diff --git a/tests/agent/test_builtin_memory_disabled_surface.py b/tests/agent/test_builtin_memory_disabled_surface.py
new file mode 100644
index 0000000000..d2771bf463
--- /dev/null
+++ b/tests/agent/test_builtin_memory_disabled_surface.py
@@ -0,0 +1,161 @@
+"""Built-in memory disabled in config must leave no dead surface behind.
+
+Setting ``memory.memory_enabled: false`` and ``memory.user_profile_enabled:
+false`` stops ``agent_init`` from building a ``MemoryStore``, so the ``memory``
+tool dispatches against ``store=None`` and every call comes back "Memory is not
+available". Before the fix the tool stayed in the schema and MEMORY_GUIDANCE
+stayed in the system prompt, so users running a third-party provider (Hindsight,
+Mem0, …) paid for both on every API call with no way to drop them — listing
+``memory`` under ``disabled_toolsets`` takes the provider's tools down too.
+
+These tests exercise the real resolution chain (config on disk → check_fn →
+``get_tool_definitions``) against a temp ``HERMES_HOME``, not mocks.
+"""
+
+import pytest
+import yaml
+
+from model_tools import get_tool_definitions
+
+
+@pytest.fixture(autouse=True)
+def _clear_caches():
+    """check_fn results and tool definitions are both cached; config written by
+    a test only takes effect once those are dropped."""
+    from model_tools import _clear_tool_defs_cache
+    from tools.registry import invalidate_check_fn_cache
+
+    invalidate_check_fn_cache()
+    _clear_tool_defs_cache()
+    yield
+    invalidate_check_fn_cache()
+    _clear_tool_defs_cache()
+
+
+def _write_memory_config(home, **memory_section):
+    home.mkdir(parents=True, exist_ok=True)
+    (home / "config.yaml").write_text(
+        yaml.safe_dump({"memory": memory_section}), encoding="utf-8"
+    )
+
+
+@pytest.fixture
+def hermes_home(tmp_path, monkeypatch):
+    home = tmp_path / ".hermes"
+    monkeypatch.setenv("HERMES_HOME", str(home))
+    return home
+
+
+def _memory_tool_names():
+    tools = get_tool_definitions(enabled_toolsets=["memory"], quiet_mode=True)
+    return {tool["function"]["name"] for tool in tools}
+
+
+class TestBuiltinMemoryToolAvailability:
+    def test_tool_hidden_when_both_stores_disabled(self, hermes_home):
+        _write_memory_config(
+            hermes_home, memory_enabled=False, user_profile_enabled=False
+        )
+        assert "memory" not in _memory_tool_names()
+
+    def test_tool_present_when_only_user_profile_enabled(self, hermes_home):
+        _write_memory_config(
+            hermes_home, memory_enabled=False, user_profile_enabled=True
+        )
+        assert "memory" in _memory_tool_names()
+
+    def test_tool_present_when_only_memory_enabled(self, hermes_home):
+        _write_memory_config(
+            hermes_home, memory_enabled=True, user_profile_enabled=False
+        )
+        assert "memory" in _memory_tool_names()
+
+    def test_tool_present_by_default(self, hermes_home):
+        """No config file at all must not strip a working tool."""
+        assert "memory" in _memory_tool_names()
+
+    def test_unreadable_config_fails_open(self, hermes_home, monkeypatch):
+        """A config read error must not silently remove the tool."""
+        from tools import memory_tool as memory_tool_module
+
+        def _boom():
+            raise RuntimeError("config unreadable")
+
+        monkeypatch.setattr(
+            "hermes_cli.config.load_config_readonly", _boom, raising=False
+        )
+        assert memory_tool_module.check_memory_requirements() is True
+
+
+class TestExternalProviderSurvivesBuiltinDisable:
+    """Dropping the built-in tool must not drop the external provider's tools.
+
+    ``memory_provider_tools_enabled`` short-circuits on the built-in tool being
+    present, so hiding that tool moves the decision onto the toolset gate. The
+    provider must still be reachable for every way a caller can ask for memory.
