diff --git a/core/auth.py b/core/auth.py
index 1e68a72..0ed140f 100644
--- a/core/auth.py
+++ b/core/auth.py
@@ -266,7 +266,8 @@ class AuthManager:
         renamed_sessions = 0
         with self._sessions_lock:
             for sess in self._sessions.values():
-                if (sess or {}).get("username") == old_username:
+                sess_user = str((sess or {}).get("username") or "").strip().lower()
+                if sess_user == old_username:
                     sess["username"] = new_username
                     renamed_sessions += 1
         if renamed_sessions:
diff --git a/routes/auth_routes.py b/routes/auth_routes.py
index 2f9b0b1..5728d3e 100644
--- a/routes/auth_routes.py
+++ b/routes/auth_routes.py
@@ -297,6 +297,7 @@ def setup_auth_routes(auth_manager: AuthManager) -> APIRouter:
         # owner-scoped DB rows before changing auth so the account keeps
         # access to its sessions, docs, email accounts, tasks, etc.
         try:
+            from sqlalchemy import func
             from core.database import Base, SessionLocal
             db = SessionLocal()
             try:
@@ -306,7 +307,7 @@ def setup_auth_routes(auth_manager: AuthManager) -> APIRouter:
                         continue
                     (
                         db.query(model)
-                        .filter(model.owner == old_username)
+                        .filter(func.lower(model.owner) == old_username)
                         .update({"owner": new_username}, synchronize_session=False)
                     )
                 db.commit()
@@ -324,9 +325,15 @@ def setup_auth_routes(auth_manager: AuthManager) -> APIRouter:
             from routes.prefs_routes import _load as _load_prefs, _save as _save_prefs
             prefs = _load_prefs()
             users = prefs.get("_users") if isinstance(prefs, dict) else None
-            if isinstance(users, dict) and old_username in users and new_username not in users:
-                users[new_username] = users.pop(old_username)
-                _save_prefs(prefs)
+            if isinstance(users, dict):
+                prefs_key = next(
+                    (k for k in users if str(k).strip().lower() == old_username),
+                    None,
+                )
+                new_taken = any(str(k).strip().lower() == new_username for k in users)
+                if prefs_key is not None and not new_taken:
+                    users[new_username] = users.pop(prefs_key)
+                    _save_prefs(prefs)
         except Exception as e:
             logger.warning("Failed to rename user prefs %s -> %s: %s", old_username, new_username, e)
 
diff --git a/tests/test_rename_user_case_insensitive.py b/tests/test_rename_user_case_insensitive.py
new file mode 100644
index 0000000..624bc87
--- /dev/null
+++ b/tests/test_rename_user_case_insensitive.py
@@ -0,0 +1,86 @@
+"""Regression: username rename must migrate mixed-case legacy owner keys.
+
+Before lowercasing was enforced everywhere, rows could be stored with
+owner='Admin' while auth usernames are normalized to 'admin'. A case-
+sensitive filter would skip those rows during rename (issue #1165).
+"""
+
+import importlib
+import sys
+import time
+import types
+from pathlib import Path
+from unittest.mock import MagicMock
+
+import pytest
+
+
+def _real_core_package():
+    root = Path(__file__).resolve().parent.parent
+    core_path = str(root / "core")
+    core = sys.modules.get("core")
+    if core is None:
+        core = types.ModuleType("core")
+        sys.modules["core"] = core
+    core.__path__ = [core_path]
+    if hasattr(core, "auth"):
+        delattr(core, "auth")
+    sys.modules.pop("core.auth", None)
+    return core
+
+
+def _fresh_auth_manager(tmp_path):
+    auth_mod = importlib.import_module("core.auth", package=_real_core_package())
+    auth_mod._hash_password = lambda password: f"hash:{password}"
+    auth_mod._verify_password = lambda password, hashed: hashed == f"hash:{password}"
+    return auth_mod.AuthManager(str(tmp_path / "auth.json"))
+
+
+def test_rename_user_updates_mixed_case_session_username(tmp_path):
+    mgr = _fresh_auth_manager(tmp_path)
+    assert mgr.create_user("admin", "pw-123456", is_admin=True) is True
+    assert mgr.create_user("bob", "pw-123456") is True
+    with mgr._sessions_lock:
+        mgr._sessions["tok1"] = {"username": "Bob", "expiry": time.time() + 3600}
+    assert mgr.rename_user("bob", "robert", "admin") is True
+    with mgr._sessions_lock:
+        assert mgr._sessions["tok1"]["username"] == "robert"
+
+
+def _has_real_sqlalchemy():
+    mod = sys.modules.get("sqlalchemy")
+    if mod is None or isinstance(mod, MagicMock):
+        return False
+    return hasattr(mod, "create_engine")
+
+
+@pytest.mark.skipif(not _has_real_sqlalchemy(), reason="sqlalchemy not installed")
+def test_rename_owner_db_filter_is_case_insensitive():
+    from sqlalchemy import create_engine, func
+    from sqlalchemy.orm import sessionmaker
+
+    from core.database import Base, Session as DbSession
+
+    engine = create_engine("sqlite:///:memory:")
+    Base.metadata.create_all(bind=engine)
+    db = sessionmaker(bind=engine)()
+    db.add(
+        DbSession(
+            id="s1",
+            name="chat",
+            endpoint_url="http://localhost:8000",
+            model="gpt-4",
+            owner="Bob",
+        )
+    )
+    db.commit()
+
+    old_username = "bob"
+    new_username = "robert"
+    db.query(DbSession).filter(func.lower(DbSession.owner) == old_username).update(
+        {"owner": new_username},
+        synchronize_session=False,
+    )
+    db.commit()
+
+    assert db.query(DbSession).first().owner == "robert"