fix(security): restrict subagent toolsets to parent's enabled set (#3269)

The delegate_task tool accepts a toolsets parameter directly from the
LLM's function call arguments. When provided, these toolsets are passed
through _strip_blocked_tools but never intersected with the parent
agent's enabled_toolsets. A model can request toolsets the parent does
not have (e.g., web, browser, rl), granting the subagent tools that
were explicitly disabled for the parent.

Intersect LLM-requested toolsets with the parent's enabled set before
applying the blocked-tool filter, so subagents can only receive a
subset of the parent's tools.

Co-authored-by: dieutx <dangtc94@gmail.com>

tests/tools/test_delegate_toolset_scope.py

@@ -0,0 +1,66 @@
+"""Tests for delegate_tool toolset scoping.
+
+Verifies that subagents cannot gain tools that the parent does not have.
+The LLM controls the `toolsets` parameter — without intersection with the
+parent's enabled_toolsets, it can escalate privileges by requesting
+arbitrary toolsets.
+"""
+
+from unittest.mock import MagicMock, patch
+from types import SimpleNamespace
+
+from tools.delegate_tool import _strip_blocked_tools
+
+
+class TestToolsetIntersection:
+    """Subagent toolsets must be a subset of parent's enabled_toolsets."""
+
+    def test_requested_toolsets_intersected_with_parent(self):
+        """LLM requests toolsets parent doesn't have — extras are dropped."""
+        parent = SimpleNamespace(enabled_toolsets=["terminal", "file"])
+
+        # Simulate the intersection logic from _build_child_agent
+        parent_toolsets = set(parent.enabled_toolsets)
+        requested = ["terminal", "file", "web", "browser", "rl"]
+        scoped = [t for t in requested if t in parent_toolsets]
+
+        assert sorted(scoped) == ["file", "terminal"]
+        assert "web" not in scoped
+        assert "browser" not in scoped
+        assert "rl" not in scoped
+
+    def test_all_requested_toolsets_available_on_parent(self):
+        """LLM requests subset of parent tools — all pass through."""
+        parent = SimpleNamespace(enabled_toolsets=["terminal", "file", "web", "browser"])
+
+        parent_toolsets = set(parent.enabled_toolsets)
+        requested = ["terminal", "web"]
+        scoped = [t for t in requested if t in parent_toolsets]
+
+        assert sorted(scoped) == ["terminal", "web"]
+
+    def test_no_toolsets_requested_inherits_parent(self):
+        """When toolsets is None/empty, child inherits parent's set."""
+        parent_toolsets = ["terminal", "file", "web"]
+        child = _strip_blocked_tools(parent_toolsets)
+        assert "terminal" in child
+        assert "file" in child
+        assert "web" in child
+
+    def test_strip_blocked_removes_delegation(self):
+        """Blocked toolsets (delegation, clarify, etc.) are always removed."""
+        child = _strip_blocked_tools(["terminal", "delegation", "clarify", "memory"])
+        assert "delegation" not in child
+        assert "clarify" not in child
+        assert "memory" not in child
+        assert "terminal" in child
+
+    def test_empty_intersection_yields_empty_toolsets(self):
+        """If parent has no overlap with requested, child gets nothing extra."""
+        parent = SimpleNamespace(enabled_toolsets=["terminal"])
+
+        parent_toolsets = set(parent.enabled_toolsets)
+        requested = ["web", "browser"]
+        scoped = [t for t in requested if t in parent_toolsets]
+
+        assert scoped == []

tools/delegate_tool.py

@@ -174,8 +174,10 @@ def _build_child_agent(
 
     # When no explicit toolsets given, inherit from parent's enabled toolsets
     # so disabled tools (e.g. web) don't leak to subagents.
+    parent_toolsets = set(getattr(parent_agent, "enabled_toolsets", None) or DEFAULT_TOOLSETS)
     if toolsets:
-        child_toolsets = _strip_blocked_tools(toolsets)
+        # Intersect with parent — subagent must not gain tools the parent lacks
+        child_toolsets = _strip_blocked_tools([t for t in toolsets if t in parent_toolsets])
     elif parent_agent and getattr(parent_agent, "enabled_toolsets", None):
         child_toolsets = _strip_blocked_tools(parent_agent.enabled_toolsets)
     else: