diff --git a/s08_context_compact/README.en.md b/s08_context_compact/README.en.md
index d738b946..dcf254cc 100644
--- a/s08_context_compact/README.en.md
+++ b/s08_context_compact/README.en.md
@@ -140,7 +140,9 @@ This triggers **reactive_compact**: more aggressive than compact_history, it ret
 def reactive_compact(messages):
     transcript = write_transcript(messages)
     tail_start = max(0, len(messages) - 5)
-    if _is_tool_result_message(messages[tail_start]) and _message_has_tool_use(messages[tail_start - 1]):
+    if (tail_start > 0 and tail_start < len(messages)
+            and _is_tool_result_message(messages[tail_start])
+            and _message_has_tool_use(messages[tail_start - 1])):
         tail_start -= 1
     summary = summarize_history(messages[:tail_start])
     return [{"role": "user",
diff --git a/s08_context_compact/README.ja.md b/s08_context_compact/README.ja.md
index 89f55ee2..72cae428 100644
--- a/s08_context_compact/README.ja.md
+++ b/s08_context_compact/README.ja.md
@@ -140,7 +140,9 @@ API がまだ `prompt_too_long`（413）を返すことがある。コンテキ
 def reactive_compact(messages):
     transcript = write_transcript(messages)
     tail_start = max(0, len(messages) - 5)
-    if _is_tool_result_message(messages[tail_start]) and _message_has_tool_use(messages[tail_start - 1]):
+    if (tail_start > 0 and tail_start < len(messages)
+            and _is_tool_result_message(messages[tail_start])
+            and _message_has_tool_use(messages[tail_start - 1])):
         tail_start -= 1
     summary = summarize_history(messages[:tail_start])
     return [{"role": "user",
diff --git a/s08_context_compact/README.md b/s08_context_compact/README.md
index db71a6cf..771257d4 100644
--- a/s08_context_compact/README.md
+++ b/s08_context_compact/README.md
@@ -140,7 +140,9 @@ def compact_history(messages):
 def reactive_compact(messages):
     transcript = write_transcript(messages)
     tail_start = max(0, len(messages) - 5)
-    if _is_tool_result_message(messages[tail_start]) and _message_has_tool_use(messages[tail_start - 1]):
+    if (tail_start > 0 and tail_start < len(messages)
+            and _is_tool_result_message(messages[tail_start])
+            and _message_has_tool_use(messages[tail_start - 1])):
         tail_start -= 1
     summary = summarize_history(messages[:tail_start])
     return [{"role": "user",