I tried using Claude Code for a few weeks and quietly stopped. Not because it was bad — it's great — but because I'd hesitate before kicking off any non-trivial task, doing the math on what a 30-minute debugging session would cost. That's the wrong incentive for an agent. The whole pitch of "let it run" depends on the bill not scaring you. So I went the other direction. I built Reasonix — same shape as Claude Code or Aider, runs in your terminal, plan mode, tool calls, MCP — but it only talks to DeepSeek, and the entire loop is engineered around one invariant: The prompt prefix must be byte-identical to the previous turn's prefix. That's it. That's the whole architectural constraint. Everything else falls out of it. This post is about why that constraint matters, what silently breaks it, and what the loop ends up looking like when you take it seriously. The mechanic DeepSeek's API has prefix caching.…