Reasonix - Deepseek: A Terminal Coding Agent Built Around the Thing Everyone Else Ignores

Most terminal coding agents are architecturally similar: a loop, a tool registry, some context management, a TUI. Reasonix picks a different thing to optimize for, and it is a thing that shows up on your bill rather than in a demo video. The tagline is "engineered around prefix-cache stability — leave it running." That phrase is doing a lot of work, so let's unpack it. Why prefix caching is the whole pitch DeepSeek's API, like several others, caches the prefix of your prompt. If the next request starts with the exact same token sequence as the previous one, the provider serves those tokens from cache and bills them at a small fraction of the normal input rate. Cache hits are dramatically cheaper than cache misses. Here is the catch: it is a prefix cache. The match has to start at token zero and run forward. Change one character near the top of your context and every token after it is a miss. Now think about what a typical agent harness does over a long session. It re-summarizes the conversation. It injects a fresh timestamp or a re-scanned directory tree at the top. It reorders tool definitions. It rewrites the system prompt when you switch modes. Every one of those is a mutation near the front of the context, and every one of them silently invalidates the entire cache. The result is an agent that feels fine and costs several times what it should. You do not notice, because nothing errors. You just watch the number go up. Reasonix's central design constraint is: don't do that. Keep the front of the context stable, append rather than mutate, and put churn where it costs least. What that looks like in practice A small, stable environment summary is injected at startup rather than regenerated each turn. Stale tool output gets snipped and pruned before summary compaction kicks in, so a giant cat result from twenty turns ago is not still sitting in your prefix. The built-in tool schema contract is documented and regression-reviewed, because a silent tool-definition reshu