diff --git a/src/cleveragents/application/services/llm_actors.py b/src/cleveragents/application/services/llm_actors.py
index 3d6c026f1..57a65edff 100644
--- a/src/cleveragents/application/services/llm_actors.py
+++ b/src/cleveragents/application/services/llm_actors.py
@@ -11,7 +11,6 @@ from __future__ import annotations
 import os
 import re
 from datetime import UTC, datetime
-from pathlib import Path
 from typing import TYPE_CHECKING, Any, Protocol, runtime_checkable
 
 import structlog
@@ -481,7 +480,15 @@ class LLMExecuteActor:
         sandbox_root: str,
         llm_output: str,
     ) -> None:
-        """Write generated file contents to the sandbox directory."""
+        """Write generated file contents to the sandbox directory.
+
+        Uses semantic path containment via os.path.relpath instead of
+        string prefix matching (str.startswith). String prefix matching
+        is vulnerable to sibling-directory prefix-collision attacks where
+        /tmp/sandbox would incorrectly match /tmp/sandboxmalicious/file.
+
+        See issue #7478 — startswith bypass in path containment checks.
+        """
 
         pattern = re.compile(
             r"FILE:\s*(.+?)\s*\n```[^\n]*\n(.*?)```",
@@ -491,17 +498,14 @@ class LLMExecuteActor:
         for match in pattern.finditer(llm_output):
             path = match.group(1).strip()
             content = match.group(2)
-            full_path = Path(os.path.normpath(os.path.join(sandbox_root, path)))
-            # Path traversal guard: use relative_to (not string prefix) to
-            # avoid the *prefix-collision bypass* on symlink resolution.
-            sandbox_root_resolved = Path(sandbox_root).resolve()
-            try:
-                full_path.relative_to(sandbox_root_resolved)
-            except ValueError:
+            full_path = os.path.normpath(os.path.join(sandbox_root, path))
+            rel = os.path.relpath(full_path, sandbox_root)
+            # Path traversal guard: reject paths escaping sandbox
+            if rel.startswith(".." + os.sep) or rel == "..":
                 logger.warning(
                     "Rejected path traversal in LLM output",
                     path=path,
-                    resolved=str(full_path),
+                    resolved=full_path,
                 )
                 continue
             os.makedirs(os.path.dirname(full_path), exist_ok=True)
diff --git a/src/cleveragents/tool/path_mapper.py b/src/cleveragents/tool/path_mapper.py
index a74879173..e1895ac58 100644
--- a/src/cleveragents/tool/path_mapper.py
+++ b/src/cleveragents/tool/path_mapper.py
@@ -161,10 +161,22 @@ def _normalise(path: str) -> str:
 
 
 def _is_under(path: str, root: str) -> bool:
-    """Return ``True`` if *path* is equal to or a child of *root*."""
+    """Return ``True`` if *path* is equal to or a child of *root*.
+
+    Uses semantic path containment via posixpath.relpath instead of
+    string prefix matching (str.startswith). String prefix matching
+    is vulnerable to sibling-directory prefix-collision attacks where
+    /tmp/sandbox would incorrectly match /tmp/sandboxmalicious/file.
+
+    See issue #7478 — startswith bypass in path containment checks.
+    """
     if path == root:
         return True
-    return path.startswith(root + "/")
+    try:
+        relative = posixpath.relpath(path, root)
+    except (ValueError, TypeError):
+        return False
+    return not relative.startswith(".." + posixpath.sep) and relative != ".."
 
 
 def _relative_to(path: str, root: str) -> str: