fix: avoid double rebasing legacy tool cards

static/sessions.js

@@ -1310,7 +1310,7 @@ let _messagesTruncated = false;
 // Older messages are loaded on-demand via _loadOlderMessages().
 const _INITIAL_MSG_LIMIT = 30;
 
-function _syncToolCallsForLoadedMessages(messages, sessionToolCalls, windowOffset=0){
+function _syncToolCallsForLoadedMessages(messages, sessionToolCalls){
   const msgs=Array.isArray(messages)?messages:[];
   const hasMessageToolMetadata=msgs.some(m=>{
     if(!m) return false;
@@ -1319,15 +1319,7 @@ function _syncToolCallsForLoadedMessages(messages, sessionToolCalls, windowOffse
     return hasTc||hasTu;
   });
   if(!hasMessageToolMetadata&&Array.isArray(sessionToolCalls)&&sessionToolCalls.length){
-    const offset=Number.isFinite(Number(windowOffset))?Number(windowOffset):0;
-    S.toolCalls=sessionToolCalls.map(tc=>{
-      const copy={...tc,done:true};
-      const idx=copy.assistant_msg_idx;
-      if(Number.isInteger(idx)&&idx>=offset&&idx<offset+msgs.length){
-        copy.assistant_msg_idx=idx-offset;
-      }
-      return copy;
-    });
+    S.toolCalls=sessionToolCalls.map(tc=>({...tc,done:true}));
   }else{
     S.toolCalls=[];
   }
@@ -1350,7 +1342,7 @@ async function _ensureMessagesLoaded(sid) {
   // toast on every mobile message (SSE/visibility events trigger this reload path
   // more aggressively on mobile).
   let msgs = (data.session.messages || []).filter(m => m && m.role);
-  _syncToolCallsForLoadedMessages(msgs, data.session.tool_calls, data.session._messages_offset||0);
+  _syncToolCallsForLoadedMessages(msgs, data.session.tool_calls);
   clearLiveToolCards();
   // #3018: preserve client-side ephemeral turn fields (_turnUsage, _turnDuration,
   // _turnTps, _gatewayRouting, _statusCard) across the loadSession replace.
@@ -1513,7 +1505,7 @@ async function _loadOlderMessages() {
     const container = $('messages');
     const prevScrollH = container ? container.scrollHeight : 0;
     S.messages = nextMessages;
-    _syncToolCallsForLoadedMessages(nextMessages, responseSession.tool_calls, responseSession._messages_offset||0);
+    _syncToolCallsForLoadedMessages(nextMessages, responseSession.tool_calls);
     // renderMessages() windows long transcripts from the end. If we do not
     // expand that window before rendering, the newly prepended page stays
     // hidden and the "hidden" counter rises while the viewport appears stuck.
@@ -1598,7 +1590,7 @@ async function _ensureAllMessagesLoaded() {
     S.messages = msgs;
     _messagesTruncated = false;
     _oldestIdx = 0;
-    _syncToolCallsForLoadedMessages(msgs, data.session.tool_calls, data.session._messages_offset||0);
+    _syncToolCallsForLoadedMessages(msgs, data.session.tool_calls);
     if (S.session && S.session.session_id === sid) {
       S.session.message_count = Number(data.session.message_count || msgs.length);
     }

tests/test_issue401.py

@@ -28,10 +28,10 @@ def test_loadsession_preserves_tool_rows():
 
 def test_loadsession_uses_session_toolcalls_only_as_fallback():
     """Session summaries are the fallback, not the primary reload source."""
-    assert "function _syncToolCallsForLoadedMessages(messages, sessionToolCalls, windowOffset=0)" in SESSIONS_JS
+    assert "function _syncToolCallsForLoadedMessages(messages, sessionToolCalls)" in SESSIONS_JS
     assert "if(!hasMessageToolMetadata&&Array.isArray(sessionToolCalls)&&sessionToolCalls.length)" in SESSIONS_JS
-    assert "const offset=Number.isFinite(Number(windowOffset))?Number(windowOffset):0;" in SESSIONS_JS
-    assert "copy.assistant_msg_idx=idx-offset;" in SESSIONS_JS
+    assert "windowOffset" not in SESSIONS_JS
+    assert "copy.assistant_msg_idx=idx-offset;" not in SESSIONS_JS
     assert "S.toolCalls=[];" in SESSIONS_JS
 
