diff --git a/tools/repair_post_modularization_patcher.py b/tools/repair_post_modularization_patcher.py
new file mode 100644
index 0000000..99c0bd0
--- /dev/null
+++ b/tools/repair_post_modularization_patcher.py
@@ -0,0 +1,68 @@
+#!/usr/bin/env python3
+from pathlib import Path
+
+path = Path(__file__).resolve().with_name("apply_post_modularization_fixes.py")
+text = path.read_text(encoding="utf-8")
+
+helper_anchor = '''def replace_once(text, old, new, label):
+    count = text.count(old)
+    if count != 1:
+        raise RuntimeError(f"{label}: expected one match, got {count}")
+    return text.replace(old, new, 1)
+'''
+helper_replacement = helper_anchor + '''\n\ndef replace_in_function(text, function_name, old, new, label):
+    marker = f"def {function_name}("
+    start = text.find(marker)
+    if start < 0:
+        raise RuntimeError(f"{label}: function {function_name!r} not found")
+    next_function = text.find("\ndef ", start + len(marker))
+    end = len(text) if next_function < 0 else next_function + 1
+    function_text = text[start:end]
+    count = function_text.count(old)
+    if count != 1:
+        raise RuntimeError(
+            f"{label}: expected one match in {function_name}, got {count}"
+        )
+    return text[:start] + function_text.replace(old, new, 1) + text[end:]
+'''
+if "def replace_in_function(" not in text:
+    if text.count(helper_anchor) != 1:
+        raise RuntimeError("replace helper anchor is not unique")
+    text = text.replace(helper_anchor, helper_replacement, 1)
+
+old_duck_call = '''text = replace_once(text, old_duck_body, '            combined = normalize_mail_body(detail)\\n', "DuckMail body normalization")'''
+new_duck_call = '''text = replace_in_function(
+    text,
+    "duckmail_get_oai_code",
+    old_duck_body,
+    '            combined = normalize_mail_body(detail)\\n',
+    "DuckMail body normalization",
+)'''
+if old_duck_call in text:
+    text = text.replace(old_duck_call, new_duck_call, 1)
+elif new_duck_call not in text:
+    raise RuntimeError("DuckMail replacement call anchor not found")
+
+old_yyds_call = '''text = replace_once(text, old_yyds_body, '            combined = normalize_mail_body(detail)\\n', "YYDS body normalization")'''
+new_yyds_call = '''text = replace_in_function(
+    text,
+    "yyds_get_oai_code",
+    old_yyds_body,
+    '            combined = normalize_mail_body(detail)\\n',
+    "YYDS body normalization",
+)'''
+if old_yyds_call in text:
+    text = text.replace(old_yyds_call, new_yyds_call, 1)
+elif new_yyds_call not in text:
+    raise RuntimeError("YYDS replacement call anchor not found")
+
+# Guard against reintroducing ambiguous full-file replacements for provider bodies.
+for forbidden in (
+    "replace_once(text, old_duck_body",
+    "replace_once(text, old_yyds_body",
+):
+    if forbidden in text:
+        raise RuntimeError(f"ambiguous provider replacement remains: {forbidden}")
+
+path.write_text(text, encoding="utf-8")
+print("post-modularization patcher repaired with function-scoped anchors")