# Hallucination Despite Retrieved Context

## The Problem

LLM adds fabricated details even when relevant context is provided, mixing real retrieved information with invented facts.

### Symptoms

* ❌ Adds details not in context
* ❌ Embellishes with plausible but false info
* ❌ Correct facts + wrong details combined
* ❌ Cannot distinguish source of claims
* ❌ Confident delivery of mixed truth/fiction

### Real-World Example

```
Retrieved context:
"Premium plan includes 5 team members and 100GB storage"

User query: "What's in premium plan?"

AI response: "Premium plan includes 5 team members, 100GB storage,
priority email support (24h response), and access to beta features."

Context ONLY mentioned: 5 members, 100GB
AI INVENTED: Priority support, beta access
```

***

## Deep Technical Analysis

### Retrieval-Generation Gap

**Incomplete Context:**

```
Context silent on some aspects:
→ User asks about support
→ Context doesn't mention support
→ LLM fills gap with "typical" support model
→ Hallucinates based on training data patterns
```

**The Helpful Assistant Dilemma:**

```
LLM trained to:
→ Be complete and helpful
→ Answer fully
→ Avoid "I don't know"

Conflicts with:
→ "Only use retrieved context"
→ Admit knowledge gaps

Helpfulness bias → hallucination
```

### Pattern Completion

**Training Data Influence:**

```
LLM saw thousands of "Premium plan" descriptions:
→ Usually include: Support, features, storage
→ Pattern: Premium = better support

Applies pattern even if not in YOUR docs:
→ Invents "priority support"
→ Sounds plausible
→ But factually wrong for your product
```

### Weak Grounding

**Instruction Adherence Limits:**

```
System prompt: "Only use provided context"

But:
→ LLM follows ~85-90% of time
→ 10-15% drifts to training knowledge
→ Cannot 100% guarantee grounding

Stronger models (GPT-4) better than weaker (GPT-3.5)
```

**Citation as Constraint:**

```
Forcing citations helps:
"For each claim, cite source: [chunk_id]"

AI must justify each fact:
→ "5 members [chunk_12]"
→ "100GB storage [chunk_12]"
→ Cannot cite invented facts
→ Reduces hallucination
```

***

## How to Solve

**Require citations for all claims + use explicit prompts: "If not in context, say 'not available in documentation'" + implement two-stage: extract facts first, then answer using only extracted + use models fine-tuned for RAG (instruction-following) + apply post-generation fact-checking against context + penalize hallucination in eval metrics.** See [Hallucination Prevention](/rag-scenarios-and-solutions/accuracy/hallucination.md).


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