diff --git a/learning_loop_node/tests/unit/test_subprocess.py b/learning_loop_node/tests/unit/test_subprocess.py
index b197cc9b..1b2c6418 100644
--- a/learning_loop_node/tests/unit/test_subprocess.py
+++ b/learning_loop_node/tests/unit/test_subprocess.py
@@ -1,5 +1,16 @@
+import asyncio
+import multiprocessing
+import os
+import queue
+import signal
+import sys
+from collections.abc import Callable, Iterator
+from typing import Any
+
 import pytest
 
+from ...trainer import subprocess as iterator_module
+from ...trainer.exceptions import UnexpectedWorkerExitError
 from ...trainer.subprocess import iterator_cpu_bound
 
 
@@ -51,3 +62,80 @@ async def test_leaving_early_does_not_leave_the_process_running():
             if item == 2:
                 break
     # reaching here without hanging is the test
+
+
+@pytest.mark.parametrize(('mode', 'exit_code'), [
+    ('system_exit', 1),
+    ('system_exit_zero', 0),
+    ('native_exit', 7),
+    ('kill', -signal.SIGKILL),
+])
+async def test_iterator_rejects_exit_without_completion(mode: str, exit_code: int) -> None:
+    with pytest.raises(UnexpectedWorkerExitError, match=rf'exited with code {exit_code} .*IteratorDone'):
+        await asyncio.wait_for(_collect_worker(_exit_without_completion, mode), timeout=10)
+
+
+@pytest.mark.parametrize('error', [SystemExit(1), ValueError('worker failed')])
+async def test_iterator_reports_failure_after_progress(error: BaseException) -> None:
+    seen = []
+
+    async def consume() -> None:
+        async with iterator_cpu_bound(_fail_after_progress, error) as results:
+            async for item in results:
+                seen.append(item)
+
+    expected_error = UnexpectedWorkerExitError if isinstance(error, SystemExit) else ValueError
+    with pytest.raises(expected_error):
+        await asyncio.wait_for(consume(), timeout=10)
+    assert seen == [0]
+
+
+async def test_iterator_accepts_completion_arriving_after_queue_timeout(monkeypatch: pytest.MonkeyPatch) -> None:
+    context = multiprocessing.get_context('spawn')
+    make_process = context.Process
+    processes = []
+
+    def record_process(*args: Any, **kwargs: Any) -> Any:
+        process = make_process(*args, **kwargs)
+        processes.append(process)
+        return process
+
+    to_thread = asyncio.to_thread
+    first_poll = True
+
+    async def delayed_timeout(func: Callable[..., Any], *args: Any, **kwargs: Any) -> Any:
+        nonlocal first_poll
+        if first_poll:
+            first_poll = False
+            await to_thread(processes[0].join, 5)
+            assert processes[0].exitcode == 0
+            raise queue.Empty
+        return await to_thread(func, *args, **kwargs)
+
+    monkeypatch.setattr(context, 'Process', record_process)
+    monkeypatch.setattr(iterator_module.asyncio, 'to_thread', delayed_timeout)
+
+    assert await asyncio.wait_for(_collect_worker(counting, 0), timeout=10) == []
+
+
+async def _collect_worker(it: Callable[..., Iterator[int]], *args: Any) -> list[int]:
+    async with iterator_cpu_bound(it, *args) as results:
+        return [item async for item in results]
+
+
+def _exit_without_completion(mode: str) -> Iterator[int]:
+    yield from ()
+    if mode == 'system_exit':
+        sys.exit(1)
+    if mode == 'system_exit_zero':
+        sys.exit(0)
+    if mode == 'native_exit':
+        os._exit(7)
+    if mode == 'kill':
+        os.kill(os.getpid(), signal.SIGKILL)
+    raise AssertionError(f'Unexpected exit mode: {mode}')
+
+
+def _fail_after_progress(error: BaseException) -> Iterator[int]:
+    yield 0
+    raise error
diff --git a/learning_loop_node/trainer/exceptions.py b/learning_loop_node/trainer/exceptions.py
index b77548cd..6eec5fa8 100644
--- a/learning_loop_node/trainer/exceptions.py
+++ b/learning_loop_node/trainer/exceptions.py
@@ -9,6 +9,10 @@ class InsufficientMemoryError(RuntimeError):
     """Raised when not even the smallest unit of work fits in memory."""
 
 
+class UnexpectedWorkerExitError(RuntimeError):
+    """Raised when an iterator worker exits without reporting completion."""
+
+
 class NodeNeedsRestartError(Exception):
     """
     NodeNeedsRestartError is raised when the node needs to be restarted.
diff --git a/learning_loop_node/trainer/subprocess.py b/learning_loop_node/trainer/subprocess.py
index 118fbf5d..50a9c7b9 100644
--- a/learning_loop_node/trainer/subprocess.py
+++ b/learning_loop_node/trainer/subprocess.py
@@ -16,6 +16,8 @@
 from multiprocessing.queues import Queue as MPQueue
 from typing import Any, ParamSpec, TypeVar
 
+from .exceptions import UnexpectedWorkerExitError
+
 logger = logging.getLogger(__name__)
 
 T = TypeVar('T')
@@ -55,9 +57,15 @@ async def _iterator_cpu_bound_inner(
             try:
                 item = await asyncio.to_thread(state_queue.get, True, 0.5)
             except queue.Empty:
-                if not process.is_alive():
-                    break
-                continue
+                if process.is_alive():
+                    continue
+                # Completion may have arrived between the timeout and the exit check.
+                try:
+                    item = await asyncio.to_thread(state_queue.get_nowait)
+                except queue.Empty as e:
+                    raise UnexpectedWorkerExitError(
+                        f'{process.name} exited with code {process.exitcode} without sending IteratorDone'
+                    ) from e
             match item:
                 case IteratorDone():
                     break