
+// the nav interleave and the feature-provider seam. Storing them is not the kind
+// of accepting stub the server's registries refused to be: nothing is discarded
+// here, so a module that registers nav in this core gets it back from `navFor`.
+
+const routes = { public: [], admin: [], player: [] }
+const nav = { public: [], admin: [], player: [] }
+const featureProviders = new Map()
+const registered = new Set()
+
+const AREAS = ['public', 'admin', 'player']
+
+function assertArea(area, call) {
+  if (!AREAS.includes(area)) throw new Error(`${call}: unknown area "${area}"`)
+}
+
+/**
+ * Route components, by area.
+ *
+ * @param {string} id  the module id — the URL segment its routes are namespaced under
+ * @param {{public?: Array, admin?: Array, player?: Array}} byArea
+ *   each entry `{ path, element, gate? }`. `path` is relative to the module's
+ *   namespace; core prefixes it and mounts it inside the area's existing wrapper
+ *   (`/<id>/…` under MaintenanceGate, `/admin/<id>/…` under RequireAuth +
+ *   AdminLayout, `/player/<id>/…` under RequirePlayer + PlayerPortalLayout).
+ *   `gate` is an optional `{ roles: [...] }` that core applies as its own
+ *   RoleGate — a module cannot supply an auth wrapper, because the sidebar and
+ *   the route table have to agree about who may see what (§3.3).
+ */
+export function registerRoutes(id, byArea) {
+  for (const [area, list] of Object.entries(byArea || {})) {
+    assertArea(area, 'registerRoutes')
+    for (const route of list || []) {
+      // Prefixed HERE rather than by the module: a module cannot claim a path
+      // outside its own namespace however it spells `path` — a leading `/`, a
+      // trailing one, or several — because it never gets to write the segment
+      // its routes hang under.
+      const path = `${id}/${String(route.path || '').replace(/^\/+/, '')}`.replace(/\/+$/, '')
+      routes[area].push({ ...route, path, moduleId: id })
+    }
+  }
+  registered.add(id)
+}
+
+/**
+ * Nav entries, interleaved into CORE groups rather than appended as a block.
+ *
+ * Today's UO items sit inside core's own Moderation and System groups; a "UO"
+ * group at the bottom of the sidebar would be a visible regression on the day
+ * the module is extracted (MODULE_SYSTEM.md §1.4). `group` names an existing
+ * core group, `order` sorts within it, and an unknown group name appends rather
+ * than dropping the item — a mis-typed group must cost a position, never a link.
+ *
+ * @param {string} id
+ * @param {{area: string, items: Array<{label, to, group?, order?, roles?, feature?}>}} spec
+ */
+export function registerNav(id, spec) {
+  const { area, items } = spec || {}
+  assertArea(area, 'registerNav')
+  for (const item of items || []) nav[area].push({ ...item, moduleId: id })
+  registered.add(id)
+}
+
+/**
+ * The hook that answers "which of this module's features may this viewer see".
+ *
+ * Core keeps a generic flag context and owns none of the semantics — `uo` fills
+ * its namespace with today's `useShardFeatures` (MODULE_SYSTEM.md §1.5). With no
+ * module installed the nav filter is a correct no-op, because no core nav item
+ * carries a `feature` today.
+ */
+export function registerFeatureProvider(id, namespace, hook) {
+  featureProviders.set(namespace, { id, hook })
+  registered.add(id)
+}
+
+export const routesFor = (area) => routes[area] || []
+
+// Sorted by the `order` a module asked for. Array#sort is stable in every engine
+// this ships to, so two modules asking for the same slot keep load order —
+// which is alphabetical by id, the same order the server scans in (§4.2).
+export const navFor = (area) =>
+  [...(nav[area] || [])].sort((a, b) => (a.order ?? 100) - (b.order ?? 100))
+
+export const featureProviderFor = (namespace) => featureProviders.get(namespace)
+
+export const registeredIds = () => [...registered]
+
+/** Test seam. Nothing in the app calls this — there is no unregistering. */
+export function _reset() {
+  for (const area of AREAS) {
+    routes[area].length = 0
+    nav[area].length = 0
+  }
+  featureProviders.clear()
+  registered.clear()
+}
+
+// The object handed to modules on window.__rg.registry. Deliberately the write
+// calls plus the read ones: a module reading `routesFor` is how it finds out
+// another module is installed, which is the only supported form of module-to-
+// module awareness (there is no dependency resolution).
+export const registry = {
+  registerRoutes,
+  registerNav,
+  registerFeatureProvider,
+  routesFor,
+  navFor,
+  featureProviderFor,
+  registeredIds,
+}
diff --git a/client/src/modules/shared.js b/client/src/modules/shared.js
new file mode 100644
index 0000000..03a4ad7
--- /dev/null
+++ b/client/src/modules/shared.js
@@ -0,0 +1,96 @@
+// ── window.__rg — the shared-dependency global ─────────────────────────────
+//
+// Phase 2, PR 7 of docs/website/MODULE_SYSTEM.md §2.7; the normative shape is
+// docs/website/MODULE_API.md §3.2.
+//
+// A module's client half is a PREBUILT ESM chunk — the operator never builds
+// anything (MODULE_SYSTEM.md §1.14) — served same-origin and loaded under
+// `script-src 'self'` with no 'unsafe-inline'. That combination is what rules out
+// an import map: an import map has to be an inline `<script type="importmap">`,
+// and the policy forbids inline scripts outright. So the shared dependencies ride
+// on a global, and the module's Rollup externals are aliased to two-line shims
+// that re-export from it (§3.6).
+//
+// **There is exactly one React in the page and core owns it.** A module that
+// bundled its own would get a second hook dispatcher and fail at its first
+// useState. That is the same rule the server half enforces for `express` and
+// `express-validator` on `ctx`, and for the same reason: anything shared between
+// core and a module is owned by core and HANDED OVER, never resolved by the
+// module.
+
+import * as react from 'react'
+import * as reactDom from 'react-dom/client'
+import * as router from 'react-router-dom'
+// The automatic JSX runtime, and it is not decoration. A module's bundler
+// compiles every .jsx file to imports from `react/jsx-runtime` under the modern
+// default, and those have to resolve to CORE's React like every other import.
+// Without it here a module would have to build with `jsxRuntime: 'classic'`;
+// with it, a module uses the default its tooling already assumes.
