[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)

src/mcp/server/lowlevel/server.py の 560 行目と 575 行目付近にある call_tool デコレーターのハンドラーを読み、modelcontextprotocol/typescript-sdk#654 / PR #655 にある対応する TypeScript SDK の修正と比較してください。エラーパスが outputSchema の検証を回避し、構造化されていないツールエラーに対して isError レスポンスを保持することを、報告された例のリグレッションチェックとともに確認してください。