@@ -7,7 +7,10 @@
 import ctypes.util
 import gc
 import os
+import pathlib
+import subprocess
 import sys
+import textwrap
 
 import numpy as np
 import pyglet
@@ -18,6 +21,7 @@
     select_headless_egl_device_for_cuda,
 )
 
+import cuda_python_test_helpers
 from cuda.core import (
     Buffer,
     Device,
@@ -71,6 +75,39 @@ def _configure_pyglet_headless():
             pyglet.options["headless_device"] = egl_device
 
 
+@pytest.mark.agent_authored(model="claude-fable-5-1")
+def test_egl_device_probe_does_not_import_pyglet_gl():
+    """The probe runs before ``headless_device`` is set.
+
+    Importing ``pyglet.gl`` there creates pyglet's shadow window, which in
+    headless mode opens EGL device 0 and keeps it for the whole session: the
+    failure that #2865 fixed. A subprocess gives a clean ``sys.modules``.
+    """
+    if not sys.platform.startswith("linux") or ctypes.util.find_library("EGL") is None:
+        pytest.skip("needs a Linux EGL runtime")
+    code = textwrap.dedent(
+        """
+        import sys
+
+        import pyglet
+        from cuda_python_test_helpers.graphics import select_headless_egl_device_for_cuda
+
+        pyglet.options["headless"] = True
+        select_headless_egl_device_for_cuda(0)
+        assert "pyglet.gl" not in sys.modules, "the EGL device probe imported pyglet.gl"
+        """
+    )
+    # The child inherits neither this process's sys.path nor conftest's
+    # source-checkout fallback, so point it at the package the parent imported.
+    helpers_root = pathlib.Path(cuda_python_test_helpers.__file__).parents[1]
+    env = dict(os.environ)
+    env["PYTHONPATH"] = os.pathsep.join(p for p in (str(helpers_root), env.get("PYTHONPATH")) if p)
+    proc = subprocess.run(  # noqa: S603 - trusted argv: this interpreter
+        [sys.executable, "-c", code], capture_output=True, text=True, env=env
+    )
+    assert proc.returncode == 0, proc.stderr
+
+
 def _allocate_gl_buffer(win, nbytes):
     """Allocate a GL buffer. Caller must have a current GL context.
 
 
@@ -12,13 +12,27 @@
 This module intentionally does **not** import ``pyglet`` at module load time:
 importing ``pyglet.gl`` / ``pyglet.window`` triggers pyglet's shadow-window
 creation, which fails on headless machines before the test has had a chance to
-set ``pyglet.options["headless"]``.
+set ``pyglet.options["headless"]``. For the same reason,
+``select_headless_egl_device_for_cuda`` must not import ``pyglet.gl``: that
+import creates the shadow window, and in headless mode the shadow window opens
+the EGL display on ``pyglet.options["headless_device"]`` before the caller has
+had a chance to set it.
 """
 
 import contextlib
 import ctypes
 
 
+def _is_pyglet_missing_function_exception(exc: BaseException) -> bool:
+    """True for ``pyglet.gl.lib.MissingFunctionException``.
+
+    Matched by name, like ``is_gl_context_unavailable``, so that the check does
+    not import ``pyglet.gl``.
+    """
+    exc_type = type(exc)
+    return exc_type.__module__.startswith("pyglet") and exc_type.__name__ == "MissingFunctionException"
+
+
 def select_headless_egl_device_for_cuda(cuda_device_ordinal: int) -> int | None:
     """Return the pyglet ``headless_device`` index matching *cuda_device_ordinal*.
 
@@ -39,7 +53,6 @@ def select_headless_egl_device_for_cuda(cuda_device_ordinal: int) -> int | None:
     """
     egl_cuda_device_nv = 0x323A
 
-    from pyglet.gl.lib import MissingFunctionException
     from pyglet.libs.egl import egl, eglext
     from pyglet.libs.egl.lib import link_EGL
 
@@ -50,8 +63,11 @@ def select_headless_egl_device_for_cuda(cuda_device_ordinal: int) -> int | None:
     )
 
     # link_EGL returns a stub for an unresolvable entry point that raises
-    # MissingFunctionException when called, so that is the only "extension not
-    # available" signal. Anything else is a real bug and must propagate.
+    # pyglet.gl.lib.MissingFunctionException when called, so that is the only
+    # "extension not available" signal. Anything else is a real bug and must
+    # propagate. The exception is matched by name: importing pyglet.gl here
+    # would create the shadow window on EGL device 0 before the caller has set
+    # pyglet.options["headless_device"] (see the module docstring).
     try:
         num_devices = egl.EGLint()
         if not eglext.eglQueryDevicesEXT(0, None, ctypes.byref(num_devices)) or num_devices.value <= 0:
@@ -66,8 +82,10 @@ def select_headless_egl_device_for_cuda(cuda_device_ordinal: int) -> int | None:
             found = egl_query_device_attrib_ext(devices[index], egl_cuda_device_nv, ctypes.byref(queried_ordinal))
             if found and queried_ordinal.value == cuda_device_ordinal:
                 return index
-    except MissingFunctionException:
-        return None
+    except Exception as exc:
+        if _is_pyglet_missing_function_exception(exc):
+            return None
+        raise
 
     return None