Strands harness is a thin composition layer, not a framework of its own. The substance lives in the [Strands Harness SDK](/docs/user-guide/sdk/index.md): the model loop, tools, context management, sessions, memory, and hooks. Strands harness’s job is to assemble those building blocks into one agent with tested defaults and a tuned system prompt, so you do not wire them up yourself. This page is for readers who know the Strands Harness SDK, or want to, and need to see exactly where Strands harness sits and how to reach past it.

## The composition model

The factory does three things in order: it resolves your options into concrete Strands Harness SDK objects (a `Model` from a `provider/name` string, a memory manager, intervention handlers), it assembles the built-in tools, plugins, subagents, and behaviors, and it constructs a plain `Agent` from the result. Nothing wraps that agent afterward. The built-in tools are ordinary registered tools, `todos` and `environment` are ordinary plugins, and sessions and memory use the Strands Harness SDK’s own managers.

## The return value is a plain Agent

`create_harness()` returns a plain Strands `Agent`, the same class you get from the Strands Harness SDK directly. There is no Strands harness type around it. Because you hold a normal `Agent`, the full Strands Harness SDK is available and nothing about Strands harness blocks a feature the Strands Harness SDK offers.

## Compose Strands Harness SDK features alongside Strands harness’s defaults

Any argument the factory does not name is passed straight through to the `Agent` constructor, and your explicit value wins over the default it corresponds to. This is the seam for Strands Harness SDK-native features. Pass your own `plugins` or `interventions` alongside Strands harness’s and they compose:

(( tab "Python" ))
```python
from strands_harness import create_harness
from strands.vended_interventions.hitl import HumanInTheLoop

from my_plugins import MyMetricsPlugin  # your own Plugin subclass

agent = create_harness(
    context_manager="agentic",
    plugins=[MyMetricsPlugin()],       # runs next to Strands harness's built-in plugins
    interventions=[HumanInTheLoop()],  # a Strands Harness SDK feature, wired straight through
)
```
(( /tab "Python" ))

(( tab "TypeScript" ))
```typescript
import { createHarness } from '@strands-agents/harness'
import { HumanInTheLoop } from '@strands-agents/sdk/vended-interventions/hitl'

import { MyMetricsPlugin } from './my-plugins' // your own Plugin implementation

const agent = await createHarness({
  contextManager: 'agentic',
  plugins: [new MyMetricsPlugin()], // runs next to Strands harness's built-in plugins
  interventions: [new HumanInTheLoop()], // a Strands Harness SDK feature, wired through
})
```
(( /tab "TypeScript" ))

## Modify the agent after construction

You can also change the agent once you have it. The system prompt, tools, and every other field are yours to adjust:

(( tab "Python" ))
```python
from strands_harness import create_harness

agent = create_harness()
agent.system_prompt += "\n\nAlways cite file paths as file:line."
```
(( /tab "Python" ))

(( tab "TypeScript" ))
```typescript
import { createHarness } from '@strands-agents/harness'

const agent = await createHarness()
agent.systemPrompt += '\n\nAlways cite file paths as file:line.'
```
(( /tab "TypeScript" ))

## The building blocks are exported too

Strands harness exports the pieces its factory composes, so you can reuse them without building the whole agent:

-   `HARNESS_CONTRACT`: the model-neutral behavioral contract, as a string.
-   `build_system_prompt` / `buildSystemPrompt`: concatenates the contract, your instructions, and any request-scoped context.
-   `resolve_memory` (Python only): Strands harness’s memory manager over a store.
-   `resolve_interventions` (Python only): the intervention sugar coerced into Strands Harness SDK handlers.

If you build an agent from Strands Harness SDK building blocks directly, you can still borrow Strands harness’s tuned prompt:

(( tab "Python" ))
```python
from strands import Agent
from strands_harness import HARNESS_CONTRACT, build_system_prompt

agent = Agent(system_prompt=build_system_prompt("You are a migration assistant."))
```
(( /tab "Python" ))

(( tab "TypeScript" ))
```typescript
import { Agent } from '@strands-agents/sdk'
import { buildSystemPrompt } from '@strands-agents/harness'

const agent = new Agent({ systemPrompt: buildSystemPrompt('You are a migration assistant.') })
```
(( /tab "TypeScript" ))

## When to reach for which

Use Strands harness when you want a capable agent fast and its defaults fit. Reach for the Strands Harness SDK directly when you are building something whose shape Strands harness does not assume: a bespoke tool set with no shell, a custom agent loop, or an [orchestration of many agents](/docs/user-guide/sdk/multi-agent/multi-agent-patterns/index.md). The two are not a fork in the road. Start on Strands harness, and because it returns a plain `Agent`, you can drop to the raw Strands Harness SDK building blocks for the parts that need it without rewriting what already works.

When a behavior looks wrong, it helps to confirm whether it originates in Strands harness or the underlying Strands Harness SDK, since Strands harness is mostly composition over Strands Harness SDK behavior.

## Next steps

-   [SDK overview](/docs/user-guide/sdk/index.md): the building blocks Strands harness composes.
-   [Configuration reference](/docs/user-guide/harness/reference/configuration/index.md): the factory options, including the passthrough to `Agent`.
-   [What the default harness does](/docs/user-guide/harness/index.md#what-the-default-harness-does): the defaults it ships with and where to tune each one.

## Implementation

### Python

- [harness-sdk/harness-py/src/strands_harness/agent.py](https://github.com/strands-agents/harness-sdk/blob/main/harness-py/src/strands_harness/agent.py)
- [harness-sdk/harness-py/src/strands_harness/__init__.py](https://github.com/strands-agents/harness-sdk/blob/main/harness-py/src/strands_harness/__init__.py)

### TypeScript

- [harness-sdk/harness-ts/src/agent.ts](https://github.com/strands-agents/harness-sdk/blob/main/harness-ts/src/agent.ts)
- [harness-sdk/harness-ts/src/index.ts](https://github.com/strands-agents/harness-sdk/blob/main/harness-ts/src/index.ts)