diff --git a/crates/hiroz-py/pyproject.toml b/crates/hiroz-py/pyproject.toml
index 8aedb5298..78a6870ae 100644
--- a/crates/hiroz-py/pyproject.toml
+++ b/crates/hiroz-py/pyproject.toml
@@ -27,6 +27,13 @@ dependencies = [
     "hiroz-msgs-py>=0.1.0",
 ]
 
+[project.optional-dependencies]
+# numpy backs the tests that prove it can consume ZPayloadView's exported
+# buffer without adding a copy. This file declares numpy. The test script does
+# not install it imperatively. That keeps `pip install -e ".[test]"` a true
+# instruction, which is what the tests' own failure message points at.
+test = ["numpy>=1.21"]
+
 [tool.maturin]
 features = ["pyo3/extension-module", "jazzy"]
 python-source = "python"
diff --git a/crates/hiroz-py/tests/test_payload_view.py b/crates/hiroz-py/tests/test_payload_view.py
index 8a814e4dc..ec412476c 100644
--- a/crates/hiroz-py/tests/test_payload_view.py
+++ b/crates/hiroz-py/tests/test_payload_view.py
@@ -163,12 +163,32 @@ def test_recv_raw_view_timeout(self, byte_array_pubsub):
 
 
 class TestZPayloadViewNumpy:
-    """Test ZPayloadView with numpy (if available)."""
+    """Test that numpy can consume ZPayloadView without adding a copy.
+
+    numpy is a declared test dependency. Its absence is a failure, not a skip.
+    These tests previously guarded with ``importorskip``. No job installed
+    numpy, so both tests had *always* skipped. The summary line shows a
+    permanently skipped test and a passing test in the same way.
+
+    Scope, precisely: ``arr.flags["OWNDATA"] is False`` proves that numpy
+    borrowed the buffer ZPayloadView exported. It does *not* prove that the
+    transport was zero-copy. ``ZPayloadView::new`` falls back to
+    ``PayloadBytes::Owned`` for a fragmented payload. numpy borrows that owned
+    copy in the same way. ``test_payload_view_is_zero_copy`` asserts
+    ``payload.is_zero_copy_py``, which is the transport-level check.
+    """
 
     @pytest.fixture(autouse=True)
     def check_numpy(self):
-        """Skip tests if numpy is not available."""
-        pytest.importorskip("numpy")
+        """Fail the test when numpy is missing. Do not skip silently."""
+        try:
+            import numpy  # noqa: F401
+        except ImportError:  # pragma: no cover - env misconfiguration
+            pytest.fail(
+                "numpy is a declared test dependency but is not installed, so "
+                "the ZPayloadView zero-copy assertions cannot run. Install it "
+                "with the test extra; do not restore importorskip here."
+            )
 
     def test_numpy_frombuffer(self, byte_array_pubsub):
         """Test that numpy.frombuffer works with ZPayloadView."""
diff --git a/scripts/test-python.nu b/scripts/test-python.nu
index 7a21507af..6eef66df1 100755
--- a/scripts/test-python.nu
+++ b/scripts/test-python.nu
@@ -42,6 +42,14 @@ def setup-venv [] {
     run-cmd "cd crates/hiroz-py; source .venv/bin/activate && pip install -e ../hiroz-msgs/python/" --shell bash --distro (get-distro)
     print "  Installed hiroz-msgs-py (message types)"
 
+    # Test-only dependency. `TestZPayloadViewNumpy` checks that numpy consumes
+    # ZPayloadView's exported buffer without adding a copy. Plain `python -m
+    # venv` creates this venv, so it inherits no package from the surrounding
+    # devShell. Without this install the tests skip. The suite then still
+    # reports success. See #266.
+    run-cmd "cd crates/hiroz-py; source .venv/bin/activate && pip install -e '.[test]'" --shell bash --distro (get-distro)
+    print "  Installed the test extra (numpy, for the zero-copy assertions)"
+
     # Install hiroz-py in editable mode using maturin
     run-cmd "cd crates/hiroz-py; source .venv/bin/activate && RUSTFLAGS='-D warnings' maturin develop" --shell bash --distro (get-distro)
     print "  Installed hiroz-py (Rust bindings)"