diff --git a/src/google/adk/artifacts/artifact_util.py b/src/google/adk/artifacts/artifact_util.py
index 8227bbb92c..f7efcd55fb 100644
--- a/src/google/adk/artifacts/artifact_util.py
+++ b/src/google/adk/artifacts/artifact_util.py
@@ -43,6 +43,24 @@ class ParsedArtifactUri(NamedTuple):
 )
 
 
+def normalize_session_id(session_id: str | None) -> str | None:
+  """Normalizes a caller-supplied session id.
+
+  Strips surrounding whitespace the same way the session services do, so an
+  artifact saved against a padded session id lands in the same storage
+  namespace as the (normalized) session it belongs to, instead of a sibling
+  namespace no caller using the trimmed id can ever reach.
+
+  Args:
+      session_id: The caller-supplied session id, or None for a user-scoped
+        artifact.
+
+  Returns:
+      The stripped session id, or None if `session_id` was None.
+  """
+  return session_id.strip() if session_id is not None else None
+
+
 def parse_artifact_uri(uri: str) -> ParsedArtifactUri | None:
   """Parses an artifact URI.
 
@@ -60,7 +78,7 @@ def parse_artifact_uri(uri: str) -> ParsedArtifactUri | None:
     return ParsedArtifactUri(
         app_name=match.group(1),
         user_id=match.group(2),
-        session_id=match.group(3),
+        session_id=normalize_session_id(match.group(3)),
         filename=match.group(4),
         version=int(match.group(5)),
     )
@@ -97,6 +115,7 @@ def get_artifact_uri(
   Returns:
       The constructed artifact URI.
   """
+  session_id = normalize_session_id(session_id)
   if session_id:
     return f"artifact://apps/{app_name}/users/{user_id}/sessions/{session_id}/artifacts/{filename}/versions/{version}"
   else:
diff --git a/src/google/adk/artifacts/file_artifact_service.py b/src/google/adk/artifacts/file_artifact_service.py
index 9c53754bd5..e66c5bcae9 100644
--- a/src/google/adk/artifacts/file_artifact_service.py
+++ b/src/google/adk/artifacts/file_artifact_service.py
@@ -247,6 +247,7 @@ def _user_artifacts_dir(base_root: Path) -> Path:
 
 def _session_artifacts_dir(base_root: Path, session_id: str) -> Path:
   """Returns the path that stores session-scoped artifacts."""
+  session_id = session_id.strip()
   artifact_util.validate_path_segment(session_id, "session_id")
   return base_root / "sessions" / session_id / "artifacts"
 
diff --git a/src/google/adk/artifacts/gcs_artifact_service.py b/src/google/adk/artifacts/gcs_artifact_service.py
index 26a3ea21db..a92fb22e4a 100644
--- a/src/google/adk/artifacts/gcs_artifact_service.py
+++ b/src/google/adk/artifacts/gcs_artifact_service.py
@@ -213,6 +213,7 @@ def _get_blob_prefix(
     if self._file_has_user_namespace(filename):
       return f"{app_name}/{user_id}/user/{filename}"
 
