TL;DR : I shipped FIVE , an open-source MCP server that generates JSON personality constraints for any LLM. Drop the JSON into your system prompt and the character stops drifting. Different approach from typical guardrails — this one filters input via cognitive primitives, not output via moderation. Harness is MIT, constraint generation is $1 /call. The unusual part is where the cognitive model came from: a decade of teaching kids in Japan. The problem most builders eventually hit If you've shipped an LLM-powered character — a game NPC, a customer service persona, a roleplay companion — you know this failure mode: You write a careful system prompt. "You are a gruff weapon shop owner. You lost your daughter in the war and never speak of it. You're rude to adults but soft with children." The character works for the first few exchanges. Then somewhere around turn 8–12, the character starts apologizing. Or volunteering its tragic backstory. Or being suddenly nice to everyone. The seal breaks.…