diff --git a/tests/support.py b/tests/support.py
index c3d7244..ea0e329 100644
--- a/tests/support.py
+++ b/tests/support.py
@@ -9,6 +9,8 @@
 from collections.abc import Callable
 from datetime import datetime, timedelta, timezone
 import json
+import os
+import sys
 import threading
 import time
 from typing import Any
@@ -60,6 +62,27 @@ def wait_until(
         time.sleep(0.005)
 
 
+def real_interpreter() -> str:
+    """The Python interpreter itself, for a child a test places in a job object.
+
+    Inside a Windows virtual environment ``sys.executable`` is a small
+    redirector that starts the base interpreter as its own child, in a job
+    object of its own. A test that puts ``sys.executable`` in a job and asks the
+    child which job it is in then reads the redirector's job, and a job limited
+    to one process refuses the interpreter as the second. The workers are not
+    affected: ``multiprocessing`` starts the base interpreter directly for the
+    same reason. The base interpreter does not see the virtual environment's
+    packages, so run only standard-library code with it. Outside a virtual
+    environment this is ``sys.executable``.
+    """
+
+    if os.name == "nt":
+        base = getattr(sys, "_base_executable", None)
+        if base and os.path.isfile(base):
+            return str(base)
+    return sys.executable
+
+
 def live_cortex_threads(baseline: set[threading.Thread]) -> list[threading.Thread]:
     """Return the ``cortex-*`` threads alive now that were not in ``baseline``."""
 
diff --git a/tests/test_brokered_process.py b/tests/test_brokered_process.py
index 0efd632..9ceef16 100644
--- a/tests/test_brokered_process.py
+++ b/tests/test_brokered_process.py
@@ -28,6 +28,7 @@
     CodeExecutionError,
 )
 import job_probe
+from support import real_interpreter
 
 windows_only = pytest.mark.skipif(os.name != "nt", reason="uses Windows job objects")
 
@@ -187,7 +188,7 @@ def test_a_brokered_process_reads_back_the_job_it_was_placed_in():
     """Run through the real broker: the child's first act is to ask which job it is in."""
 
     result = code_execution._run_brokered_process(
-        [sys.executable, str(Path(job_probe.__file__))], workspace=Path.cwd(), timeout=60
+        [real_interpreter(), str(Path(job_probe.__file__))], workspace=Path.cwd(), timeout=60
     )
 
     assert result["returncode"] == 0
diff --git a/tests/test_worker_isolation.py b/tests/test_worker_isolation.py
index 80a48d2..36b77c3 100644
--- a/tests/test_worker_isolation.py
+++ b/tests/test_worker_isolation.py
@@ -55,7 +55,7 @@
 from cortex_backend.execution.local_scratch_attempt import SCRATCH_WORKER_JOB_LIMITS, LocalScratchAttempt
 from cortex_backend.execution.recipe_coordinator import RecipeExecutionError
 from cortex_backend.execution.recipes import parse_image_transform
-from support import wait_until
+from support import real_interpreter, wait_until
 import job_probe
 import worker_probes
 
@@ -94,10 +94,12 @@ def _run_in_job(limits: JobLimits, *arguments: str) -> str:
 
     The child waits for one line on stdin before it does anything, so the job is
     attached first; that is the whole point of holding a worker at a checkpoint.
+    The child is the interpreter itself, not a virtual environment's redirector
+    (see ``real_interpreter``), so the job is attached to the process that runs.
     """
 
     child = subprocess.Popen(
-        [sys.executable, *arguments],
+        [real_interpreter(), *arguments],
         stdin=subprocess.PIPE,
         stdout=subprocess.PIPE,
         text=True,
@@ -114,6 +116,23 @@ def _run_in_job(limits: JobLimits, *arguments: str) -> str:
             child.wait(timeout=30)
 
 
+def test_the_interpreter_the_job_tests_start_is_the_process_that_runs():
+    """The process handed to the job is the one that executes the test's code.
+
+    Under a Windows virtual environment ``sys.executable`` fails this: the
+    process started is a redirector and the code runs in its child.
+    """
+
+    child = subprocess.Popen(
+        [real_interpreter(), "-c", "import os; print(os.getpid())"],
+        stdout=subprocess.PIPE,
+        text=True,
+    )
+    output, _ = child.communicate(timeout=60)
+
+    assert int(output) == child.pid
+
+
 @windows_only
 def test_a_real_job_carries_exactly_the_limits_it_was_given():
     limits = JobLimits(