diff --git a/static/messages.js b/static/messages.js
index 8295794d..220ab1a6 100644
--- a/static/messages.js
+++ b/static/messages.js
@@ -7092,7 +7092,7 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
         // identical turns make BOTH comparisons succeed, preferring the prefix
         // splices at the wrong offset and silently drops/duplicates rows
         // (#7628). Never prefer prefix when truncation is active.
-        const _truncatedRecovery=!!_messagesTruncated||!!(_oldestIdx>0);
+        const _truncatedRecovery=(typeof _messagesTruncated!=='undefined'&&!!_messagesTruncated)||(typeof _oldestIdx!=='undefined'&&!!(_oldestIdx>0));
         const _preserveCurrentTranscript=preserveVisibleOnShorterTerminalSnapshot&&(_truncatedRecovery?_stagedMatchesCurrentSuffix:_stagedMatchesCurrentPrefix);
         const _resolvedMessages=_preserveCurrentTranscript
           ? [..._stagedMessages,..._currentVisibleMessages.slice(_truncatedRecovery?_stagedSuffixStart+_stagedMessages.length:_stagedMessages.length)]
diff --git a/tests/test_session_default_window.py b/tests/test_session_default_window.py
index b10e63bb..3676d061 100644
--- a/tests/test_session_default_window.py
+++ b/tests/test_session_default_window.py
@@ -158,7 +158,7 @@ def test_settle_truncation_signal_decides_match_strategy():
     BOTH comparisons succeed; the prefix branch then splices at the wrong offset
     and silently drops/duplicates rows) (#7628)."""
     restore = _restore_settled_session_body()
-    assert "_truncatedRecovery=!!_messagesTruncated||!!(_oldestIdx>0)" in restore
+    assert "_truncatedRecovery=(typeof _messagesTruncated!=='undefined'&&!!_messagesTruncated)||(typeof _oldestIdx!=='undefined'&&!!(_oldestIdx>0))" in restore
     # The splice offset must follow the SAME truncation decision as the match
     # strategy — no fallthrough to prefix when both match.
     assert "_truncatedRecovery?_stagedSuffixStart+_stagedMessages.length:_stagedMessages.length" in restore