diff --git a/integrations/hermes/compartment/__init__.py b/integrations/hermes/compartment/__init__.py
index c8777d3..6bfe861 100644
--- a/integrations/hermes/compartment/__init__.py
+++ b/integrations/hermes/compartment/__init__.py
@@ -358,3 +358,16 @@ def shutdown(self) -> None:
                 self._vault.save()
             except Exception as exc:
                 logger.warning("compartment shutdown save failed: %s", exc)
+
+
+def register(ctx) -> None:
+    """Plugin entry point: hand Hermes the provider instance.
+
+    Hermes's memory loader calls ``register(ctx)`` first and only falls back
+    to scanning the module for a ``MemoryProvider`` subclass when it is
+    missing, so this changes nothing for existing installs. It is required by
+    the Nous plugin catalog's admission probe (``hermes plugins validate``),
+    which imports the plugin in a bare interpreter and fails with "no
+    register() function" without it.
+    """
+    ctx.register_memory_provider(CompartmentMemoryProvider())
diff --git a/src/compartment/data/hermes-plugin/__init__.py b/src/compartment/data/hermes-plugin/__init__.py
index c8777d3..6bfe861 100644
--- a/src/compartment/data/hermes-plugin/__init__.py
+++ b/src/compartment/data/hermes-plugin/__init__.py
@@ -358,3 +358,16 @@ def shutdown(self) -> None:
                 self._vault.save()
             except Exception as exc:
                 logger.warning("compartment shutdown save failed: %s", exc)
+
+
+def register(ctx) -> None:
+    """Plugin entry point: hand Hermes the provider instance.
+
+    Hermes's memory loader calls ``register(ctx)`` first and only falls back
+    to scanning the module for a ``MemoryProvider`` subclass when it is
+    missing, so this changes nothing for existing installs. It is required by
+    the Nous plugin catalog's admission probe (``hermes plugins validate``),
+    which imports the plugin in a bare interpreter and fails with "no
+    register() function" without it.
+    """
+    ctx.register_memory_provider(CompartmentMemoryProvider())
diff --git a/tests/test_hermes_plugin_register.py b/tests/test_hermes_plugin_register.py
new file mode 100644
index 0000000..818769f
--- /dev/null
+++ b/tests/test_hermes_plugin_register.py
@@ -0,0 +1,83 @@
+"""The Hermes provider plugin exposes the ``register(ctx)`` entry point.
+
+Hermes discovers a user-installed memory provider by scanning its
+``__init__.py`` for the ``MemoryProvider`` contract and loads it through
+``register(ctx)`` first, falling back to the subclass scan. The Nous plugin
+catalog's admission gate (``hermes plugins validate``) has no fallback: it
+imports the plugin in a bare interpreter, calls ``register`` against a
+recording context, and fails the entry with "no register() function" when
+it is absent. Both shipped copies must carry it.
+"""
+import importlib.util
+import pathlib
+import sys
+import types
+
+import pytest
+
+ROOT = pathlib.Path(__file__).resolve().parents[1]
+PLUGIN_INITS = (
+    ROOT / "integrations" / "hermes" / "compartment" / "__init__.py",
+    ROOT / "src" / "compartment" / "data" / "hermes-plugin" / "__init__.py",
+)
+
+
+class _RecordingContext:
+    """The shape of Hermes's probe context: every registration is a no-op."""
+
+    def __init__(self):
+        self.memory_providers = []
+
+    def register_memory_provider(self, provider):
+        self.memory_providers.append(provider)
+
+    def __getattr__(self, _name):
+        return lambda *args, **kwargs: None
+
+
+def _load_plugin(path: pathlib.Path, monkeypatch):
+    """Import the plugin by file path the way Hermes does, with the one
+    Hermes module it imports (``agent.memory_provider``) stubbed out."""
+    agent_pkg = types.ModuleType("agent")
+    memory_provider = types.ModuleType("agent.memory_provider")
+
+    class MemoryProvider:  # the ABC the plugin subclasses
+        pass
+
+    memory_provider.MemoryProvider = MemoryProvider
+    agent_pkg.memory_provider = memory_provider
+    monkeypatch.setitem(sys.modules, "agent", agent_pkg)
+    monkeypatch.setitem(sys.modules, "agent.memory_provider", memory_provider)
+
+    name = f"compartment_hermes_plugin_{path.parent.name.replace('-', '_')}"
+    spec = importlib.util.spec_from_file_location(
+        name, path, submodule_search_locations=[str(path.parent)])
+    module = importlib.util.module_from_spec(spec)
+    monkeypatch.setitem(sys.modules, name, module)
+    spec.loader.exec_module(module)
+    return module, MemoryProvider
+
+
+@pytest.mark.parametrize("path", PLUGIN_INITS, ids=lambda p: p.parent.name)
+def test_register_hands_hermes_the_provider(path, monkeypatch):
+    module, base = _load_plugin(path, monkeypatch)
+    ctx = _RecordingContext()
+
+    module.register(ctx)
+
+    assert len(ctx.memory_providers) == 1
+    provider = ctx.memory_providers[0]
+    assert isinstance(provider, base)
+    assert provider.name == "compartment"
+    # The picker's "no setup needed" hint comes from an empty schema.
+    assert provider.get_config_schema() == []
+
+
+@pytest.mark.parametrize("path", PLUGIN_INITS, ids=lambda p: p.parent.name)
+def test_register_never_imports_compartment_at_load(path, monkeypatch):
+    """Importing the plugin and registering must not need the compartment
+    package: Hermes enumerates providers before anything is installed, and
+    the catalog probe runs in a bare interpreter."""
+    monkeypatch.setitem(sys.modules, "compartment", None)  # import -> ImportError
+    module, _ = _load_plugin(path, monkeypatch)
+    module.register(_RecordingContext())