diff --git a/docs/config.md b/docs/config.md
index ef8e3b5..b767c09 100644
--- a/docs/config.md
+++ b/docs/config.md
@@ -37,7 +37,8 @@ Merge rules: objects merge key-by-key; **arrays and scalars replace** wholesale.
 | `features.push` | bool | Web Push notifications — the Settings row **and** the re-subscribe-on-connect. |
 | `features.imageUpload` | bool | Composer image button **and** paste / drag-drop upload. |
 | `features.register` | bool | Account self-service: the "Create account" button, the "Forgot password" link, and their FAQ entries. |
-| `plugins` | string[] | Plugin script URLs loaded at startup. See [Plugins](https://orbit.tchatou.fr/docs/plugins/). |
+| `plugins` | string[] | Plugin script URLs loaded at startup — a bare URL runs in-page, or `{url, sandbox: true, permissions: […]}` runs isolated. See [Plugins](https://orbit.tchatou.fr/docs/plugins/). |
+| `builtins` | string[] | Built-in **sandboxed** features to enable by name (opt-in; default `[]`). Currently `"dice"`. See [The sandbox](https://orbit.tchatou.fr/docs/sandbox/). |
 
 > `defaults.*` only seed a user's preferences the **first** time — once someone changes a setting,
 > it's stored in their browser and the config no longer overrides it.
@@ -61,6 +62,7 @@ can still switch in **Settings**). The bundled plugins are localized too — see
   "report":   { "service": "ReportServ", "target": "#staff" },
   "defaults": { "theme": "dark", "compact": true, "lang": "" },
   "features": { "push": true, "imageUpload": true, "register": false },
-  "plugins":  []
+  "plugins":  [],
+  "builtins": []
 }
 ```
diff --git a/docs/plugins.md b/docs/plugins.md
index fa9ff3a..c576800 100644
--- a/docs/plugins.md
+++ b/docs/plugins.md
@@ -169,3 +169,7 @@ they run with the same trust as the app itself. There is no user-uploaded plugin
 mechanism. Orbit deliberately does **not** expose internal modules or runtime
 component replacement — that would couple plugins to internals that are still
 moving. The API above is the stable surface.
+
+For code you don't fully trust — a community or third-party plugin — mark the
+entry `sandbox: true` to run it in an isolated, capability-gated iframe instead.
+See [The sandbox](https://orbit.tchatou.fr/docs/sandbox/).
diff --git a/docs/sandbox.md b/docs/sandbox.md
new file mode 100644
index 0000000..8be734d
--- /dev/null
+++ b/docs/sandbox.md
@@ -0,0 +1,119 @@
+Section: Plugins
+Order: 220
+
+# The sandbox
+
+Most plugins run **in-page** — a trusted `<script>` on the app origin with the
+full `Orbit` API. That is fine for code an operator wrote or fully vets, but it
+also means such a plugin can read anything on the page: cookies, `localStorage`
+(including the SASL handoff password), the store. For code you **don't** fully
+trust — a community or third-party plugin — Orbit has a real isolation boundary.
+
+The **sandbox** runs code in an **opaque-origin iframe**
+(`sandbox="allow-scripts"`, no `allow-same-origin`) that can reach the app **only**
+through a capability-gated message bridge. Sandboxed code cannot touch the page
+DOM, cookies, `localStorage`, or the store — it can do only what its declared
+`permissions` allow, and nothing else.
+
+> **Experimental.** The sandbox API is small and may change between releases. It
+> is a core subsystem (`src/sandbox/`), not a plugin itself.
+
+## Two ways it is used
+
+### Built-in features
+
+Orbit ships a few small features that run **sandboxed by default** — bundled into
+the app and mounted by the app itself, no external file to load. They are
+**opt-in per deployment**: a built-in only mounts if its name is listed in
+`config.json`.
+
+```jsonc
+{
+  "builtins": ["dice"]     // enables the sandboxed 🎲 / 🪙 footer widget
+}
+```
+
+The default is `[]` (no built-ins). Currently the only built-in is `dice`.
