diff --git a/hermes_cli/plugins_cmd_install.py b/hermes_cli/plugins_cmd_install.py
index b7fd9556d0..3f06aced8f 100644
--- a/hermes_cli/plugins_cmd_install.py
+++ b/hermes_cli/plugins_cmd_install.py
@@ -385,6 +385,15 @@ def _install_plugin_core(
         manifest = _read_manifest_for_install(tmp_target)
         plugin_name = manifest.get("name") or (
             subdir.rstrip("/").rsplit("/", 1)[-1] if subdir else _pc()._repo_name_from_url(git_url))
+        link = plugins_dir / str(plugin_name)
+        from pm.filesystem import is_junction
+        if "/" not in str(plugin_name) and (link.is_symlink() or is_junction(link)):
+            # A provider's own installer (e.g. `mnemosyne-hermes install`) links its package here; the
+            # name is fine, the slot is taken. Say so instead of blaming the manifest.
+            raise _pc().PluginOperationError(
+                f"Plugin '{plugin_name}' is already installed outside the catalog: {link} is a link to "
+                f"{os.path.realpath(link)}. Delete that link to install the catalog version.",
+                failure_class="already_installed")
         try:
             target = _pc()._sanitize_plugin_name(plugin_name, plugins_dir)
         except ValueError as e:
diff --git a/tests/hermes_cli/test_plugin_install_manifest_version.py b/tests/hermes_cli/test_plugin_install_manifest_version.py
index 4234f40b5b..460247c9f1 100644
--- a/tests/hermes_cli/test_plugin_install_manifest_version.py
+++ b/tests/hermes_cli/test_plugin_install_manifest_version.py
@@ -85,3 +85,21 @@ def test_manifest_version_above_shared_support_is_refused_cleanly(
 
     assert not (home / "plugins" / "demo").exists()
     assert not (home / "plugins" / ".install-metadata.json").exists()
+
+
+def test_a_linked_plugin_slot_is_already_installed_not_a_bad_manifest(monkeypatch, tmp_path):
+    """A provider's own installer can leave ``plugins/<name>`` as a link into a venv (mnemosyne-hermes
+    did). The catalog install must say the slot is taken, not record ``manifest_invalid``."""
+    from hermes_cli import plugins_cmd
+
+    monkeypatch.setattr(plugins_cmd, "_scan_on_install_enabled", lambda: False)
+    repo = _plugin_repo(tmp_path, {"name": "demo", "version": "1.0.0"})
+    home = tmp_path / "home"
+    monkeypatch.setenv("HERMES_HOME", str(home))
+    (home / "plugins").mkdir(parents=True)
+    (tmp_path / "elsewhere").mkdir()
+    (home / "plugins" / "demo").symlink_to(tmp_path / "elsewhere", target_is_directory=True)
+
+    with pytest.raises(plugins_cmd.PluginOperationError, match="already installed outside the catalog") as exc:
+        plugins_cmd._install_plugin_core(repo.as_uri(), force=False)
+    assert exc.value.failure_class == "already_installed"