diff --git a/agent/codex_responses_adapter.py b/agent/codex_responses_adapter.py
index 8a6831144f..60e9f49ef2 100644
--- a/agent/codex_responses_adapter.py
+++ b/agent/codex_responses_adapter.py
@@ -165,9 +165,11 @@ def _neutralize_harmony_tokens(text: str) -> str:
     """Keep Harmony source readable without emitting reserved wire tokens."""
     if not text or "<" not in text or "|" not in text:
         return text
-    # No ASCII code point is a Unicode format control (Cf), and str.isascii() is an
-    # O(1) flag check, so ASCII text skips the per-character category scan entirely.
-    if text.isascii() or not any(unicodedata.category(char) == "Cf" for char in text):
+    # No ASCII code point is a Unicode format control (Cf): str.isascii() is an O(1) flag
+    # check, and other text only needs each distinct non-ASCII character categorised once.
+    if text.isascii() or not any(
+        unicodedata.category(char) == "Cf" for char in set(text) if char > "\x7f"
+    ):
         return _HARMONY_CONTROL_TOKEN_RE.sub(rf"<{_FULLWIDTH_PIPE}\1{_FULLWIDTH_PIPE}>", text)
     # The backend strips Unicode format controls (e.g. U+200B) before its reserved-token
     # check, so match on the visible text and rewrite the original spans.