diff --git a/src/google/adk/code_executors/unsafe_local_code_executor.py b/src/google/adk/code_executors/unsafe_local_code_executor.py
index f4b52145..1f5c552a 100644
--- a/src/google/adk/code_executors/unsafe_local_code_executor.py
+++ b/src/google/adk/code_executors/unsafe_local_code_executor.py
@@ -14,15 +14,12 @@
 
 from __future__ import annotations
 
-from contextlib import redirect_stdout
-import io
 import logging
-import multiprocessing
 import os
-import queue
 import re
 import signal
-import traceback
+import subprocess
+import sys
 from typing import Any
 
 from pydantic import Field
@@ -39,82 +36,83 @@ logger = logging.getLogger('google_adk.' + __name__)
 # escalating to SIGKILL, so that the timeout itself cannot block forever.
 _TERMINATE_GRACE_SECONDS = 5
 
+# Runs one program in the child interpreter.
+#
+# The program arrives on stdin rather than in argv because a single argument is
+# capped (at 128 KiB on Linux) and generated programs can carry their own data.
+# Reading it leaves stdin at end-of-file, which is what the program would have
+# seen before.
+#
+# The traceback is printed here, without the frame this wrapper contributes, so
+# that a failure shows the model its own code and not a file inside this
+# package -- a frame it can do nothing about, which then stays in the
+# conversation for every later request.
+_RUNNER = """
+import sys, traceback
 
-def _execute_in_process(
-    code: str,
-    globals_: dict[str, Any],
-    result_queue: multiprocessing.Queue[tuple[str, str | None]],
-) -> None:
-  """Executes code in a separate process and puts result in queue."""
-  # Detach into a new session/process group before running anything, so that a
-  # timed-out execution can be killed together with everything it spawned.
-  if hasattr(os, 'setsid'):
-    try:
-      os.setsid()
-    except OSError:
-      logger.debug('Could not detach the execution process group.')
+_run_name = sys.argv[1]
+del sys.argv[1:]
 
-  stdout = io.StringIO()
-  error = None
-  try:
-    with redirect_stdout(stdout):
-      exec(code, globals_, globals_)
-  except BaseException:
-    error = traceback.format_exc()
-  result_queue.put((stdout.getvalue(), error))
+_globals = {'__name__': _run_name} if _run_name else {}
+_source = sys.stdin.buffer.read().decode('utf-8')
+
+try:
+  exec(compile(_source, '<code>', 'exec'), _globals, _globals)
+except SystemExit:
+  # The program chose its own exit status, so let it stand rather than
+  # reporting a deliberate clean exit as a failure.
+  raise
+except BaseException as exc:
+  _tb = exc.__traceback__
+  traceback.print_exception(
+      type(exc), exc, _tb.tb_next if _tb else None, file=sys.stderr
+  )
+  sys.exit(1)
+"""
 
 
-def _execution_group(
-    process: multiprocessing.process.BaseProcess,
-) -> int | None:
-  """Returns the group the execution detached into, or None if it has not."""
-  if process.pid is None or not hasattr(os, 'killpg'):
-    return None
-  try:
-    group = os.getpgid(process.pid)
-    # Only report the group once the execution has detached into its own;
-    # otherwise the group is still ours and signalling it would take down the
-    # agent along with the code it is running.
-    return group if group != os.getpgid(0) else None
-  except OSError:
-    return None
+def _run_name(code: str) -> str:
+  """Returns the `__name__` the code should run under, or '' for none."""
+  if re.search(r"if\s+__name__\s*==\s*['\"]__main__['\"]", code):
+    return '__main__'
+  return ''
 
 
 def _signal_group(group: int, sig: int) -> None:
   """Signals every process left in a group, tolerating an empty one."""
+  if not hasattr(os, 'killpg'):
+    return
   try:
     os.killpg(group, sig)
   except OSError:
     logger.debug('Could not signal the execution process group.')
 