+import * as jsxRuntime from 'react/jsx-runtime'
+
+import { registry } from './registry.js'
+import { MODULE_API_VERSION } from './version.js'
+
+import PublicLayout from '../components/PublicLayout.jsx'
+import PageHeader from '../components/PageHeader.jsx'
+import { Loading, ErrorState, EmptyState } from '../components/PageState.jsx'
+import { useAsync } from '../lib/useAsync.js'
+import { useAuth } from '../contexts/AuthContext.jsx'
+import { useSite } from '../contexts/SiteContext.jsx'
+import { request, ApiError } from '../api/client.js'
+
+// The UI kit is CURATED AND CLOSED (§3.4), not a re-export of components/. These
+// seven are what the smallest UO page already needs beyond React and the router:
+// without them a module either reaches into core's tree — violating the
+// zero-import rule the whole boundary rests on — or ships its own copies, which
+// means a module page that does not look like the site it is installed in, and
+// that drifts further every time core's layout changes.
+//
+// Adding a member is a MINOR MODULE_API_VERSION bump; changing a member's props
+// is a MAJOR one. That is a real constraint on core's own refactoring and it is
+// the price of the boundary being worth anything.
+//
+// `AdminPage` appears in §3.4's table and is deliberately absent: core has no
+// such component — admin views are plain markup inside AdminLayout — and
+// inventing one to satisfy a table would be a core change with no consumer until
+// Phase 3. The contract is amended rather than the code padded, and adding it
+// later costs a minor bump, which is exactly the case the versioning is for.
+const ui = {
+  PublicLayout,
+  PageHeader,
+  Loading,
+  ErrorState,
+  EmptyState,
+  useAsync,
+  useAuth,
+  useSite,
+}
+
+// The request PRIMITIVE, not the `api` object (§3.5). `api.atlas` and `api.shard`
+// are module bindings that only still live in core's client because Phase 3 has
+// not moved them; a module builds its own namespace over `request` and owns the
+// paths it calls — which is right, because it owns the routes at the other end.
+const api = { request, ApiError }
+
+/**
+ * Publish `window.__rg`. Called by main.jsx before it renders, and before any
+ * module chunk evaluates.
+ *
+ * Frozen, one level down as well as at the top: the object a module reaches for
+ * its React is not somewhere a module gets to leave something for the next one.
+ * Cross-module communication is a thing the contract does not have, and an
+ * unfrozen global is how a codebase acquires one by accident.
+ */
+export function publishSharedDependencies() {
+  window.__rg = Object.freeze({
+    version: MODULE_API_VERSION,
+    react,
+    reactDom,
+    router,
+    jsxRuntime,
+    registry,
+    ui: Object.freeze(ui),
+    api: Object.freeze(api),
+  })
+  return window.__rg
+}
diff --git a/client/src/modules/version.js b/client/src/modules/version.js
new file mode 100644
index 0000000..1d4ff98
--- /dev/null
+++ b/client/src/modules/version.js
@@ -0,0 +1,14 @@
+// The client's copy of MODULE_API_VERSION. It must equal the server's
+// (server/src/modules/version.js) — the two halves version ONE contract
+// (docs/website/MODULE_API.md §1.1), and a module checks whichever half it is
+// talking to: `coreApi` against the server's at load time, `window.__rg.version`
+// against the client's before it registers anything.
+//
+// Duplicated rather than fetched, and that is deliberate. The value has to be on
+// `window.__rg` before the first module chunk evaluates, which is earlier than
+// any network round trip could answer — a fetched version would mean either an
+// await before render or a module reading `undefined`. The cost of the copy is
+// that the two files can drift, so a test asserts they agree
+// (client/test/moduleRegistry.test.js) rather than trusting a bump to remember
+// both.
+export const MODULE_API_VERSION = '1.0.0'
diff --git a/client/test/moduleRegistry.test.js b/client/test/moduleRegistry.test.js
new file mode 100644
index 0000000..2c5740a
--- /dev/null
+++ b/client/test/moduleRegistry.test.js
@@ -0,0 +1,164 @@
+import { test, beforeEach } from 'node:test'
+import assert from 'node:assert/strict'
+import fs from 'node:fs'
+import path from 'node:path'
+import { fileURLToPath } from 'node:url'
+
+import {
+  registry,
+  registerRoutes,
+  registerNav,
+  registerFeatureProvider,
+  routesFor,
+  navFor,
+  featureProviderFor,
+  registeredIds,
+  _reset,
+} from '../src/modules/registry.js'
+import { MODULE_API_VERSION } from '../src/modules/version.js'
+
+// The client-side module registry (docs/website/MODULE_API.md §3.3). Tested in
+// isolation from React, like the nav-override merge next door, because the
+// property worth proving has nothing to do with rendering: a module gets exactly
+// the URL namespace core gave it, however it spells the paths it registers.
+//
+// window.__rg itself (modules/shared.js) is not tested here — it imports .jsx and
+// there is no DOM in this runner. What it publishes is React, the router and
+// core components: a wiring test would assert that an import statement imported
+// something. Phase 1's spike proved the half that can actually fail, which is a
+// real chunk resolving its externals against the global in a browser under an
+// enforced CSP.
+
+beforeEach(() => _reset())
+
+test('a module route is namespaced under the module id', () => {
+  registerRoutes('uo', { public: [{ path: 'atlas', element: 'ATLAS' }] })
+  assert.deepEqual(
+    routesFor('public').map((r) => r.path),
+    ['uo/atlas'],
+  )
+})
+
+test('a module cannot spell its way out of its namespace', () => {
+  // Whatever the module writes, the segment it lands under is core's to choose:
+  // leading slashes, several of them, a trailing one, or nothing at all.
+  registerRoutes('uo', {
+    public: [
+      { path: '/atlas' },
+      { path: '//atlas/creatures' },
+      { path: 'atlas/' },
+      { path: '' },
+    ],
+  })
+  assert.deepEqual(
+    routesFor('public').map((r) => r.path),
+    ['uo/atlas', 'uo/atlas/creatures', 'uo/atlas', 'uo'],
+  )
+})
+
+test('a path is namespaced, not sanitised — traversal stays a literal segment', () => {
+  // `..` is not stripped, and does not need to be: React Router matches path
+  // patterns literally, so `/uo/../admin` is a route nothing navigates to rather
+  // than a route that resolves somewhere else. Asserted so that a future
+  // "cleanup" that starts resolving these knows it changed a behaviour.
