diff --git a/src/cleveragents/tool/path_mapper.py b/src/cleveragents/tool/path_mapper.py
index e1895ac58..30a31f238 100644
--- a/src/cleveragents/tool/path_mapper.py
+++ b/src/cleveragents/tool/path_mapper.py
@@ -182,8 +182,14 @@ def _is_under(path: str, root: str) -> bool:
 def _relative_to(path: str, root: str) -> str:
     """Return the part of *path* relative to *root*.
 
+    Uses :func:`posixpath.relpath` for canonical relative-path extraction,
+    consistent with :func:`_is_under`.  This avoids string-slicing prefix-collision
+    attacks where an attacker filename is a literal prefix of the sandbox root
+    (see issue #7478).
+
     Assumes :func:`_is_under` has already been checked.
     """
     if path == root:
         return ""
-    return path[len(root) + 1 :]
+    rel = posixpath.relpath(path, root)
+    return rel.replace("\\", "/")