+
+### Sandboxed plugins
+
+Any `config.json` plugin entry can opt into the sandbox with `sandbox: true` and a
+list of `permissions`. Use this for community / third-party / less-trusted plugins.
+
+```jsonc
+"plugins": [
+  "/app/plugins/orbit-clock.js",                     // in-page (trusted)
+  { "url": "/app/plugins/community-thing.js",         // sandboxed (isolated)
+    "sandbox": true, "permissions": ["irc", "storage"] }
+]
+```
+
+## Capabilities
+
+A sandboxed plugin can do nothing privileged unless the operator grants the
+matching permission. Reading a cached state snapshot and rendering UI inside its
+own iframe need no grant — they are already contained.
+
+| permission | unlocks |
+|---|---|
+| `irc`     | `orbit.irc.say / msg / send / join / part / list` — acts as the user |
+| `notify`  | `orbit.notify(title, body)` |
+| `storage` | `orbit.storage.set(key, value)` (namespaced, persisted for the plugin) |
+
+Ungranted or unknown calls are refused by the host (fail-closed).
+
+## The sandboxed API
+
+A sandboxed plugin registers the same way as any plugin —
+`Orbit.plugin(name, fn)` — but `fn` receives a **constrained** `orbit`:
+
+| member | notes |
+|---|---|
+| `orbit.log(...)` | namespaced console log |
+| `orbit.on(event, fn)` | forwarded events: `connected`, `message`, `buffer.active`, `status` |
+| `orbit.irc.*` | `say / msg / send / join / part / list` (needs `irc`) |
+| `orbit.notify(title, body?)` | desktop notification (needs `notify`) |
+| `orbit.state.active() / nick() / account() / buffers()` | synchronous, from a pushed snapshot |
+| `orbit.storage.get(k, fallback?) / set(k, v)` | `set` needs `storage` |
+| `orbit.ui(slot, build)` | `build(el)` fills the plugin's own iframe; the host sizes the frame to the content |
+
+The app's theme variables (`--bg`, `--ink`, `--accent`, `--muted`, `--border`,
+`--panel`, `--green-soft`) are mirrored into the sandbox and kept in sync on theme
+change, so sandboxed UI can use `var(--accent)` and look native in light and dark.
+
+## Writing one
+
+```js
+Orbit.plugin('coinflip', function (orbit) {
+  orbit.ui('footer_item', function (root) {
+    var b = document.createElement('button');
+    b.textContent = '🪙';
+    b.style.cssText = 'border:1px solid var(--border);background:var(--panel);' +
+      'color:var(--ink);border-radius:9px;padding:.3rem .5rem;cursor:pointer';
+    b.onclick = function () { orbit.irc.say('🪙 ' + (Math.random() < 0.5 ? 'heads' : 'tails')); };
+    root.appendChild(b);
+  });
+});
+```
+
+With `"permissions": ["irc"]` this can post to the channel but has no access to the
+page, cookies, `localStorage`, or the store. UI is built with plain DOM inside the
+plugin's own iframe — the app's React is not shared across the isolation boundary.
+
+## Self-hosting
+
+The sandbox iframe needs two things from your web server, both contained (the
+sandbox document is opaque-origin, so its looser script policy cannot touch the
+app):
+
+1. Add `'self'` to the app's `frame-src` in its Content-Security-Policy, so the app
+   may embed the sandbox document.
+2. Serve `/app/plugin-sandbox.html` with its **own** tight CSP that lets the
+   bootstrap run but blocks all network egress:
+
+   ```
+   Content-Security-Policy: default-src 'none'; script-src 'unsafe-inline' 'unsafe-eval'; style-src 'unsafe-inline'; img-src data:; connect-src 'none'; frame-ancestors 'self'
+   ```
+
+Until (1) is applied the sandbox iframe is blocked, so sandboxed code is simply
+inert — in-page plugins are unaffected.