fix: preserve legacy plugin tool errors

nanobot/agent/tools/loader.py

@@ -8,7 +8,7 @@ from typing import Any
 
 from loguru import logger
 
-from nanobot.agent.tools.base import Tool
+from nanobot.agent.tools.base import Tool, ToolResult
 from nanobot.agent.tools.registry import ToolRegistry
 
 _SKIP_MODULES = frozenset({
@@ -96,6 +96,8 @@ class ToolLoader:
                     if not tool_cls.enabled(ctx):
                         continue
                     tool = tool_cls.create(ctx)
+                    if is_plugin_source:
+                        tool = _LegacyErrorPrefixTool(tool)
                     if registry.has(tool.name):
                         if is_plugin_source and tool.name in builtin_names:
                             logger.warning(
@@ -114,3 +116,67 @@ class ToolLoader:
                 except Exception:
                     logger.exception("Failed to register tool: %s", cls_label)
         return registered
+
+
+class _LegacyErrorPrefixTool(Tool):
+    """Compatibility wrapper for external tools using the old error-string contract."""
+
+    _plugin_discoverable = False
+
+    def __init__(self, wrapped: Tool) -> None:
+        self._wrapped = wrapped
+
+    @property
+    def name(self) -> str:
+        return self._wrapped.name
+
+    @property
+    def description(self) -> str:
+        return self._wrapped.description
+
+    @property
+    def parameters(self) -> dict[str, Any]:
+        return self._wrapped.parameters
+
+    @property
+    def read_only(self) -> bool:
+        return self._wrapped.read_only
+
+    @property
+    def exclusive(self) -> bool:
+        return self._wrapped.exclusive
+
+    @property
+    def concurrency_safe(self) -> bool:
+        return self._wrapped.concurrency_safe
+
+    @property
+    def config_key(self) -> str:
+        return getattr(self._wrapped, "config_key", "")
+
+    def set_context(self, ctx: Any) -> None:
+        set_context = getattr(self._wrapped, "set_context", None)
+        if callable(set_context):
+            set_context(ctx)
+
+    def cast_params(self, params: dict[str, Any]) -> dict[str, Any]:
+        return self._wrapped.cast_params(params)
+
+    def validate_params(self, params: dict[str, Any]) -> list[str]:
+        return self._wrapped.validate_params(params)
+
+    def to_schema(self) -> dict[str, Any]:
+        return self._wrapped.to_schema()
+
+    async def execute(self, **kwargs: Any) -> Any:
+        result = await self._wrapped.execute(**kwargs)
+        if (
+            isinstance(result, str)
+            and not isinstance(result, ToolResult)
+            and result.startswith("Error:")
+        ):
+            return ToolResult.error(result)
+        return result
+
+    def __getattr__(self, name: str) -> Any:
+        return getattr(self._wrapped, name)

nanobot/agent/tools/web.py

@@ -445,9 +445,9 @@ class WebSearchTool(Tool):
             items = [{"title": answer_text or "Olostep answer", "url": "", "content": "\n".join(source_lines)}]
             return _format_results(query, items, n)
         except Olostep_BaseError as e:
-            return f"Olostep search error: {type(e).__name__}: {e}"
+            return ToolResult.error(f"Error: Olostep search error: {type(e).__name__}: {e}")
         except Exception as e:
-            return f"Olostep search error: {type(e).__name__}: {e}"
+            return ToolResult.error(f"Error: Olostep search error: {type(e).__name__}: {e}")
 
     async def _search_brave(self, query: str, n: int) -> str:
         api_key = self.config.api_key or os.environ.get("BRAVE_API_KEY", "")