diff --git a/contributing/samples/gepa/experiment.py b/contributing/samples/gepa/experiment.py
index f3751206a8..2710c3894c 100644
--- a/contributing/samples/gepa/experiment.py
+++ b/contributing/samples/gepa/experiment.py
@@ -43,7 +43,6 @@
 from tau_bench.types import EnvRunResult
 from tau_bench.types import RunConfig
 import tau_bench_agent as tau_bench_agent_lib
-
 import utils
 
 
diff --git a/contributing/samples/gepa/run_experiment.py b/contributing/samples/gepa/run_experiment.py
index d857da9635..e31db15788 100644
--- a/contributing/samples/gepa/run_experiment.py
+++ b/contributing/samples/gepa/run_experiment.py
@@ -25,7 +25,6 @@
 from absl import flags
 import experiment
 from google.genai import types
-
 import utils
 
 _OUTPUT_DIR = flags.DEFINE_string(
diff --git a/src/google/adk/skills/_utils.py b/src/google/adk/skills/_utils.py
index cab70a8d4b..e3185135e7 100644
--- a/src/google/adk/skills/_utils.py
+++ b/src/google/adk/skills/_utils.py
@@ -17,6 +17,7 @@
 from __future__ import annotations
 
 import logging
+import os
 import pathlib
 from typing import Union
 
@@ -401,7 +402,7 @@ def _load_skill_from_gcs_dir(
         f" name '{skill_name_expected}'."
     )
 
-  def _load_files_in_dir(subdir: str) -> Dict[str, Union[str, bytes]]:
+  def _load_files_in_dir(subdir: str) -> dict[str, str | bytes]:
     prefix = f"{skill_dir_prefix}{subdir}/"
     blobs = bucket.list_blobs(prefix=prefix)
     result = {}
@@ -411,10 +412,26 @@ def _load_files_in_dir(subdir: str) -> Dict[str, Union[str, bytes]]:
       if not relative_path:
         continue
 
+      # Use PurePosixPath for platform-independent GCS path validation
+      p = pathlib.PurePosixPath(relative_path)
+
+      # Reject absolute paths and traversal sequences
+      if p.is_absolute() or ".." in p.parts:
+        raise ValueError(f"Unsafe path in skill resource: {relative_path!r}")
+
+      normalized = p.as_posix()
+
+      # Prevent silent file overwrites via path aliasing
+      if normalized in result:
+        raise ValueError(f"Duplicate normalized path detected: {normalized!r}")
+
+      # NOTE: Final path safety enforced during materialization
+      # via realpath + commonpath checks in skill_toolset.py
       try:
-        result[relative_path] = blob.download_as_text()
+        result[normalized] = blob.download_as_text()
       except UnicodeDecodeError:
-        result[relative_path] = blob.download_as_bytes()
+        result[normalized] = blob.download_as_bytes()
+
     return result
 
   references = _load_files_in_dir("references")
diff --git a/src/google/adk/tools/skill_toolset.py b/src/google/adk/tools/skill_toolset.py
index a7a2c683ca..9f7fef11e6 100644
--- a/src/google/adk/tools/skill_toolset.py
+++ b/src/google/adk/tools/skill_toolset.py
@@ -520,6 +520,7 @@ def _build_wrapper_code(
       )
 
     # Build the boilerplate extract string
+
     code_lines = [
         "import os",
         "import tempfile",
@@ -531,8 +532,14 @@ def _build_wrapper_code(
         "def _materialize_and_run():",
         "  _orig_cwd = os.getcwd()",
         "  with tempfile.TemporaryDirectory() as td:",
+        "    _real_base = os.path.realpath(td)",
         "    for rel_path, content in _files.items():",
-        "      full_path = os.path.join(td, rel_path)",
+        "      if os.path.isabs(rel_path):",
+        "        raise ValueError(f'Absolute path rejected: {rel_path!r}')",
+        "      _safe = os.path.realpath(os.path.join(td, rel_path))",
+        "      if os.path.commonpath([_real_base, _safe]) != _real_base:",
+        "        raise ValueError(f'Path traversal detected: {rel_path!r}')",
+        "      full_path = _safe",
         "      os.makedirs(os.path.dirname(full_path), exist_ok=True)",
         "      mode = 'wb' if isinstance(content, bytes) else 'w'",
         "      with open(full_path, mode) as f:",