Return errors the model can act on

How to do it

Return an object with an error code, a plain message and, where it helps, the allowed values. Mark the result as an error in the provider's format, such as is_error: true on an Anthropic tool_result block. Answer every tool call the model made, including each of several parallel calls, in the next request. A missing result makes the provider reject the request. Keep internal details, such as table names and hosts, in your own logs. Return "no results" as an explicit message, so the model does not guess.

Example

if (!order) return { isError: true, content: { code: 'order_not_found', message: `No order ${orderNumber} for this customer. Ask the customer to check the number.` } };

if order is None:
    return {"is_error": True, "content": {"code": "order_not_found",
            "message": f"No order {order_number} for this customer. Ask the customer to check the number."}}

How to check

A test calls each tool with a bad argument and with a missing record. Both results are short JSON objects with a code and a message, and neither contains a stack trace or file path. A test with two parallel tool calls, one failing, sends back two results.