OpenFeature

OpenFeature is a CNCF specification for a vendor-neutral feature flag API. feat ships a Provider for the OpenFeature web SDK and exports a Provider class from @feathq/js-sdk for the server SDK. If you write your code against OpenFeature, you can swap feat for any other OpenFeature-compatible vendor without changing application code.

Why OpenFeature

Two practical reasons:

• Vendor portability. Your application code calls OpenFeature.getClient().getBooleanValue(...). Swapping the Provider is a one-line change. The rest of your app does not care.
• Standard surface. Engineers familiar with OpenFeature do not have to learn a new client API.

Spec reference: openfeature.dev/specification.

Server-side: @feathq/js-sdk

The FeatProvider class is bundled with @feathq/js-sdk. No extra install required.

import { OpenFeature } from "@openfeature/server-sdk";
import { FeatClient, FeatProvider } from "@feathq/js-sdk";

const featClient = new FeatClient({
  apiKey: process.env.FEAT_SERVER_KEY!,
  dataPlaneUrl: "https://data.feat.so",
});

await OpenFeature.setProviderAndWait(new FeatProvider(featClient));

const client = OpenFeature.getClient();

const enabled = await client.getBooleanValue("checkout-v2", false, {
  targetingKey: "user-123",
  plan: "pro",
});

See Node.js SDK for everything else the client supports.

Web: @feathq/openfeature-web

The web Provider is a separate package because the OpenFeature web spec separates “evaluation context lives on the provider” from “context is passed per call.” The web Provider routes context through OpenFeature.setContext.

npm install @feathq/web-sdk @feathq/openfeature-web @openfeature/web-sdk

import { OpenFeature } from "@openfeature/web-sdk";
import { FeatWebClient } from "@feathq/web-sdk";
import { FeatWebProvider } from "@feathq/openfeature-web";

const featClient = new FeatWebClient({
  apiKey: "feat_cs_…",
  dataPlaneUrl: "https://data.feat.so",
});

await OpenFeature.setProviderAndWait(new FeatWebProvider(featClient));
await OpenFeature.setContext({ targetingKey: "user-123" });

const client = OpenFeature.getClient();
const enabled = client.getBooleanValue("checkout-v2", false);    // sync

@feathq/web-sdk and @openfeature/web-sdk are peer dependencies. Use OpenFeature.setContext (not the per-call context) for the web Provider; it routes through to client.setContext on the underlying feat client.

Type coercion and reasons

The Provider returns the supplied default value with TYPE_MISMATCH if you ask a boolean flag for a string, etc. The reason field carries through TARGETING_MATCH, SPLIT, FALLTHROUGH, DEFAULT, DISABLED, and ERROR; see Evaluation order for what each means.

OpenFeature’s reason vocabulary is close but not identical to feat’s. The Provider maps:

feat reason	OpenFeature reason
TARGETING_MATCH	TARGETING_MATCH
SPLIT	SPLIT
FALLTHROUGH	DEFAULT
DEFAULT	DEFAULT
DISABLED	DISABLED
ERROR	ERROR

Other languages

The Go, Python, and Ruby SDKs expose the feat-native API documented in their per-language pages. A Provider for each is planned; use the native API in the meantime.

Related

• SDKs overview
• Contexts
• OpenFeature specification