diff --git a/scripts/ci/materialize_base_javascript_packages.py b/scripts/ci/materialize_base_javascript_packages.py
index 7defba36f..2e394ddb5 100644
--- a/scripts/ci/materialize_base_javascript_packages.py
+++ b/scripts/ci/materialize_base_javascript_packages.py
@@ -1,5 +1,12 @@
 #!/usr/bin/env python3
-"""Materialize pnpm locks from a validated pull-request base commit."""
+"""Materialize pnpm locks from a validated pull-request base commit.
+
+A ``pnpm-lock.yaml`` whose sibling ``package.json`` does not pin an exact pnpm
+``packageManager`` is treated as a genuine pnpm project only when no sibling
+``package-lock.json`` exists; otherwise it is a vestigial second lockfile in an
+npm-managed project and is skipped so the downstream npm install path handles it
+instead of failing coverage evidence.
+"""
 
 from __future__ import annotations
 
@@ -97,6 +104,14 @@ def base_pnpm_projects(
         if not isinstance(package_manager, str) or not PNPM_SPEC_RE.fullmatch(
             package_manager
         ):
+            if str(project_root / "package-lock.json") in regular_paths:
+                # A sibling package-lock.json means npm owns this project and
+                # the pnpm-lock.yaml is a vestigial second lockfile. Skip pnpm
+                # materialization so the downstream npm (package-lock.json)
+                # install path handles it, instead of failing the whole
+                # coverage-evidence job. A genuine pnpm-only project (no sibling
+                # package-lock.json) still must pin an exact pnpm packageManager.
+                continue
             raise ValueError(
                 f"trusted base package manifest {package_path} must declare an exact pnpm packageManager version"
             )
diff --git a/tests/test_materialize_base_javascript_packages.py b/tests/test_materialize_base_javascript_packages.py
index 2534f906f..039673a67 100644
--- a/tests/test_materialize_base_javascript_packages.py
+++ b/tests/test_materialize_base_javascript_packages.py
@@ -208,6 +208,34 @@ def fake_git(_repo_root: Path, _command: str, object_spec: str) -> bytes:
         materializer.base_pnpm_projects(tmp_path, "a" * 40)
 
 
+def test_skips_npm_project_with_vestigial_pnpm_lock(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """An npm project's stray pnpm-lock.yaml is skipped, not fail-closed.
+
+    A base tree with a ``pnpm-lock.yaml`` plus a sibling ``package-lock.json``
+    and no exact pnpm ``packageManager`` is npm-managed, so pnpm materialization
+    is skipped (the downstream npm install path owns it) rather than failing the
+    whole coverage-evidence job.
+    """
+    regular_paths = {
+        "frontend/package.json",
+        "frontend/pnpm-lock.yaml",
+        "frontend/package-lock.json",
+    }
+    monkeypatch.setattr(
+        materializer, "_regular_base_paths", lambda *_args: regular_paths
+    )
+
+    def fake_git(_repo_root: Path, _command: str, object_spec: str) -> bytes:
+        if object_spec.endswith(":frontend/package.json"):
+            return b"{}"
+        raise AssertionError(f"unexpected git object: {object_spec}")
+
+    monkeypatch.setattr(materializer, "_git", fake_git)
+    assert materializer.base_pnpm_projects(tmp_path, "a" * 40) == []
+
+
 def test_rejects_mutable_or_non_pnpm_package_manager(tmp_path: Path) -> None:
     """Only an exact pnpm runner specification may populate the trusted store."""
     repo, base_sha = fixture_repo(tmp_path)