tntrenjin · Sep 30, 2026 · Sep 30, 2026
diff --git a/strix/agents/factory.py b/strix/agents/factory.py
@@ -599,18 +599,21 @@ def _finish_tool_use_behavior(
     list_sitemap,
     view_sitemap_entry,
     scope_rules,
-    list_mcps,
-    search_mcp_tools,
-    get_mcp_tool_schema,
-    describe_mcp,
-    call_mcp,
     view_agent_graph,
     send_message_to_agent,
     wait_for_agents,
     create_agent,
     stop_agent,
 )
 
+_MCP_TOOLS: tuple[Tool, ...] = (
+    list_mcps,
+    search_mcp_tools,
+    get_mcp_tool_schema,
+    describe_mcp,
+    call_mcp,
+)
+
 
 # Extra tools registered for scan agents. Mirrors
 # ``strix.runtime.backends.register_backend``: register before the first
@@ -694,14 +697,18 @@ def build_strix_agent(
             system_prompt_context=system_prompt_context,
         )
 
+    base_tools: list[Tool] = list(_BASE_TOOLS)
+    if system_prompt_context and system_prompt_context.get("mcp_available"):
+        base_tools.extend(_MCP_TOOLS)
+
     agent_tools = [*_EXTRA_TOOLS, *(extra_tools or [])]
     if interactive:
         # Yielding to the user is only meaningful when one is attached.
         agent_tools.append(respond_to_user)
     if is_root:
-        tools: list[Tool] = [*_BASE_TOOLS, *agent_tools, finish_scan]
+        tools: list[Tool] = [*base_tools, *agent_tools, finish_scan]
     else:
-        tools = [*_BASE_TOOLS, *agent_tools, agent_finish]
+        tools = [*base_tools, *agent_tools, agent_finish]
     _ensure_unique_tool_names(tools)
     tools = [
         _with_bounded_result(_with_strictness(_with_coerced_arguments(tool), strict_tool_schemas))

diff --git a/tests/test_agent_factory_shell.py b/tests/test_agent_factory_shell.py
@@ -115,3 +115,39 @@ def test_function_tools_are_result_bounded() -> None:
     by_name = {t.name: t for t in agent.tools}
 
     assert getattr(by_name["think"], "_strix_bounded", False) is True
+
+
+_MCP_TOOL_NAMES = {
+    "list_mcps",
+    "search_mcp_tools",
+    "get_mcp_tool_schema",
+    "describe_mcp",
+    "call_mcp",
+}
+
+
+def _tool_names(*, is_root: bool, **kwargs: Any) -> set[str]:
+    return {t.name for t in factory.build_strix_agent(is_root=is_root, **kwargs).tools}
+
+
+@pytest.mark.parametrize("is_root", [True, False])
+@pytest.mark.parametrize(
+    "system_prompt_context",
+    [None, {}, {"mcp_available": False}],
+)
+def test_mcp_tools_absent_without_mcp_configured(
+    is_root: bool, system_prompt_context: dict[str, Any] | None
+) -> None:
+    names = _tool_names(is_root=is_root, system_prompt_context=system_prompt_context)
+    assert _MCP_TOOL_NAMES.isdisjoint(names)
+
+
+@pytest.mark.parametrize("is_root", [True, False])
+def test_mcp_tools_present_when_mcp_is_configured(is_root: bool) -> None:
+    names = _tool_names(is_root=is_root, system_prompt_context={"mcp_available": True})
+    assert names >= _MCP_TOOL_NAMES
+
+
+def test_mcp_tools_do_not_leak_into_a_later_agent_without_mcp() -> None:
+    _tool_names(is_root=True, system_prompt_context={"mcp_available": True})
+    assert _MCP_TOOL_NAMES.isdisjoint(_tool_names(is_root=True))
diff --git a/tests/test_mcp_client.py b/tests/test_mcp_client.py
@@ -890,8 +890,9 @@ async def test_call_mcp_flags_an_errored_result_failed_for_the_tui() -> None:
 def test_agent_carries_exactly_the_generic_mcp_tools_regardless_of_connections() -> None:
     """No matter how many MCP connections a run makes, an agent's tool list gains
     exactly the generic MCP tools and never a per-connection provider tool."""
-    root = factory.build_strix_agent(is_root=True)
-    child = factory.build_strix_agent(is_root=False)
+    context = {"mcp_available": True}
+    root = factory.build_strix_agent(is_root=True, system_prompt_context=context)
+    child = factory.build_strix_agent(is_root=False, system_prompt_context=context)
 
     root_names = [t.name for t in root.tools]
     child_names = [t.name for t in child.tools]
@@ -919,7 +920,9 @@ def test_agent_carries_exactly_the_generic_mcp_tools_regardless_of_connections()
     # The tool list does not grow with connection count: it is the same set of
     # names whether or not any connection exists, because connections never
     # contribute tools.
-    assert root_names == [t.name for t in factory.build_strix_agent(is_root=True).tools]
+    assert root_names == [
+        t.name for t in factory.build_strix_agent(is_root=True, system_prompt_context=context).tools
+    ]
 
 
 # --- prompt guidance replaces the old per-connection inventory ---------------

diff --git a/tests/test_runner_root_prompt.py b/tests/test_runner_root_prompt.py
@@ -91,7 +91,12 @@ def _build_strix_agent(**kwargs: Any) -> object:
         return object()
 
     monkeypatch.setattr(runner, "build_strix_agent", _build_strix_agent)
-    monkeypatch.setattr(runner, "make_child_factory", lambda **_kwargs: lambda **_k: object())
+
+    def _make_child_factory(**kwargs: Any) -> Any:
+        captured["child_factory_kwargs"] = kwargs
+        return lambda **_k: object()
+
+    monkeypatch.setattr(runner, "make_child_factory", _make_child_factory)
     monkeypatch.setattr(runner, "open_agent_session", lambda _root_id, _db: object())
 
     async def _raise_rate_limit(*_args: Any, **kwargs: Any) -> None:
@@ -225,6 +230,8 @@ async def test_mcp_available_flag_set_when_a_connection_attaches(
     assert kwargs["system_prompt_context"]["mcp_connections"] == [
         {"name": "fs", "purpose": "local files", "tool_count": 0}
     ]
+    child_context = captured["child_factory_kwargs"]["system_prompt_context"]
+    assert child_context["mcp_available"] is True
 
 
 @pytest.mark.asyncio
@@ -249,6 +256,8 @@ async def test_mcp_available_flag_absent_without_a_connection(
     kwargs = captured["kwargs"]
     assert "mcp_available" not in kwargs["system_prompt_context"]
     assert "mcp_connections" not in kwargs["system_prompt_context"]
+    child_context = captured["child_factory_kwargs"]["system_prompt_context"]
+    assert "mcp_available" not in child_context
 
 
 @pytest.mark.asyncio