diff --git a/app/components/demo_toolbar.py b/app/components/demo_toolbar.py
new file mode 100644
index 0000000..829c25c
--- /dev/null
+++ b/app/components/demo_toolbar.py
@@ -0,0 +1,76 @@
+"""Demo toolbar — tier simulation for DEMO_MODE instances.
+
+Renders a slim full-width bar above the Streamlit nav showing
+Free / Paid / Premium pills. Clicking a pill sets a prgn_demo_tier
+cookie (for persistence across reloads) and st.session_state.simulated_tier
+(for immediate use within the current render pass).
+
+Only ever rendered when DEMO_MODE=true.
+"""
+from __future__ import annotations
+
+import os
+
+import streamlit as st
+import streamlit.components.v1 as components
+
+_VALID_TIERS = ("free", "paid", "premium")
+_DEFAULT_TIER = "paid"  # most compelling first impression
+
+_DEMO_MODE = os.environ.get("DEMO_MODE", "").lower() in ("1", "true", "yes")
+
+_COOKIE_JS = """
+<script>
+(function() {{
+  document.cookie = 'prgn_demo_tier={tier}; path=/; SameSite=Lax';
+}})();
+</script>
+"""
+
+
+def get_simulated_tier() -> str:
+    """Return the current simulated tier, defaulting to 'paid'."""
+    return st.session_state.get("simulated_tier", _DEFAULT_TIER)
+
+
+def set_simulated_tier(tier: str) -> None:
+    """Set simulated tier in session state + cookie. Reruns the page."""
+    if tier not in _VALID_TIERS:
+        return
+    st.session_state["simulated_tier"] = tier
+    components.html(_COOKIE_JS.format(tier=tier), height=0)
+    st.rerun()
+
+
+def render_demo_toolbar() -> None:
+    """Render the demo mode toolbar.
+
+    Shows a dismissible info bar with tier-selection pills.
+    Call this at the TOP of app.py's render pass, before pg.run().
+    """
+    current = get_simulated_tier()
+
+    labels = {
+        "free": "Free",
+        "paid": "Paid ✓" if current == "paid" else "Paid",
+        "premium": "Premium ✓" if current == "premium" else "Premium",
+    }
+
+    with st.container():
+        cols = st.columns([3, 1, 1, 1, 2])
+        with cols[0]:
+            st.caption("🎭 **Demo mode** — exploring as:")
+        for i, tier in enumerate(_VALID_TIERS):
+            with cols[i + 1]:
+                is_active = tier == current
+                if st.button(
+                    labels[tier],
+                    key=f"_demo_tier_{tier}",
+                    type="primary" if is_active else "secondary",
+                    use_container_width=True,
+                ):
+                    if not is_active:
+                        set_simulated_tier(tier)
+        with cols[4]:
+            st.caption("[Get your own →](https://circuitforge.tech/software/peregrine)")
+        st.divider()
diff --git a/tests/test_demo_toolbar.py b/tests/test_demo_toolbar.py
new file mode 100644
index 0000000..fdeebb0
--- /dev/null
+++ b/tests/test_demo_toolbar.py
@@ -0,0 +1,63 @@
+"""Tests for app/components/demo_toolbar.py."""
+import sys, os
+from pathlib import Path
+import pytest
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+# Ensure DEMO_MODE is set so the module initialises correctly
+os.environ["DEMO_MODE"] = "true"
+
+
+def test_set_simulated_tier_updates_session_state(monkeypatch):
+    """set_simulated_tier writes to st.session_state.simulated_tier."""
+    session = {}
+    injected = []
+    monkeypatch.setattr("streamlit.components.v1.html", lambda h, height=0: injected.append(h))
+    monkeypatch.setattr("streamlit.session_state", session, raising=False)
+    monkeypatch.setattr("streamlit.rerun", lambda: None)
+
+    from unittest.mock import patch
+    with patch('app.components.demo_toolbar._DEMO_MODE', True):
+        from importlib import reload
+        import app.components.demo_toolbar as m
+        reload(m)
+        m.set_simulated_tier("premium")
+
+    assert session.get("simulated_tier") == "premium"
+    assert any("prgn_demo_tier=premium" in h for h in injected)
+
+
+def test_set_simulated_tier_invalid_ignored(monkeypatch):
+    """Invalid tier strings are rejected."""
+    session = {}
+    monkeypatch.setattr("streamlit.components.v1.html", lambda h, height=0: None)
+    monkeypatch.setattr("streamlit.session_state", session, raising=False)
+    monkeypatch.setattr("streamlit.rerun", lambda: None)
+
+    from unittest.mock import patch
+    with patch('app.components.demo_toolbar._DEMO_MODE', True):
+        from importlib import reload
+        import app.components.demo_toolbar as m
+        reload(m)
+        m.set_simulated_tier("ultramax")
+
+    assert "simulated_tier" not in session
+
+
+def test_get_simulated_tier_defaults_to_paid(monkeypatch):
+    """Returns 'paid' when no tier is set yet."""
+    monkeypatch.setattr("streamlit.session_state", {}, raising=False)
+    monkeypatch.setattr("streamlit.query_params", {}, raising=False)
+
+    from app.components.demo_toolbar import get_simulated_tier
+    assert get_simulated_tier() == "paid"
+
+
+def test_get_simulated_tier_reads_session(monkeypatch):
+    """Returns tier from st.session_state when set."""
+    monkeypatch.setattr("streamlit.session_state", {"simulated_tier": "free"}, raising=False)
+    monkeypatch.setattr("streamlit.query_params", {}, raising=False)
+
+    from app.components.demo_toolbar import get_simulated_tier
+    assert get_simulated_tier() == "free"