diff --git a/src/services/reddit-account-runtime-builder.ts b/src/services/reddit-account-runtime-builder.ts
index 799d95b..b91da1e 100644
--- a/src/services/reddit-account-runtime-builder.ts
+++ b/src/services/reddit-account-runtime-builder.ts
@@ -17,7 +17,12 @@ import { MonitorService } from './monitor.js';
 import { ProcessorService, isSkip } from './processor.js';
 import { ResponseCheckerService } from './response-checker.js';
 import { StrategySelector } from './strategy-selector.js';
-import { AccountAuthContext } from './account-auth-context.js';
+import {
+  AccountAuthContext,
+  normalizeRedditAccountId,
+  type AuthenticatedAccount,
+  type RedditAccountIdentity,
+} from './account-auth-context.js';
 import { ChatCache } from './chat/cache.js';
 import {
   ChatService, MatrixClient, MatrixMediaDownloader, MatrixTokenRefresher,
@@ -75,16 +80,12 @@ async function buildRuntime(
     }
 
     const accountAuth = new AccountAuthContext(logger);
-    accountAuth.markAuthenticated({
-      accountId: session.accountId,
-      redditUsername: identity.username,
-      matrixUserId: session.matrixUserId,
-    });
+    accountAuth.markAuthenticated(runtimeAuthenticatedAccount(session, identity));
     accountAuth.setAuthenticator(async () => {
       const current = await browserClient.getAuthenticatedIdentity();
       if (!current || matrixRedditId(session.matrixUserId) !== current.accountId.toLowerCase().replace(/^t2_/, '')) return null;
       return await matrixClient.whoami() === session.matrixUserId
-        ? { accountId: session.accountId, redditUsername: current.username, matrixUserId: session.matrixUserId }
+        ? runtimeAuthenticatedAccount(session, current)
         : null;
     });
 
@@ -153,6 +154,18 @@ async function buildRuntime(
   }
 }
 
+/** Keeps the Reddit auth fullname separate from the Matrix-shaped runtime/database key. */
+export function runtimeAuthenticatedAccount(
+  session: Pick<VerifiedAccountSession, 'matrixUserId'>,
+  identity: RedditAccountIdentity,
+): AuthenticatedAccount {
+  return {
+    accountId: normalizeRedditAccountId(identity.accountId),
+    redditUsername: identity.username,
+    matrixUserId: session.matrixUserId,
+  };
+}
+
 class AccountRuntimeLoop {
   private controller: AbortController | null = null;
   private loop: Promise<void> | null = null;
diff --git a/tests/reddit-account-runtime.test.ts b/tests/reddit-account-runtime.test.ts
index 48f1242..5e28e04 100644
--- a/tests/reddit-account-runtime.test.ts
+++ b/tests/reddit-account-runtime.test.ts
@@ -10,6 +10,7 @@ import {
   RedditAccountRuntimeManager,
   type RedditAccountRuntimeResources,
 } from '../src/services/reddit-account-runtime.js';
+import { runtimeAuthenticatedAccount } from '../src/services/reddit-account-runtime-builder.js';
 
 function account(
   key: string,
@@ -219,6 +220,17 @@ describe('RedditAccountRuntimeManager', () => {
 });
 
 describe('account runtime isolation', () => {
+  it('uses the Reddit fullname for auth while retaining the Matrix runtime owner', () => {
+    expect(runtimeAuthenticatedAccount(
+      { matrixUserId: '@t2_1u57iqxxbv:reddit.com' },
+      { username: 'Upset_Marsupial1006', accountId: 't2_1u57iqxxbv' },
+    )).toEqual({
+      accountId: 't2_1u57iqxxbv',
+      redditUsername: 'Upset_Marsupial1006',
+      matrixUserId: '@t2_1u57iqxxbv:reddit.com',
+    });
+  });
+
   it('derives distinct deterministic browser profiles from account IDs', () => {
     const first = accountBrowserProfileDir('./data/browser', '@t2_first:reddit.com');
     const second = accountBrowserProfileDir('./data/browser', '@t2_second:reddit.com');