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What are hallucinations and what causes them?

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01

Understand the problem

Why models state falsehoods fluently, and the failure modes every AI engineer must anticipate.

hallucinationgroundingreliability
02

Attempt it yourself

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03

Study the solution

A hallucination is fluent output that is factually wrong or fabricated — invented citations, APIs that do not exist, confident wrong answers. They happen because the model optimizes plausibility (next-token likelihood), not truth: when the training data or provided context lacks the answer, the most statistically plaus

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04

Read the code

The abstain-or-cite pattern
const system = [
  "Answer ONLY from the provided context.",
  "Cite the doc id for every claim, like [doc-3].",
  "If the context does not contain the answer, reply exactly:",
  '"I could not find this in the provided documents."',
].join("\n");

// downstream: reject answers whose citations do not resolve
const ok = extractCitations(reply).every((id) => contextIds.has(id));
05

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