fix(tests): wrap Path.expanduser() to never raise on Windows runner

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Set the unit-test hermetic HOME at conftest import time so a single xdist worker on the `windows-2025-vs2026` runner can no longer race fixture setup and re-trigger the 53 `Path.home()` failures the session-scoped autouse fixture was supposed to prevent. (#1271)
 - Override `Path.home()` itself in the root test conftest so the 46 remaining Windows `RuntimeError: Could not determine home directory` failures on xdist worker `gw2` cannot recur regardless of which conftest the worker imports first; per-test `monkeypatch.setenv("HOME", ...)` continues to work because the override consults env vars before falling back to the hermetic tmp dir. (#1272)
 - Retry the `apm mcp search` and `apm mcp show` integration tests on the documented "Could not reach MCP registry" transient (with backoff and a final skip) so a brief `api.mcp.github.com` outage no longer red-marks the Windows integration job. (#1274)
+- Also wrap `Path.expanduser()` in the root test conftest so the `windows-2025-vs2026` runner cannot raise `RuntimeError("Could not determine home directory.")` from `ntpath.expanduser` when production code (e.g. `install.package_resolution.user_scope_rejection_reason`) calls `Path("~/pkg").expanduser()`. Falls back to the hermetic tmp dir; assertions about `~/pkg` being absolute still hold. (#1276)
 
 ## [0.13.0] - 2026-05-11
 

tests/conftest.py

@@ -70,6 +70,29 @@ def _hermetic_home(_cls=Path) -> Path:
 Path.home = classmethod(_hermetic_home)  # type: ignore[method-assign]
 
 
+# Same problem, different code path: Path("~/pkg").expanduser() goes through
+# ntpath.expanduser, which raises RuntimeError("Could not determine home
+# directory.") on the windows-2025-vs2026 runner when USERPROFILE and
+# HOMEPATH are both absent. Production code (e.g. install.package_resolution
+# user_scope_rejection_reason) relies on expanduser to detect that ~/pkg is
+# absolute. Wrap Path.expanduser so the RuntimeError can never surface.
+_ORIGINAL_EXPANDUSER = Path.expanduser
+
+
+def _hermetic_expanduser(self):
+    try:
+        return _ORIGINAL_EXPANDUSER(self)
+    except RuntimeError:
+        parts = self.parts
+        if not parts or not parts[0].startswith("~"):
+            return self
+        remainder = parts[1:]
+        return _TMP_HOME.joinpath(*remainder) if remainder else _TMP_HOME
+
+
+Path.expanduser = _hermetic_expanduser  # type: ignore[method-assign]
+
+
 @pytest.fixture(autouse=True, scope="session")
 def _validate_primitive_coverage():
     """Fail fast if KNOWN_TARGETS has primitives without dispatch handlers."""

tests/unit/test_path_home_override.py

@@ -31,3 +31,20 @@ def test_path_home_honors_per_test_home_setenv(monkeypatch, tmp_path):
     monkeypatch.setenv("HOME", str(target))
 
     assert Path.home() == target
+
+
+def test_path_expanduser_does_not_raise_with_cleared_env(monkeypatch):
+    """Path('~/pkg').expanduser() must not raise on the windows-2025-vs2026 runner.
+
+    ntpath.expanduser raises RuntimeError when USERPROFILE and HOMEPATH are
+    both absent. Production code in install.package_resolution depends on
+    expanduser to detect that `~/pkg` is absolute. The root conftest wraps
+    Path.expanduser so this case falls back to the hermetic tmp dir.
+    """
+    for key in ("HOME", "USERPROFILE", "HOMEDRIVE", "HOMEPATH"):
+        monkeypatch.delenv(key, raising=False)
+
+    expanded = Path("~/pkg").expanduser()
+
+    assert expanded.is_absolute()
+    assert expanded.name == "pkg"