DeepSeek Harness developer preview: everything is a plugin

DeepSeek has released Harness, its agent harness, in developer preview for harness developers worldwide — with the source code included. In the company's framing, the model is the soul of an agent, while the harness is what lets an agent understand its environment, use tools and keep working in real-world settings.

Every capability is a plugin

Harness is built on the Cordis plugin system. Models, tools, skills, sessions, sandboxes, storage, loops, scheduling and the UI are each separate plugins, and Cordis services and events let them work together. Developers can select, swap or extend any capability in configuration without changing the Harness source code. Getting started takes one command: npx @deepseek-ai/dsh web, and the repository is published on GitHub.

Every run is traceable

Everything the model sees is recorded in an append-only session log: system prompts, reasoning, tool calls and their results, subagent scheduling and every context injection. A Trajectory view lets developers inspect those records by source, while resume, fork, search and replay all operate on the same event stream.

Four runtime modes

Standard mode ships the full toolset: file editing, shell, file and web search, skills, planning, goals, subagents and workflows. Code mode exposes the same capabilities through the Code Mode SDK so the model can combine multi-step operations into a single TypeScript program. Minimal mode keeps only two tools — a persistent bash and str_replace_editor — for benchmarking models in a minimal environment. Creator mode adds runtime inspection, in-memory Cordis plugin experiments and preset-authoring guidance so developers can compose new modes of their own.