+    session_id = artifact_util.normalize_session_id(session_id)
     if session_id is None:
       raise InputValidationError(
           "Session ID must be provided for session-scoped artifacts."
@@ -254,6 +255,7 @@ def _save_artifact(
       custom_metadata: Optional[dict[str, Any]] = None,
   ) -> int:
     artifact = ensure_part(artifact)
+    session_id = artifact_util.normalize_session_id(session_id)
     versions = self._list_versions(
         app_name=app_name,
         user_id=user_id,
@@ -337,6 +339,7 @@ def _load_artifact(
       filename: str,
       version: Optional[int] = None,
   ) -> Optional[types.Part]:
+    session_id = artifact_util.normalize_session_id(session_id)
     if version is None:
       versions = self._list_versions(
           app_name=app_name,
@@ -417,6 +420,7 @@ def _list_artifact_keys(
   ) -> list[str]:
     artifact_util.validate_path_segment(app_name, "app_name")
     artifact_util.validate_path_segment(user_id, "user_id")
+    session_id = artifact_util.normalize_session_id(session_id)
     if session_id is not None:
       artifact_util.validate_path_segment(session_id, "session_id")
     filenames = set()
@@ -622,6 +626,7 @@ def _get_authenticated_url_sync(
       max_depth: int = _MAX_ARTIFACT_REFERENCE_DEPTH,
   ) -> Optional[str]:
     """Generates an authenticated browser URL for an artifact."""
+    session_id = artifact_util.normalize_session_id(session_id)
     if version is None:
       versions = self._list_versions(
           app_name=app_name,
@@ -729,6 +734,7 @@ def _get_signed_url_sync(
       max_depth: int = _MAX_ARTIFACT_REFERENCE_DEPTH,
   ) -> Optional[str]:
     """Generates a time-limited signed URL for an artifact."""
+    session_id = artifact_util.normalize_session_id(session_id)
     if version is None:
       versions = self._list_versions(
           app_name=app_name,
diff --git a/src/google/adk/artifacts/in_memory_artifact_service.py b/src/google/adk/artifacts/in_memory_artifact_service.py
index 2ed4e0a9ac..259a536951 100644
--- a/src/google/adk/artifacts/in_memory_artifact_service.py
+++ b/src/google/adk/artifacts/in_memory_artifact_service.py
@@ -91,6 +91,7 @@ def _artifact_path(
     if self._file_has_user_namespace(filename):
       return f"{app_name}/{user_id}/user/{filename}"
 
+    session_id = artifact_util.normalize_session_id(session_id)
     if session_id is None:
       raise InputValidationError(
           "Session ID must be provided for session-scoped artifacts."
@@ -110,6 +111,7 @@ async def save_artifact(
       custom_metadata: Optional[dict[str, Any]] = None,
   ) -> int:
     artifact = ensure_part(artifact)
+    session_id = artifact_util.normalize_session_id(session_id)
     path = self._artifact_path(app_name, user_id, filename, session_id)
     if path not in self.artifacts:
       self.artifacts[path] = []
@@ -167,6 +169,7 @@ async def load_artifact(
       session_id: Optional[str] = None,
       version: Optional[int] = None,
   ) -> Optional[types.Part]:
+    session_id = artifact_util.normalize_session_id(session_id)
     path = self._artifact_path(app_name, user_id, filename, session_id)
     versions = self.artifacts.get(path)
     if not versions:
@@ -222,6 +225,7 @@ async def list_artifact_keys(
   ) -> list[str]:
     artifact_util.validate_path_segment(app_name, "app_name")
     artifact_util.validate_path_segment(user_id, "user_id")
+    session_id = artifact_util.normalize_session_id(session_id)
     if session_id is not None:
       artifact_util.validate_path_segment(session_id, "session_id")
     usernamespace_prefix = f"{app_name}/{user_id}/user/"
diff --git a/tests/unittests/artifacts/test_artifact_service.py b/tests/unittests/artifacts/test_artifact_service.py
index a6dd4b642c..a17e853004 100644
--- a/tests/unittests/artifacts/test_artifact_service.py
+++ b/tests/unittests/artifacts/test_artifact_service.py
@@ -286,6 +286,125 @@ async def test_save_load_delete(service_type, artifact_service_factory):
   )
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "service_type",
+    [
+        ArtifactServiceType.IN_MEMORY,
+        ArtifactServiceType.GCS,
+        ArtifactServiceType.FILE,
+    ],
+)
+async def test_padded_session_id_lands_in_same_namespace_as_trimmed(
+    service_type, artifact_service_factory
+):
+  """An artifact saved against a whitespace-padded session id must be
+  reachable, listable, and deletable using the trimmed id, since that is the
+  id the corresponding session is actually stored under (session services
+  normalize session_id the same way)."""
+  artifact_service = artifact_service_factory(service_type)
+  artifact = types.Part(text="hello")
+
+  await artifact_service.save_artifact(
+      app_name="app0",
+      user_id="user0",
+      session_id="sess0\n",
+      filename="report.txt",
+      artifact=artifact,
+  )
+
+  # Reachable under the trimmed id, the one the session itself is keyed on.
+  assert (
+      await artifact_service.load_artifact(
+          app_name="app0",
+          user_id="user0",
+          session_id="sess0",
+          filename="report.txt",
+      )
+      == artifact
+  )
+  assert "report.txt" in await artifact_service.list_artifact_keys(
+      app_name="app0", user_id="user0", session_id="sess0"
+  )
+
+  # Also reachable under the original padded id: both forms must resolve to
+  # the same stored artifact rather than the padded id silently shadowing it
+  # in a namespace only the padded id itself could ever reach again.
+  assert (
+      await artifact_service.load_artifact(
+          app_name="app0",
+          user_id="user0",
+          session_id="sess0\n",
+          filename="report.txt",
+      )
+      == artifact
+  )
+
+  await artifact_service.delete_artifact(
+      app_name="app0",
+      user_id="user0",
+      session_id="sess0",
+      filename="report.txt",
+  )
+  assert not await artifact_service.load_artifact(
+      app_name="app0",
+      user_id="user0",
+      session_id="sess0\n",
+      filename="report.txt",
+  )
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "service_type",
+    [
+        ArtifactServiceType.IN_MEMORY,
+        ArtifactServiceType.GCS,
+    ],
+)
+async def test_artifact_reference_allows_padded_session_id_at_call_site(
+    service_type, artifact_service_factory
+):
+  """A caller that consistently uses a padded session id must still be able
+  to save and resolve an artifact reference within that session: the scope
+  check must compare normalized ids on both sides, not the caller's raw
+  string against a URI minted from the (already normalized) stored id."""
+  artifact_service = artifact_service_factory(service_type)
+
+  await artifact_service.save_artifact(
+      app_name="app0",
+      user_id="user0",
+      session_id="sess0\n",
+      filename="source.txt",
+      artifact=types.Part(text="hello"),
+  )
+
+  ref = types.Part(
+      file_data=types.FileData(
+          file_uri=(
+              "artifact://apps/app0/users/user0/sessions/sess0/"
+              "artifacts/source.txt/versions/0"
+          ),
+          mime_type="text/plain",
+      )
+  )
+  await artifact_service.save_artifact(
+      app_name="app0",
+      user_id="user0",
+      session_id="sess0\n",
+      filename="ref.txt",
+      artifact=ref,
+  )
+
+  loaded = await artifact_service.load_artifact(
+      app_name="app0",
+      user_id="user0",
+      session_id="sess0\n",
+      filename="ref.txt",
+  )
+  assert loaded == types.Part(text="hello")
+
+
 @pytest.mark.asyncio
 async def test_in_memory_loads_nested_artifact_reference(
     artifact_service_factory,
diff --git a/tests/unittests/artifacts/test_artifact_util.py b/tests/unittests/artifacts/test_artifact_util.py
index d5b79242d3..43986b1a62 100644
--- a/tests/unittests/artifacts/test_artifact_util.py
+++ b/tests/unittests/artifacts/test_artifact_util.py
@@ -112,6 +112,40 @@ def test_get_user_scoped_artifact_uri():
   assert uri == "artifact://apps/app2/users/user2/artifacts/file2/versions/456"
 
