diff --git a/__tests__/unit/profile/neutral-username.test.ts b/__tests__/unit/profile/neutral-username.test.ts
new file mode 100644
index 000000000..a76a106f7
--- /dev/null
+++ b/__tests__/unit/profile/neutral-username.test.ts
@@ -0,0 +1,73 @@
+/**
+ * Public handles must not be minted from email addresses.
+ *
+ * `/profiles/<username>` is served with no auth and robots.txt has no
+ * `/profiles` rule. Every profile-creation path used to set the handle to
+ * `email.split('@')[0]`, which published 77 people's email local parts as
+ * crawlable identifiers — reported through OrangeCat's own feedback widget.
+ */
+import { readFileSync, readdirSync, statSync } from 'node:fs';
+import { join } from 'node:path';
+import { neutralUsernameFor } from '@/lib/profile/neutral-username';
+import { isValidUsername } from '@/lib/validation/base';
+
+describe('neutralUsernameFor', () => {
+  const id = '3f2a1b9c-7d4e-4a52-9c8b-0e1f2a3b4c5d';
+
+  it('carries nothing about the person', () => {
+    expect(neutralUsernameFor(id)).toBe('user_3f2a1b9c7d4e');
+  });
+
+  it('is accepted by the app own username rules', () => {
+    expect(isValidUsername(neutralUsernameFor(id))).toBe(true);
+  });
+
+  it('mirrors the SQL trigger: user_ + 12 hex of the id, dashes stripped', () => {
+    // supabase/migrations/20260826130000: 'user_' || left(replace(id::text,'-',''), 12)
+    expect(neutralUsernameFor(id)).toBe(`user_${id.replace(/-/g, '').slice(0, 12)}`);
+  });
+
+  it('differs per user', () => {
+    expect(neutralUsernameFor(id)).not.toBe(
+      neutralUsernameFor('99999999-7d4e-4a52-9c8b-0e1f2a3b4c5d')
+    );
+  });
+});
+
+/**
+ * The class, closed. Fixing the DB trigger alone was NOT enough — `ensureProfile`
+ * and two form pre-fills each derived the handle from the email independently,
+ * and the count rose 72 -> 77 while the trigger fix was being written. A comment
+ * would not have caught that; this does.
+ */
+describe('no code derives a public handle from an email', () => {
+  const SRC = join(process.cwd(), 'src');
+  const ALLOWED = join('lib', 'profile', 'neutral-username.ts'); // documents the bug in prose
+
+  function sourceFiles(dir: string, out: string[] = []): string[] {
+    for (const name of readdirSync(dir)) {
+      const full = join(dir, name);
+      if (statSync(full).isDirectory()) sourceFiles(full, out);
+      else if (/\.tsx?$/.test(name)) out.push(full);
+    }
+    return out;
+  }
+
+  it('has no site assigning a username from an email local part', () => {
+    const offenders: string[] = [];
+    for (const file of sourceFiles(SRC)) {
+      if (file.endsWith(ALLOWED)) continue;
+      readFileSync(file, 'utf8')
+        .split('\n')
+        .forEach((line, i) => {
+          const trimmed = line.trim();
+          if (trimmed.startsWith('//') || trimmed.startsWith('*')) return;
+          // Same line mentions a handle AND splits an address on '@'.
+          if (/username/i.test(line) && /split\(\s*['"]@['"]\s*\)/.test(line)) {
+            offenders.push(`${file}:${i + 1}: ${trimmed}`);
+          }
+        });
+    }
+    expect(offenders).toEqual([]);
+  });
+});
diff --git a/scripts/check-data-invariants.mjs b/scripts/check-data-invariants.mjs
index 809ce0ca6..7083dbd38 100644
--- a/scripts/check-data-invariants.mjs
+++ b/scripts/check-data-invariants.mjs
@@ -289,7 +289,13 @@ async function checkSilentlyDroppedCatTurns() {
  * number RISES, which means a new write path started deriving handles from
  * emails again.
  */
-const EMAIL_DERIVED_USERNAME_BASELINE = 72; // measured on production 2026-08-26
+// Measured on production 2026-08-26 AFTER the trigger fix landed. The first
+// reading taken while writing that fix said 72; five accounts signed up in the
+// hour between, so a baseline of 72 would have failed on its very first
+// nightly run — a gate red about code that is fine, which is the habit this
+// ratchet exists to avoid. The number is a floor now rather than a moving
+// target: handle_new_user no longer mints these, so nothing can add to it.
+const EMAIL_DERIVED_USERNAME_BASELINE = 77;
 
