diff --git a/docs/SANDBOX.md b/docs/SANDBOX.md
index c01dc4d..633b2f5 100644
--- a/docs/SANDBOX.md
+++ b/docs/SANDBOX.md
@@ -45,8 +45,11 @@ The sandboxed API (`src/modules/sandbox/host.ts` + `public/plugin-sandbox.html`)
 `orbit.log`, `orbit.on(event, fn)` (forwarded `connected` / `message` /
 `buffer.active` / `status`), `orbit.irc.*`, `orbit.notify`,
 `orbit.state.active/nick/account/buffers` (from a pushed snapshot, synchronous),
-`orbit.storage.get/set`, and `orbit.ui(slot, build)` where `build(el)` populates the
-plugin's own iframe and the host sizes the frame to the content.
+`orbit.server.network/isupport/hasCap/caps` (also from the snapshot, synchronous —
+gate cap-dependent behaviour so a plugin never 421s a lean server; no `numeric()`
+here since the sandbox gets no `raw` events), `orbit.storage.get/set`, and
+`orbit.ui(slot, build)` where `build(el)` populates the plugin's own iframe and the
+host sizes the frame to the content.
 
 The app's theme CSS vars (`--bg`, `--ink`, `--accent`, `--muted`, `--border`,
 `--panel`, `--green-soft`) are mirrored into the sandbox and kept in sync on theme
diff --git a/public/plugin-sandbox.html b/public/plugin-sandbox.html
index b9f8673..45193c2 100644
--- a/public/plugin-sandbox.html
+++ b/public/plugin-sandbox.html
@@ -22,7 +22,7 @@
  * served with its own strict CSP.
  */
 (function () {
-  var port, snap = { active: '', nick: '', account: '', buffers: [] }, store = {};
+  var port, snap = { active: '', nick: '', account: '', buffers: [], network: '', isupport: {}, caps: [] }, store = {};
   var listeners = {};                 // event name -> [fn]
   var pending = new Map(), rpcId = 0;  // in-flight RPCs
 
@@ -87,6 +87,13 @@
         account: function () { return snap.account; },
         buffers: function () { return snap.buffers.slice(); },
       },
+      // Read-only server / capability info, from the snapshot (no RPC, no grant).
+      server: {
+        network: function () { return snap.network || ''; },
+        isupport: function () { var o = {}, m = snap.isupport || {}, k; for (k in m) o[k] = m[k]; return o; },
+        hasCap: function (cap) { return (snap.caps || []).some(function (c) { return c.name === cap && c.enabled; }); },
+        caps: function () { return (snap.caps || []).map(function (c) { return { name: c.name, available: c.available, enabled: c.enabled }; }); },
+      },
       storage: {
         get: function (k, fallback) { return k in store ? store[k] : fallback; },
         set: function (k, v) { store[k] = v; return rpc('storage.set', k, v); },
diff --git a/src/modules/sandbox/host.ts b/src/modules/sandbox/host.ts
index c691344..d2b4be1 100644
--- a/src/modules/sandbox/host.ts
+++ b/src/modules/sandbox/host.ts
@@ -26,7 +26,13 @@ const STORE_PREFIX = 'orbit-sbx:';
 
 function snapshot(): StateSnapshot {
   const s = activeStore().getState();
-  return { active: s.active, nick: s.nick, account: s.account, buffers: Object.keys(s.buffers) };
+  const cl = s.client;
+  return {
+    active: s.active, nick: s.nick, account: s.account, buffers: Object.keys(s.buffers),
+    network: cl?.server.network ?? '',
+    isupport: { ...(cl?.server.isupport ?? {}) },
+    caps: cl?.ircv3.listCaps() ?? [],
+  };
 }
 
 // Current values of the app's themeable CSS vars, mirrored into the sandbox.
diff --git a/src/modules/sandbox/protocol.ts b/src/modules/sandbox/protocol.ts
index 345a207..07a7580 100644
--- a/src/modules/sandbox/protocol.ts
+++ b/src/modules/sandbox/protocol.ts
@@ -70,7 +70,16 @@ export const THEME_VARS = ['--bg', '--ink', '--accent', '--muted', '--border', '
 // guest -> host
 export interface RpcMsg { type: 'rpc'; id: number; method: string; args: unknown[]; }
 
-export interface StateSnapshot { active: string; nick: string; account: string; buffers: string[]; }
+export interface StateSnapshot {
+  active: string; nick: string; account: string; buffers: string[];
+  // Read-only server / capability info (client.server + client.ircv3), pushed in the
+  // snapshot so a sandboxed plugin can gate cap-dependent behaviour synchronously —
+  // never a raw command that 421s a lean server. (No numeric map: the sandbox gets
+  // no 'raw' events, so there are no numeric replies to name.)
+  network: string;
+  isupport: Record<string, string>;
+  caps: { name: string; available: boolean; enabled: boolean }[];
+}
 
 export type HostToGuest = InitMsg | EventMsg | SnapshotMsg | ThemeMsg | RpcReplyMsg;
 export type GuestToHost = RpcMsg;