This is the lookup layer for the Strands harness factory (`create_harness` in Python, `createHarness` in TypeScript). Every argument is optional. The two libraries expose the same surface, each spelled for its language (`snake_case` in Python, `camelCase` in TypeScript). For how each option is used in practice, follow the links into the configure and tools guides.

## Options

| Python | TypeScript | Default | Purpose |
| --- | --- | --- | --- |
| `model` | `model` | `bedrock/global.anthropic.claude-opus-4-8` | A `provider/name` string, a bare Amazon Bedrock id, or a `Model` instance. See [choose a model](/docs/user-guide/harness/configure/model/index.md). |
| `effort` | `effort` | `"auto"` | Reasoning effort: `"auto"`, `"low"`, `"medium"`, `"high"`, or off. Ignored for a `Model` instance. |
| `instructions` | `instructions` | none | A domain block appended after the harness contract. Ignored when a full system prompt is passed. |
| `tools` | `tools` | none | Consumer tools, added alongside the built-ins. See [add tools](/docs/user-guide/harness/configure/tools-and-instructions/index.md). |
| `plugins` | `plugins` | none | Consumer Strands Harness SDK plugins, added alongside the built-in plugins. |
| `mcp_servers` | `mcpServers` | none | MCP servers to connect: a JSON file path or a mapping. See [MCP servers](/docs/user-guide/harness/configure/mcp-servers/index.md). |
| `builtin_tools` | `builtinTools` | `shell`, `read`, `write`, `edit`, `web_fetch`, `web_search`, `programmatic_tool_caller`, `subagent` | Built-in tools to enable, or `[]` for none. |
| `builtin_tools={"web_fetch": {"model": ...}}` | `builtinTools: { web_fetch: { model } }` | provider default | The summarizer model `web_fetch` runs on. See [web access](/docs/user-guide/harness/tools/web-access/index.md). |
| `caching` | `caching` | `"auto"` (on) | Prompt caching where the provider supports it; off disables what Strands harness configures. |
| `context_manager` | `contextManager` | `"auto"` | `"auto"`, `"agentic"`, or off. Enables context management and offloading. |
| `session={"id": ...}` | `session: { id }` | none | Persist and resume this conversation by id. |
| `session={"dir": ...}` | `session: { dir }` | `./.agent/sessions` | Where session state and offloaded artifacts live. |
| `skills` | `skills` | `./.agent/skills` | Directory (or list) scanned for Agent Skills; off to disable. |
| `builtin_plugins` | `builtinPlugins` | `["todos", "environment"]` | Built-in feature plugins to enable, or `[]` for none. |
| `memory` | `memory` | on | File-based long-term memory; off to disable. |
| `memory={"dir": ...}` | `memory: { dir }` | `./.agent/memory` | Where the default memory store’s files live. |
| `memory={"stores": [...]}` | `memory: { stores }` | none | Swap the memory backend while keeping Strands harness’s policy. |
| `interventions` | `interventions` | none | Gate tool calls behind approval or a policy. See [interventions](/docs/user-guide/harness/configure/interventions/index.md). |
| `background_tasks` | `backgroundTasks` | `{ agentic: ['*'] }` | Background-tasks policy. See [background tasks](/docs/user-guide/harness/configure/background-tasks/index.md). |

## Value notes

-   **`effort`** accepts `"auto"` (provider recommended), `"low"`/`"medium"`/`"high"` (explicit; some providers accept finer levels), and `"off"`. A level the resolved provider rejects fails at construction.
-   **`caching`** accepts `"auto"`/`True` (on), `False`/`None` (off). Off has no effect where caching is automatic (OpenAI, Google, bedrock-mantle). Enabling it explicitly on a `Model` instance is ignored with a warning.
-   **`context_manager`** accepts `"auto"`, `"agentic"`, and `False`/`None`. Off also disables context offloading.
-   **`memory`** accepts `True` (on) and `False`/`None` (off). Ignored when a `memory_manager` is passed through.
-   **`builtin_tools`**, **`builtin_plugins`** each take a list of names; an unknown name fails at construction with the valid names.

## Passthrough to the Agent

Any keyword the factory does not name is forwarded to the underlying `Agent` constructor. An explicit passthrough value takes precedence over the default it corresponds to (for example, passing `system_prompt` overrides the prompt Strands harness would build from `instructions`, and passing `memory_manager` overrides `memory`). In TypeScript, `HarnessAgentOptions` extends the Strands Harness SDK’s `AgentConfig` (minus the fields Strands harness manages), so any other `AgentConfig` field, such as `retryStrategy`, passes through verbatim. See [compose with the Strands Harness SDK](/docs/user-guide/harness/composing-with-sdk/index.md).

## Exported building blocks

Beyond the factory, Strands harness exports the pieces it composes: `HARNESS_CONTRACT`, `build_system_prompt` / `buildSystemPrompt`, `resolve_memory`, and `resolve_interventions` (Python only). These are documented in [compose with the Strands Harness SDK](/docs/user-guide/harness/composing-with-sdk/index.md).

## 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/defaults.py](https://github.com/strands-agents/harness-sdk/blob/main/harness-py/src/strands_harness/defaults.py)

### TypeScript

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