diff --git a/nanobot/agent/tools/web.py b/nanobot/agent/tools/web.py
index 335fc9be..aa5645ac 100644
--- a/nanobot/agent/tools/web.py
+++ b/nanobot/agent/tools/web.py
@@ -705,10 +705,10 @@ class WebSearchTool(Tool):
             return _format_results(query, items, n)
         except httpx.HTTPStatusError as e:
             if e.response.status_code == 429:
-                return "Error: Serper search rate limited. Try again later or reduce search frequency."
-            return f"Error: Serper search failed ({e.response.status_code}): {e}"
+                return ToolResult.error("Error: Serper search rate limited. Try again later or reduce search frequency.")
+            return ToolResult.error(f"Error: Serper search failed ({e.response.status_code}): {e}")
         except Exception as e:
-            return f"Error: Serper search failed: {e}"
+            return ToolResult.error(f"Error: Serper search failed: {e}")
 
     async def _search_volcengine(
         self,
diff --git a/tests/tools/test_web_search_tool.py b/tests/tools/test_web_search_tool.py
index 18dbf160..b7ad7d60 100644
--- a/tests/tools/test_web_search_tool.py
+++ b/tests/tools/test_web_search_tool.py
@@ -3,6 +3,7 @@
 import httpx
 import pytest
 
+from nanobot.agent.tools.registry import is_tool_error_result
 from nanobot.agent.tools.web import WebSearchTool
 from nanobot.config.schema import WebSearchConfig
 
@@ -271,6 +272,7 @@ async def test_serper_search_http_error(monkeypatch):
     tool = _tool(provider="serper", api_key="bad-serper-key")
     result = await tool.execute(query="serper")
     assert "Error: Serper search failed (403)" in result
+    assert is_tool_error_result(tool.name, result)
 
 
 @pytest.mark.asyncio
@@ -282,6 +284,7 @@ async def test_serper_search_rate_limited(monkeypatch):
     tool = _tool(provider="serper", api_key="serper-key")
     result = await tool.execute(query="serper")
     assert "Serper search rate limited" in result
+    assert is_tool_error_result(tool.name, result)
 
 
 @pytest.mark.asyncio