 
-def _kill_execution(process: multiprocessing.process.BaseProcess) -> None:
-  """Kills a timed-out execution along with any process it spawned."""
-  # Resolved up front: once the execution process has been reaped its group can
-  # no longer be looked up through it, and the group is what holds whatever the
-  # code spawned.
-  group = _execution_group(process)
+def _kill_execution(process: subprocess.Popen[str]) -> tuple[str, str]:
+  """Kills a timed-out execution, returning what it wrote before it died."""
+  # The execution leads its own process group, so its pid is the group that
+  # holds whatever it spawned. `terminate` and `kill` below reach the execution
+  # itself on the platforms that have no group to signal.
+  group = process.pid
 
   # SIGTERM first, so the code and its children get the same grace period the
   # execution process itself gets before anything is killed outright.
-  if group is not None:
-    _signal_group(group, signal.SIGTERM)
+  _signal_group(group, signal.SIGTERM)
   process.terminate()
-  process.join(_TERMINATE_GRACE_SECONDS)
+  try:
+    process.wait(_TERMINATE_GRACE_SECONDS)
+  except subprocess.TimeoutExpired:
+    pass
 
   # Escalate unconditionally: the execution process exiting says nothing about
-  # a child of it that is ignoring SIGTERM.
-  if group is not None:
-    _signal_group(group, signal.SIGKILL)
-  if process.is_alive():
-    process.kill()
-    process.join()
-
-
-def _prepare_globals(code: str, globals_: dict[str, Any]) -> None:
-  """Prepare globals for code execution, injecting __name__ if needed."""
-  if re.search(r"if\s+__name__\s*==\s*['\"]__main__['\"]", code):
-    globals_['__name__'] = '__main__'
+  # a child of it that is ignoring SIGTERM, and such a child holds the output
+  # pipes open besides.
+  _signal_group(group, signal.SIGKILL)
+  process.kill()
+  try:
+    return process.communicate(timeout=_TERMINATE_GRACE_SECONDS)
+  except subprocess.TimeoutExpired:
+    return '', ''
 
 
 class UnsafeLocalCodeExecutor(BaseCodeExecutor):
@@ -144,33 +142,56 @@ class UnsafeLocalCodeExecutor(BaseCodeExecutor):
       code_execution_input: CodeExecutionInput,
   ) -> CodeExecutionResult:
     logger.debug('Executing code:\n```\n%s\n```', code_execution_input.code)
-    # Execute the code.
-    globals_: dict[str, Any] = {}
-    _prepare_globals(code_execution_input.code, globals_)
 
-    ctx = multiprocessing.get_context('spawn')
-    result_queue: multiprocessing.Queue[tuple[str, str | None]] = ctx.Queue()
-    process = ctx.Process(
-        target=_execute_in_process,
-        args=(code_execution_input.code, globals_, result_queue),
-        daemon=True,
+    env = dict(os.environ)
+    # A plain child interpreter starts with its own import path, so pass the
+    # agent's along: code importing a module from the application resolved
+    # before this and still does.
+    env['PYTHONPATH'] = os.pathsep.join(p for p in sys.path if p)
+    # Pin the child's output encoding. Its stdout is a pipe, so it would
+    # otherwise encode with the host locale and a program printing non-ASCII
+    # under a non-UTF-8 locale would die encoding its own output.
+    env['PYTHONIOENCODING'] = 'utf-8'
+
+    process = subprocess.Popen(
+        [sys.executable, '-c', _RUNNER, _run_name(code_execution_input.code)],
+        stdin=subprocess.PIPE,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        encoding='utf-8',
+        errors='replace',
+        env=env,
+        # Its own session, so a timeout can take down everything the code
+        # started and nothing else. Ignored on Windows, as `os.killpg` is.
+        start_new_session=True,
     )
-    process.start()
 
-    output = ''
-    error = ''
+    timed_out = False
     try:
-      output, err = result_queue.get(timeout=self.timeout_seconds)
-      process.join()
-      if err:
-        error = err
-    except queue.Empty:
-      _kill_execution(process)
-      error = f'Code execution timed out after {self.timeout_seconds} seconds.'
+      output, error = process.communicate(
+          input=code_execution_input.code, timeout=self.timeout_seconds
+      )
+    except subprocess.TimeoutExpired:
+      output, error = _kill_execution(process)
+      timed_out = True
+
+    if timed_out:
+      # Appended rather than assigned: whatever the code wrote before it was
+      # killed is still the useful diagnostic, but on its own it would hide
+      # the fact that the run was cut short.
+      note = f'Code execution timed out after {self.timeout_seconds} seconds.'
+      error = f'{error}\n{note}' if error else note
+    elif process.returncode == 0:
+      # A non-empty stderr is what marks the result failed and drives the retry
+      # counter, so it has to mean the program failed rather than that the
+      # program wrote a warning. The exit status is what says which happened.
+      error = ''
+    elif not error:
+      # The code died without saying why: a signal, or a call to `os._exit`.
+      # Reporting nothing would show the model a clean run.
+      error = f'Code execution exited with status {process.returncode}.'
 
