Truncation Information Loss

Issue: Critical Information Lost When Context is Truncated

Frequency: Very Common

Symptoms

• Agent forgets earlier conversation context
• Important instructions get cut off
• References to truncated content fail
• Response quality degrades in long conversations
• Agent contradicts earlier statements

Root Cause When context exceeds the window limit, truncation removes content - typically from the beginning (oldest first). If critical information (user preferences, key facts, early instructions) is in truncated portions, the agent loses access to it. Simple FIFO truncation doesn’t consider information importance.

Example

Context window: 8K tokens
Conversation reaches: 12K tokens

Truncation (FIFO - first in, first out):
  [TRUNCATED] User: "Always respond in Spanish"
  [TRUNCATED] User: "My account number is 12345"
  [KEPT] ... recent messages ...
  [KEPT] User: "What's my account number?"

Agent: "I don't have your account number. 
        Could you provide it?"  (in English)

Two failures:
1. Forgot account number (truncated)
2. Forgot language preference (truncated)

Contributing Factors

• Simple FIFO truncation
• No importance weighting
• No summary of truncated content
• Large system prompts leaving less room
• No semantic deduplication
• Truncation at arbitrary boundaries

Eval Recipes

Test Cases

Metrics

Mitigation Strategies

Prevention

1. Importance scoring: Weight content by relevance
2. Summarization: Summarize truncated content
3. Key extraction: Extract and preserve key facts
4. Sliding window with summary: Keep summary of old content
5. Semantic deduplication: Remove redundant content first
6. Protected sections: Never truncate critical instructions

Architecture Pattern

[System Prompt - PROTECTED]
[Key Facts Summary - PROTECTED]
[Summarized History - Compressible]
[Recent Context - Full detail]
[Current Turn - Full detail]

Production Signals

Key Metrics

Alerts

References

• Lost in the Middle
• LongLLMLingua