Glossary
Hallucinated arguments
Tool-call arguments the model invented rather than took from the user or the data, such as a made-up id, amount or recipient.
Checks run on our GPUs in Switzerland. Request payloads are not stored.Security
What Hallucinated arguments means
A hallucinated argument is a value in a tool call that has no basis in what the user said or what the agent has seen: an order id that does not exist, a refund amount nobody mentioned, an email address that looks plausible but was never provided. The tool name can be right and the call still harmful, because the arguments point at the wrong thing.
These errors are easy to miss because the call is well-formed and type-checks. Schema validation catches a string where a number should be, not a confident wrong number.
MCP Guard’s args_grounded head scores whether the arguments are supported by what the user asked. By default, a score below 0.5 turns the verdict into ask. The check works best when you send the user’s request (intent or user_message) and the relevant recent turns, so the model can see where each value should have come from.
Related terms
How Hallucinated arguments connects to the rest of the vocabulary.