     # Collect the final result.
-    result_queue.close()
-    result_queue.join_thread()
     return CodeExecutionResult(
         stdout=output,
         stderr=error,
diff --git a/tests/unittests/code_executors/test_unsafe_local_code_executor.py b/tests/unittests/code_executors/test_unsafe_local_code_executor.py
index 49b54d89..b5256dfe 100644
--- a/tests/unittests/code_executors/test_unsafe_local_code_executor.py
+++ b/tests/unittests/code_executors/test_unsafe_local_code_executor.py
@@ -12,9 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import multiprocessing
 import os
 import signal
+import subprocess
 import textwrap
 import time
 from unittest.mock import MagicMock
@@ -49,6 +49,31 @@ def _is_alive(pid: int) -> bool:
   return state != "Z"
 
 
+def _execute_within(
+    executor: UnsafeLocalCodeExecutor,
+    invocation_context: InvocationContext,
+    code: str,
+    seconds: float,
+) -> CodeExecutionResult:
+  """Executes `code` under a wall-clock bound.
+
+  Several callers below cover code that dies without reporting a result, which
+  is precisely the case the old executor could wait on forever. The bound is
+  the executor's own timeout rather than a watchdog around it: a child that
+  dies closes its pipes, so the wait ends on its own, and passing the timeout
+  through also exercises the path that enforces it.
+  """
+  started = time.monotonic()
+  result = executor.execute_code(
+      invocation_context, CodeExecutionInput(code=code)
+  )
+  elapsed = time.monotonic() - started
+  assert (
+      elapsed < seconds * 4
+  ), f"the executor took {elapsed:.1f}s against a {seconds}s timeout"
+  return result
+
+
 @pytest.fixture
 def mock_invocation_context() -> InvocationContext:
   """Provides a mock InvocationContext."""
@@ -156,6 +181,185 @@ class TestUnsafeLocalCodeExecutor:
     assert result.stdout == ""
     assert "Code execution timed out after 1 seconds." in result.stderr
 
