Why does my MCP tool error show only 'Error executing tool X' without my message?

How it was checked: Verified on mcp 2.3.0 over a real stdio session: RuntimeError showed a generic error; ToolError delivered the message.

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Fix

In the Python MCP SDK v2, only ToolError messages are passed to the model. Any other exception is treated as a crash and its text stays on the server. Raise from mcp.server.mcpserver.exceptions import ToolError; raise ToolError('clear, actionable message') for expected failures (missing key, insufficient balance, 404). Keep unexpected bugs as normal exceptions so internal details aren't leaked.

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