DeepSeek open-sourced its own agent harness where every capability—models, tools, sandboxes, loops, scheduling, even the UI—is a swappable Cordis plugin, plus an append-only session log capturing system prompts, reasoning, tool calls, and context injections that you can resume, fork, search, and replay. This gives you a fully inspectable, model-agnostic scaffold for building coding agents, with a Code mode that lets the model orchestrate multi-step tool calls as a single TypeScript program instead of round-tripping each call—useful if you want a debuggable alternative to opaque proprietary harnesses.