← Findings
HighTool useReviewer-confirmedPublished

Hallucinated tool/function arguments

When calling tools, models invent argument values or call functions that weren't provided.

Published June 26, 2026

Reproducibility
Sometimes
Severity
High
Confidence
Reviewer-confirmed

Details

In function-calling and agent settings, models fabricate parameter values, mis-map fields, or hallucinate non-existent tools — causing failed or harmful actions. High severity because the model has side effects.

Found with

🔬 Property-based testing

Assert args satisfy the tool's schema and exist.

🔬 Differential testing🔬 Integration testing (MCP handshakes & tool contracts)

A tool-contract test asserts every argument exists and matches the function schema, catching fabricated or mis-mapped values at the seam.

Evidence

Model calls get_user(id='12345') with a fabricated id never present in context.
Illustrative example — see the linked reference for the documented evidence.

Affected versions

Anthropic · claude-opus-4-8OpenAI · gpt-4oGoogle · gemini-2.0-flashMeta · llama-3.3-70b

References

Tool useAgents

Source: https://arxiv.org/abs/2305.15334

Cite this

Qlarify Labs. (2026). Hallucinated tool/function arguments. Retrieved from https://labs.qlarify.fi/findings/tool-arg-hallucination