diff --git a/internal/adapter/openai/tool_history_sanitize.go b/internal/adapter/openai/tool_history_sanitize.go
index 6a2e80a..126414a 100644
--- a/internal/adapter/openai/tool_history_sanitize.go
+++ b/internal/adapter/openai/tool_history_sanitize.go
@@ -5,10 +5,13 @@ import (
 )
 
 var leakedToolHistoryPattern = regexp.MustCompile(`(?is)\[TOOL_CALL_HISTORY\][\s\S]*?\[/TOOL_CALL_HISTORY\]|\[TOOL_RESULT_HISTORY\][\s\S]*?\[/TOOL_RESULT_HISTORY\]`)
+var emptyJSONFencePattern = regexp.MustCompile("(?is)```json\\s*```")
 
 func sanitizeLeakedToolHistory(text string) string {
 	if text == "" {
 		return text
 	}
-	return leakedToolHistoryPattern.ReplaceAllString(text, "")
+	out := leakedToolHistoryPattern.ReplaceAllString(text, "")
+	out = emptyJSONFencePattern.ReplaceAllString(out, "")
+	return out
 }
diff --git a/internal/adapter/openai/tool_history_sanitize_test.go b/internal/adapter/openai/tool_history_sanitize_test.go
index 02128c9..7c10ad2 100644
--- a/internal/adapter/openai/tool_history_sanitize_test.go
+++ b/internal/adapter/openai/tool_history_sanitize_test.go
@@ -43,6 +43,14 @@ func TestSanitizeLeakedToolHistoryPreservesChunkWhitespace(t *testing.T) {
 	}
 }
 
+func TestSanitizeLeakedToolHistoryRemovesEmptyJSONFence(t *testing.T) {
+	raw := "before\n```json\n```\nafter"
+	got := sanitizeLeakedToolHistory(raw)
+	if got != "before\n\nafter" {
+		t.Fatalf("unexpected sanitized empty json fence: %q", got)
+	}
+}
+
 func TestFlushToolSieveDropsToolHistoryLeak(t *testing.T) {
 	var state toolStreamSieveState
 	chunk := "[TOOL_CALL_HISTORY]\nstatus: already_called\nfunction.name: exec\nfunction.arguments: {}\n[/TOOL_CALL_HISTORY]"