+  registerRoutes('uo', { public: [{ path: '../admin' }] })
+  assert.deepEqual(routesFor('public')[0].path, 'uo/../admin')
+})
+
+test('routes keep their gate and carry the owning module id', () => {
+  registerRoutes('uo', {
+    admin: [{ path: 'shard-ops', element: 'OPS', gate: { roles: ['admin', 'moderator'] } }],
+  })
+  const [route] = routesFor('admin')
+  assert.deepEqual(route.gate, { roles: ['admin', 'moderator'] })
+  assert.equal(route.moduleId, 'uo')
+  assert.equal(route.element, 'OPS')
+})
+
+test('the three areas are kept apart', () => {
+  registerRoutes('uo', {
+    public: [{ path: 'atlas' }],
+    admin: [{ path: 'link' }],
+    player: [{ path: 'chars' }],
+  })
+  assert.equal(routesFor('public').length, 1)
+  assert.equal(routesFor('admin').length, 1)
+  assert.equal(routesFor('player').length, 1)
+  // An area nobody registered is an empty list, never undefined: App.jsx maps
+  // over all three unconditionally.
+  _reset()
+  for (const area of ['public', 'admin', 'player']) assert.deepEqual(routesFor(area), [])
+})
+
+test('an unknown area throws rather than being dropped', () => {
+  // Loudly, because the alternative is a module whose pages simply never appear
+  // and no indication anywhere of why.
+  assert.throws(() => registerRoutes('uo', { publik: [{ path: 'atlas' }] }), /unknown area/)
+  assert.throws(() => registerNav('uo', { area: 'sidebar', items: [] }), /unknown area/)
+  assert.equal(registeredIds().length, 0)
+})
+
+test('nav items sort by order, and equal orders keep load order', () => {
+  registerNav('aa', { area: 'admin', items: [{ label: 'Second', to: '/a', order: 30 }] })
+  registerNav('zz', { area: 'admin', items: [{ label: 'Third', to: '/z', order: 30 }] })
+  registerNav('mm', { area: 'admin', items: [{ label: 'First', to: '/m', order: 10 }] })
+  assert.deepEqual(
+    navFor('admin').map((i) => i.label),
+    ['First', 'Second', 'Third'],
+  )
+})
+
+test('a nav item with no order sorts after the ones that asked for a place', () => {
+  registerNav('uo', {
+    area: 'public',
+    items: [{ label: 'Unordered', to: '/u' }, { label: 'Early', to: '/e', order: 5 }],
+  })
+  assert.deepEqual(
+    navFor('public').map((i) => i.label),
+    ['Early', 'Unordered'],
+  )
+})
+
+test('a feature provider is stored under its namespace, with its owner', () => {
+  const hook = () => ({ atlas: true })
+  registerFeatureProvider('uo', 'shard', hook)
+  assert.deepEqual(featureProviderFor('shard'), { id: 'uo', hook })
+  assert.equal(featureProviderFor('nothing'), undefined)
+})
+
+test('every registration marks the module registered', () => {
+  registerRoutes('a', { public: [{ path: 'x' }] })
+  registerNav('b', { area: 'public', items: [] })
+  registerFeatureProvider('c', 'ns', () => {})
+  assert.deepEqual(registeredIds().sort(), ['a', 'b', 'c'])
+})
+
+test('the registry object handed to modules exposes the whole surface', () => {
+  // window.__rg.registry is the ONLY way a module reaches any of this, so a
+  // member missing from the object is a member that does not exist.
+  assert.deepEqual(Object.keys(registry).sort(), [
+    'featureProviderFor',
+    'navFor',
+    'registerFeatureProvider',
+    'registerNav',
+    'registerRoutes',
+    'registeredIds',
+    'routesFor',
+  ])
+})
+
+test('the client and server halves declare the same MODULE_API_VERSION', () => {
+  // The value is duplicated because it has to be on window.__rg before the first
+  // module chunk evaluates, which is earlier than a fetch could answer. This is
+  // the test that pays for the copy: a bump that edits one file fails here
+  // instead of shipping a core whose two halves disagree about the contract they
+  // implement.
+  const here = path.dirname(fileURLToPath(import.meta.url))
+  const server = fs.readFileSync(
+    path.join(here, '..', '..', 'server', 'src', 'modules', 'version.js'),
+    'utf8',
+  )
+  const match = server.match(/MODULE_API_VERSION\s*=\s*'([^']+)'/)
+  assert.ok(match, 'server/src/modules/version.js no longer declares MODULE_API_VERSION as a literal')
+  assert.equal(MODULE_API_VERSION, match[1])
+})
diff --git a/server/src/app.js b/server/src/app.js
index 9d79377..8b9b8c9 100644
--- a/server/src/app.js
+++ b/server/src/app.js
@@ -186,6 +186,47 @@ modules.load({
 
 app.use('/api', (req, res) => res.status(404).json({ message: 'Not found' }))
 
+// Installed modules' prebuilt client chunks, at /modules/<id>/ — same-origin, so
+// `script-src 'self'` admits them with no nonce and no inline script
+// (docs/website/MODULE_API.md §3.1). Three properties, each load-bearing:
+//
+//   • The static root is the directory the ENTRY sits in, never the module root.
+//     One express.static over a module root would publish its server source, its
+//     module.json and its schema fragment; the loader rejects an entry that would
+//     make those the same directory.
+//   • Behind the module's own state guard, so a failed module's chunk is 503 and
+//     a disabled one's is 404 — the same answers its API gives, for the same
+//     reason: the browser should not be running the client half of something the
+//     server half has stopped serving.
+//   • `fallthrough: false`, so a missing file is a 404 here rather than falling
+//     through to the SPA catch-all and answering a `<script src>` with the index
+//     shell, which the browser then rejects on its MIME type instead.
+//
+// Vite's library build emits an unhashed `entry.js`, so `no-cache` (revalidate,
+// not "do not store") is what stops an upgraded module serving yesterday's chunk
+// out of the disk cache.
+for (const chunk of modules.clientChunks()) {
+  app.use(
+    chunk.url,
+    chunk.guard,
+    express.static(chunk.dir, {
+      fallthrough: false,
+      setHeaders: (res) => {
+        res.set('Cache-Control', 'no-cache')
+        res.set('X-Content-Type-Options', 'nosniff')
+      },
+    }),
+  )
+}
+
+// Everything else under /modules is a 404, not the SPA shell. The namespace
+// belongs to installed modules' chunks — an unknown module id or a file a module
+// does not ship is a missing file, and answering a `<script src>` with an HTML
+// page turns that into a MIME-type refusal in the console with a 200 in the
+// network tab. It also keeps the namespace's boundary a fact of the app rather
+// than of whichever catch-all happens to be mounted after it.
