diff --git a/steward/templates/base.html b/steward/templates/base.html
index 7b7a95d..2fdd8ea 100644
--- a/steward/templates/base.html
+++ b/steward/templates/base.html
@@ -51,37 +51,76 @@ code { font-family: ui-monospace, monospace; font-size: 0.875em; background: var
     radial-gradient(ellipse 50% 40% at 20% 80%, rgba(200, 168, 64, 0.025) 0%, transparent 65%);
 }
 
-/* Nav */
-nav {
-  background: var(--bg-card);
-  padding: 0 1.5rem;
-  display: flex;
-  align-items: center;
-  gap: 0;
-  border-bottom: 1px solid var(--border-mid);
-  height: 48px;
-  position: relative;
-  z-index: 10;
+/* ── App shell: persistent left sidebar + content column ───────────────────── */
+.app-shell { display: flex; min-height: 100vh; position: relative; z-index: 1; }
+.sidebar {
+  width: 220px; flex-shrink: 0; background: var(--bg-card);
+  border-right: 1px solid var(--border-mid);
+  display: flex; flex-direction: column;
+  position: sticky; top: 0; height: 100vh; overflow-y: auto; z-index: 20;
 }
-nav .brand {
-  font-family: var(--font-serif);
-  font-weight: 700;
-  font-size: 1.1rem;
-  margin-right: 2rem;
-  letter-spacing: 0.01em;
-  display: flex;
-  align-items: center;
-  gap: 0.45rem;
-  color: var(--gold);
-  text-decoration: none;
+.sidebar .brand {
+  font-family: var(--font-serif); font-weight: 700; font-size: 1.15rem;
+  letter-spacing: 0.01em; color: var(--gold); text-decoration: none;
+  display: flex; align-items: center; gap: 0.5rem;
+  padding: 0.85rem 1.25rem; border-bottom: 1px solid var(--border);
 }
-nav .brand svg { flex-shrink: 0; }
-nav a { color: var(--text-muted); font-size: 0.875rem; padding: 0 0.875rem; height: 48px; display: flex; align-items: center; border-bottom: 2px solid transparent; transition: color .15s, border-color .15s; }
-nav a:hover { color: var(--text); border-bottom-color: var(--border-mid); }
-nav a.nav-end { margin-left: auto; }
+.sidebar .brand svg { flex-shrink: 0; }
+.side-nav { flex: 1; padding: 0.75rem 0; }
+.nav-group { margin-bottom: 0.9rem; }
+.nav-group-label {
+  font-size: 0.66rem; text-transform: uppercase; letter-spacing: 0.09em;
+  color: var(--text-dim); font-weight: 600; padding: 0 1.25rem; margin-bottom: 0.3rem;
+}
+.side-nav a {
+  display: flex; align-items: center; gap: 0.5rem;
+  padding: 0.4rem 1.25rem; font-size: 0.9rem; color: var(--text-muted);
+  border-left: 2px solid transparent; text-decoration: none;
+  transition: color .12s, background .12s, border-color .12s;
+}
+.side-nav a:hover { color: var(--text); background: var(--bg-elevated); }
+.side-nav a.active {
+  color: var(--gold); border-left-color: var(--gold);
+  background: color-mix(in srgb, var(--gold) 8%, transparent);
+}
+.side-user { border-top: 1px solid var(--border); padding: 0.7rem 1.25rem; }
+.side-user a {
+  color: var(--text-muted); font-size: 0.84rem; text-decoration: none;
+  display: flex; align-items: center; gap: 0.4rem;
+}
+.side-user a:hover { color: var(--text); }
+.app-main { flex: 1; min-width: 0; display: flex; flex-direction: column; }
+.topbar { display: none; }   /* slim mobile bar; shown only under 900px */
+.nav-scrim { display: none; }
 
 /* Layout */
-main { padding: 1.5rem 2rem; max-width: 1600px; margin: 0 auto; width: 100%; box-sizing: border-box; position: relative; z-index: 1; }
+main { padding: 1.5rem 2rem; max-width: 1400px; margin: 0 auto; width: 100%; box-sizing: border-box; position: relative; z-index: 1; }
+
+/* Responsive: sidebar slides off-canvas under 900px, toggled by ☰ */
+@media (max-width: 900px) {
+  .sidebar {
+    position: fixed; left: 0; top: 0; bottom: 0; height: 100%;
+    transform: translateX(-100%); transition: transform .2s ease;
+    box-shadow: 0 0 30px rgba(0,0,0,0.5);
+  }
+  body.nav-open .sidebar { transform: translateX(0); }
+  .topbar {
+    display: flex; align-items: center; gap: 0.75rem; height: 48px; padding: 0 1rem;
+    background: var(--bg-card); border-bottom: 1px solid var(--border-mid);
+    position: sticky; top: 0; z-index: 15;
+  }
+  .sidebar-toggle {
+    background: none; border: none; color: var(--text-muted); font-size: 1.3rem;
+    cursor: pointer; line-height: 1; padding: 0.25rem;
+  }
+  .topbar .brand-sm {
+    font-family: var(--font-serif); color: var(--gold); font-weight: 700;
+    font-size: 1.05rem; text-decoration: none;
+  }
+  body.nav-open .nav-scrim {
+    display: block; position: fixed; inset: 0; background: rgba(0,0,0,0.5); z-index: 18;
+  }
+}
 
