diff --git a/src/google/adk/artifacts/artifact_util.py b/src/google/adk/artifacts/artifact_util.py
index 8227bbb92c4..57a2a2c2f96 100644
--- a/src/google/adk/artifacts/artifact_util.py
+++ b/src/google/adk/artifacts/artifact_util.py
@@ -143,6 +143,31 @@ def _is_drive_qualified(value: str) -> bool:
   return _WINDOWS_DRIVE_RE.match(value) is not None
 
 
+def validate_session_id_segment(session_id: str) -> None:
+  """Validates a session_id that will be used as a literal storage segment.
+
+  In addition to the checks in `validate_path_segment`, rejects the literal
+  value "user". Backends that lay out session-scoped and user-scoped
+  artifacts in the same flat namespace (in-memory, GCS) use that exact
+  string as a reserved segment marking user-scoped artifacts, so a session
+  actually named "user" would silently write into -- and read out of -- that
+  reserved namespace instead of its own.
+
+  Args:
+    session_id: The caller-supplied session id.
+
+  Raises:
+    InputValidationError: If `session_id` fails `validate_path_segment`, or
+      is the reserved value "user".
+  """
+  validate_path_segment(session_id, "session_id")
+  if session_id == "user":
+    raise input_validation_error.InputValidationError(
+        "session_id must not be the reserved value 'user', which this"
+        " backend uses internally to mark user-scoped artifacts."
+    )
+
+
 def validate_path_segment(value: str, field_name: str) -> None:
   """Rejects values that could alter the constructed path.
 
diff --git a/src/google/adk/artifacts/gcs_artifact_service.py b/src/google/adk/artifacts/gcs_artifact_service.py
index 26a3ea21db3..57afaad1022 100644
--- a/src/google/adk/artifacts/gcs_artifact_service.py
+++ b/src/google/adk/artifacts/gcs_artifact_service.py
@@ -217,7 +217,7 @@ def _get_blob_prefix(
       raise InputValidationError(
           "Session ID must be provided for session-scoped artifacts."
       )
-    artifact_util.validate_path_segment(session_id, "session_id")
+    artifact_util.validate_session_id_segment(session_id)
     return f"{app_name}/{user_id}/{session_id}/{filename}"
 
   def _get_blob_name(
@@ -418,7 +418,7 @@ def _list_artifact_keys(
     artifact_util.validate_path_segment(app_name, "app_name")
     artifact_util.validate_path_segment(user_id, "user_id")
     if session_id is not None:
-      artifact_util.validate_path_segment(session_id, "session_id")
+      artifact_util.validate_session_id_segment(session_id)
     filenames = set()
 
     if session_id:
diff --git a/src/google/adk/artifacts/in_memory_artifact_service.py b/src/google/adk/artifacts/in_memory_artifact_service.py
index 2ed4e0a9ac6..49683e08955 100644
--- a/src/google/adk/artifacts/in_memory_artifact_service.py
+++ b/src/google/adk/artifacts/in_memory_artifact_service.py
@@ -95,7 +95,7 @@ def _artifact_path(
       raise InputValidationError(
           "Session ID must be provided for session-scoped artifacts."
       )
-    artifact_util.validate_path_segment(session_id, "session_id")
+    artifact_util.validate_session_id_segment(session_id)
     return f"{app_name}/{user_id}/{session_id}/{filename}"
 