 async function checkEmailDerivedUsernames() {
   const count = Number(await rpc('count_email_derived_usernames'));
diff --git a/src/components/profile/ProfileWizard/hooks/useProfileWizard.ts b/src/components/profile/ProfileWizard/hooks/useProfileWizard.ts
index a48ff45cc..ccc798082 100644
--- a/src/components/profile/ProfileWizard/hooks/useProfileWizard.ts
+++ b/src/components/profile/ProfileWizard/hooks/useProfileWizard.ts
@@ -31,11 +31,11 @@ export function useProfileWizard(
     resolver: zodResolver(profileSchema),
     mode: 'onChange',
     defaultValues: {
-      username:
-        profile.username ||
-        (typeof userEmail === 'string' && userEmail.includes('@')
-          ? userEmail.split('@')[0]
-          : userEmail || ''),
+      // No email fallback: username is NOT NULL and always set by a creation
+      // path, so this only ever fired on a half-loaded profile — and when it
+      // did it pre-filled the PUBLIC handle field with the user's email local
+      // part, one Save away from publishing it.
+      username: profile.username || '',
       name: profile.name || '',
       bio: profile.bio || '',
       location_country: profile.location_country || '',
diff --git a/src/components/profile/hooks/useProfileEditor.ts b/src/components/profile/hooks/useProfileEditor.ts
index e8629452a..ff706d81f 100644
--- a/src/components/profile/hooks/useProfileEditor.ts
+++ b/src/components/profile/hooks/useProfileEditor.ts
@@ -73,11 +73,11 @@ export function useProfileEditor({
     mode: 'onChange',
     reValidateMode: 'onChange',
     defaultValues: {
-      username:
-        profile.username ||
-        (typeof userEmail === 'string' && userEmail.includes('@')
-          ? userEmail.split('@')[0]
-          : userEmail || ''),
+      // No email fallback: username is NOT NULL and always set by a creation
+      // path, so this only ever fired on a half-loaded profile — and when it
+      // did it pre-filled the PUBLIC handle field with the user's email local
+      // part, one Save away from publishing it.
+      username: profile.username || '',
       name: profile.name || '',
       bio: profile.bio || '',
       location_country: profile.location_country || '',
diff --git a/src/lib/profile/neutral-username.ts b/src/lib/profile/neutral-username.ts
new file mode 100644
index 000000000..150760526
--- /dev/null
+++ b/src/lib/profile/neutral-username.ts
@@ -0,0 +1,24 @@
+/**
+ * The handle a new account gets before its owner picks one.
+ *
+ * It must carry no personal information. `/profiles/<username>` is served with
+ * no auth and robots.txt has no `/profiles` rule, so whatever goes here is
+ * published and crawlable. Deriving it from the email address — as every
+ * creation path used to — put 77 people's email local parts on the public web,
+ * reconstructable into real addresses with a handful of common domains.
+ *
+ * MIRRORS the SQL in
+ * supabase/migrations/20260826130000_stop_deriving_usernames_from_email.sql.
+ * Profiles are created from two independent places — the `handle_new_user`
+ * trigger on auth.users, and `ensureProfile()` when a profile is missing — and
+ * they must not disagree about what a fresh handle looks like. If you change
+ * the shape here, change it there and vice versa; the invariant gate
+ * (`count_email_derived_usernames`) catches the email case, not a cosmetic
+ * drift between the two.
+ */
+
+/** `user_` + 12 hex chars of the id. Unique by construction, says nothing about
+ *  the person, and satisfies the app's username pattern (^[a-zA-Z0-9_-]+$). */
+export function neutralUsernameFor(userId: string): string {
+  return `user_${String(userId).replace(/-/g, '').slice(0, 12)}`;
+}
diff --git a/src/services/profile/server.ts b/src/services/profile/server.ts
index db6d6de5f..c44c0199c 100644
--- a/src/services/profile/server.ts
+++ b/src/services/profile/server.ts
@@ -13,6 +13,7 @@ import { DATABASE_TABLES } from '@/config/database-tables';
 import { getTableName } from '@/config/entity-registry';
 import { getOrCreateUserActor } from '@/services/actors/getOrCreateUserActor';
 import { STATUS } from '@/config/database-constants';
+import { neutralUsernameFor } from '@/lib/profile/neutral-username';
 
 type ProfileRow = Database['public']['Tables']['profiles']['Row'];
 type ProfileInsert = Database['public']['Tables']['profiles']['Insert'];
@@ -138,18 +139,17 @@ export class ProfileServerService {
 
       // Profile doesn't exist, create it
       const safeEmail = typeof userEmail === 'string' ? userEmail : null;
-      const emailName = safeEmail && safeEmail.includes('@') ? safeEmail.split('@')[0] : null;
-      // Sanitize: replace dots and other invalid chars with underscores, keep only letters/numbers/underscores/hyphens
-      const sanitizedEmailName = emailName ? emailName.replace(/[^a-zA-Z0-9_-]/g, '_') : null;
-      const username =
-        sanitizedEmailName && sanitizedEmailName.length > 0
-          ? sanitizedEmailName
-          : `user_${String(userId).slice(0, 8)}`;
+      // Never the email local part. This path creates profiles independently of
+      // the handle_new_user trigger, so fixing only the trigger left this one
+      // still publishing people's email prefixes as crawlable handles — which
+      // is what took the count from 72 to 77 while the trigger fix was being
+      // written. The display name gets no email fallback either: a name quietly
+      // set to someone's email prefix is the same leak wearing another label.
+      const username = neutralUsernameFor(userId);
       const name =
         (userMetadata?.full_name as string | undefined) ||
         (userMetadata?.name as string | undefined) ||
         (userMetadata?.display_name as string | undefined) ||
-        (emailName && emailName.length > 0 ? emailName : null) ||
         'User';
 
       const profileData: ProfileInsert = {