[v1.x] Server-side outputSchema validation blocks tool error reporting (isError: true)

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

When a tool handler returns unstructured content for an error case, the low-level server's call_tool decorator validates the output against outputSchema before checking if the response is an error.

This prevents tools with outputSchema from reporting errors via isError: true, as the validation fails with "outputSchema defined but no structured output returned", replacing the original error message.

This was already fixed in the TypeScript SDK via modelcontextprotocol/typescript-sdk#654 / PR #655 (2025-06-24), but the equivalent fix has not been applied to the Python SDK v1.x branch.

The issue is in src/mcp/server/lowlevel/server.py, the call_tool decorator handler:

1. Line ~560: outputSchema validation runs unconditionally — should skip when the result is an error
2. Line ~575: isError=False is hardcoded — there is no path for the handler to signal an error through unstructured content while outputSchema is defined

Example Code

from mcp.server.lowlevel import Server
from mcp import types

server = Server("test")

@server.call_tool()
async def call_tool(name: str, arguments: dict):
    # Tool has outputSchema but needs to return an error
    # This gets blocked by outputSchema validation
    return [types.TextContent(type="text", text="Resource not found")]

Python & MCP Python SDK

- Python: 3.11 / 3.12
- MCP SDK: v1.26.0+ (v1.x branch)

Đọc trình xử lý decorator call_tool trong src/mcp/server/lowlevel/server.py quanh các dòng 560 và 575, sau đó so sánh bản sửa tương ứng của TypeScript SDK trong modelcontextprotocol/typescript-sdk#654 / PR #655. Xác nhận rằng nhánh lỗi tránh việc xác thực outputSchema và giữ nguyên phản hồi isError cho các lỗi công cụ không có cấu trúc, kèm theo một kiểm tra hồi quy cho ví dụ đã được báo cáo.