+app.use('/modules', (req, res) => res.status(404).json({ message: 'Not found' }))
+
 // ── /.well-known ──────────────────────────────────────────────────────
 // Android App Links verification file at the web root (M9 follow-up). Mounted
 // before the SPA catch-all so it returns JSON, not the index shell. 404s unless
diff --git a/server/src/modules/loader.js b/server/src/modules/loader.js
index bbc3aa1..9ab8139 100644
--- a/server/src/modules/loader.js
+++ b/server/src/modules/loader.js
@@ -17,9 +17,11 @@
 // past its own try/catch — a bad module must cost the site its routes, never its
 // boot (§4.4).
 //
-// What is deliberately NOT here yet, each landing with the PR that first calls
-// it (§2.7): GET /api/v1/public/modules and the client chunk's static mount
-// (PRs 6-7).
+// PR 7 added the client half's server-side end: validating `client.entry` and
+// publishing where the chunk lives (`clientChunks()`), so app.js can serve it and
+// utils/htmlShell.js can inject its script tag. The loader resolves and validates;
+// it does not mount, because the chunk hangs off the ROOT app rather than a tier
+// router, and app.js is where core's own static mounts live.
 //
 // PR 3 added the fragment half of the schema story: this file VALIDATES a
 // fragment (statement by statement, at load time, before anything is mounted)
@@ -322,6 +324,58 @@ function ownedByCore(tierRouter, prefix) {
   )
 }
 
+// ── The client chunk ───────────────────────────────────────────────────────
+
+// A chunk filename, and the same character set utils/htmlShell.js will accept in
+// a script src. Two copies of the rule, deliberately: this one rejects the module
+// at load time, that one refuses to write the tag. A validator three files away
+// staying strict is not something an HTML attribute should depend on.
+const CHUNK_FILE = /^[A-Za-z0-9][A-Za-z0-9._-]*\.js$/
+
+/**
+ * Resolve and validate `client.entry` — where a module's prebuilt chunk lives on
+ * disk, and the URL it is served at (§3.1).
+ *
+ * The rule that matters most is the last one, and it is the one a reviewer would
+ * not think to ask for: the static mount is rooted at the DIRECTORY THE ENTRY IS
+ * IN, not at the module root. One `express.static` over a module root would
+ * publish its server source, its `module.json` and its schema fragment to the
+ * internet. So an entry sitting directly in the module root is rejected rather
+ * than quietly turning the whole module into a public directory.
+ *
+ * @returns {{dir: string, url: string, entryUrl: string}|null} null when the
+ *   module ships no client half — a server-only module is perfectly normal.
+ */
+function resolveClient(dir, id, manifest) {
+  // Absent `client` is a server-only module. Present but empty is not the same
+  // thing: it states a client half and delivers none, which would be a module
+  // whose pages never load and nothing anywhere saying why.
+  if (manifest.client === undefined) return null
+  const { entry } = manifest.client
+  if (typeof entry !== 'string' || !entry.trim()) fail('manifest', 'client.entry must be a path')
+
+  const file = path.resolve(dir, entry)
+  // Containment before anything else: `../../server/src/config` resolves to a
+  // real, readable directory, and every check below it would pass.
+  if (file !== dir && !file.startsWith(dir + path.sep)) {
+    fail('manifest', `client.entry "${entry}" escapes the module directory`)
+  }
+  if (!CHUNK_FILE.test(path.basename(file))) {
+    fail('manifest', `client.entry "${entry}" must name a .js file`)
+  }
+  const chunkDir = path.dirname(file)
+  if (chunkDir === dir) {
+    fail('manifest', `client.entry "${entry}" must be in a subdirectory — its directory is served`)
+  }
+  if (!fs.existsSync(file)) fail('manifest', `client.entry "${entry}" is missing`)
+
+  return {
+    dir: chunkDir,
+    url: `/modules/${id}`,
+    entryUrl: `/modules/${id}/${path.basename(file)}`,
+  }
+}
+
 function readManifest(dir, id, tierRouters) {
   const file = path.join(dir, 'module.json')
   const manifest = JSON.parse(fs.readFileSync(file, 'utf8'))
@@ -358,6 +412,15 @@ function readManifest(dir, id, tierRouters) {
     if (!registries.hasSlot(slot)) fail('extensions', `unknown extension slot "${slot}"`)
   }
 
+  if (manifest.client !== undefined) {
+    if (typeof manifest.client !== 'object' || manifest.client === null || Array.isArray(manifest.client)) {
+      fail('manifest', 'client must be an object')
+    }
+    for (const key of Object.keys(manifest.client)) {
+      if (key !== 'entry') fail('manifest', `unknown key "client.${key}" in module.json`)
+    }
+  }
+
   if (manifest.schema && !manifest.purge) {
     // A module that can create tables and cannot drop them leaves an operator
     // with orphaned data and no supported way to remove it.
@@ -434,6 +497,7 @@ function load(tierRouters) {
       tables: new Set(),
       called: new Set(),
       hooks: { onBoot: null, onShutdown: null },
+      client: null,
       ctx: null,
       state: 'installed',
       stage: null,
@@ -446,6 +510,7 @@ function load(tierRouters) {
     let stage = 'manifest'
     try {
       record.manifest = readManifest(dir, id, tierRouters)
+      record.client = resolveClient(dir, id, record.manifest)
       stage = 'schema'
       record.tables = tablesOf(dir, record.manifest)
       checkTableNames(id, record.tables)
@@ -522,17 +587,35 @@ function load(tierRouters) {
 function mount(record, tierRouters) {
   for (const tier of TIERS) {
     for (const [prefix, router] of record.routes[tier]) {
-      tierRouters[tier].use(prefix, (req, res, next) => {
-        if (record.state === 'startup_failed') {
-          return res.status(503).json({ message: 'Module unavailable' })
-        }
-        if (record.state === 'disabled') return res.status(404).json({ message: 'Not found' })
-        return next()
-      }, router)
+      tierRouters[tier].use(prefix, stateGuard(record), router)
     }
   }
 }
 
+/**
+ * The dispatch guard, as a middleware over the LIVE record.
+ *
+ * A closure over the record rather than over its state: everything mounts once,
+ * at boot, and the states that matter here are reached afterwards — the schema
+ * replay fails, `onBoot` throws, an admin disables the module. A guard that read
+ * the state at mount time would answer for the state a module was in before any
+ * of that happened.
+ *
+ * Used for a module's API routes and, since PR 7, for its client chunk: a module
+ * answering 503 on its API must not also be handing the browser the script that
+ * calls it, and one an admin has disabled should be as absent from the page as it
+ * is from the nav.
