Errors the model can use

When a tool fails, its message is the model's only clue.

In MCP's Python SDK, a plain exception reaches the model as just “Error executing tool”. Raise ToolError and your message gets through.

An error the model can't usegemini-3-flash-previewRecorded real run

Task Where is my order A12?

You

task

Model1.1s

get_order

find_orders

track_shipment

Model call 1. It read 191 tokens and wrote 20. It asked for a tool, so it isn't done.

get_order({"order_id":"A12"})

stop_reason: tool_use

Context window

191 tokens sent

0.02% of the 10,48,576-token window. Every call sends all of it again.

Context window191 tokens sent

Be the modelAn error the model can't use

0/1 best

You are gemini-3-flash-preview. This is all you know: 262 tokens.

System prompt
You are QuickCart's support agent. Use the tools to look up real order data. Never guess. Be brief and kind.
Your tools
- get_order Look up an order by its id. Returns status, tracking id and the promised delivery date.
- find_orders List the ids of this customer's recent orders.
- track_shipment Where a shipment is right now, by tracking id. Returns status, location and the latest delivery estimate.
User
Where is my order A12?
You called
get_order("A12")
get_order returned
Error executing tool get_order

Your move?

The same server raising ToolError. The model is told what's wrong and what to call next:

An error the model can usegemini-3-flash-previewRecorded real run