+  def test_execute_code_main_guard_runs(
+      self, mock_invocation_context: InvocationContext
+  ):
+    """Code guarded on `__main__` runs, as it did in the previous child."""
+    executor = UnsafeLocalCodeExecutor()
+    code_input = CodeExecutionInput(code=textwrap.dedent("""
+        if __name__ == '__main__':
+          print('guarded')
+        """))
+
+    result = executor.execute_code(mock_invocation_context, code_input)
+
+    assert result.stdout == "guarded\n"
+    assert result.stderr == ""
+
+  def test_execute_code_without_main_guard_is_not_main(
+      self, mock_invocation_context: InvocationContext
+  ):
+    """Code that does not ask to be `__main__` is not given the name."""
+    executor = UnsafeLocalCodeExecutor()
+    code_input = CodeExecutionInput(code="print(globals().get('__name__'))")
+
+    result = executor.execute_code(mock_invocation_context, code_input)
+
+    assert result.stdout == "None\n"
+
+  def test_execute_code_separates_stdout_from_stderr(
+      self, mock_invocation_context: InvocationContext
+  ):
+    """Each stream lands in its own field rather than being interleaved."""
+    executor = UnsafeLocalCodeExecutor()
+    code_input = CodeExecutionInput(code=textwrap.dedent("""
+        import sys
+        sys.stdout.write('to out')
+        sys.stderr.write('to err')
+        sys.exit(0)
+        """))
+
+    result = executor.execute_code(mock_invocation_context, code_input)
+
+    assert result.stdout == "to out"
+    # The program wrote to stderr but succeeded. A non-empty stderr is what
+    # marks a result failed and drives the retry counter, so a warning must not
+    # be reported as a failure.
+    assert result.stderr == ""
+
+  def test_execute_code_nonzero_exit_is_reported_as_a_failure(
+      self, mock_invocation_context: InvocationContext
+  ):
+    """A program that exits non-zero fails even when it says nothing."""
+    executor = UnsafeLocalCodeExecutor()
+    code_input = CodeExecutionInput(code="import sys\nsys.exit(3)")
+
+    result = executor.execute_code(mock_invocation_context, code_input)
+
+    assert result.stdout == ""
+    assert result.stderr == "Code execution exited with status 3."
+
+  def test_execute_code_reports_a_crash_rather_than_hanging(
+      self, mock_invocation_context: InvocationContext
+  ):
+    """Code that exits without reporting anything still returns a result."""
+    executor = UnsafeLocalCodeExecutor()
+
+    result = _execute_within(
+        executor,
+        mock_invocation_context,
+        "import os\nprint('before', flush=True)\nos._exit(3)",
+        seconds=30,
+    )
+
+    assert result.stdout == "before\n"
+    assert result.stderr == "Code execution exited with status 3."
+
+  @pytest.mark.skipif(
+      not hasattr(signal, "SIGKILL"),
+      reason="Death by signal is checked on POSIX only.",
+  )
+  def test_execute_code_reports_death_by_signal(
+      self, mock_invocation_context: InvocationContext
+  ):
+    """Code killed outright -- by a segfault or the OOM killer -- returns."""
+    executor = UnsafeLocalCodeExecutor()
+
+    result = _execute_within(
+        executor,
+        mock_invocation_context,
+        "import os, signal\nos.kill(os.getpid(), signal.SIGKILL)",
+        seconds=30,
+    )
+
+    assert result.stdout == ""
+    assert result.stderr == (
+        f"Code execution exited with status {-signal.SIGKILL}."
+    )
+
+  def test_execute_code_traceback_omits_this_module(
+      self, mock_invocation_context: InvocationContext
+  ):
+    """A failure shows the model its own code, not a frame from this package."""
+    executor = UnsafeLocalCodeExecutor()
+    code_input = CodeExecutionInput(
+        code="def divide():\n  return 1 / 0\n\ndivide()"
+    )
+
+    result = executor.execute_code(mock_invocation_context, code_input)
+
+    assert "ZeroDivisionError" in result.stderr
+    assert "unsafe_local_code_executor" not in result.stderr
+    # Both frames of the executed code survive; only the wrapper's is dropped.
+    assert result.stderr.count('File "<code>"') == 2
+
+  def test_execute_code_preserves_unicode(
+      self, mock_invocation_context: InvocationContext
+  ):
+    executor = UnsafeLocalCodeExecutor()
+    code_input = CodeExecutionInput(code="print('你好, café')")
+
+    result = executor.execute_code(mock_invocation_context, code_input)
+
+    assert result.stdout == "你好, café\n"
+    assert result.stderr == ""
+
+  def test_execute_code_output_encoding_does_not_follow_the_host_locale(
+      self, mock_invocation_context: InvocationContext
+  ):
+    """Otherwise a program printing non-ASCII dies encoding its own output."""
+    executor = UnsafeLocalCodeExecutor()
+    code_input = CodeExecutionInput(
+        code="import sys\nprint(sys.stdout.encoding)"
+    )
+
+    result = executor.execute_code(mock_invocation_context, code_input)
+
+    assert result.stdout.strip().lower() == "utf-8"
+
+  def test_execute_code_large_output(
+      self, mock_invocation_context: InvocationContext
+  ):
+    """Output far larger than a pipe buffer is read out rather than deadlocked."""
+    executor = UnsafeLocalCodeExecutor()
+    code_input = CodeExecutionInput(code="print('x' * 1000000)")
+
+    result = _execute_within(
+        executor, mock_invocation_context, code_input.code, seconds=60
+    )
+
+    assert result.stdout == "x" * 1000000 + "\n"
+    assert result.stderr == ""
+
+  def test_execute_code_large_program(
+      self, mock_invocation_context: InvocationContext
+  ):
+    """A program carrying its own data is not capped by an argument limit."""
+    executor = UnsafeLocalCodeExecutor()
+    payload = "a" * 300000
+    code_input = CodeExecutionInput(
+        code=f"data = {payload!r}\nprint(len(data))"
+    )
+
+    result = executor.execute_code(mock_invocation_context, code_input)
+
+    assert result.stdout == f"{len(payload)}\n"
+    assert result.stderr == ""
+
+  def test_execute_code_imports_resolve_from_the_agent_path(
+      self, mock_invocation_context: InvocationContext
+  ):
+    """Code importing what the application can import still resolves."""
+    executor = UnsafeLocalCodeExecutor()
+    code_input = CodeExecutionInput(code="import google.adk\nprint('imported')")
+
+    result = _execute_within(
+        executor, mock_invocation_context, code_input.code, seconds=120
+    )
+
+    assert result.stdout == "imported\n"
+    assert result.stderr == ""
+
   def test_kill_execution_signals_group_before_killing_it(self, monkeypatch):
     """The group gets SIGTERM and its grace period before SIGKILL."""
     signalled = []
@@ -163,35 +367,51 @@ class TestUnsafeLocalCodeExecutor:
         unsafe_local_code_executor.os,
         "killpg",
         lambda group, sig: signalled.append((group, sig)),
-    )
-    monkeypatch.setattr(
-        unsafe_local_code_executor,
-        "_execution_group",
-        lambda process: 4321,
+        raising=False,
     )
     process = MagicMock()
-    process.is_alive.return_value = False
+    process.pid = 4321
     process.terminate.side_effect = lambda: signalled.append(("child", "term"))
+    process.kill.side_effect = lambda: signalled.append(("child", "kill"))
+    process.communicate.return_value = ("out", "err")
 
