Put critical instructions in the first 20% of system…
Position your system prompt's most important instructions in the first 20% of tokens, because transformer attention mechanisms allocate disproportionate processing to early sequence positions through positional encoding and causal masking.
When This Fires
- Writing or designing system prompts for LLM-based tools
- When AI assistants don't follow instructions that are present in the system prompt
- Optimizing prompt performance for specific behaviors
- During any prompt engineering where instruction priority matters
Common Failure Mode
Writing system prompts like documents — background context first, instructions at the end: "You are an assistant that... [200 tokens of context]. When responding, please... [critical instruction buried at position 300]." The critical instruction competes with 200 tokens of context for attention. Invert: critical instructions first, then context.
The Protocol
When writing system prompts: (1) Identify the 3-5 most critical instructions — the behaviors that must be followed above all else. (2) Place them in the first 20% of the prompt's token budget. (3) Follow with secondary instructions, context, and examples. (4) If a critical instruction isn't being followed despite being present → check its position. Moving it to the first 20% often resolves the compliance issue without changing the wording.