diff --git a/src/google/adk/cli/agent_test_runner.py b/src/google/adk/cli/agent_test_runner.py
index 88a77d247f..d749e91428 100644
--- a/src/google/adk/cli/agent_test_runner.py
+++ b/src/google/adk/cli/agent_test_runner.py
@@ -963,8 +963,8 @@ def get_next_fc_id():
       session_data.pop("lastUpdateTime", None)
 
       # Write back to file
-      with open(test_file, "w") as f:
-        json.dump(session_data, f, indent=2, sort_keys=True)
+      with open(test_file, "w", encoding="utf-8") as f:
+        json.dump(session_data, f, indent=2, sort_keys=True, ensure_ascii=False)
         f.write("\n")
 
       print(f"Successfully rebuilt {test_file}")
diff --git a/tests/unittests/cli/test_agent_test_runner.py b/tests/unittests/cli/test_agent_test_runner.py
new file mode 100644
index 0000000000..3e37e30022
--- /dev/null
+++ b/tests/unittests/cli/test_agent_test_runner.py
@@ -0,0 +1,90 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import annotations
+
+import json
+from types import SimpleNamespace
+from unittest import mock
+
+import google.adk.cli.agent_test_runner as agent_test_runner
+from google.adk.events.event import Event
+from google.genai import types
+
+
+def test_rebuild_tests_preserves_non_ascii_event_text(tmp_path, monkeypatch):
+  """Rebuilt test files preserve non-ASCII event text."""
+  agent_dir = tmp_path / "test_agent"
+  tests_dir = agent_dir / "tests"
+  tests_dir.mkdir(parents=True)
+  (agent_dir / "agent.py").write_text("", encoding="utf-8")
+  test_file = tests_dir / "unicode.json"
+  session_data = {
+      "events": [{
+          "author": "user",
+          "content": {
+              "role": "user",
+              "parts": [{"text": "日本語の質問"}],
+          },
+      }]
+  }
+  test_file.write_text(
+      json.dumps(session_data, ensure_ascii=False), encoding="utf-8"
+  )
+
+  class _Runner:
+
+    def __init__(self):
+      self.session = SimpleNamespace(user_id="test_user", id="test_session")
+      self.runner = self
+
+    async def run_async(self, **kwargs):
+      del kwargs
+      yield Event(
+          author="test_agent",
+          invocation_id="live-invocation",
+          content=types.Content(
+              role="model",
+              parts=[types.Part.from_text(text="日本語の回答")],
+          ),
+      )
+
+  loader = mock.create_autospec(
+      agent_test_runner.AgentLoader, instance=True, spec_set=True
+  )
+  loader.load_agent.return_value = object()
+  loader_factory = mock.create_autospec(
+      agent_test_runner.AgentLoader, spec_set=True, return_value=loader
+  )
+  monkeypatch.setattr(
+      agent_test_runner,
+      "AgentLoader",
+      loader_factory,
+  )
+  runner_factory = mock.create_autospec(
+      agent_test_runner.InMemoryRunner,
+      spec_set=True,
+      return_value=_Runner(),
+  )
+  monkeypatch.setattr(
+      agent_test_runner,
+      "InMemoryRunner",
+      runner_factory,
+  )
+
+  agent_test_runner.rebuild_tests(str(agent_dir))
+
+  rebuilt = test_file.read_text(encoding="utf-8")
+  assert "日本語の質問" in rebuilt
+  assert "日本語の回答" in rebuilt