 
@@ -114,3 +114,27 @@ def test_reload_uses_session_summary_when_messages_have_no_tool_metadata():
     assert result["has_metadata"] is False
     assert result["fallback_len"] == 1
     assert result["done_flag"] is True
+
+
+def test_rebased_legacy_toolcalls_stay_distinct_in_paged_window():
+    """Backend-rebased legacy tool calls must not be rebased a second time."""
+    result = _run_js("""
+        const messages = [
+            { role: 'assistant', content: 'first legacy page row' },
+            { role: 'assistant', content: 'second legacy page row' }
+        ];
+        const sessionToolCalls = [
+            { name: 'first_tool', assistant_msg_idx: 0, snippet: 'first' },
+            { name: 'second_tool', assistant_msg_idx: 1, snippet: 'second' }
+        ];
+        const loaded = loadSessionShape(messages, sessionToolCalls);
+        const renderedRawIdxs = messages.map((_, idx) => idx);
+        process.stdout.write(JSON.stringify({
+            indices: loaded.toolCalls.map(tc => tc.assistant_msg_idx),
+            distinct_indices: new Set(loaded.toolCalls.map(tc => tc.assistant_msg_idx)).size,
+            anchors_visible: loaded.toolCalls.every(tc => renderedRawIdxs.includes(tc.assistant_msg_idx))
+        }));
+    """)
+    assert result["indices"] == [0, 1]
+    assert result["distinct_indices"] == 2
+    assert result["anchors_visible"] is True

tests/test_session_tail_payload.py

@@ -117,11 +117,12 @@ def test_msg_before_window_keeps_only_that_page_session_tool_calls():
     session = _FakeSession([
         {"role": "user", "content": "first"},
         {"role": "assistant", "content": "second legacy message"},
-        {"role": "user", "content": "third"},
+        {"role": "assistant", "content": "third legacy message"},
         {"role": "assistant", "content": "fourth legacy message"},
     ])
     session.tool_calls = [
         {"name": "first-page-tool", "snippet": "kept", "assistant_msg_idx": 1},
+        {"name": "second-page-tool", "snippet": "also kept", "assistant_msg_idx": 2},
         {"name": "tail-tool", "snippet": "not in page", "assistant_msg_idx": 3},
         {"name": "unindexed-tool", "snippet": "cannot place"},
     ]
@@ -133,7 +134,9 @@ def test_msg_before_window_keeps_only_that_page_session_tool_calls():
 
     assert payload["messages"] == session.messages[1:3]
     assert payload["tool_calls"] == [
-        {"name": "first-page-tool", "snippet": "kept", "assistant_msg_idx": 0}
+        {"name": "first-page-tool", "snippet": "kept", "assistant_msg_idx": 0},
+        {"name": "second-page-tool", "snippet": "also kept", "assistant_msg_idx": 1},
     ]
     assert session.tool_calls[0]["assistant_msg_idx"] == 1
+    assert session.tool_calls[1]["assistant_msg_idx"] == 2
     assert payload["_messages_offset"] == 1

tests/test_tool_call_history_paging.py

@@ -13,15 +13,16 @@ def test_sessions_js_resyncs_tool_calls_after_history_window_replacement():
     can keep stale anchors and show unloaded/thinking placeholders while the
     user scrolls through history.
     """
-    assert "function _syncToolCallsForLoadedMessages(messages, sessionToolCalls, windowOffset=0)" in SESSIONS_JS
-    assert "_syncToolCallsForLoadedMessages(msgs, data.session.tool_calls, data.session._messages_offset||0);" in SESSIONS_JS
-    assert "S.messages = nextMessages;\n    _syncToolCallsForLoadedMessages(nextMessages, responseSession.tool_calls, responseSession._messages_offset||0);" in SESSIONS_JS
-    assert "S.messages = msgs;\n    _messagesTruncated = false;\n    _oldestIdx = 0;\n    _syncToolCallsForLoadedMessages(msgs, data.session.tool_calls, data.session._messages_offset||0);" in SESSIONS_JS
+    assert "function _syncToolCallsForLoadedMessages(messages, sessionToolCalls)" in SESSIONS_JS
+    assert "_syncToolCallsForLoadedMessages(msgs, data.session.tool_calls);" in SESSIONS_JS
+    assert "S.messages = nextMessages;\n    _syncToolCallsForLoadedMessages(nextMessages, responseSession.tool_calls);" in SESSIONS_JS
+    assert "S.messages = msgs;\n    _messagesTruncated = false;\n    _oldestIdx = 0;\n    _syncToolCallsForLoadedMessages(msgs, data.session.tool_calls);" in SESSIONS_JS
 
 
 def test_sessions_js_clears_session_tool_calls_when_messages_have_own_metadata():
     assert "const hasTc=Array.isArray(m.tool_calls)&&m.tool_calls.length>0;" in SESSIONS_JS
     assert "const hasTu=Array.isArray(m.content)&&m.content.some(p=>p&&p.type==='tool_use');" in SESSIONS_JS
-    assert "const offset=Number.isFinite(Number(windowOffset))?Number(windowOffset):0;" in SESSIONS_JS
-    assert "const copy={...tc,done:true};" in SESSIONS_JS
+    assert "windowOffset" not in SESSIONS_JS
+    assert "copy.assistant_msg_idx=idx-offset;" not in SESSIONS_JS
+    assert "S.toolCalls=sessionToolCalls.map(tc=>({...tc,done:true}));" in SESSIONS_JS
     assert "S.toolCalls=[];" in SESSIONS_JS