diff --git a/gui/chatblockmodel.cpp b/gui/chatblockmodel.cpp
index 126abb4..e9d9e61 100644
--- a/gui/chatblockmodel.cpp
+++ b/gui/chatblockmodel.cpp
@@ -131,29 +131,33 @@ static constexpr int LOAD_MORE_COUNT = 30;
 
 void ChatBlockModel::reparseIncremental()
 {
-    // Only re-parse from where the last block started — O(tail) not O(full text)
+    // Fast path: just update the last block's content from the raw text tail.
+    // Only do a full reparse if a new block header appeared.
     QString tail = m_rawText.mid(m_lastBlockTextOffset);
-    QList<ChatBlock> tailBlocks = parseBlocks(tail);
 
-    // Replace the last block (which may have grown) and append any new blocks
-    if (!m_allBlocks.isEmpty())
-        m_allBlocks.removeLast();
-    m_allBlocks.append(tailBlocks);
+    // Check if a new block header appeared in the NEW portion (after old last-block content)
+    int oldLastBlockLen = m_allBlocks.isEmpty() ? 0 : m_allBlocks.last().content.size();
+    // The header itself is at m_lastBlockTextOffset, content starts after it + newline
+    // Simple heuristic: if the tail contains another header pattern, do full reparse
+    static const QRegularExpression headerRe(
+        R"(^\[(user|assistant|reasoning|call|tool|error|info)(?::([^\]]*))\])",
+        QRegularExpression::MultilineOption);
+    auto it = headerRe.globalMatch(tail);
+    int headerCount = 0;
+    while (it.hasNext()) { it.next(); headerCount++; }
 
-    // Update offset: last block starts at the last header in the tail
-    if (tailBlocks.size() > 1) {
-        // Multiple blocks parsed from tail — advance offset to where the last one starts
-        // Find the last header boundary in m_rawText
-        static const QRegularExpression headerRe(
-            R"(^\[(user|assistant|reasoning|call|tool|error|info)(?::([^\]]*))\])",
-            QRegularExpression::MultilineOption);
-        auto it = headerRe.globalMatch(m_rawText);
-        int lastPos = 0;
-        while (it.hasNext()) {
-            auto match = it.next();
-            lastPos = match.capturedStart();
+    if (headerCount > 1) {
+        // New block boundary appeared — fall back to full reparse
+        reparse();
+        return;
+    }
+
+    // Common case: just the last block grew. Update its content directly.
+    if (!m_allBlocks.isEmpty()) {
+        QList<ChatBlock> tailBlocks = parseBlocks(tail);
+        if (!tailBlocks.isEmpty()) {
+            m_allBlocks.last() = tailBlocks.last();
         }
-        m_lastBlockTextOffset = lastPos;
     }
 
     updateVisibleWindow();