diff --git a/crates/hiroz-py/pyproject.toml b/crates/hiroz-py/pyproject.toml
index 8aedb5298..b251d7e9c 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. Declared here rather than installed
+# imperatively by the test script so that `pip install -e ".[test]"` is a
+# true instruction -- the tests' own failure message points at it.
+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..6b9a2ee31 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, so its absence is a failure rather
+    than a skip. These tests previously guarded with ``importorskip`` and
+    numpy was never installed, so both had *always* skipped. A permanently
+    skipped test is indistinguishable from a passing one in the summary line.
+
+    Scope, precisely: ``arr.flags["OWNDATA"] is False`` proves numpy borrowed
+    the buffer ZPayloadView exported rather than copying it. It does *not*
+    prove the transport was zero-copy — ``ZPayloadView::new`` falls back to
+    ``PayloadBytes::Owned`` for a fragmented payload, and numpy borrows that
+    owned copy just as happily. ``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 loudly if numpy is missing, rather than skipping 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..cd41bc45d 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, and the venv is
+    # created with plain `python -m venv`, so it inherits nothing from the
+    # surrounding devShell. Without this the tests skip and the suite 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)"