# Hallucinated Tools

> Trusting the model to only call tools you gave it — then watching it invoke functions that don't exist.

- **Category**: Agentic AI
- **Subcategory**: Anti-Patterns
- **Canonical URL**: https://designpattern.fyi/patterns/hallucinated_tools/

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## Description
**Intent**: Assuming the model will only call tools it was shown in the prompt — without validating the name at dispatch time.

**Context**: An agent is configured with a registered tool palette. The host accepts whatever name the model emits and dispatches it without checking against the palette. The model confidently calls things that don''t exist — and the host silently fails or mis-routes to a similarly named tool that does.

**Solution**: Validate every tool call against the registered palette before dispatch. Reject unknown names with a typed error the agent loop can react to. See tool-use, structured-output.



## Use Cases
- Never use this; treat any model-emitted tool name as untrusted input.
- Validate every tool call against the registered tool palette before dispatch.
- Reject unknown tool names with a typed error the agent loop can handle.






## Trade-offs




### Considerations & Drawbacks

- Silent failures when unrecognized tool names just disappear

- Mis-dispatches to similarly named tools that do exist — wrong actions run silently

- Debugging is confusing — the model looks correct, the host is at fault







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**Reference**: [Original Source](https://www.agentpatternscatalog.org/patterns/hallucinated-tools/)