+ */
+function stateGuard(record) {
+  return (req, res, next) => {
+    if (record.state === 'startup_failed') {
+      return res.status(503).json({ message: 'Module unavailable' })
+    }
+    if (record.state === 'disabled') return res.status(404).json({ message: 'Not found' })
+    return next()
+  }
+}
+
 // ── State ──────────────────────────────────────────────────────────────────
 
 // The states a loaded record may hold, deliberately a hardcoded subset rather
@@ -657,7 +740,58 @@ function fragments() {
     .map((r) => ({ id: r.id, file: path.join(r.dir, r.manifest.schema) }))
 }
 
+/**
+ * Every module that ships a client chunk, with where to serve it from and the
+ * guard to serve it behind — in scan order.
+ *
+ * Listed regardless of state, because mounting happens once at boot and the
+ * guard is what answers for the state at request time (the same arrangement the
+ * API routes have). A module that failed VALIDATION never reaches here at all:
+ * `record.client` is only resolved once the manifest passed.
+ *
+ * `dir` is the directory the entry sits in, never the module root — see
+ * resolveClient. app.js does the mounting; this file does not know about the
+ * root app.
+ *
+ * @returns {{id: string, dir: string, url: string, entryUrl: string, guard: Function}[]}
+ */
+function clientChunks() {
+  assertLoaded('clientChunks')
+  return [...modules.values()]
+    .filter((r) => r.client)
+    .map((r) => ({ id: r.id, ...r.client, guard: stateGuard(r) }))
+}
+
+/**
+ * The script URLs the HTML shell should inject, in scan order.
+ *
+ * `started` only, and that is the difference between this and clientChunks():
+ * the mount is a standing offer answered by a guard, while the tag is a decision
+ * taken per page render, when the state is already known. A module whose `onBoot`
+ * failed keeps its URLs and answers 503 on them — loading its client half would
+ * render its pages against a backend that cannot serve them.
+ *
+ * @returns {string[]}
+ */
+function clientEntryUrls() {
+  assertLoaded('clientEntryUrls')
+  return [...modules.values()]
+    .filter((r) => r.client && r.state === 'started')
+    .map((r) => r.client.entryUrl)
+}
+
 /** Absolute path of the modules directory. */
 const dir = () => MODULES_DIR
 
-module.exports = { load, list, setState, fragments, bootable, shutdownHooks, isLoaded, dir }
+module.exports = {
+  load,
+  list,
+  setState,
+  fragments,
+  bootable,
+  shutdownHooks,
+  clientChunks,
+  clientEntryUrls,
+  isLoaded,
+  dir,
+}
diff --git a/server/src/utils/htmlShell.js b/server/src/utils/htmlShell.js
index ab2a525..762ba9e 100644
--- a/server/src/utils/htmlShell.js
+++ b/server/src/utils/htmlShell.js
@@ -81,8 +81,9 @@ function absolutize(url) {
  * byte-identical property without a database.
  *
  * @param {string} html  the built index.html
- * @param {{logo?: string, favicon?: string, theme?: object|null}} [overrides]
- *   effective brand assets and theme; anything absent falls back to BRAND_* env
+ * @param {{logo?: string, favicon?: string, theme?: object|null, moduleEntries?: string[]}} [overrides]
+ *   effective brand assets and theme; anything absent falls back to BRAND_* env.
+ *   `moduleEntries` are the same-origin URLs of installed modules' client chunks.
  * @returns {string}
  */
 function render(html, overrides = {}) {
@@ -105,10 +106,43 @@ function render(html, overrides = {}) {
   ]
     .filter(Boolean)
     .join('\n    ')
-  return html
+  const scripts = moduleScriptTags(overrides.moduleEntries)
+  const withHead = html
     .replace(/<title>[\s\S]*?<\/title>/i, `<title>${title}</title>`)
     .replace(/(<meta\s+name="description"\s+content=")[\s\S]*?("\s*\/?>)/i, `$1${desc}$2`)
     .replace(/<\/head>/i, `    ${tags}\n  </head>`)
+  if (scripts.length === 0) return withHead
+  return withHead.replace(/<\/body>/i, `    ${scripts.join('\n    ')}\n  </body>`)
+}
+
+// Installed modules' prebuilt client chunks (docs/website/MODULE_API.md §3.1).
+//
+// `type="module"` with a `src`, never inline: `script-src 'self'` admits a
+// same-origin src with no nonce, and an inline tag would be blocked outright —
+// which is also why the shared dependencies ride on window.__rg rather than an
+// import map, since an import map has to be inline.
+//
+// **Injected before `</body>`, not into `</head>`, and the position is the
+// contract.** Module scripts are deferred, so they execute in document order
+// after core's own bundle — which is where `window.__rg` is published, and what
+// every one of a module's imports resolves against. Vite happens to hoist core's
+// entry script into `<head>` today, which would make a `</head>` injection work
+// too; that is a bundler's emit choice, and if it ever changed, every module in
+// the wild would break on its first import with nothing in this repo having been
+// edited. Last in the body is after core's script wherever core's script is.
+//
+// The path is built by the loader from the module id and the entry's basename,
+// both already validated, so nothing operator-supplied reaches the attribute.
+// It is re-checked here anyway: what may appear in an HTML attribute should be a
+// property of the code that writes the HTML, not of a validator two files away
+// staying strict.
+const MODULE_ENTRY_PATH = /^\/modules\/[a-z][a-z0-9-]{1,31}\/[A-Za-z0-9][A-Za-z0-9._-]*\.js$/
+
+function moduleScriptTags(entries) {
+  if (!Array.isArray(entries)) return []
+  return entries
+    .filter((src) => typeof src === 'string' && MODULE_ENTRY_PATH.test(src))
+    .map((src) => `<script type="module" src="${htmlEscape(src)}"></script>`)
 }
 
 // The admin theme as a :root block, or '' when this instance has never been
@@ -170,7 +204,26 @@ async function get() {
       // mean a failing query per page view.
       overrides = {}
     }
-    const html = render(template, overrides)
+    // The module list is in-memory and filesystem-derived, so unlike the brand
+    // read above it cannot fail on a DB fault and needs no fallback of its own.
+    // Required lazily for the same reason the settings model is: app.js requires
+    // this file, and the loader would otherwise be pulled into that chain.
+    let moduleEntries = []
+    try {
+      // eslint-disable-next-line global-require
+      moduleEntries = require('../modules/loader').clientEntryUrls()
+    } catch {
+      // The only reachable throw is §7.6's guard — the shell rendered before
+      // modules.load() ran, which app.js's ordering makes impossible and a test
+      // that renders in isolation makes possible. A page with no module scripts
+      // is the right answer either way; it is what a bare core serves.
