diff --git a/CHANGELOG.md b/CHANGELOG.md
index 231f94f7b3c..080fb8e5a17 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- **PR #2408** by @Michaelyklam (fixes #2404) — Auto-compression cards now close the current live Activity burst before rendering, so post-compression tools start a fresh `Activity` row instead of joining the pre-compression tool group across a real timeline/context boundary.
+
 ## [v0.51.77] — 2026-05-16 — Release BA (stage-370 — 1-PR follow-up — live Activity grouping boundary fix)
 
 ### Fixed
diff --git a/static/ui.js b/static/ui.js
index 1152a0b0e7c..297072d248c 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -4718,6 +4718,13 @@ function ensureActivityGroup(inner, opts){
   if(live) _startActivityElapsedTimer(group);
   return group;
 }
+function closeCurrentLiveActivityGroup(){
+  const turn=$('liveAssistantTurn');
+  if(!turn) return;
+  turn.querySelectorAll('.tool-call-group[data-live-tool-call-group="1"][data-live-activity-current="1"]').forEach(group=>{
+    group.removeAttribute('data-live-activity-current');
+  });
+}
 function _compressionStateForCurrentSession(){
   const state=window._compressionUi;
   if(!state||!S.session||state.sessionId!==S.session.session_id) return null;
@@ -4845,6 +4852,7 @@ function appendLiveCompressionCard(state){
   }
   const inner=_assistantTurnBlocks(turn);
   if(!inner) return false;
+  closeCurrentLiveActivityGroup();
   const node=_compressionCardsNode(state);
   if(!node) return false;
   node.setAttribute('data-live-compression-card','1');
diff --git a/tests/test_ui_tool_call_cleanup.py b/tests/test_ui_tool_call_cleanup.py
index cee72ab3dc4..e647b35392c 100644
--- a/tests/test_ui_tool_call_cleanup.py
+++ b/tests/test_ui_tool_call_cleanup.py
@@ -327,6 +327,19 @@ def test_live_visible_interim_text_splits_tool_bursts_not_thinking(self):
             "Tool starts must not split consecutive tools into one-tool Activity rows."
         )
 
+    def test_live_compression_card_splits_current_tool_activity_burst(self):
+        compression_fn = _function_body(UI_JS, "appendLiveCompressionCard")
+        close_fn = _function_body(UI_JS, "closeCurrentLiveActivityGroup")
+        assert "closeCurrentLiveActivityGroup();" in compression_fn, (
+            "Auto-compression cards should close the current live Activity burst so later tools start a fresh group."
+        )
+        assert "data-live-activity-current" in close_fn, (
+            "The live compression boundary helper must clear the current Activity marker."
+        )
+        assert "removeAttribute('data-live-activity-current')" in close_fn, (
+            "Closing a live Activity burst should leave the row rendered but stop later tools from reusing it."
+        )
+
 
 class TestToolCardDesignTokens:
     def test_root_defines_shared_layout_design_tokens(self):