 /* Alerts */
 .alert { padding: 0.75rem 1rem; border-radius: 6px; margin-bottom: 1rem; font-size: 0.9rem; }
@@ -224,8 +263,10 @@ body.dash-editing .widget-drawer { transform:translateY(0); }
 
 <div id="candle-glow"></div>
 
+{% set _p = request.path %}
+<div class="app-shell">
 {% if session.user_id is defined %}
-<nav>
+  <aside class="sidebar">
     <a href="/" class="brand">
       <svg width="22" height="22" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg" aria-hidden="true">
         <circle cx="12" cy="12" r="10.5" fill="none" stroke="#c8a840" stroke-width="1.5"/>
@@ -237,18 +278,45 @@ body.dash-editing .widget-drawer { transform:translateY(0); }
       </svg>
       Steward
     </a>
-    <a href="/">Dashboard</a>
-    <a href="/status">Status</a>
-    <a href="/hosts/">Hosts</a>
-    <a href="/monitors/">Monitors</a>
-    <a href="/alerts/">Alerts</a>
-    <a href="/ansible/">Ansible</a>
-    {% if session.user_role == 'admin' %}
-    <a href="/settings/">Settings</a>
-    <a href="/audit/">Audit</a>
-    {% endif %}
-    <a href="/logout" class="nav-end">{{ session.username }}</a>
-</nav>
+    <nav class="side-nav" aria-label="Primary">
+      <div class="nav-group">
+        <div class="nav-group-label">Overview</div>
+        <a href="/" {% if _p == '/' %}class="active" aria-current="page"{% endif %}>Dashboard</a>
+        <a href="/status" {% if _p.startswith('/status') %}class="active" aria-current="page"{% endif %}>Status</a>
+      </div>
+      <div class="nav-group">
+        <div class="nav-group-label">Infrastructure</div>
+        <a href="/hosts/" {% if _p.startswith('/hosts') %}class="active" aria-current="page"{% endif %}>Hosts</a>
+        {# plugin links land here in slice 2 (get_nav hook) #}
+      </div>
+      <div class="nav-group">
+        <div class="nav-group-label">Monitoring</div>
+        <a href="/monitors/" {% if _p.startswith('/monitors') %}class="active" aria-current="page"{% endif %}>Monitors</a>
+        <a href="/alerts/" {% if _p.startswith('/alerts') %}class="active" aria-current="page"{% endif %}>Alerts</a>
+      </div>
+      <div class="nav-group">
+        <div class="nav-group-label">Automation</div>
+        <a href="/ansible/" {% if _p.startswith('/ansible') %}class="active" aria-current="page"{% endif %}>Ansible</a>
+      </div>
+      {% if session.user_role == 'admin' %}
+      <div class="nav-group">
+        <div class="nav-group-label">Admin</div>
+        <a href="/settings/" {% if _p.startswith('/settings') %}class="active" aria-current="page"{% endif %}>Settings</a>
+        <a href="/audit/" {% if _p.startswith('/audit') %}class="active" aria-current="page"{% endif %}>Audit</a>
+      </div>
+      {% endif %}
+    </nav>
+    <div class="side-user">
+      <a href="/logout" title="Log out">⏻ {{ session.username }}</a>
+    </div>
+  </aside>
+{% endif %}
+  <div class="app-main">
+{% if session.user_id is defined %}
+    <div class="topbar">
+      <button class="sidebar-toggle" onclick="document.body.classList.toggle('nav-open')" aria-label="Toggle navigation">☰</button>
+      <a href="/" class="brand-sm">Steward</a>
+    </div>
 {% endif %}
 
 <script>
@@ -314,6 +382,11 @@ function setTimeRange(val) {
         {% block breadcrumb %}{% endblock %}
         {% block content %}{% endblock %}
     </main>
+  </div>{# .app-main #}
+</div>{# .app-shell #}
+{% if session.user_id is defined %}
+<div class="nav-scrim" onclick="document.body.classList.remove('nav-open')"></div>
+{% endif %}
 
 {% block extra_scripts %}{% endblock %}
 
diff --git a/tests/test_templates_parse.py b/tests/test_templates_parse.py
new file mode 100644
index 0000000..46e6f25
--- /dev/null
+++ b/tests/test_templates_parse.py
@@ -0,0 +1,22 @@
+"""Syntax-parse every first-party template so a broken tag fails the unit lane.
+
+The app only renders a handful of templates in unit tests (e.g. the login page),
+so a Jinja syntax error elsewhere would otherwise ship green. env.parse() checks
+syntax without resolving extends/includes/imports — enough to catch an unbalanced
+or mistyped tag across the whole template tree (e.g. the base.html layout).
+"""
+import pathlib
+
+import jinja2
+
+import steward
+
+_ROOT = pathlib.Path(steward.__file__).parent / "templates"
+
+
+def test_all_steward_templates_parse():
+    env = jinja2.Environment()
+    files = sorted(_ROOT.rglob("*.html"))
+    assert files, "no steward templates found"
+    for f in files:
+        env.parse(f.read_text())  # raises TemplateSyntaxError on a bad tag