diff --git a/packages/client/src/discovery.ts b/packages/client/src/discovery.ts
index 1215cdb..163d8ab 100644
--- a/packages/client/src/discovery.ts
+++ b/packages/client/src/discovery.ts
@@ -40,16 +40,18 @@
  */
 
 import * as inbox from "./inbox";
-import { ensureAccount } from "./store-registry";
+import { ensureAccount, reservedAccount } from "./store-registry";
 import type { Nuri, PrincipalId } from "./types";
 
 /**
  * The reserved SPECIAL ACCOUNT that owns the global discovery index in the
- * polyfill. A normal shim account (username), so its scope documents are created
- * on first sight like any other account — but it is never a real user; it only
- * hosts the index document. Disappears at migration (see file header).
+ * polyfill. It hosts the index document but is never a real user. It lives in
+ * the registry's RESERVED namespace ({@link reservedAccount}), whose key
+ * `normalizeUser` can never produce — so a user named "index"/"@index" can NOT
+ * hijack it (they would normalize to "index", a disjoint key). Disappears at
+ * migration (see file header).
  */
-export const INDEX_ACCOUNT = "@index";
+export const INDEX_ACCOUNT = reservedAccount("index");
 
 /** One entry as materialized from the discovery index. */
 export interface IndexEntry {
diff --git a/packages/client/src/store-registry.ts b/packages/client/src/store-registry.ts
index 7029178..3b0ec4b 100644
--- a/packages/client/src/store-registry.ts
+++ b/packages/client/src/store-registry.ts
@@ -65,8 +65,43 @@ function accountSubject(username: string): string {
   // The username is UNTRUSTED and lands in an IRI position. Percent-encode it
   // (escapeIri) so no `>` / `"` / whitespace / control char can break out of
   // the `<...>` and inject triples into the shim graph (the account→doc trust
-  // root). normalize() runs first so the subject stays stable per shim key.
-  return `${SHIM}:account:${escapeIri(normalize(username))}`;
+  // root). accountKey() runs first so the subject stays stable per shim key.
+  return `${SHIM}:account:${escapeIri(accountKey(username))}`;
+}
+
+// --- reserved accounts -----------------------------------------------------
+//
+// Some accounts are internal to the lib (e.g. the discovery index owner) and
+// must NOT collide with any user-chosen username. A reserved account is created
+// via {@link reservedAccount}, which marks the name with a sentinel PREFIX that
+// `normalizeUser` (consumer-injected) can never produce: it strips a leading
+// `@`, trims, and lowercases, so a NUL prefix is unreachable. Reserved
+// keys therefore live in a disjoint namespace from every normalized username —
+// a real user named "index"/"@index" can never resolve to the reserved
+// `reservedAccount("index")` account.
+const RESERVED_PREFIX = "\u0000reserved:";
+
+/**
+ * Wrap an internal account name so it occupies a key that no user input can
+ * produce (see {@link RESERVED_PREFIX}). Pass the result to {@link ensureAccount}
+ * (and the other registry calls) instead of a bare username.
+ */
+export function reservedAccount(name: string): string {
+  return `${RESERVED_PREFIX}${name}`;
+}
+
+/** Whether a name is a reserved-account sentinel (from {@link reservedAccount}). */
+function isReserved(username: string): boolean {
+  return username.startsWith(RESERVED_PREFIX);
+}
+
+/**
+ * The shim/cache key for an account. Reserved accounts bypass `normalizeUser`
+ * entirely and key on their sentinel-prefixed name, so they cannot collide with
+ * a normalized username; everyone else normalizes as usual.
+ */
+function accountKey(username: string): string {
+  return isReserved(username) ? username : normalize(username);
 }
 
 // --- session / normalization access (injected by the consumer) ------------
@@ -147,7 +182,7 @@ export async function loadShim(): Promise<Map<string, AccountRecord>> {
     for (const row of readBindings(result)) {
       const username = bindingValue(row, "username");
       if (!username) continue;
-      map.set(normalize(username), {
+      map.set(accountKey(username), {
         username,
         docPublic: bindingValue(row, "docPublic"),
         docProtected: bindingValue(row, "docProtected"),
@@ -180,7 +215,7 @@ async function createDoc(): Promise<Nuri> {
  */
 export async function ensureAccount(username: string): Promise<AccountRecord> {
   const map = await loadShim();
-  const key = normalize(username);
+  const key = accountKey(username);
   const existing = map.get(key);
   if (existing) return existing;
 
diff --git a/packages/client/test/discovery.test.ts b/packages/client/test/discovery.test.ts
index 234a8a5..ddf1b06 100644
--- a/packages/client/test/discovery.test.ts
+++ b/packages/client/test/discovery.test.ts
@@ -8,7 +8,7 @@ import {
   resetConfig,
   setCurrentUser,
 } from "../src/polyfill";
-import { resetRegistryCache } from "../src/store-registry";
+import { resetRegistryCache, ensureAccount } from "../src/store-registry";
 import type { RegistrySession } from "../src/store-registry";
 
 // discovery.ts submits to / reads from a global index owned by a RESERVED
@@ -209,8 +209,34 @@ test("from: null makes an anonymous submission", async () => {
   expect(entries[0]!.from).toBeNull();
 });
 
-test("INDEX_ACCOUNT is the reserved special account", () => {
-  expect(INDEX_ACCOUNT).toBe("@index");
+test("INDEX_ACCOUNT lives in the reserved namespace (no typed username can equal it)", () => {
+  // The index account occupies a key no user input can produce: it is prefixed
+  // with a NUL control char, which a user cannot type into a username field and
+  // which no `normalizeUser` output (a typeable value) contains. So it is
+  // disjoint from the keys "index" / "@index" a hostile user would submit.
+  expect(INDEX_ACCOUNT.startsWith("\u0000")).toBe(true); // unreachable-by-typing sentinel
+  expect(INDEX_ACCOUNT).not.toBe("index");
+  expect(INDEX_ACCOUNT).not.toBe("@index");
+});
+
+test("a user named 'index'/'@index' does NOT resolve to the index account's document", async () => {
+  // The discovery index lives on INDEX_ACCOUNT. A hostile (or unlucky) user who
+  // registers as "index" or "@index" normalizes to key "index" — which must be
+  // a DISJOINT key from the reserved index account, so they get their own
+  // documents and cannot hijack / read-write the global index document.
+  const indexRecord = await ensureAccount(INDEX_ACCOUNT);
+
+  // A real user "index" — same normalized form as "@index".
+  const userIndex = await ensureAccount("index");
+  expect(userIndex.docPublic).not.toBe(indexRecord.docPublic);
+  expect(userIndex.docProtected).not.toBe(indexRecord.docProtected);
+  expect(userIndex.docPrivate).not.toBe(indexRecord.docPrivate);
+
+  // "@index" must land on the SAME account as "index" (both normalize to
+  // "index") — and still NOT on the reserved index account.
+  const userAtIndex = await ensureAccount("@index");
+  expect(userAtIndex.docPublic).toBe(userIndex.docPublic);
+  expect(userAtIndex.docPublic).not.toBe(indexRecord.docPublic);
 });
 
 test("watchIndex fires immediately then when a submission arrives", async () => {