diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 01dc4c1a..5b4a2c5a 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -175,7 +175,7 @@ jobs:
       - name: 🔏 Sigstore sign artifacts
         uses: sigstore/gh-action-sigstore-python@790bc6befb9d733738f18d8f895854b453640ec9  # v3.5.0
         with:
-          inputs: dist/*.whl dist/*.tar.gz
+          inputs: dist/*.whl dist/*.tar.gz sbom-python.cdx.json
       - name: ⬆️ Attach artifacts to release
         env:
           GH_TOKEN: ${{ github.token }}
diff --git a/scripts/sbom_from_wheel.sh b/scripts/sbom_from_wheel.sh
index 69c1cf61..520cec85 100755
--- a/scripts/sbom_from_wheel.sh
+++ b/scripts/sbom_from_wheel.sh
@@ -46,15 +46,16 @@ uvx --from "cyclonedx-bom==${CYCLONEDX_BOM_VERSION}" cyclonedx-py environment "$
   --mc-type "$MC_TYPE" \
   --output-format json -o "$OUT"
 
-# Installed *after* the SBOM is generated, and that order is load-bearing: this
-# venv is the environment the document describes, so anything added to it before
-# the previous step would be published as a component of the release.
-uv pip install --quiet --python "$VENV/bin/python" "packaging==${PACKAGING_VERSION}"
-
-# Run under the venv's interpreter rather than the runner's. The check below
-# evaluates environment markers, and the only environment whose answers mean
-# anything here is the one the SBOM describes.
-"$VENV/bin/python" - "$DIST_DIR" "$OUT" <<'PYEOF'
+# Overlaid, not installed. This venv is the environment the document describes,
+# so installing into it would publish packaging as a component of the release --
+# and where packaging is already in the closure, would silently replace a
+# version the document has just recorded. `--with` layers it onto the
+# interpreter for this one call and leaves the venv byte-identical, while the
+# check still sees the venv's own site-packages: the only environment whose
+# marker answers mean anything here.
+uv run --no-project --python "$VENV/bin/python" \
+  --with "packaging==${PACKAGING_VERSION}" \
+  python - "$DIST_DIR" "$OUT" <<'PYEOF'
 """Complete the root component from the wheel, then verify the SBOM.
 
 Every project here declares `dynamic = ["version"]`, so cyclonedx-py reads the
@@ -69,6 +70,7 @@ import re
 import sys
 import zipfile
 
+from packaging.markers import UndefinedComparison, UndefinedEnvironmentName
 from packaging.requirements import InvalidRequirement, Requirement
 
 dist, out = pathlib.Path(sys.argv[1]), pathlib.Path(sys.argv[2])
@@ -91,6 +93,26 @@ if wheel is None:
 raw_name, version = wheel.name.split("-")[:2]
 expected = normalize(raw_name)
 
+
+def installed_here(marker):
+    """True if a dependency carrying this marker is installed in this venv.
+
+    The wheel above was installed without extras, so anything gated behind one
+    is legitimately absent. Two probes decide that: if the marker's answer
+    changes with `extra`, it is extra-gated whatever the operator. Markers
+    raise at evaluate time, not parse time, so both probes are guarded.
+    """
+    if marker is None:
+        return True
+    results = set()
+    for probe in ("", "\x00no-such-extra"):
+        try:
+            results.add(marker.evaluate({"extra": probe}))
+        except (UndefinedComparison, UndefinedEnvironmentName) as exc:
+            fail(f"cannot evaluate the environment marker {str(marker)!r}: {exc}")
+    return False if len(results) > 1 else results.pop()
+
+
 # Requires-Dist from the wheel's own metadata, narrowed to what this
 # environment should actually hold. A marker decides that, so a marker is what
 # has to be evaluated -- not a substring of one.
@@ -104,8 +126,11 @@ expected = normalize(raw_name)
 # dependency today; the check is fixed here so that the first one added does not
 # break a release to discover it.
 #
-# An empty `extra` is what makes `extra == "cli"` false: no extra was requested
-# when the wheel was installed above.
+# Extras are decided by whether the marker's answer depends on `extra` at all,
+# not by evaluating it against an empty one. `extra == "cli"` is false for an
+# empty extra, but `extra in "cli"` is *true* -- "" is a substring of every
+# string -- so the sentinel alone would demand an extra-gated dependency that
+# was correctly never installed, and abort the release blaming the venv.
 requires = set()
 with zipfile.ZipFile(wheel) as zf:
     metadata_name = next((n for n in zf.namelist() if n.endswith(".dist-info/METADATA")), None)
@@ -119,7 +144,7 @@ with zipfile.ZipFile(wheel) as zf:
             req = Requirement(spec)
         except InvalidRequirement as exc:
             fail(f"{wheel.name} has an unparsable Requires-Dist {spec!r}: {exc}")
-        if req.marker is not None and not req.marker.evaluate({"extra": ""}):
+        if not installed_here(req.marker):
             continue
         requires.add(normalize(req.name))