diff --git a/common/chat-peg-parser.cpp b/common/chat-peg-parser.cpp
index 0f7e2d74..a2865d94 100644
--- a/common/chat-peg-parser.cpp
+++ b/common/chat-peg-parser.cpp
@@ -785,19 +785,17 @@ common_peg_parser common_chat_peg_builder::prefix(const std::string & s, const s
     if (delimiter.empty()) {
         return literal(s);
     }
-    auto pos = s.find(delimiter);
+    auto pos = s.rfind(delimiter);
     if (pos == std::string::npos) {
         // The generation prompt may force-open the reasoning block without the
         // whitespace that surrounds the detected tag (e.g. a prompt ending in
         // '<think>' while history renders '<think>\n'). Only strip when the
         // prompt ends exactly with the trimmed tag, so prompts with trailing
         // whitespace after the tag (e.g. '<think>\n') keep their behavior.
-        size_t b = delimiter.find_first_not_of(" \t\n\r");
-        size_t e = delimiter.find_last_not_of(" \t\n\r");
-        if (b != std::string::npos) {
-            const std::string trimmed = delimiter.substr(b, e - b + 1);
-            if (s.size() >= trimmed.size() &&
-                s.compare(s.size() - trimmed.size(), trimmed.size(), trimmed) == 0) {
+        if (auto b = delimiter.find_first_not_of(" \t\n\r"); b != std::string::npos) {
+            auto e = delimiter.find_last_not_of (" \t\n\r");
+            auto trimmed = delimiter.substr(b, e - b + 1);
+            if (s.size() >= trimmed.size() && s.compare(s.size() - trimmed.size(), trimmed.size(), trimmed) == 0) {
                 pos = s.size() - trimmed.size();
             }
         }