TraceLayer Harness developer preview

Everything is a plugin

TraceLayer Harness is now in developer preview for agent harness developers worldwide — source code included.

Every capability is a plugin that can be swapped or recomposed: models, tools, skills, sessions, sandboxes, storage, loops, scheduling, and the UI.

View on GitHub Developer docs Community plugins Cordis paper 
$ npx @tracelayer/tlh web
Agent = Model + Harness

Harness
keeps agents working in real-world environments

The model is the soul of an agent.

A harness lets an agent understand its environment, use tools, and keep working in real-world settings.

Cordis kernel

The Cordis kernel manages plugin mounting, unmounting, and dependencies. Agent capabilities live in the plugins.

Capabilities as plugins

Plugins provide every agent capability, including models, tools, skills, sessions, sandboxes, storage, loops, scheduling, and the UI. Cordis services and events let the plugins work together.

Compose with configuration

Developers can select, swap, or extend any capability in configuration without changing the TraceLayer Harness source code.

Design approach

Everything is a plugin. Every run is traceable.

Everything is a plugin

TraceLayer Harness is built on Cordis's plugin system. Plugins provide every agent capability, including models, tools, skills, sessions, sandboxes, storage, loops, scheduling, and the UI. Cordis services and events let the plugins work together. Developers can select, swap, or extend any capability in configuration without changing the TraceLayer Harness source code.

Every run is traceable

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. In the Trajectory view, you can inspect these records by source. Resume, fork, search, and replay all operate on the same event stream.

Multiple runtime modes

Standard mode includes the full toolset. Code mode uses model-generated code to orchestrate multiple rounds of tool calls. Minimal mode keeps only a shell tool and a file editor for benchmarking models in a minimal environment. Creator mode lets you inspect the current runtime, test Cordis plugins in memory, and combine them into new modes.

TraceLayer Harness settings showing installed plugins and their status
Reconstruct a complete run from a single session log
tlh-demo Creator mode
Describe what you want to build
Standard mode
Full coding agent with file editing, shell, file and web search, skills, planning, goals, subagents, and workflows.
Code mode
All Standard mode capabilities, with tools exposed through the Code Mode SDK so the model can combine multi-step operations in one TypeScript program.
Minimal mode
Two-tool coding agent with persistent bash and str_replace_editor.
Creator mode
Built for creating custom agent presets, with all Standard mode capabilities plus runtime inspection, plugin experiments, and preset-authoring guidance.

Customize your TraceLayer Harness

demo.mp4 — product feature demo
Get started

Try it now or install from source

Quick start

Install Node.js, then launch the Web UI with npx.

$ npx @tracelayer/tlh web

Install from source

Clone the full source and follow the setup instructions in the repository.

$ git clone https://github.com/tracelayer/harness

Join the TLH plugin ecosystem

TraceLayer Harness remains in developer preview and is still being tested by developers building agent harnesses. Its core plugins and APIs will continue to evolve. We look forward to exploring the limits of intelligence with developers worldwide using open-source infrastructure that is reusable and composable.

View on GitHub Developer docs Community plugins