+      moduleEntries = []
+    }
+    // Note for whoever builds the admin Modules screen: a state change after boot
+    // (an operator disabling a module) has to call invalidate(), exactly as a
+    // brand-asset write does. The TTL converges on its own within five minutes;
+    // the explicit call is what makes the toggle feel like it did something.
+    const html = render(template, { ...overrides, moduleEntries })
     // An invalidation that landed while this read was in flight means the value
     // we just read may already be stale. Serve it, but do not cache it.
     if (generation === startedAt) cached = { html, at: Date.now() }
diff --git a/server/test/htmlShell.test.js b/server/test/htmlShell.test.js
index 03b5b4d..dbb55af 100644
--- a/server/test/htmlShell.test.js
+++ b/server/test/htmlShell.test.js
@@ -227,3 +227,86 @@ test('an invalidation during a render is not overwritten by the stale result', a
   await inflight
   assert.match(await htmlShell.get(), /new\.png/, 'the pre-write value must not have been cached')
 })
+
+// ── Installed modules' client chunks (MODULE_API.md §3.1) ──────────────────
+
+// The built shell as Vite actually emits it: core's entry is a module script in
+// <head>, and the injection has to land AFTER it wherever it is.
+const BUILT_TEMPLATE = `<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <title>Vite App</title>
+    <meta name="description" content="placeholder" />
+    <script type="module" crossorigin src="/assets/index-abc123.js"></script>
+    <link rel="stylesheet" crossorigin href="/assets/index-abc123.css" />
+  </head>
+  <body><div id="root"></div></body>
+</html>`
+
+test('a module chunk is injected as a same-origin module script', () => {
+  const html = htmlShell.render(TEMPLATE, { moduleEntries: ['/modules/uo/entry.js'] })
+  assert.match(html, /<script type="module" src="\/modules\/uo\/entry\.js"><\/script>/)
+})
+
+test('the injection lands after core’s own bundle, not before it', () => {
+  // The property the whole client contract rests on: module scripts are deferred
+  // and execute in document order, so core's bundle must run first — it is what
+  // publishes window.__rg, and every import in the chunk resolves against it.
+  // Injecting into </head> would work today only because Vite hoists core's
+  // entry there; before </body> is after it wherever a bundler decides to put it.
+  const html = htmlShell.render(BUILT_TEMPLATE, { moduleEntries: ['/modules/uo/entry.js'] })
+  assert.ok(
+    html.indexOf('/assets/index-abc123.js') < html.indexOf('/modules/uo/entry.js'),
+    'the module chunk must come after core’s bundle',
+  )
+  assert.ok(html.indexOf('/modules/uo/entry.js') < html.indexOf('</body>'))
+  assert.ok(html.indexOf('</head>') < html.indexOf('/modules/uo/entry.js'))
+})
+
+test('several modules keep the order they were given', () => {
+  const html = htmlShell.render(TEMPLATE, {
+    moduleEntries: ['/modules/aaa/entry.js', '/modules/zzz/entry.js'],
+  })
+  assert.ok(html.indexOf('/modules/aaa/') < html.indexOf('/modules/zzz/'))
+})
+
+test('no installed modules leaves the shell byte-identical', () => {
+  // A bare core must serve exactly what it served before this PR — including
+  // when the list is absent rather than empty, which is what a render before
+  // modules.load() produces.
+  const baseline = legacyRenderIndexHtml(TEMPLATE)
+  assert.equal(htmlShell.render(TEMPLATE, { moduleEntries: [] }), baseline)
+  assert.equal(htmlShell.render(TEMPLATE, {}), baseline)
+})
+
+test('anything that is not a module chunk URL is refused, not escaped into the page', () => {
+  // The loader builds these from a validated id and a validated basename, so
+  // none of this is reachable today. It is enforced here anyway: what may appear
+  // in a script src should be a property of the code writing the HTML, not of a
+  // validator two files away staying strict.
+  const html = htmlShell.render(TEMPLATE, {
+    moduleEntries: [
+      'https://evil.example/entry.js',        // off-origin
+      '/modules/uo/../../etc/passwd',         // traversal
+      '/modules/UO/entry.js',                 // not a valid module id
+      '/modules/uo/entry.js"></script><script>alert(1)</script>', // attribute break-out
+      '/modules/uo/../secrets.js',
+      '/uploads/entry.js',                    // right shape, wrong root
+      42,
+      null,
+    ],
+  })
+  assert.ok(!html.includes('<script type="module" src='), html)
+  assert.equal(htmlShell.render(TEMPLATE, { moduleEntries: [] }), legacyRenderIndexHtml(TEMPLATE))
+})
+
+test('get() renders without module scripts when the loader has never scanned', async () => {
+  // db/seed.js's problem, one layer up: this file is required by app.js, and a
+  // render that reached the loader before modules.load() would throw §7.6's
+  // guard on a request path. A bare shell is the right answer.
+  settings.getShellBrand = async () => ({ logo: brand.logo, favicon: brand.favicon, theme: null })
+  htmlShell.init(TEMPLATE)
+  const html = await htmlShell.get()
+  assert.ok(!html.includes('/modules/'))
+})
diff --git a/server/test/moduleClientChunk.test.js b/server/test/moduleClientChunk.test.js
new file mode 100644
index 0000000..b80bcc1
--- /dev/null
+++ b/server/test/moduleClientChunk.test.js
@@ -0,0 +1,180 @@
+// ── A module's client chunk, served by the real app ────────────────────────
+//
+// Phase 2, PR 7 of docs/website/MODULE_SYSTEM.md §2.7; the contract is
+// MODULE_API.md §3.1. moduleLoader.test.js proves the loader resolves and
+// validates the chunk; this file proves what the app does with the answer, and
+// it boots the REAL app.js to do it — because the three properties worth locking
+// are properties of the mount, not of the loader:
+//
+//   1. the module's own dist directory is published, and nothing above it;
+//   2. the chunk is served behind the module's state guard, so a failed or
+//      disabled module's client half is as absent as its API;
+//   3. a miss is a 404 and never the SPA shell, which a browser would reject on
+//      its MIME type after the request appeared to succeed.
+//
+// The modules directory is written and MODULES_DIR is set BEFORE app.js is
+// required, because the scan is synchronous and happens during that require.
+// Node's test runner gives each file its own process, so this cannot disturb
+// another test's view of the loader.
