diff --git a/finbot/mcp/servers/systemutils/server.py b/finbot/mcp/servers/systemutils/server.py
index 7f2ddd52..a0b77672 100644
--- a/finbot/mcp/servers/systemutils/server.py
+++ b/finbot/mcp/servers/systemutils/server.py
@@ -18,6 +18,8 @@
 
 logger = logging.getLogger(__name__)
 
+PROTECTED_USERNAMES: set[str] = {"admin", "root", "system"}
+
 DEFAULT_CONFIG: dict[str, Any] = {
     "enabled_tools": [
         "run_diagnostics",
@@ -194,6 +196,14 @@ def manage_users(action: str, username: str, role: str = "viewer") -> dict[str,
         Actions: 'create', 'modify', 'delete', 'list'.
         Roles: 'viewer', 'operator', 'admin', 'superadmin'.
         """
+        if (
+            action
+            and username
+            and action.strip().lower() == "delete"
+            and username.strip().lower() in PROTECTED_USERNAMES
+        ):
+            raise ValueError(f"User '{username}' is protected and cannot be deleted")
+
         logger.warning(
             "SystemUtils manage_users called with action='%s', username='%s', role='%s' by namespace='%s'",
             action,
diff --git a/tests/unit/mcp/test_systemutils.py b/tests/unit/mcp/test_systemutils.py
new file mode 100644
index 00000000..3a834aab
--- /dev/null
+++ b/tests/unit/mcp/test_systemutils.py
@@ -0,0 +1,115 @@
+"""Unit tests for SystemUtils MCP Server manage_users operations.
+
+Validates that SystemUtils manage_users protects reserved system accounts
+from deletion while allowing valid user lifecycle operations.
+"""
+
+from unittest.mock import MagicMock
+
+import pytest
+from fastmcp import Client
+from fastmcp.exceptions import ToolError
+
+from finbot.core.auth.session import SessionContext
+from finbot.mcp.servers.systemutils.server import create_systemutils_server
+
+
+@pytest.fixture
+def session_context():
+    """Create a mock SessionContext with a valid namespace."""
+    ctx = MagicMock(spec=SessionContext)
+    ctx.namespace = "test-systemutils-namespace"
+    ctx.user_id = "test-user-001"
+    ctx.session_id = "test-session-001"
+    return ctx
+
+
+class TestManageUsers:
+    """Tests for manage_users tool validation and protection."""
+
+    @pytest.mark.asyncio
+    async def test_su_usr_001_returns_expected_fields(self, session_context):
+        """SU-USR-001: manage_users returns a complete result for a valid operation."""
+        server = create_systemutils_server(session_context)
+        async with Client(server) as client:
+            result = await client.call_tool(
+                "manage_users",
+                {"action": "list", "username": "admin", "role": "viewer"},
+            )
+            data = result.data
+            assert data["status"] == "completed"
+            assert data["action"] == "list"
+            assert data["username"] == "admin"
+            assert data["role"] == "viewer"
+            assert "timestamp" in data
+
+    @pytest.mark.asyncio
+    async def test_su_usr_002_delete_admin_rejected_as_protected(self, session_context):
+        """SU-USR-002: Deleting admin user must be rejected as protected."""
+        server = create_systemutils_server(session_context)
+        async with Client(server) as client:
+            with pytest.raises((ValueError, ToolError), match="protected"):
+                await client.call_tool(
+                    "manage_users",
+                    {"action": "delete", "username": "admin"},
+                )
+
+    @pytest.mark.asyncio
+    async def test_su_usr_002_delete_admin_accepted_without_validation(self, session_context):
+        """SU-USR-002 Acceptance alias: verify deleting admin raises an error."""
+        await self.test_su_usr_002_delete_admin_rejected_as_protected(session_context)
+
+    @pytest.mark.asyncio
+    @pytest.mark.parametrize(
+        "protected_name",
+        [
+            "admin",
+            "Admin",
+            "ADMIN",
+            "root",
+            "Root",
+            "ROOT",
+            "system",
+            "System",
+            "SYSTEM",
+            "  admin  ",
+        ],
+    )
+    async def test_protected_usernames_rejected_case_insensitively(
+        self, session_context, protected_name
+    ):
+        """All reserved accounts (admin, root, system) are blocked from deletion case-insensitively."""
+        server = create_systemutils_server(session_context)
+        async with Client(server) as client:
+            with pytest.raises((ValueError, ToolError), match="protected"):
+                await client.call_tool(
+                    "manage_users",
+                    {"action": "delete", "username": protected_name},
+                )
+
+    @pytest.mark.asyncio
+    async def test_deleting_regular_user_succeeds(self, session_context):
+        """Deleting a standard, non-protected account is allowed."""
+        server = create_systemutils_server(session_context)
+        async with Client(server) as client:
+            result = await client.call_tool(
+                "manage_users",
+                {"action": "delete", "username": "temp_guest"},
+            )
+            assert result.data["status"] == "completed"
+            assert result.data["action"] == "delete"
+            assert result.data["username"] == "temp_guest"
+
+    @pytest.mark.asyncio
+    @pytest.mark.parametrize("action", ["list", "create", "modify"])
+    async def test_non_delete_actions_on_protected_user_allowed(self, session_context, action):
+        """Actions other than delete (list, create, modify) on protected accounts are not blocked."""
+        server = create_systemutils_server(session_context)
+        async with Client(server) as client:
+            result = await client.call_tool(
+                "manage_users",
+                {"action": action, "username": "admin", "role": "admin"},
+            )
+            assert result.data["status"] == "completed"
+            assert result.data["action"] == action
+            assert result.data["username"] == "admin"