DeepSeek has opened up its agent stack. The company announced that DeepSeek Harness is now in developer preview for agent harness developers worldwide, with source code included. The core idea: every capability is a plugin that can be swapped or recomposed, including models, tools, skills, sessions, sandboxes, storage, loops, scheduling, and even the UI. Built on Cordis's plugin system, developers can select, swap, or extend any capability in configuration without touching the DeepSeek Harness source code.
Harness is designed to keep agents working in real-world environments. Every run is traceable: everything the model sees is recorded in an append-only session log, including system prompts, reasoning, tool calls and results, subagent scheduling, and context injections. A Trajectory view lets developers inspect these records by source, with the ability to resume, fork, search, and replay operations.
The preview ships with four modes. Standard mode includes the full toolset for a complete coding agent. Code mode exposes tools through a Code Mode SDK so the model can combine multiple tool calls. Minimal mode keeps only a shell tool and a file editor for benchmarking models, and Creator mode lets developers inspect the runtime and test Cordis plugins live. DeepSeek says core plugins and APIs will continue to evolve during the preview period.