diff --git a/cuda_core/build_hooks.py b/cuda_core/build_hooks.py
index 626d50355ab..09ad41bacfb 100644
--- a/cuda_core/build_hooks.py
+++ b/cuda_core/build_hooks.py
@@ -26,7 +26,31 @@
 build_sdist = _build_meta.build_sdist
 get_requires_for_build_sdist = _build_meta.get_requires_for_build_sdist
 
-COMPILE_FOR_COVERAGE = bool(int(os.environ.get("CUDA_PYTHON_COVERAGE", "0")))
+
+def env_int(name: str, default: int) -> int:
+    """Read an integer build knob from the environment.
+
+    Unset and empty mean the same thing. ``CUDA_PYTHON_COVERAGE= pip install .``
+    (and the equivalent empty value in a Dockerfile ``ENV`` or a CI job spec) is
+    how a variable gets neutralised, and it must not abort the build -- a bare
+    ``int()`` here raises while the PEP 517 backend module is still being
+    imported, so the failure arrives before any build output.
+
+    A value that is set to something non-integer is a typo worth stopping for:
+    silently ignoring ``CUDA_PYTHON_COVERAGE=yes`` would hand back a build with
+    no coverage instrumentation. It is reported with the variable's name rather
+    than as an anonymous ``invalid literal for int()``.
+    """
+    raw = os.environ.get(name, "").strip()
+    if not raw:
+        return default
+    try:
+        return int(raw)
+    except ValueError:
+        raise ValueError(f"environment variable {name}={os.environ[name]!r} must be an integer") from None
+
+
+COMPILE_FOR_COVERAGE = bool(env_int("CUDA_PYTHON_COVERAGE", 0))
 
 
 # Please keep in sync with the copy in cuda_bindings/build_hooks.py.
@@ -219,7 +243,9 @@ def get_sources(mod_name):
         for mod in module_names()
     )
 
-    nthreads = int(os.environ.get("CUDA_PYTHON_PARALLEL_LEVEL", os.cpu_count() // 2))
+    # ``os.cpu_count()`` is documented to return None when it cannot be
+    # determined; fall back to a serial build rather than a TypeError.
+    nthreads = env_int("CUDA_PYTHON_PARALLEL_LEVEL", (os.cpu_count() or 1) // 2)
     compile_time_env = {"CUDA_CORE_BUILD_MAJOR": int(_determine_cuda_major_version())}
     compiler_directives = {"embedsignature": True, "warn.deprecated.IF": False, "freethreading_compatible": True}
     _CythonOptions.warning_errors = True
diff --git a/cuda_core/setup.py b/cuda_core/setup.py
index e0d745b1f21..8f9a76d3a42 100644
--- a/cuda_core/setup.py
+++ b/cuda_core/setup.py
@@ -10,8 +10,10 @@
 from setuptools.command.build_ext import build_ext as _build_ext
 from setuptools.command.build_py import build_py as _build_py
 
-nthreads = int(os.environ.get("CUDA_PYTHON_PARALLEL_LEVEL", os.cpu_count() // 2))
-coverage_mode = bool(int(os.environ.get("CUDA_PYTHON_COVERAGE", "0")))
+# Shared with build_hooks so the two build entry points parse these knobs
+# identically (see build_hooks.env_int for why a bare int() is not enough).
+nthreads = build_hooks.env_int("CUDA_PYTHON_PARALLEL_LEVEL", (os.cpu_count() or 1) // 2)
+coverage_mode = bool(build_hooks.env_int("CUDA_PYTHON_COVERAGE", 0))
 _ROOT_DIR = Path(__file__).resolve().parent
 _AOTI_SHIM_DEF_FILE = _ROOT_DIR / "cuda" / "core" / "_include" / "aoti_shim.def"
 _AOTI_SHIM_LIB_FILE = _ROOT_DIR / "build" / "aoti_shim.lib"
diff --git a/cuda_core/tests/test_build_hooks.py b/cuda_core/tests/test_build_hooks.py
index c08ad4cd3c5..6bbca879afb 100644
--- a/cuda_core/tests/test_build_hooks.py
+++ b/cuda_core/tests/test_build_hooks.py
@@ -165,3 +165,49 @@ def test_missing_cuda_path_raises_error(self):
             pytest.raises(RuntimeError, match="CUDA_PATH or CUDA_HOME"),
         ):
             build_hooks._determine_cuda_major_version()
+
+
+class TestEnvInt:
+    """Integer build knobs read from the environment."""
+
+    @pytest.mark.agent_authored(model="claude-opus-5")
+    @pytest.mark.parametrize(
+        ("value", "expected"),
+        [
+            pytest.param(None, 7, id="unset"),
+            # `VAR= pip install .` is how a shell neutralizes a variable, and
+            # the same shape appears in Dockerfile ENV and CI job specs.
+            pytest.param("", 7, id="empty"),
+            pytest.param("   ", 7, id="whitespace-only"),
+            pytest.param("0", 0, id="zero"),
+            pytest.param("4", 4, id="positive"),
+            pytest.param(" 4 ", 4, id="padded"),
+        ],
+    )
+    def test_env_int_reads_the_value_or_falls_back(self, value, expected):
+        env = {} if value is None else {"CUDA_PYTHON_TEST_KNOB": value}
+        with mock.patch.dict(os.environ, env, clear=True):
+            assert build_hooks.env_int("CUDA_PYTHON_TEST_KNOB", 7) == expected
+
+    @pytest.mark.agent_authored(model="claude-opus-5")
+    def test_env_int_names_the_variable_for_a_non_integer(self):
+        with (
+            mock.patch.dict(os.environ, {"CUDA_PYTHON_TEST_KNOB": "yes"}, clear=True),
+            pytest.raises(ValueError, match=r"CUDA_PYTHON_TEST_KNOB='yes' must be an integer"),
+        ):
+            build_hooks.env_int("CUDA_PYTHON_TEST_KNOB", 7)
+
+    @pytest.mark.agent_authored(model="claude-opus-5")
+    @pytest.mark.parametrize("value", ["", "   "], ids=["empty", "whitespace-only"])
+    def test_backend_imports_with_an_empty_coverage_flag(self, value):
+        """An empty CUDA_PYTHON_COVERAGE must not break the PEP 517 backend.
+
+        COMPILE_FOR_COVERAGE is evaluated at module scope, so a bare int() there
+        raised while the build backend was still being imported -- pip reported
+        `ValueError: invalid literal for int() with base 10: ''` before any
+        build output appeared.
+        """
+        with mock.patch.dict(os.environ, {"CUDA_PYTHON_COVERAGE": value}, clear=True):
+            reloaded = _load_build_hooks()
+
+        assert reloaded.COMPILE_FOR_COVERAGE is False