fix(tools): isolate plugin runtime state

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/agent/test_subagent.py

@@ -28,3 +28,27 @@ async def test_subagent_uses_tool_loader():
     assert tools.has("glob")
     assert not tools.has("message")
     assert not tools.has("spawn")
+
+
+@pytest.mark.asyncio
+async def test_subagent_build_tools_isolates_file_read_state(tmp_path):
+    """Each spawned subagent needs a fresh file-state cache."""
+    (tmp_path / "note.txt").write_text("hello\n", encoding="utf-8")
+    provider = MagicMock(spec=LLMProvider)
+    provider.get_default_model.return_value = "test"
+    sm = SubagentManager(
+        provider=provider,
+        workspace=tmp_path,
+        bus=MessageBus(),
+        model="test",
+        max_tool_result_chars=16_000,
+    )
+
+    first_read = sm._build_tools().get("read_file")
+    second_read = sm._build_tools().get("read_file")
+
+    assert first_read is not second_read
+    assert (await first_read.execute(path="note.txt")).startswith("1| hello")
+    second_result = await second_read.execute(path="note.txt")
+    assert second_result.startswith("1| hello")
+    assert "File unchanged" not in second_result