diff --git a/server/src/services/adminService.ts b/server/src/services/adminService.ts
index 7cce69c3..a3e067b3 100644
--- a/server/src/services/adminService.ts
+++ b/server/src/services/adminService.ts
@@ -141,6 +141,16 @@ export function updateUser(id: string, data: { username?: string; email?: string
   }
   const passwordHash = password ? bcrypt.hashSync(password, BCRYPT_COST) : null;
 
+  // Don't let the admin UI demote the last remaining admin — that would leave the
+  // instance with no one able to manage it (and on OIDC-only setups, no recovery). #1274
+  if (role && role !== 'admin') {
+    const current = db.prepare('SELECT role FROM users WHERE id = ?').get(id) as { role?: string } | undefined;
+    if (current?.role === 'admin') {
+      const adminCount = (db.prepare("SELECT COUNT(*) as count FROM users WHERE role = 'admin'").get() as { count: number }).count;
+      if (adminCount <= 1) return { error: 'Cannot remove the last admin', status: 400 };
+    }
+  }
+
   db.prepare(`
     UPDATE users SET
       username = COALESCE(?, username),
diff --git a/server/src/services/oidcService.ts b/server/src/services/oidcService.ts
index 774faa83..558b3a12 100644
--- a/server/src/services/oidcService.ts
+++ b/server/src/services/oidcService.ts
@@ -385,8 +385,20 @@ export function findOrCreateUser(
     if (process.env.OIDC_ADMIN_VALUE) {
       const newRole = resolveOidcRole(userInfo, false);
       if (user.role !== newRole) {
-        db.prepare('UPDATE users SET role = ? WHERE id = ?').run(newRole, user.id);
-        user = { ...user, role: newRole } as User;
+        // Never let the claim-based downgrade strip the last admin. The bootstrap
+        // admin (first SSO user) usually doesn't carry the admin claim, so a forced
+        // re-login — e.g. after a JWT-secret rotation — would otherwise demote it and
+        // lock an OIDC-only instance out for good. #1274
+        const demotingLastAdmin =
+          user.role === 'admin' &&
+          newRole !== 'admin' &&
+          (db.prepare("SELECT COUNT(*) as count FROM users WHERE role = 'admin'").get() as { count: number }).count <= 1;
+        if (demotingLastAdmin) {
+          console.warn(`[OIDC] Kept admin role for user ${user.id}: their OIDC claims map to '${newRole}', but they are the only admin — demoting would lock the instance out.`);
+        } else {
+          db.prepare('UPDATE users SET role = ? WHERE id = ?').run(newRole, user.id);
+          user = { ...user, role: newRole } as User;
+        }
       }
     }
     return { user };