Practical MCP with FastMCP & LangChain

Masking Internal Error Details

To prevent leaking sensitive information through error messages, FastMCP provides a mask_error_details option when instantiating the FastMCP class. When enabled, any standard Python exception (like ValueError or TypeError) that occurs during tool execution is replaced with a generic error message such as Error executing tool 'tool_name'. However, ToolError messages always pass through to the client, giving you explicit control over what the LLM sees. This is particularly useful in production environments where you want to avoid exposing internal implementation details or sensitive data.

Here is an example of how internal error details are masked while ToolError messages remain visible:

mcp = FastMCP(name="SecureServer", mask_error_details=True)

@mcp.tool()
def risky_operation(param: str)