diff --git a/pm/environments_adopt.py b/pm/environments_adopt.py
index 2df1bdf6f2..6c3b9a003e 100644
--- a/pm/environments_adopt.py
+++ b/pm/environments_adopt.py
@@ -83,3 +83,35 @@ def adopt(previous: Path, selected: Path, running: Path) -> bool:
     os.environ["PATH"] = _replace(os.environ.get("PATH", ""), venv_bin_dir(previous), venv_bin_dir(selected))
     importlib.invalidate_caches()
     return True
+
+
+def _running_and_selected(project_root: Path) -> tuple[Path, Path] | None:
+    running = running_environment(project_root)
+    if running is None:
+        return None
+    try:
+        return running, selected_venv(project_root)
+    except (OSError, RuntimeError, ValueError):
+        return None
+
+
+def restart_needed(project_root: Path) -> str | None:
+    """Why this process must restart to load the selected generation, or None when it runs it.
+
+    Also None when the process does not run from one of this install's generations (a developer
+    venv, Nix): it never loads a PM selection, so a restart would not change what it imports.
+    The reason names the selected generation, so it changes with every new publication.
+    """
+    pair = _running_and_selected(project_root)
+    if pair is None or pair[0].resolve() == pair[1].resolve():
+        return None
+    running, selected = pair
+    return (f"dependency generation {selected.parent.name} was published after this process "
+            f"loaded {running.parent.name}")
+
+
+def adopt_selected(project_root: Path) -> bool:
+    """Move this process onto the selected generation. True when it already runs it, adopted it,
+    or runs from no generation of this install at all (there is nothing to adopt)."""
+    pair = _running_and_selected(project_root)
+    return pair is None or adopt(pair[0], pair[1], pair[0])
diff --git a/tests/hermes_cli/plugin_worker_support.py b/tests/hermes_cli/plugin_worker_support.py
index b0a9c6fcdc..09ff622cd1 100644
--- a/tests/hermes_cli/plugin_worker_support.py
+++ b/tests/hermes_cli/plugin_worker_support.py
@@ -114,6 +114,49 @@ class PluginWorld:
         assert result.returncode == 0, result.stderr
 
 
+def publish_plugins(world: PluginWorld, plugins: dict[str, list[str]]) -> Path:
+    """Enable exactly ``plugins`` (name → declared requirements) and publish the generation PM
+    builds for them. Each plugin imports its requirements, leaves an ``imported`` marker, and
+    ``request(specs)`` asks ``install_specs`` for more on its own behalf."""
+    from pm import client
+
+    for name, requirements in plugins.items():
+        plugin = world.home / "plugins" / name
+        if plugin.is_dir():
+            continue
+        plugin.mkdir(parents=True)
+        (plugin / "plugin.yaml").write_text(yaml.safe_dump(
+            {"name": name, "version": "1.0", "python_dependencies": requirements}), encoding="utf-8")
+        modules = [spec.split("=")[0].split("<")[0].split(">")[0].replace("-", "_") for spec in requirements]
+        (plugin / "__init__.py").write_text(
+            "".join(f"import {module}\n" for module in modules)
+            + "from pathlib import Path\nPath(__file__).with_name('imported').touch()\n"
+            "def register(ctx):\n    pass\n"
+            "def request(specs):\n    from tools.lazy_deps import install_specs\n    return install_specs(specs)\n",
+            encoding="utf-8")
+    (world.home / "config.yaml").write_text(yaml.safe_dump(
+        {"plugins": {"enabled": sorted(plugins), "disabled": []}}), encoding="utf-8")
+    client.sync_venv(explicit=True)
+    return world.selected()
+
+
+@pytest.fixture
+def boot(plugin_world, monkeypatch):
+    """``boot(environment)``: this test process imports from ``environment`` the way a Hermes process
+    booted on it does. Adoption rewrites sys.path and PATH (monkeypatch restores both); modules
+    imported from the world are forgotten afterwards, so a later test imports its own."""
+    from pm.environments import site_packages, venv_bin_dir
+
+    def run_from(environment: Path) -> None:
+        monkeypatch.setenv("PATH", os.pathsep.join([str(venv_bin_dir(environment)), os.defpath]))
+        monkeypatch.syspath_prepend(str(site_packages(environment)))
+
+    yield run_from
+    for name, module in list(sys.modules.items()):
+        if str(getattr(module, "__file__", None) or "").startswith(str(plugin_world.root)):
+            del sys.modules[name]
+
+
 @pytest.fixture
 def plugin_world(tmp_path, monkeypatch, isolated_python):
     from pm import client, paths
diff --git a/tests/pm/test_restart_needed.py b/tests/pm/test_restart_needed.py
new file mode 100644
index 0000000000..06eeeaf19b
--- /dev/null
+++ b/tests/pm/test_restart_needed.py
@@ -0,0 +1,23 @@
+"""restart_needed / adopt_selected against real generations: real uv, offline wheels, temp HERMES_HOME."""
+from __future__ import annotations
+
+from tests.hermes_cli.plugin_worker_support import (
+    boot as boot, isolated_python as isolated_python, plugin_world as plugin_world, publish_plugins)
+
+
+def test_a_process_on_a_superseded_generation_needs_a_restart_until_it_adopts(plugin_world, boot):
+    from pm.environments_adopt import adopt_selected, restart_needed
+
+    world = plugin_world
+    first = publish_plugins(world, {"base": ["plugin-proof-dep==1.0"]})
+    # This interpreter never booted from the install: nothing it could restart into.
+    assert restart_needed(world.core) is None
+    assert adopt_selected(world.core)
+    boot(first)
+    assert restart_needed(world.core) is None
+    second = publish_plugins(world, {"base": ["plugin-proof-dep==1.0"], "adds": ["plugin-proof-other"]})
+    assert second != first
+    reason = restart_needed(world.core)
+    assert reason and second.parent.name in reason
+    assert adopt_selected(world.core)
+    assert restart_needed(world.core) is None