fix: propagate MCPError from tool handlers as JSON-RPC error

Previously, only UrlElicitationRequiredError was re-raised; any other
MCPError raised inside a tool function was caught by the bare
'except Exception' clause and wrapped into a ToolError, which then
surfaced as a CallToolResult(isError=True) instead of a structured
JSON-RPC error response.

Since UrlElicitationRequiredError is a subclass of MCPError,
broadening the guard to 'except MCPError' covers all cases including
the original one.

Fixes #2770

Author: shiwenbin1617

src/mcp/server/mcpserver/tools/base.py

@@ -10,7 +10,7 @@
 from mcp.server.mcpserver.utilities.context_injection import find_context_parameter
 from mcp.server.mcpserver.utilities.func_metadata import FuncMetadata, func_metadata
 from mcp.shared._callable_inspection import is_async_callable
-from mcp.shared.exceptions import UrlElicitationRequiredError
+from mcp.shared.exceptions import MCPError
 from mcp.shared.tool_name_validation import validate_and_warn_tool_name
 from mcp.types import Icon, ToolAnnotations
 
@@ -111,9 +111,7 @@ async def run(
                 result = self.fn_metadata.convert_result(result)
 
             return result
-        except UrlElicitationRequiredError:
-            # Re-raise UrlElicitationRequiredError so it can be properly handled
-            # as an MCP error response with code -32042
+        except MCPError:
             raise
         except Exception as e:
             raise ToolError(f"Error executing tool {self.name}: {e}") from e

tests/server/mcpserver/test_url_elicitation_error_throw.py

@@ -90,6 +90,23 @@ async def multi_auth(ctx: Context) -> str:
         assert url_error.elicitations[1].elicitation_id == "gdrive-auth"
 
 
+@pytest.mark.anyio
+async def test_mcp_error_propagates_as_json_rpc_error():
+    """Test that MCPError raised from a tool propagates as a JSON-RPC error, not isError result."""
+    mcp = MCPServer(name="McpErrorServer")
+
+    @mcp.tool(description="A tool that raises a plain MCPError")
+    async def failing_tool(ctx: Context) -> str:
+        raise MCPError(-32000, "custom MCP error")
+
+    async with Client(mcp) as client:
+        with pytest.raises(MCPError) as exc_info:
+            await client.call_tool("failing_tool", {})
+
+        assert exc_info.value.error.code == -32000
+        assert exc_info.value.error.message == "custom MCP error"
+
+
 @pytest.mark.anyio
 async def test_normal_exceptions_still_return_error_result():
     """Test that normal exceptions still return CallToolResult with is_error=True."""