diff --git a/tests/conftest.py b/tests/conftest.py
index ad2bb2e..26b1d8e 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -4,15 +4,34 @@
 import pytest
 
 
-def _engine_available():
+def _engine_available(generation=2):
+    """Whether ``needle._library_path`` would find an engine without downloading.
+
+    Every location this looks in has to be one that function looks in, or the
+    gate skips tests an installed engine could have run.
+    """
     try:
         import needle
         from needle.agent import fetch
+
+        override = os.environ.get(f"NEEDLE{generation}_LIB_PATH")
+        if generation == 2 and not override:
+            override = os.environ.get("NEEDLE_LIB_PATH")
+        if override:
+            return os.path.exists(override)
+
         here = os.path.dirname(needle.__file__)
         name = fetch._lib_name()
-        cache = os.path.join(os.path.expanduser("~"), ".cache",
-                             "cactus-needle", fetch.ENGINE_VERSION, name)
-        return os.path.exists(os.path.join(here, name)) or os.path.exists(cache)
+        stem, suffix = os.path.splitext(name)
+        local_names = [f"{stem}{generation}{suffix}"]
+        if generation == 2:
+            local_names.append(name)
+        if any(os.path.exists(os.path.join(here, local)) for local in local_names):
+            return True
+
+        cache = os.path.join(os.path.expanduser("~"), ".cache", "cactus-needle",
+                             f"v{generation}", fetch.engine_version(generation), name)
+        return os.path.exists(cache)
     except Exception:
         return False
 
diff --git a/tests/test_fetch.py b/tests/test_fetch.py
index 9c579f7..520e071 100644
--- a/tests/test_fetch.py
+++ b/tests/test_fetch.py
@@ -62,3 +62,45 @@ def test_fetch_library_creates_destination(tmp_path, monkeypatch):
 
     assert (tmp_path / "new" / "libneedle.so").read_bytes() == b"engine"
     assert out == str(tmp_path / "new" / "libneedle.so")
+
+
+def test_engine_gate_finds_the_cache_the_runtime_loads_from(tmp_path, monkeypatch):
+    import needle
+    from needle.agent import fetch
+    from conftest import _engine_available
+
+    package = tmp_path / "pkg"
+    package.mkdir()
+    monkeypatch.setattr(needle, "__file__", str(package / "__init__.py"))
+    monkeypatch.setenv("HOME", str(tmp_path))
+    monkeypatch.setenv("USERPROFILE", str(tmp_path))
+    monkeypatch.delenv("NEEDLE_LIB_PATH", raising=False)
+    monkeypatch.delenv("NEEDLE2_LIB_PATH", raising=False)
+
+    assert not _engine_available()
+
+    cache = tmp_path / ".cache" / "cactus-needle" / "v2" / fetch.engine_version(2)
+    cache.mkdir(parents=True)
+    (cache / fetch._lib_name()).write_bytes(b"")
+
+    assert _engine_available()
+
+
+def test_engine_gate_honours_the_library_override(tmp_path, monkeypatch):
+    import needle
+    from needle.agent import fetch
+    from conftest import _engine_available
+
+    package = tmp_path / "pkg"
+    package.mkdir()
+    monkeypatch.setattr(needle, "__file__", str(package / "__init__.py"))
+    monkeypatch.setenv("HOME", str(tmp_path))
+    monkeypatch.setenv("USERPROFILE", str(tmp_path))
+
+    engine = tmp_path / fetch._lib_name()
+    engine.write_bytes(b"")
+    monkeypatch.setenv("NEEDLE_LIB_PATH", str(engine))
+    assert _engine_available()
+
+    monkeypatch.setenv("NEEDLE_LIB_PATH", str(tmp_path / "gone"))
+    assert not _engine_available()