/flags-sdk

API Reference

Table of Contents

• flags (core)
• flags/react
• flags/next
• flags/sveltekit

flags (core)

verifyAccess

Verify access to the flags discovery endpoint. Returns Promise<boolean>.

import { verifyAccess } from 'flags';

const access = await verifyAccess(request.headers.get('Authorization'));
if (!access) return NextResponse.json(null, { status: 401 });

mergeProviderData

Merge provider data from multiple sources for the Flags Explorer.

import { mergeProviderData } from 'flags';

return mergeProviderData([
  getProviderData(flags),
  getStatsigProviderData({ /* ... */ }),
]);

reportValue

Report flag value to Vercel for Runtime Logs and Web Analytics.

import { reportValue } from 'flags';
reportValue('summer-sale', true);

Encryption Functions

All default to process.env.FLAGS_SECRET.

Optional secret and expirationTime (default '1y') params on encrypt functions.

verifyAccessProof

Verify an access proof token. Returns Promise<boolean>.

safeJsonStringify

XSS-safe JSON.stringify. Escapes < and other dangerous chars.

import { safeJsonStringify } from 'flags';
safeJsonStringify({ markup: '<html></html>' });
// '{"markup":"\\u003chtml>\\u003c/html>"}'

flags/react

FlagValues

Renders a <script data-flag-values> tag for the Flags Explorer.

import { FlagValues } from 'flags/react';
<FlagValues values={{ myFlag: true }} />

For confidential flags, encrypt first:

import { encryptFlagValues } from 'flags';
import { FlagValues } from 'flags/react';

async function ConfidentialFlags({ values }) {
  const encrypted = await encryptFlagValues(values);
  return <FlagValues values={encrypted} />;
}

FlagDefinitions

Renders a <script data-flag-definitions> tag with flag metadata.

import { FlagDefinitions } from 'flags/react';

<FlagDefinitions definitions={{
  myFlag: {
    options: [{ value: false }, { value: true }],
    origin: 'https://example.com/flag/myFlag',
    description: 'Example flag',
  },
}} />

flags/next

flag

Declare a feature flag for Next.js.

createFlagsDiscoveryEndpoint

Creates an App Router route handler for /.well-known/vercel/flags. Auto-verifies access and adds version header.

getProviderData

Turn flag() declarations into Flags Explorer-compatible definitions.

precompute

Evaluate multiple flags, return encoded string.

evaluate

Evaluate multiple flags in a single call. Prefer this over Promise.all([flagA(), flagB()]) — it pre-reads headers, cookies, and overrides once for the whole batch and lets adapters resolve a group through one bulkDecide call, reducing parallel promises and microtask overhead.

import { evaluate } from 'flags/next';

const [a, b] = await evaluate([flagA, flagB]); // positional
const { a, b } = await evaluate({ a: flagA, b: flagB }); // keyed

serialize

Turn evaluated flags into serialized representation.

getPrecomputed

Retrieve flag values from precomputation code.

deserialize

Retrieve all flag values as a record from code.

generatePermutations

Generate all possible precomputation codes for flag options.

dedupe

Deduplicate function calls per request.

Not available in Pages Router.

flags/sveltekit

flag

Declare a feature flag for SvelteKit.

createHandle

Server hook that establishes context for flags and handles /.well-known/vercel/flags.

Must come first when composed with sequence.

getProviderData

Turn flag declarations into Flags Explorer-compatible definitions.

precompute

Evaluate multiple flags, return encoded string.

generatePermutations

Generate all precomputation codes for prerendering.