fix: preserve legacy plugin tool errors

tests/agent/test_tool_loader_entrypoints.py

@@ -1,7 +1,11 @@
 from unittest.mock import MagicMock, patch
 
+import pytest
+
 from nanobot.agent.tools.base import Tool
+from nanobot.agent.tools.context import ToolContext
 from nanobot.agent.tools.loader import ToolLoader
+from nanobot.agent.tools.registry import ToolRegistry, is_tool_error_result
 
 
 def test_loader_discovers_entry_point_tools():
@@ -74,3 +78,67 @@ def test_loader_skips_abstract_entry_point_tools():
         discovered = loader._discover_plugins()
 
     assert "abstract_plugin" not in discovered
+
+
+@pytest.mark.asyncio
+async def test_loader_entry_point_error_wrapper_preserves_tool_api(tmp_path):
+    """Only adapt legacy plugin error strings; keep the wrapped tool API intact."""
+    mock_ep = MagicMock()
+    mock_ep.name = "api_plugin"
+
+    class _ApiPluginTool(Tool):
+        config_key = "api_plugin"
+
+        @property
+        def name(self) -> str:
+            return "api_plugin"
+
+        @property
+        def description(self) -> str:
+            return "Entry-point plugin with custom tool API methods."
+
+        @property
+        def parameters(self) -> dict:
+            return {"type": "object", "properties": {"value": {"type": "string"}}}
+
+        @property
+        def read_only(self) -> bool:
+            return True
+
+        @property
+        def concurrency_safe(self) -> bool:
+            return False
+
+        def cast_params(self, params: dict) -> dict:
+            return {"value": str(params["value"])}
+
+        def validate_params(self, params: dict) -> list[str]:
+            return [] if params == {"value": "1"} else ["bad value"]
+
+        def to_schema(self) -> dict:
+            return {"name": self.name, "custom": True}
+
+        async def execute(self, **_):
+            return "Error: plugin failed"
+
+    mock_ep.load.return_value = _ApiPluginTool
+
+    registry = ToolRegistry()
+    with patch("nanobot.agent.tools.loader.entry_points", return_value=[mock_ep]):
+        ToolLoader(test_classes=[]).load(
+            ToolContext(config=None, workspace=str(tmp_path)),
+            registry,
+        )
+
+    tool = registry.get("api_plugin")
+    assert tool is not None
+    assert tool.config_key == "api_plugin"
+    assert tool.read_only is True
+    assert tool.concurrency_safe is False
+    assert tool.cast_params({"value": 1}) == {"value": "1"}
+    assert tool.validate_params({"value": "1"}) == []
+    assert tool.to_schema() == {"name": "api_plugin", "custom": True}
+
+    result = await tool.execute(value="1")
+    assert is_tool_error_result("api_plugin", result) is True
+    assert str(result) == "Error: plugin failed"