# Hallucination

> A hallucination is fluent, confident output that is factually wrong or fabricated — plausible text unsupported by any source, the signature LLM failure mode.

**A hallucination is model output that reads confident and coherent but is factually wrong or invented — a fabricated API, a nonexistent citation, a wrong number stated smoothly.**

It's not a bug to patch but a property of how generation works: an LLM produces the most *plausible* continuation, and plausibility tracks truth only where training data was dense. The failure concentrates exactly where users least expect it — specifics. Names, versions, citations, niche APIs: the model fills gaps with statistically likely inventions, delivered in the same confident tone as real knowledge.

Mitigation is an engineering stack, not a setting. **Grounding** narrows the gap between plausible and true: [RAG](/glossary/rag) puts real sources in the prompt and confines answers to them. **Constraints** shrink the surface: [structured outputs](/guides/concepts/structured-output-2026) can't hallucinate fields that fail validation. **Measurement** keeps you honest: faithfulness metrics and [LLM-as-judge](/glossary/llm-as-judge) scoring inside a real [eval suite](/guides/evaluation/write-llm-evals) turn "it seems to hallucinate less" into a number you can gate releases on.

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_Source: https://agentscamp.com/glossary/hallucination — Term on AgentsCamp._