diff --git a/simplemem_router.py b/simplemem_router.py
index 7b1188d9..5d6fe359 100644
--- a/simplemem_router.py
+++ b/simplemem_router.py
@@ -112,7 +112,11 @@ def register(
     overwrite: bool = False,
 ) -> None:
     """
-    Register a memory backend.
+    Register a memory backend under a short *mode* name.
+
+    After registration the backend is available via :func:`create` and
+    :func:`list_modes`.  Built-in modes (``"text"``, ``"omni"``) are
+    registered automatically at import time.
 
     Args:
         mode:          Short name used in ``create(mode=...)``.
@@ -125,6 +129,30 @@ def register(
         init:          Custom constructor ``fn(cls, **kwargs) -> instance``.
                        If *None*, the default ``cls(**kwargs)`` is used.
         overwrite:     Allow replacing an existing registration.
+
+    Raises:
+        ValueError: If *mode* is already registered and *overwrite* is
+            ``False``.
+
+    Examples::
+
+        # Register a custom backend
+        register(
+            mode="custom",
+            module_path="mypackage.memory",
+            class_name="MyMemorySystem",
+            description="My custom memory backend",
+        )
+        mem = create(mode="custom")
+
+        # Replace an existing registration
+        register(
+            mode="custom",
+            module_path="mypackage.memory_v2",
+            class_name="MyMemorySystemV2",
+            description="My improved backend",
+            overwrite=True,
+        )
     """
     if mode in _registry and not overwrite:
         raise ValueError(
diff --git a/tests/test_router_register.py b/tests/test_router_register.py
new file mode 100644
index 00000000..c773dc7d
--- /dev/null
+++ b/tests/test_router_register.py
@@ -0,0 +1,113 @@
+"""
+Tests for the public ``register`` function in simplemem_router.
+"""
+import sys
+import os
+import pytest
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+import simplemem_router as simplemem
+from simplemem_router import register, list_modes, is_available
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+_TEST_MODE = "_test_register_dummy"
+_TEST_MODE_2 = "_test_register_dummy2"
+
+
+def _cleanup(*modes):
+    """Remove test modes from the registry after each test."""
+    # Access the private registry to clean up test entries.
+    import simplemem_router as _sr
+    for mode in modes:
+        _sr._registry.pop(mode, None)
+
+
+# ---------------------------------------------------------------------------
+# Tests
+# ---------------------------------------------------------------------------
+
+class TestRegister:
+    def test_register_adds_mode_to_list_modes(self):
+        """A newly registered mode appears in list_modes()."""
+        try:
+            register(
+                mode=_TEST_MODE,
+                module_path="os",
+                class_name="getcwd",
+                description="Dummy test backend",
+            )
+            assert _TEST_MODE in list_modes()
+            assert list_modes()[_TEST_MODE] == "Dummy test backend"
+        finally:
+            _cleanup(_TEST_MODE)
+
+    def test_register_duplicate_raises_value_error(self):
+        """Registering an existing mode without overwrite=True raises ValueError."""
+        try:
+            register(
+                mode=_TEST_MODE,
+                module_path="os",
+                class_name="getcwd",
+                description="First",
+            )
+            with pytest.raises(ValueError, match="already registered"):
+                register(
+                    mode=_TEST_MODE,
+                    module_path="os",
+                    class_name="getcwd",
+                    description="Second",
+                )
+        finally:
+            _cleanup(_TEST_MODE)
+
+    def test_register_overwrite_replaces_description(self):
+        """overwrite=True silently replaces the existing registration."""
+        try:
+            register(
+                mode=_TEST_MODE,
+                module_path="os",
+                class_name="getcwd",
+                description="Original",
+            )
+            register(
+                mode=_TEST_MODE,
+                module_path="os",
+                class_name="getcwd",
+                description="Replaced",
+                overwrite=True,
+            )
+            assert list_modes()[_TEST_MODE] == "Replaced"
+        finally:
+            _cleanup(_TEST_MODE)
+
+    def test_register_stores_required_deps(self):
+        """required_deps are stored on the backend descriptor."""
+        import simplemem_router as _sr
+        try:
+            register(
+                mode=_TEST_MODE,
+                module_path="os",
+                class_name="getcwd",
+                required_deps=["os"],
+            )
+            assert _sr._registry[_TEST_MODE].required_deps == ["os"]
+        finally:
+            _cleanup(_TEST_MODE)
+
+    def test_register_missing_dep_not_available(self):
+        """is_available returns False when a required package is absent."""
+        try:
+            register(
+                mode=_TEST_MODE,
+                module_path="os",
+                class_name="getcwd",
+                required_deps=["_nonexistent_package_xyz"],
+            )
+            assert is_available(_TEST_MODE) is False
+        finally:
+            _cleanup(_TEST_MODE)