   @override
@@ -223,7 +223,7 @@ async def list_artifact_keys(
     artifact_util.validate_path_segment(app_name, "app_name")
     artifact_util.validate_path_segment(user_id, "user_id")
     if session_id is not None:
-      artifact_util.validate_path_segment(session_id, "session_id")
+      artifact_util.validate_session_id_segment(session_id)
     usernamespace_prefix = f"{app_name}/{user_id}/user/"
     session_prefix = (
         f"{app_name}/{user_id}/{session_id}/" if session_id else None
diff --git a/tests/unittests/artifacts/test_artifact_service.py b/tests/unittests/artifacts/test_artifact_service.py
index a6dd4b642c9..8297319ffd3 100644
--- a/tests/unittests/artifacts/test_artifact_service.py
+++ b/tests/unittests/artifacts/test_artifact_service.py
@@ -286,6 +286,82 @@ async def test_save_load_delete(service_type, artifact_service_factory):
   )
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "service_type",
+    [
+        ArtifactServiceType.IN_MEMORY,
+        ArtifactServiceType.GCS,
+    ],
+)
+async def test_save_artifact_rejects_reserved_user_as_session_id(
+    service_type, artifact_service_factory
+):
+  """IN_MEMORY and GCS lay session-scoped and user-scoped artifacts out in
+  the same flat namespace, using the literal segment "user" to mark
+  user-scoped ones. A session actually named "user" must be rejected rather
+  than silently colliding with that reserved segment."""
+  artifact_service = artifact_service_factory(service_type)
+
+  with pytest.raises(InputValidationError, match="reserved value 'user'"):
+    await artifact_service.save_artifact(
+        app_name="app0",
+        user_id="user0",
+        session_id="user",
+        filename="report.txt",
+        artifact=types.Part(text="hello"),
+    )
+
+
+@pytest.mark.asyncio
+async def test_file_allows_reserved_user_as_session_id(
+    artifact_service_factory,
+):
+  """Unlike IN_MEMORY and GCS, FILE lays session-scoped artifacts out under
+  their own `sessions/<id>/` subtree, distinct from the user-scoped
+  `artifacts/` subtree, so a session literally named "user" cannot collide
+  with it and is not rejected."""
+  artifact_service = artifact_service_factory(ArtifactServiceType.FILE)
+
+  await artifact_service.save_artifact(
+      app_name="app0",
+      user_id="user0",
+      session_id="user",
+      filename="report.txt",
+      artifact=types.Part(text="hello"),
+  )
+  loaded = await artifact_service.load_artifact(
+      app_name="app0",
+      user_id="user0",
+      session_id="user",
+      filename="report.txt",
+  )
+  assert loaded == types.Part(text="hello")
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "service_type",
+    [
+        ArtifactServiceType.IN_MEMORY,
+        ArtifactServiceType.GCS,
+    ],
+)
+async def test_list_artifact_keys_rejects_reserved_user_as_session_id(
+    service_type, artifact_service_factory
+):
+  """A session literally named "user" must be rejected by
+  list_artifact_keys too, not just by save/load/delete -- otherwise a
+  caller's listing could silently return another session's (or the
+  user-scope's) filenames."""
+  artifact_service = artifact_service_factory(service_type)
+
+  with pytest.raises(InputValidationError, match="reserved value 'user'"):
+    await artifact_service.list_artifact_keys(
+        app_name="app0", user_id="user0", session_id="user"
+    )
+
+
 @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 d5b79242d3c..6731182aa80 100644
--- a/tests/unittests/artifacts/test_artifact_util.py
+++ b/tests/unittests/artifacts/test_artifact_util.py
@@ -192,6 +192,20 @@ def test_validate_path_segment_invalid(value, field_name):
     artifact_util.validate_path_segment(value, field_name)
 
 
+def test_validate_session_id_segment_rejects_reserved_user():
+  with pytest.raises(InputValidationError, match="reserved value 'user'"):
+    artifact_util.validate_session_id_segment("user")
+
+
+def test_validate_session_id_segment_allows_ordinary_values():
+  artifact_util.validate_session_id_segment("session1")
+
+
+def test_validate_session_id_segment_still_runs_path_segment_checks():
+  with pytest.raises(InputValidationError, match="must not be empty"):
+    artifact_util.validate_session_id_segment("")
+
+
 @pytest.mark.parametrize(
     "caller_session_id, uri_session_id",
     [