Deduplicate System Messages (Prompt Cache Prefix Consolidation)

Overview

Many multi-agent frameworks inject fresh role: "system" messages or duplicate guideline blocks into the middle of conversation histories (e.g., re-injecting system rules after every tool execution).

Scattering system messages throughout the message array causes two severe issues:

1. Breaks Prompt Caching: Modern LLM caching engines (Anthropic Prompt Caching, OpenAI Prefix Caching) require a static, uninterrupted prefix. Injecting dynamic system messages invalidates the cache on every turn.
2. Input Token Inflation: Re-sending 1,500 tokens of system instructions on every turn across 40 turns burns 60,000 redundant input tokens.

The System Prompt Consolidation Protocol enforces a single, canonical, immutable system header at Turn 0, ensuring maximum prompt cache hit rates (90% discount) and zero redundant token spend.

Fragmented System Injection vs. Consolidated Prefix Cache