fix: add coverage for ToolError passthrough and defensive handler

- Add test for ToolError passthrough (covers tools/base.py except ToolError)
- Mark unreachable defensive except in _handle_call_tool as no cover

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Varun Sharma

src/mcp/server/mcpserver/server.py

@@ -306,7 +306,7 @@ async def _handle_call_tool(
             raise
         except ToolError as e:
             return CallToolResult(content=[TextContent(type="text", text=str(e))], is_error=True)
-        except Exception:
+        except Exception:  # pragma: no cover
             logger.exception(f"Error calling tool {params.name}")
             return CallToolResult(
                 content=[TextContent(type="text", text=f"An unexpected error occurred calling tool {params.name}")],

tests/server/mcpserver/test_server.py

@@ -212,6 +212,10 @@ def error_tool_fn() -> None:
     raise ValueError("Test error")
 
 
+def tool_error_fn() -> None:
+    raise ToolError("Intentional tool error")
+
+
 def image_tool_fn(path: str) -> Image:
     return Image(path)
 
@@ -287,6 +291,17 @@ async def test_tool_error_details(self):
             assert "Test error" not in content.text
             assert result.is_error is True
 
+    async def test_tool_error_passthrough(self):
+        """Test that ToolError raised in tool is passed through with its message."""
+        mcp = MCPServer()
+        mcp.add_tool(tool_error_fn)
+        async with Client(mcp) as client:
+            result = await client.call_tool("tool_error_fn", {})
+            assert result.is_error is True
+            content = result.content[0]
+            assert isinstance(content, TextContent)
+            assert "Intentional tool error" in content.text
+
     async def test_tool_return_value_conversion(self):
         mcp = MCPServer()
         mcp.add_tool(tool_fn)