+
+process.env.DB_HOST = '127.0.0.1'
+process.env.DB_PORT = '59999'
+
+const fs = require('fs')
+const os = require('os')
+const path = require('path')
+
+const { test, before, after } = require('node:test')
+const assert = require('node:assert/strict')
+
+const CHUNK = 'export const hello = 1\n'
+
+const tmpRoot = fs.mkdtempSync(path.join(os.tmpdir(), 'rg-module-chunk-'))
+const dist = path.join(tmpRoot, 'uo', 'client', 'dist')
+fs.mkdirSync(dist, { recursive: true })
+fs.writeFileSync(path.join(dist, 'entry.js'), CHUNK)
+fs.writeFileSync(path.join(dist, 'sidecar.js'), 'export const also = 2\n')
+// The two files a static mount rooted one level too high would publish.
+fs.writeFileSync(path.join(tmpRoot, 'uo', 'secrets.js'), 'const TOKEN = "leak"\n')
+fs.writeFileSync(
+  path.join(tmpRoot, 'uo', 'module.json'),
+  JSON.stringify({
+    id: 'uo',
+    name: 'Ultima Online',
+    version: '1.0.0',
+    coreApi: '^1.0.0',
+    client: { entry: 'client/dist/entry.js' },
+  }),
+)
+process.env.MODULES_DIR = tmpRoot
+
+/* eslint-disable global-require */
+const app = require('../src/app')
+const loader = require('../src/modules/loader')
+const db = require('../src/utils/db')
+const htmlShell = require('../src/utils/htmlShell')
+const settings = require('../src/model/settings/settings.model')
+/* eslint-enable global-require */
+
+let server
+let base
+
+before(async () => {
+  server = await new Promise((resolve) => {
+    const s = app.listen(0, '127.0.0.1', () => resolve(s))
+  })
+  base = `http://127.0.0.1:${server.address().port}`
+  // The state the module would be in after a clean boot. lifecycle.js does this
+  // against the database; here it is set directly, since what is under test is
+  // what the mount does with a state, not how the state was reached.
+  loader.setState('uo', 'started')
+})
+
+after(async () => {
+  server.closeAllConnections()
+  await new Promise((resolve) => server.close(resolve))
+  await db.close()
+  fs.rmSync(tmpRoot, { recursive: true, force: true })
+})
+
+test('the chunk is served at the URL the shell injects', async () => {
+  const [entryUrl] = loader.clientEntryUrls()
+  assert.equal(entryUrl, '/modules/uo/entry.js')
+
+  const res = await fetch(base + entryUrl)
+  assert.equal(res.status, 200)
+  assert.equal(await res.text(), CHUNK)
+  // A module chunk is JavaScript to the browser or it is nothing: a `<script
+  // type="module">` whose response is not a JS MIME type is refused outright.
+  assert.match(res.headers.get('content-type'), /javascript/)
+})
+
+test('a sibling file in the same dist directory is served too', async () => {
+  // Not incidental: Rollup can split a chunk, and the entry then imports its
+  // siblings by relative URL. Publishing only the named entry would break every
+  // module that is more than one file.
+  const res = await fetch(`${base}/modules/uo/sidecar.js`)
+  assert.equal(res.status, 200)
+})
+
+test('nothing above the dist directory is reachable', async () => {
+  // The failure this rule exists to prevent: server source, module.json and the
+  // schema fragment published to the internet by one over-broad static mount.
+  for (const p of ['/modules/uo/module.json', '/modules/uo/secrets.js', '/modules/uo/../module.json']) {
+    const res = await fetch(base + p)
+    assert.notEqual(res.status, 200, `${p} must not be served`)
+    assert.ok(!(await res.text()).includes('leak'))
+  }
+})
+
+test('the chunk revalidates rather than being cached to a stale copy', async () => {
+  // Vite's library build emits an unhashed entry.js, so an upgraded module would
+  // otherwise keep serving yesterday's chunk out of the browser's disk cache.
+  const res = await fetch(`${base}/modules/uo/entry.js`)
+  assert.equal(res.headers.get('cache-control'), 'no-cache')
+  assert.equal(res.headers.get('x-content-type-options'), 'nosniff')
+})
+
+test('a missing file is a 404, not the SPA shell', async () => {
+  const res = await fetch(`${base}/modules/uo/nope.js`)
+  assert.equal(res.status, 404)
+  assert.ok(!(await res.text()).includes('<div id="root">'))
+})
+
+test('an unknown module id is not served at all', async () => {
+  const res = await fetch(`${base}/modules/nope/entry.js`)
+  assert.notEqual(res.status, 200)
+})
+
+test('a failed module’s chunk is 503 and a disabled one’s is 404', async () => {
+  // The same answers the module's API routes give, and for the same reason: the
+  // browser must not be running the client half of something the server half has
+  // stopped serving.
+  loader.setState('uo', 'startup_failed', { stage: 'boot', reason: 'onBoot threw' })
+  let res = await fetch(`${base}/modules/uo/entry.js`)
+  assert.equal(res.status, 503)
+
+  loader.setState('uo', 'disabled')
+  res = await fetch(`${base}/modules/uo/entry.js`)
+  assert.equal(res.status, 404)
+
+  // And the guard reads the LIVE state — the mount happened once, at boot, long
+  // before any of these transitions.
+  loader.setState('uo', 'started')
+  res = await fetch(`${base}/modules/uo/entry.js`)
+  assert.equal(res.status, 200)
+})
+
+test('the module is published to clients while it is started, and only then', async () => {
+  // Ties the two surfaces together: /public/modules and the injected script tag
+  // answer the same question — what is serving — and they must never disagree.
+  const seen = async () => {
+    const res = await fetch(`${base}/api/v1/public/modules`)
+    return (await res.json()).modules.map((m) => m.id)
+  }
+  assert.deepEqual(await seen(), ['uo'])
+  assert.deepEqual(loader.clientEntryUrls(), ['/modules/uo/entry.js'])
+
+  loader.setState('uo', 'disabled')
+  assert.deepEqual(await seen(), [])
+  assert.deepEqual(loader.clientEntryUrls(), [])
+  loader.setState('uo', 'started')
+})
+
+test('the shell hands the browser the tag for the chunk the app serves', async () => {
+  // The one seam htmlShell.test.js cannot cover, because it renders against a
+  // literal list: that the shell asks the LOADER, and gets back a URL this same
+  // app answers 200 on. The two are wired through a lazy require inside a
+  // try/catch, which is exactly the shape that can silently return [] forever.
