DeepSeek has piqued the interest of the developer community by releasing an early version of its open source agent harness. This happens as harnesses have become increasingly important to those working with machine learning models.
"Powered by the Cordis meta-framework, DeepSeek Harness is an agent harness built around one core idea: Everything is a plugin," the China-based AI biz said. "Models, tools, skills, sessions, sandboxes, filesystems, loops, orchestration, and UI are ALL implemented as plugins, and can be mixed, matched, replaced, and extended."
The term "harness" came into common use this year to describe a longstanding software function – middleware or a mediation layer that handles the input passed to an AI model and the output returned from it. Harnesses oversee prompts, context management, tool orchestration, the agent loop, state management, error handling, safety, permissions, and related concerns.
Claude Code serves as a harness for Anthropic's Claude model family and Codex performs a similar function for OpenAI's GPT model family. And there are many other model harnesses, including Aider, Cline, Goose, OpenCode, OpenHands, and Pi, to name a few.
The term isn't precise: It may be used to refer just to the agent loop and tools, or it may be extended to a broader set of concerns related to orchestrating different tools, services, and capabilities like sandboxing, subagents, and so on. Google Antigravity, for example, consists of the Antigravity Agent Runtime (harness) that can be accessed through the Agent SDK, the Antigravity 2.0 desktop application, and the Antigravity CLI.
Vague definitions aside, AI model harnesses are now where much of the competition is happening, particularly as models proliferate and become commoditized. The harness often implements the user interface, a source of user inertia, and once developers configure their tooling and become accustomed to doing things a certain way, it becomes more burdensome to switch to a competing product, even if the interface consists mainly of a command line.
What's more, various studies have suggested that model performance (and cost) varies significantly with the harness used, due to different design choices. For example, the Pi coding agent relies on a minimal system prompt of about 200 tokens. Claude Code by comparison uses a system prompt of around 10,000 tokens (or did until last month when Anthropic trimmed the system prompt by about 80 percent). The same model will produce different results with different harnesses.
DeepSeek Harness is noteworthy because of its innovative design, and because it shows Chinese AI labs moving to compete beyond model benchmarks and pricing.
First, it treats everything as a plugin. It uses the plugin system from its underlying Cordis framework, which is designed to make it possible to add and remove components dynamically without wreaking havoc.
"Plugins provide every agent capability, including models, tools, skills, sessions, sandboxes, storage, loops, scheduling, and the UI," the DeepSeek Harness website explains. "Cordis services and events let the plugins work together. Developers can select, swap, or extend any capability in configuration without changing the DeepSeek Harness source code."
A DeepSeek paper [PDF] by researchers Yifan Shi, Wei Zhang, and Tianyi Cui explains the function of Cordis in more detail. Cordis is designed to support dynamic composability – adding plugins and removing them on the fly without breaking the application.
The paper refers to this as temporal composability – removing a component and reverting its effect upon removal – and spatial composability – allowing components to manage dependencies upon other components.
It cites as an example the plugin system used by Microsoft's Visual Studio Code. VS Code, the authors explain, runs all of its extensions in a shared process called the extension host. Once activated, they cannot be removed on the fly; the host has to be restarted.
While VS Code provides a way for extensions to declare dependencies between extensions, it's seldom used. DeepSeek Harness supports plugin dependencies.
The DeepSeek researchers argue temporal and spatial composability are necessary in a system where modification can occur continuously with little or no human oversight. It's a way of avoiding forced restarts and crashes when components appear and disappear.
DeepSeek Harness also supports another useful feature: chain of thought traces.
"Everything the model sees is recorded in an append-only session log: system prompts, reasoning, tool calls and results, subagent scheduling, and every context injection," the DeepSeek Harness website says. "In the Trajectory view, you can inspect these records by source. Resume, fork, search, and replay all operate on the same event stream."
DeepSeek R1 made waves when it was released last year and it was trained to use chain of thought reasoning. This involves breaking down prompts into a series of "thoughts" and reflecting on those steps before emitting a final answer.
Access to this intermediate reasoning turns out to be useful for assessing whether a model is reasoning well, whether its responses are accurate, how additional "thinking" affects output, and so on.
Anthropic provides some access to thinking when extended or adaptive thinking is available (it varies by model). But increasingly the biz has been hiding model reasoning by summarizing chain of thought traces. That appears to be due in part to concerns that chain of thought traces can be used for copying models through a standard research process called distillation.
Earlier this year, Anthropic said it had implemented classifiers for the "detection of chain-of-thought elicitation used to construct reasoning training data." The company also does not display raw chain of thought. It explains that "the text in a thinking block is a summary of Claude's reasoning." Accessing raw thinking requires contacting Anthropic sales personnel.
Except for its open source models, OpenAI has also chosen to hide chain of thought reasoning, which the company uses for model monitoring. "After weighing multiple factors including user experience, competitive advantage, and the option to pursue the chain of thought monitoring, we have decided not to show the raw chains of thought to users," the biz said two years ago when it introduced its o1 reasoning model.
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With the newly released DeepSeek-V4-Pro and V4-Flash, the API provides thinking mode enabled by default. And as the open source model ecosystem matures, having access to chain of thought looks likely to become another opportunity for competitive differentiation.
"I don't think the DeepSeek Harness is perfect but this is for sure the first time I have been looking at something new in the space and felt quite inspired to revisit some of our choices," said Armin Ronacher, co-founder of AI biz Earendil, which now steers the development of the Pi agent, in a social media post. "I love that part about Open Source a lot!" ®