+    """
+
+    def test_provider_tools_enabled_when_memory_toolset_requested(self):
+        from agent.memory_manager import memory_provider_tools_enabled
+
+        assert memory_provider_tools_enabled(
+            ["memory", "file"], None, memory_tool_present=False
+        )
+
+    def test_provider_tools_enabled_for_unrestricted_toolsets(self):
+        from agent.memory_manager import memory_provider_tools_enabled
+
+        assert memory_provider_tools_enabled(None, None, memory_tool_present=False)
+
+    def test_disabled_toolsets_still_takes_everything_down(self):
+        """The heavy switch keeps its documented meaning."""
+        from agent.memory_manager import memory_provider_tools_enabled
+
+        assert not memory_provider_tools_enabled(
+            None, ["memory"], memory_tool_present=False
+        )
+
+
+class TestInjectionEndToEnd:
+    """The real ``inject_memory_provider_tools`` with no built-in memory tool."""
+
+    def test_provider_tools_injected_without_builtin_memory_tool(self):
+        from types import SimpleNamespace
+
+        from agent.memory_manager import MemoryManager, inject_memory_provider_tools
+        from agent.memory_provider import MemoryProvider
+
+        class _Provider(MemoryProvider):
+            @property
+            def name(self):
+                return "fake_hindsight"
+
+            def is_available(self):
+                return True
+
+            def initialize(self, session_id, **kwargs):
+                pass
+
+            def get_tool_schemas(self):
+                return [
+                    {
+                        "name": "hindsight_retain",
+                        "description": "retain",
+                        "parameters": {"type": "object", "properties": {}},
+                    }
+                ]
+
+        manager = MemoryManager()
+        manager.add_provider(_Provider())
+        agent = SimpleNamespace(
+            _memory_manager=manager,
+            enabled_toolsets=["memory"],
+            disabled_toolsets=None,
+            tools=[],
+            valid_tool_names=set(),
+        )
+
+        added = inject_memory_provider_tools(agent)
+
+        assert added == 1
+        assert "hindsight_retain" in agent.valid_tool_names
diff --git a/tests/run_agent/test_run_agent.py b/tests/run_agent/test_run_agent.py
index cd4836c575..8713b588fa 100644
--- a/tests/run_agent/test_run_agent.py
+++ b/tests/run_agent/test_run_agent.py
@@ -910,6 +910,35 @@ class TestBuildSystemPrompt:
     def test_memory_guidance_when_memory_tool_loaded(self, agent_with_memory_tool):
         from agent.prompt_builder import MEMORY_GUIDANCE
 
+        agent_with_memory_tool._memory_enabled = True
+        prompt = agent_with_memory_tool._build_system_prompt()
+        assert MEMORY_GUIDANCE in prompt
+
+    def test_no_memory_guidance_when_both_builtin_stores_disabled(
+        self, agent_with_memory_tool
+    ):
+        """Guidance must follow the stores, not just the tool's presence.
+
+        With both built-in stores off, ``agent_init`` never builds a
+        ``MemoryStore``, so every memory call returns "Memory is not
+        available" — telling the model to save facts there is a dead
+        instruction paid for on every API call.
+        """
+        from agent.prompt_builder import MEMORY_GUIDANCE
+
+        agent_with_memory_tool._memory_enabled = False
+        agent_with_memory_tool._user_profile_enabled = False
+        prompt = agent_with_memory_tool._build_system_prompt()
+        assert MEMORY_GUIDANCE not in prompt
+
+    def test_memory_guidance_when_only_user_profile_enabled(
+        self, agent_with_memory_tool
+    ):
+        """USER.md alone still backs the tool, so the guidance stays."""
+        from agent.prompt_builder import MEMORY_GUIDANCE
+
+        agent_with_memory_tool._memory_enabled = False
+        agent_with_memory_tool._user_profile_enabled = True
         prompt = agent_with_memory_tool._build_system_prompt()
         assert MEMORY_GUIDANCE in prompt