---
title: Shared packages
description: The runtime-neutral contracts that let the frontend and backend agree without depending on each other's internals.
sidebar:
  order: 4
---

The shared packages define the public thread-event and canonical-engine schemas.
The backend event log and each live provider stay authoritative for persisted and
native state; the packages are the contract layer on top.

## The packages

| Package | What it owns |
| --- | --- |
| `@useagent/agent-client` | The browser-side client: reconnect to SSE, reduce events into thread state, and expose selectors to the UI, plus typed API helpers. |
| `@useagent/agent-harness` | Provider-neutral canonical event, capability, session, and `ProviderDriver` lifecycle types (the engine control contract), plus the protocol-boundary OpenCode frame parsers exported under `./opencode` and `./t3-tool`, the one place vendor names are permitted. |
| `@useagent/artifact-formats` | Native DOCX, XLSX, PPTX, and PDF renderers, plus bounded Office text extraction. |
| `@useagent/artifact-workspace` | Provider-neutral workpiece kinds, actions, and the native export capability matrix shared by backend and frontend. |
| `@useagent/conformance` | Deterministic, framework-free contract tests for the client and harness packages. It does not run live providers. |

Both apps consume the packages as plain `file:../packages/...` dependencies in
`backend/package.json` and `frontend/package.json`. There is no workspace or
turbo root: each app installs with bun on its own, and the package exports point
straight at TypeScript source, so both sides type-check the contract at build
time.

## Why the contract lives in packages

<CardGroup cols={2}>
  <Card title="One schema, two consumers" icon="git-compare">
    The frontend and backend both depend on the same event and capability types,
    so a change to the contract is a change both sides see at build time.
  </Card>
  <Card title="Engine neutrality" icon="plug">
    `agent-harness` is where "any engine renders through one UI" is actually
    enforced. Adapters implement its `ProviderDriver` lifecycle.
  </Card>
</CardGroup>

## The conformance suite

`packages/conformance` holds deterministic tests for the public contracts:
reducer behavior, replay, and capability gating. It runs without a live provider,
so it is fast and stable in CI. Live provider parity is a separate,
environment-gated proof covered under [Verification](/operations/verification).

:::note
Sandbox adapters share a provider-neutral interface but keep provider-specific
tests for native capabilities. The neutral contract is verified by conformance;
the provider-specific behavior is verified per provider.
:::