> ## Documentation Index > Fetch the complete documentation index at: https://usefused.com/docs/llms.txt > Use this file to discover all available pages before exploring further. # Use a Harnest Extension > Discover a Harnest Extension, consume its compiler-owned namespace, and keep access invocation-scoped. A project-local Harnest Extension is discovered from `extensions//extension.yaml`. You do not add it to a central registry or manually attach it to the root agent. From the agent folder, install a verified public PyPI wheel by its short slug or full distribution name: ```bash theme={null} harnest extensions install warehouse # Equivalent: harnest extensions install harnest-extension-warehouse harnest env sync . ``` To review and test an unpublished extension, install its local source tree instead: ```bash theme={null} harnest extensions install ../harnest-extension-warehouse harnest env sync . ``` For a PyPI install, Harnest accepts a universal `py3-none-any` wheel and validates the manifest, wheel metadata, digest, entry point, and package layout without importing extension code. Platform- or ABI-specific wheels are not installed. For local source, it validates the manifest and layout plus any available project identity and version metadata. Both paths materialize the complete package into `extensions/` through a staged replacement and preserve its internal paths. Compilation projects only directories listed in `contributes`; it does not scatter their files into the agent root. Harnest refuses an existing extension unless you pass `--force`. Review same-process extension code and its declared capabilities before installation; installing changes dependency inputs, so refresh `harnest-runtime.lock` with `harnest env sync .`. Published wheels may declare dependencies on Harnest or its framework packages. During installation, Harnest omits those compiler-owned requirements from the materialized project and supplies its pinned runtime versions instead. Extension-owned dependencies remain in the root environment solve. Locally authored extensions must not redeclare compiler-owned packages. After compilation, Harnest exposes the extension module as `harnest.extensions.`. Import the singleton from a discovered Agent Tool and call its bounded API: ```python tools/query_warehouse.py theme={null} from harnest.extensions.warehouse import extension as warehouse from harnest.agent import tool @tool async def query_warehouse(statement: str) -> list[dict]: """Query the configured warehouse through its Harnest Extension.""" return await warehouse.query(statement) ``` The call runs inside a managed invocation, so `warehouse.context` resolves the typed `WarehouseContext` created for that request. Retaining it after the invocation ends fails closed. ## Resolve the typed context explicitly Trusted application code can ask Harnest for the same invocation-scoped view: ```python lib/warehouse_service.py theme={null} from harnest import context from harnest.extensions.warehouse import WarehouseContext async def run_query(statement: str) -> list[dict]: """Resolve the warehouse capability without exposing its SDK client.""" warehouse = context.extensions("warehouse", WarehouseContext) return await warehouse.query(statement) ``` Keep application clients and connection pools private on the singleton. Expose domain operations through the typed context instead of returning the raw SDK object. ## Find published extensions Search public PyPI for compatible `harnest-extension-*` packages: ```bash theme={null} harnest extensions search warehouse ``` Search verifies package naming, one `harnest.extensions` entry point, the bundled `extension.yaml` and `extension.py`, release identity, and the published wheel digest without importing package code. `official` means Fused controls and explicitly recognizes that package name; `community` means only that the wheel satisfies the structural contract. Neither label is a security review. Search does not install an extension, and `pip install` alone does not register one with an agent project. Use `harnest extensions install ` to verify and materialize a published wheel, then run `harnest env sync` before compiling. Define the local manifest, singleton, typed context, and dependencies. Add only the hooks and factories declared by the manifest.