DeepSeek Harness is an open source agent harness designed to keep AI agents working in real-world environments by helping them understand context, use tools, and continue operating across complex tasks. Built on the Cordis plugin system, every capability is a plugin that can be selected, swapped, extended, or recomposed through configuration without changing the core source code. Plugins provide models, tools, skills, sessions, sandboxes, storage, loops, scheduling, and the UI, while Cordis manages plugin mounting, dependencies, services, and events. Every run is traceable through an append-only session log that records system prompts, reasoning, tool calls and results, subagent scheduling, and context injections. The Trajectory view lets developers inspect these records by source, while resume, fork, search, and replay operate on the same event stream.