# `@vercel/flags-core`

The core evaluation engine for [Vercel Flags](https://vercel.com/docs/flags/vercel-flags), the feature flag platform built into Vercel. This package provides direct access to the flag evaluation client, data fetching, and an [OpenFeature](https://openfeature.dev/) provider.

For Next.js and SvelteKit applications, use the [Flags SDK](https://flags-sdk.dev/) with [`@flags-sdk/vercel`](https://flags-sdk.dev/providers/vercel) provider instead. Use `@vercel/flags-core` when you need lower-level control, are working with an unsupported framework, or want to use the OpenFeature standard.

## Installation

```bash
npm i @vercel/flags-core
```

## Usage

```ts
import { createClient } from '@vercel/flags-core';

const client = createClient(process.env.FLAGS!);

await client.initialize();

const result = await client.evaluate<boolean>('show-new-feature', false, {
  user: { id: 'user-123' },
});
```

To associate evaluation metrics with an environment, pass the
`metricEnvironment` option:

```ts
const client = createClient(process.env.FLAGS!, {
  metricEnvironment: 'preview',
});
```

This option is sent only to the metrics ingestion endpoint. It does not select
the environment used for flag evaluation.

## OpenFeature

An OpenFeature-compatible provider is available at `@vercel/flags-core/openfeature`:

```ts
import { OpenFeature } from '@openfeature/server-sdk';
import { VercelProvider } from '@vercel/flags-core/openfeature';

await OpenFeature.setProviderAndWait(new VercelProvider());
const client = OpenFeature.getClient();
```

## Documentation

- [Core Library Docs](https://vercel.com/docs/flags/vercel-flags/sdks/core)
- [OpenFeature Provider Docs](https://vercel.com/docs/flags/vercel-flags/sdks/openfeature)
- [Vercel Flags](https://vercel.com/docs/flags/vercel-flags)