diff --git a/static/js/sessions.js b/static/js/sessions.js
index cf59d478c..254a5dc21 100644
--- a/static/js/sessions.js
+++ b/static/js/sessions.js
@@ -285,10 +285,14 @@ let _sessionListFocused = false;
 function _deselectCurrentSession(sid) {
   if (currentSessionId !== sid) return;
   currentSessionId = null;
+  try { window.__odysseusLastSelectedSessionId = ''; } catch (_) {}
   uiModule.el('chat-history').innerHTML = '';
   uiModule.el('current-meta').textContent = 'Odysseus Chat';
   Storage.remove('lastSessionId');
   history.replaceState(null, '', window.location.pathname);
+  document.querySelectorAll('.list-item.active-session, .session-item.active').forEach(el => {
+    el.classList.remove('active-session', 'active');
+  });
   if (window.chatModule && window.chatModule.showWelcomeScreen) {
     window.chatModule.showWelcomeScreen();
   }
diff --git a/tests/test_new_chat_session_isolation.py b/tests/test_new_chat_session_isolation.py
new file mode 100644
index 000000000..69166b6ed
--- /dev/null
+++ b/tests/test_new_chat_session_isolation.py
@@ -0,0 +1,45 @@
+from pathlib import Path
+
+
+APP_JS = Path("static/app.js")
+CHAT_JS = Path("static/js/chat.js")
+SESSIONS_JS = Path("static/js/sessions.js")
+
+
+def _slice(source, start_marker, end_marker):
+    start = source.index(start_marker)
+    end = source.index(end_marker, start)
+    return source[start:end]
+
+
+def test_fresh_chat_clears_restore_and_active_session_state():
+    app = APP_JS.read_text(encoding="utf-8")
+    sessions = SESSIONS_JS.read_text(encoding="utf-8")
+
+    fresh_chat = _slice(app, "function _startFreshChat()", "/** Sync Research indicator")
+    deselect_current = _slice(sessions, "function _deselectCurrentSession(sid)", "// Reset send button to idle state")
+    set_current_session = _slice(sessions, "export function setCurrentSessionId(id)", "export function getSortMode()")
+
+    assert "sessionModule.setCurrentSessionId(null)" in fresh_chat
+    assert "window.__odysseusLastSelectedSessionId = id || ''" in set_current_session
+    assert "window.__odysseusLastSelectedSessionId = ''" in deselect_current
+    assert "Storage.remove('lastSessionId')" in set_current_session
+    assert "Storage.remove('lastSessionId')" in deselect_current
+    assert ".list-item.active-session, .session-item.active" in set_current_session
+    assert ".list-item.active-session, .session-item.active" in deselect_current
+
+
+def test_first_send_in_new_chat_posts_materialized_session_id():
+    chat = CHAT_JS.read_text(encoding="utf-8")
+    send_path = _slice(chat, "export async function handleChatSubmit(e)", "export function abortCurrentRequest")
+
+    materialize_positions = [
+        idx
+        for idx in range(len(send_path))
+        if send_path.startswith("sessionModule.materializePendingSession()", idx)
+    ]
+    stream_id_pos = send_path.index("const streamSessionId = sessionModule.getCurrentSessionId();")
+    post_session_pos = send_path.index("fd.append('session', streamSessionId);")
+
+    assert materialize_positions
+    assert max(materialize_positions) < stream_id_pos < post_session_pos