LangChain system prompt

A LangChain system prompt is a top-level instruction in a chat template that specifies global behavior for a large language model before any user or tool messages. Declared with SystemMessage(content=”You are a helpful paralegal…”), it anchors tone, persona, domain boundaries, and formatting rules. In LangChain, you pass a system prompt to a ChatPromptTemplate or embed it as the first element in a PromptTemplate role list, then pass in streaming user data and context snippets. Because the system prompt is immutable across sessions, it prevents jailbreaks and drift by ensuring that each subsequent response adheres to policy guidelines—“cite sources,” “skip medical advice,” or “respond in JSON.” Teams version system prompts in YAML, A/B test variants with LangChain scoring, and dynamically replaces them via environment configurations for multi-tenant SaaS. When combined with tools and memory, LangChain’s carefully crafted cueing system reduces hallucinations, increases brand voice consistency, and turns cue development into a manageable asset.