-    unsafe_local_code_executor._kill_execution(process)
+    assert unsafe_local_code_executor._kill_execution(process) == ("out", "err")
 
     assert signalled == [
         (4321, signal.SIGTERM),
         ("child", "term"),
         (4321, signal.SIGKILL),
+        ("child", "kill"),
     ]
-    process.join.assert_any_call(
+    process.wait.assert_called_once_with(
         unsafe_local_code_executor._TERMINATE_GRACE_SECONDS
     )
 
+  def test_kill_execution_gives_up_on_pipes_that_never_close(self, monkeypatch):
+    """A pipe held open by something unkillable does not block the agent."""
+    monkeypatch.setattr(
+        unsafe_local_code_executor.os,
+        "killpg",
+        lambda group, sig: None,
+        raising=False,
+    )
+    process = MagicMock()
+    process.pid = 4321
+    process.wait.side_effect = subprocess.TimeoutExpired("cmd", 5)
+    process.communicate.side_effect = subprocess.TimeoutExpired("cmd", 5)
+
+    assert unsafe_local_code_executor._kill_execution(process) == ("", "")
+
   @pytest.mark.skipif(
       not hasattr(os, "killpg")
       or not hasattr(os, "fork")
       or not os.path.isdir("/proc"),
       reason="Process-group teardown is checked on POSIX with /proc only.",
   )
-  def test_kill_execution_kills_what_the_code_spawned(self, tmp_path):
-    """Killing a live execution takes the processes it spawned with it."""
+  def test_timeout_kills_what_the_code_spawned(
+      self, mock_invocation_context: InvocationContext, tmp_path
+  ):
+    """A timed-out execution takes the processes it spawned with it."""
     pid_file = tmp_path / "spawned.pid"
     # Forked rather than spawned through `sys.executable`, so the descendant
     # exists within milliseconds and the test never waits on interpreter
@@ -208,29 +428,18 @@ class TestUnsafeLocalCodeExecutor:
           f.write(str(spawned))
         time.sleep(60)
         """)
-    ctx = multiprocessing.get_context("spawn")
-    result_queue = ctx.Queue()
-    process = ctx.Process(
-        target=unsafe_local_code_executor._execute_in_process,
-        args=(code, {}, result_queue),
-        daemon=True,
-    )
-    process.start()
+    executor = UnsafeLocalCodeExecutor(timeout_seconds=5)
+
     spawned_pid = None
     try:
-      # Waiting for the pid to be written rather than for a fixed duration:
-      # the only thing that has to have happened is the fork. The file exists
-      # from the moment it is opened, so its content is what is polled for.
-      deadline = time.time() + 30
-      while time.time() < deadline and not _written_pid(pid_file):
-        time.sleep(0.05)
+      result = _execute_within(
+          executor, mock_invocation_context, code, seconds=60
+      )
+      assert "Code execution timed out after 5 seconds." in result.stderr
+
       spawned_pid = _written_pid(pid_file)
       if spawned_pid is None:
         pytest.skip("this environment could not start the execution process")
-
-      unsafe_local_code_executor._kill_execution(process)
-
-      assert not process.is_alive()
       deadline = time.time() + 10
       while time.time() < deadline and _is_alive(spawned_pid):
         time.sleep(0.05)
@@ -241,7 +450,3 @@ class TestUnsafeLocalCodeExecutor:
           os.kill(spawned_pid, signal.SIGKILL)
         except OSError:
           pass
-      if process.is_alive():
-        process.kill()
-      process.join()
-      result_queue.close()