diff --git a/static/sessions.js b/static/sessions.js
index 485132f6..56c8d410 100644
--- a/static/sessions.js
+++ b/static/sessions.js
@@ -4766,6 +4766,28 @@ function _partitionSidebarSessionRows(allMatched, activeSidForSidebar){
   };
 }
 
+function _renderSidebarRowsFromRawSessions(sessionsRaw){
+  return _attachChildSessionsToSidebarRows(_collapseSessionLineageForSidebar(sessionsRaw), sessionsRaw);
+}
+
+function _countRenderedSidebarRows(allMatched, activeSidForSidebar, showCliOnly){
+  const sessionsRaw=[];
+  for(const s of allMatched){
+    if(!_sidebarRowHasVisibleMessages(s, activeSidForSidebar)) continue;
+    const isCli=_isCliSession(s);
+    if(showCliOnly ? !isCli : isCli) continue;
+    if(s.default_hidden&&!(_activeProject&&_activeProject!==NO_PROJECT_FILTER&&s.project_id===_activeProject)) continue;
+    if(_activeProject===NO_PROJECT_FILTER){
+      if(s.project_id) continue;
+    } else if(_activeProject){
+      if(s.project_id!==_activeProject) continue;
+    }
+    if(!_showArchived&&s.archived) continue;
+    sessionsRaw.push(s);
+  }
+  return _renderSidebarRowsFromRawSessions(sessionsRaw).length;
+}
+
 function renderSessionListFromCache(){
   // Don't re-render while user is actively renaming a session (would destroy the input)
   if(_renamingSid) return;
@@ -4795,7 +4817,13 @@ function renderSessionListFromCache(){
     sessionsRaw,
     archivedCount,
   }=_partitionSidebarSessionRows(allMatched, activeSidForSidebar);
-  const sessions=_attachChildSessionsToSidebarRows(_collapseSessionLineageForSidebar(sessionsRaw), sessionsRaw);
+  const sessions=_renderSidebarRowsFromRawSessions(sessionsRaw);
+  const renderedWebuiSessionCount=_sessionSourceFilter==='webui'
+    ? sessions.length
+    : _countRenderedSidebarRows(allMatched, activeSidForSidebar, false);
+  const renderedCliSessionCount=_sessionSourceFilter==='cli'
+    ? sessions.length
+    : _countRenderedSidebarRows(allMatched, activeSidForSidebar, true);
   _syncSidebarExpansionForActiveSession(sessions, activeSidForSidebar);
   const list=$('sessionList');
   const animateRefresh=_sessionListRefreshAnimationPending;
@@ -4830,7 +4858,7 @@ function renderSessionListFromCache(){
     const sourceTabs=document.createElement('div');
     sourceTabs.className='session-source-tabs';
     for(const filter of ['webui','cli']){
-      const count=filter==='cli'?cliSessionCount:webuiSessionCount;
+      const count=filter==='cli'?renderedCliSessionCount:renderedWebuiSessionCount;
       const btn=document.createElement('button');
       btn.type='button';
       btn.className='session-source-tab'+(_sessionSourceFilter===filter?' active':'');
diff --git a/tests/test_issue3966_session_count_label.py b/tests/test_issue3966_session_count_label.py
new file mode 100644
index 00000000..e561d0ed
--- /dev/null
+++ b/tests/test_issue3966_session_count_label.py
@@ -0,0 +1,32 @@
+"""Regression coverage for sidebar source counts using rendered rows."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+SESSIONS_JS = (ROOT / "static" / "sessions.js").read_text(encoding="utf-8")
+
+
+def test_render_counts_use_post_collapse_rows():
+    render_start = SESSIONS_JS.index("function renderSessionListFromCache()")
+    render_end = SESSIONS_JS.index("function _showProjectPicker", render_start)
+    render_body = SESSIONS_JS[render_start:render_end]
+
+    assert "const sessions=_renderSidebarRowsFromRawSessions(sessionsRaw);" in render_body
+    assert "? sessions.length" in render_body
+    assert ": _countRenderedSidebarRows(allMatched, activeSidForSidebar, false);" in render_body
+    assert ": _countRenderedSidebarRows(allMatched, activeSidForSidebar, true);" in render_body
+    assert "const count=filter==='cli'?renderedCliSessionCount:renderedWebuiSessionCount;" in render_body
+    assert "const count=filter==='cli'?cliSessionCount:webuiSessionCount;" not in render_body
+
+
+def test_rendered_count_helper_collapses_before_counting():
+    helper_start = SESSIONS_JS.index("function _countRenderedSidebarRows(")
+    helper_end = SESSIONS_JS.index("function renderSessionListFromCache()", helper_start)
+    helper_body = SESSIONS_JS[helper_start:helper_end]
+
+    assert "sessionsRaw.push(s);" in helper_body
+    assert "return _renderSidebarRowsFromRawSessions(sessionsRaw).length;" in helper_body
+    assert "function _renderSidebarRowsFromRawSessions(sessionsRaw){" in SESSIONS_JS
+    assert "_attachChildSessionsToSidebarRows(_collapseSessionLineageForSidebar(sessionsRaw), sessionsRaw)" in SESSIONS_JS
diff --git a/tests/test_sidebar_session_partition.py b/tests/test_sidebar_session_partition.py
index c99f66b4..aa54b61f 100644
--- a/tests/test_sidebar_session_partition.py
+++ b/tests/test_sidebar_session_partition.py
@@ -20,6 +20,7 @@ def test_render_uses_single_pass_partition_helper():
     render_body = SESSIONS_JS[render_start:render_end]
 
     assert "_partitionSidebarSessionRows(allMatched, activeSidForSidebar)" in render_body
+    assert "_renderSidebarRowsFromRawSessions(sessionsRaw)" in render_body
     assert "withMessages.filter(" not in render_body
 
 
@@ -35,3 +36,14 @@ def test_partition_helper_applies_message_source_project_and_archive_gates():
     assert "return {" in block
     assert "profileFiltered," in block
     assert "sessionsRaw," in block
+
+
+def test_partition_helper_keeps_raw_source_counts_while_render_owns_visible_counts():
+    render_start = SESSIONS_JS.index("function renderSessionListFromCache()")
+    render_end = SESSIONS_JS.index("function _showProjectPicker", render_start)
+    render_body = SESSIONS_JS[render_start:render_end]
+
+    assert "webuiSessionCount," in _partition_block()
+    assert "cliSessionCount," in _partition_block()
+    assert "const renderedWebuiSessionCount=" in render_body
+    assert "const renderedCliSessionCount=" in render_body