 
+def test_normalize_session_id_strips_whitespace():
+  assert artifact_util.normalize_session_id("  sess0\n") == "sess0"
+
+
+def test_normalize_session_id_passes_none_through():
+  assert artifact_util.normalize_session_id(None) is None
+
+
+def test_get_artifact_uri_normalizes_padded_session_id():
+  """A padded session id must not leak into the constructed URI, since the
+  session it points at is stored under the trimmed id."""
+  uri = artifact_util.get_artifact_uri(
+      app_name="app1",
+      user_id="user1",
+      session_id="  session1\n",
+      filename="file1",
+      version=123,
+  )
+  assert (
+      uri
+      == "artifact://apps/app1/users/user1/sessions/session1/artifacts/file1/versions/123"
+  )
+
+
+def test_parse_artifact_uri_normalizes_a_legacy_padded_session_id():
+  """A URI minted before this fix could carry a padded session id in its own
+  path segment; parsing it must still yield the trimmed id so downstream
+  scope checks compare like with like."""
+  uri = "artifact://apps/app1/users/user1/sessions/session1\n/artifacts/file1/versions/123"
+  parsed = artifact_util.parse_artifact_uri(uri)
+  assert parsed is not None
+  assert parsed.session_id == "session1"
+
+
 def test_is_artifact_ref_true():
   """Tests is_artifact_ref with a valid artifact reference."""
   artifact = types.Part(