diff --git a/docker/Dockerfile.base b/docker/Dockerfile.base
index 39982b343d08..c3ae1cb67079 100644
--- a/docker/Dockerfile.base
+++ b/docker/Dockerfile.base
@@ -103,6 +103,7 @@ RUN touch /bin/nvidia-smi && \
     mkdir -p /var/run/nvidia-persistenced && \
     touch /var/run/nvidia-persistenced/socket
 
+# Cache-bust marker (PR #5655 validation: force fresh pip resolve to pick up cmeel-boost 1.90.0).
 # On arm64, pre-install nlopt 2.6.2 (transitively pinned by isaacteleop[retargeters]).
 # There is no aarch64 wheel for this version, and pip's isolated build env hides the
 # system numpy from nlopt's cmake-based build, so we install it manually with
diff --git a/source/conftest.py b/source/conftest.py
new file mode 100644
index 000000000000..e15dcb34658d
--- /dev/null
+++ b/source/conftest.py
@@ -0,0 +1,23 @@
+# Copyright (c) 2022-2026, The Isaac Lab Project Developers (https://github.com/isaac-sim/IsaacLab/blob/main/CONTRIBUTORS.md).
+# All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+
+"""Dep-manifest diagnostic: prints numpy version + bundled OpenBLAS hash at pytest session start.
+
+Loaded by pytest when collecting tests under ``source/``. Importing numpy here registers
+its vendored OpenBLAS ``pthread_atfork`` handler in pytest's process, which is the same
+process that later calls ``fork()`` via ``SimulationApp()``. The output identifies which
+numpy + OpenBLAS bundle actually landed in each CI test container.
+"""
+
+import os
+
+import numpy
+
+print(f"\n[dep-manifest] numpy {numpy.__version__}", flush=True)
+_libs_dir = os.path.join(os.path.dirname(numpy.__file__), os.pardir, "numpy.libs")
+if os.path.isdir(_libs_dir):
+    for _f in sorted(os.listdir(_libs_dir)):
+        if "openblas" in _f.lower():
+            print(f"[dep-manifest] bundled openblas: {_f}", flush=True)