DeepSeek Harness

DeepSeek Harness is an open-source agent runtime framework released by DeepSeek built around the architectural thesis Agent = Model + Harness. It provides a modular, traceable execution harness designed to keep autonomous agents functioning reliably in real-world environments without hardcoded core logic.

Core Architectural Design

The system is designed around two primary engineering principles: Everything is a plugin and Every run is traceable.

1. The Microkernel & Capabilities as Plugins

DeepSeek Harness builds upon the Cordis kernel, delegating all functional capabilities into plugins that register typed services:

• Core Subsystems as Plugins: Models (ctx.llm), tool execution (ctx.tools), sessions (ctx.sessions), agent swarms (ctx.agents), skills, sandboxes, storage, loops, scheduling, and UI are all decoupled plugins.
• Dynamic Reconfiguration: Developers can swap, compose, or extend capabilities purely through configuration without modifying the harness source code.

2. Append-Only Traceability & Trajectory Replay

Every interaction the model observes is captured in an append-only event stream:

• System prompts, reasoning thoughts, tool calls, tool results, subagent schedules, and context injections are recorded.
• In the Trajectory View, developers inspect runs by source.
• The primitives resume, fork, search, and replay all operate natively on the unified session event log.

Multiple Runtime Modes

DeepSeek Harness includes 4 preset operational modes tailored for different development and benchmarking workflows:

1. Standard Mode: Full coding agent equipped with file editing, shell execution, search, skills, planning, goals, subagents, and workflows.
2. Code Mode: Exposes tools via the Code Mode SDK, enabling the model to write TypeScript programs that orchestrate complex, multi-step tool calls in a single execution step.
3. Minimal Mode: Constrained two-tool coding agent (bash and str_replace_editor) designed for clean model benchmarking.
4. Creator Mode: Metaprogramming environment designed for inspecting active runtimes, testing in-memory Cordis plugins, and authoring custom agent presets.

Related Concepts

• cordis framework
• agent harness engineering
• agent native infrastructure
• subagents and context management
• context engineering
• deepseek