+  //
+  // The settings read is stubbed rather than left to fail: the pool points at a
+  // dead port, and its ten-second connect timeout would be paid here for a
+  // fallback the test does not care about.
+  settings.getShellBrand = async () => ({ logo: '', favicon: '', theme: null })
+  htmlShell.init('<!doctype html><html><head><title>t</title></head><body><div id="root"></div></body></html>')
+  const html = await htmlShell.get()
+
+  assert.match(html, /<script type="module" src="\/modules\/uo\/entry\.js"><\/script>/)
+  const res = await fetch(`${base}/modules/uo/entry.js`)
+  assert.equal(res.status, 200)
+})
diff --git a/server/test/moduleLoader.test.js b/server/test/moduleLoader.test.js
index 70e234f..9161b9a 100644
--- a/server/test/moduleLoader.test.js
+++ b/server/test/moduleLoader.test.js
@@ -640,3 +640,105 @@ test('a module colliding with an already-registered name fails alone, unmounted'
   assert.equal(claims('/first'), true)
   assert.equal(claims('/second'), false)
 })
+
+// ── The client chunk (MODULE_API.md §3.1) ──────────────────────────────────
+
+/** A module shipping a prebuilt chunk at the conventional client/dist/entry.js. */
+function withChunk(id, { entry = 'client/dist/entry.js', write = true, body = 'export default 1' } = {}) {
+  const dir = writeModule(id, { manifest: { client: { entry } } })
+  if (write) {
+    const file = path.join(dir, entry)
+    fs.mkdirSync(path.dirname(file), { recursive: true })
+    fs.writeFileSync(file, body)
+  }
+  return dir
+}
+
+test('a module with a chunk publishes where to serve it from and its URL', () => {
+  const dir = withChunk('uo')
+  const loader = freshLoader(tmpRoot)
+  const [chunk] = loader.clientChunks()
+
+  assert.equal(chunk.id, 'uo')
+  assert.equal(chunk.url, '/modules/uo')
+  assert.equal(chunk.entryUrl, '/modules/uo/entry.js')
+  // The DIRECTORY THE ENTRY IS IN, never the module root: one express.static over
+  // a module root would publish its server source, its module.json and its schema
+  // fragment.
+  assert.equal(chunk.dir, path.join(dir, 'client', 'dist'))
+  assert.equal(typeof chunk.guard, 'function')
+})
+
+test('a server-only module contributes no chunk', () => {
+  writeModule('plain', { server: 'module.exports = () => {}' })
+  const loader = freshLoader(tmpRoot)
+  assert.deepEqual(loader.clientChunks(), [])
+  assert.deepEqual(loader.clientEntryUrls(), [])
+})
+
+test('an entry directly in the module root is refused — its directory is served', () => {
+  // The rule with the largest blast radius in this file. Accepting it would root
+  // the static mount at the module root and publish everything in it.
+  const dir = writeModule('uo', { manifest: { client: { entry: 'entry.js' } } })
+  fs.writeFileSync(path.join(dir, 'entry.js'), 'export default 1')
+  const loader = freshLoader(tmpRoot)
+  assert.equal(stateOf(loader, 'uo').state, 'startup_failed')
+  assert.match(stateOf(loader, 'uo').reason, /must be in a subdirectory/)
+  assert.deepEqual(loader.clientChunks(), [])
+})
+
+test('an entry that escapes the module directory is refused before anything else', () => {
+  // `../../server/src/config/csp.js` is a real, readable file, and every check
+  // after containment would have passed.
+  withChunk('uo', { entry: '../../server/src/config/csp.js', write: false })
+  const loader = freshLoader(tmpRoot)
+  assert.equal(stateOf(loader, 'uo').state, 'startup_failed')
+  assert.match(stateOf(loader, 'uo').reason, /escapes the module directory/)
+})
+
+test('an entry that is not a .js file, or is missing, is refused', () => {
+  withChunk('aaa', { entry: 'client/dist/entry.mjs' })
+  withChunk('bbb', { entry: 'client/dist/entry.js', write: false })
+  const loader = freshLoader(tmpRoot)
+  assert.match(stateOf(loader, 'aaa').reason, /must name a \.js file/)
+  assert.match(stateOf(loader, 'bbb').reason, /is missing/)
+})
+
+test('a malformed client key is a loud failure, not an ignored setting', () => {
+  writeModule('aaa', { manifest: { client: 'client/dist/entry.js' } })
+  writeModule('bbb', { manifest: { client: { entry: 'x/e.js', chunks: ['a.js'] } } })
+  writeModule('ccc', { manifest: { client: {} } })
+  const loader = freshLoader(tmpRoot)
+  assert.match(stateOf(loader, 'aaa').reason, /client must be an object/)
+  assert.match(stateOf(loader, 'bbb').reason, /unknown key "client\.chunks"/)
+  assert.match(stateOf(loader, 'ccc').reason, /client\.entry must be a path/)
+  for (const id of ['aaa', 'bbb', 'ccc']) assert.equal(stateOf(loader, id).stage, 'manifest')
+})
+
+test('only a STARTED module gets a script tag, though every one keeps its mount', () => {
+  // The mount is a standing offer answered by a guard; the tag is a decision
+  // taken per render, when the state is known. A module answering 503 on its API
+  // must not also be handing the browser the script that calls it.
+  withChunk('uo')
+  const loader = freshLoader(tmpRoot)
+  assert.deepEqual(loader.clientEntryUrls(), [], 'registered is not yet serving')
+
+  loader.setState('uo', 'started')
+  assert.deepEqual(loader.clientEntryUrls(), ['/modules/uo/entry.js'])
+
+  loader.setState('uo', 'startup_failed', { stage: 'boot', reason: 'nope' })
+  assert.deepEqual(loader.clientEntryUrls(), [])
+  assert.equal(loader.clientChunks().length, 1, 'the mount stays; the guard answers for it')
+
+  loader.setState('uo', 'disabled')
+  assert.deepEqual(loader.clientEntryUrls(), [])
+})
+
+test('the chunk accessors throw before load(), like every other one', () => {
+  process.env.MODULES_DIR = tmpRoot
+  delete require.cache[require.resolve('../src/modules/loader')]
+  // eslint-disable-next-line global-require
+  const loader = require('../src/modules/loader')
+  assert.throws(() => loader.clientChunks(), /modules\.clientChunks\(\) before modules\.load\(\)/)
+  assert.throws(() => loader.clientEntryUrls(), /modules\.clientEntryUrls\(\) before modules\.load\(\)/)
+})