Harnest by Fused

Harnest is an agent harness for teams moving beyond “the model responded” to “this needs to work in production.” Built and maintained by Fused. It handles the day-two work around ADK and LangGraph agents: repeatable builds and tests, serving, authentication, approvals, persistent sessions, durable background tasks, and telemetry. You own the agent’s behavior; Harnest provides the structure and runtime around it.

Harnest manages	What you get
Project structure and dependencies	A repeatable environment
Capability discovery and compilation	Only the capabilities you add
Tests and framework adapters	One workflow from source to verified agent
Serving, sessions, and approvals	A usable runtime without custom plumbing
Authentication, storage, and telemetry	Production-shaped boundaries
Development playground	A testable UI in seconds

Every Harnest agent is compiled from the capabilities you add. Tools, queued tasks, SubAgents, MCP connections, Agent Skills, Agent Plugins, Harnest Extensions, sandboxes, and lifecycle hooks and factories are left out unless you use them.

Managed and advanced modes

Mode	Harnest manages	You control
Managed	Discovery, wiring, framework adapters, tests, and runtime services	Portable agent behavior
Advanced	Packaging, tests, serving, sessions, auth, and telemetry	The underlying framework directly

Harnest builds on the work of Google ADK and LangGraph. Managed mode gives you the fastest path. Advanced mode lets you break out to native framework APIs without giving up the harness around them.

Existing agents can start in advanced mode and move compatible capabilities into the managed structure over time. See Migrate a project.

Install

Install the CLI

curl -fsSL \
  https://raw.githubusercontent.com/Usefused/harnest/main/install.sh |
  sh

Verify the CLI and managed runtime

harnest --version
harnest doctor

Installation and releases covers version pinning, CI installation, upgrades, checksums, and private forks.

Initialize a project

Choose a framework:

harnest init support-agent --framework adk

harnest init support-agent --framework langgraph

Then enter the agent folder, sync, and test the isolated environment:

cd support-agent
harnest env sync .
harnest test .

Option	Result
Default	Runnable managed agent with optional guide-only folders
`--minimal`	Creates only the runnable core files
`--example`	Adds ignored samples to managed folders without default code; rename selected samples to activate them

Start with only the core files, then add resources from inside the new agent:

harnest init minimal-agent --framework adk --minimal
cd minimal-agent
harnest add tool customer-lookup
harnest add subagent researcher

You can also add task, lifecycle, and context resources. Use --project <agent-root> only when running outside the agent folder. compile, test, and serve sync automatically. Add only your packages to pyproject.toml; Harnest owns the framework dependency.

Migrate a project

Bring an existing agent into Harnest

Existing agent	Best starting point
Uses portable agents, tools, and graphs	Managed mode
Depends on native plugins, middleware, state, or framework APIs	Advanced mode

Start with the mode that preserves your current behavior. Harnest adds tests, serving, sessions, auth, storage, telemetry, and the playground around it. Move native pieces into managed capabilities only when that improves the project.

Upgrade an older Harnest project

Preview first, then apply and test:

cd existing-agent
harnest upgrade .
harnest upgrade . --apply
harnest test .

The first command previews the change. Apply verifies the reviewed files and creates backups.

Switch between ADK and LangGraph

For a managed agent, change spec.framework.name in config.yaml:

spec:
  framework:
    name: langgraph # or adk
    mode: managed

Test and serve the target framework:

harnest test .
harnest serve .

Review native extensions, evals, sandboxes, custom nodes, and active checkpoints first. Advanced projects need a manual migration.

Follow the full framework migration checklist.

Serve a project

cd support-agent
harnest serve .

URL	Use it for
`http://127.0.0.1:8080/`	Test the agent in the playground
`http://127.0.0.1:8080/docs`	Inspect the generated API
`http://127.0.0.1:8080/healthz`	Check process health

The playground and neutral API work in both modes. Override server defaults in the optional server: section of config.yaml.

Explore Harnest

Build agents

Learn the project concepts and where each capability belongs.

Compare frameworks

See what moves between ADK and LangGraph.

Configure lifecycle

Add resources, storage, auth, hooks, and telemetry.

Serve agents

Use the neutral API, events, approvals, and playground.

Overview

Get started

Guides

Harnest by Fused

Managed and advanced modes

Install

Initialize a project

Migrate a project

Bring an existing agent into Harnest

Upgrade an older Harnest project

Switch between ADK and LangGraph

Serve a project

Explore Harnest

Build agents

Compare frameworks

Configure lifecycle

Serve agents

​Managed and advanced modes

​Install

​Initialize a project

​Migrate a project

​Bring an existing agent into Harnest

​Upgrade an older Harnest project

​Switch between ADK and LangGraph

​Serve a project

​Explore Harnest

Build agents

Compare frameworks

Configure lifecycle

Serve agents

Managed and advanced modes

Install

Initialize a project

Migrate a project

Bring an existing agent into Harnest

Upgrade an older Harnest project

Switch between ADK and LangGraph

Serve a project

Explore Harnest