plugin-feature-flags

@frontmcp/plugin-feature-flags

Gate MCP capabilities behind feature flags — hide tools, branch behaviour, and roll out changes per user without redeploying.

Install

npm install @frontmcp/plugin-feature-flags

Usage

import { FeatureFlagPlugin } from '@frontmcp/plugin-feature-flags';
import { FrontMcp } from '@frontmcp/sdk';

@FrontMcp({
  info: { name: 'my-server', version: '1.0.0' },
  apps: [MyApp],
  plugins: [
    FeatureFlagPlugin.configure({
      provider: 'static',
      flags: { 'new-search': true, 'beta-export': false },
    }),
  ],
})
class Server {}

Then read flags from any tool through this.featureFlags:

@Tool({ name: 'search', inputSchema: { q: z.string() } })
export default class SearchTool extends ToolContext {
  async execute({ q }: { q: string }) {
    if (await this.featureFlags.isEnabled('new-search')) {
      return newSearch(q);
    }
    return legacySearch(q);
  }
}

Providers

Provider	provider	Notes
Static	'static'	Flags from config. Good for local dev and tests.
Split.io	'splitio'	Requires an SDK key.
LaunchDarkly	'launchdarkly'	Requires an SDK key.
Unleash	'unleash'	Requires a URL + API token.
Custom	'custom'	Supply your own FeatureFlagAdapter.

FeatureFlagPlugin.configure({
  provider: 'launchdarkly',
  sdkKey: process.env.LD_SDK_KEY,
  // Which identity the flag is evaluated for. Defaults to the session's user.
  userIdResolver: (ctx) => ctx.authInfo?.clientId,
  attributesResolver: (ctx) => ({ plan: ctx.authInfo?.extra?.plan }),
});

Custom adapter

import { FeatureFlagPlugin, type FeatureFlagAdapter } from '@frontmcp/plugin-feature-flags';

const adapter: FeatureFlagAdapter = {
  async isEnabled(key, ctx) {
    return myBackend.check(key, ctx.userId);
  },
};

FeatureFlagPlugin.configure({ provider: 'custom', adapter });

API

this.featureFlags (an injected FeatureFlagAccessor):

Method	Returns	Purpose
isEnabled(key, default?)	Promise<boolean>	Evaluate one boolean flag
getVariant(key)	Promise<FeatureFlagVariant>	Multivariate flag value
evaluateFlags(keys)	Promise<Map<string, boolean>>	Batch evaluation in one round trip
resolveRef(ref)	Promise<boolean>	Resolve a flag reference ({ flag, negate })

Outside a tool, use getFeatureFlags() / tryGetFeatureFlags().

Failure behaviour

If the provider is unreachable, isEnabled returns the defaultValue you pass (or false). Always pass an explicit default for a flag that gates something important, so an outage degrades the way you intend rather than silently disabling a feature.

Full guide: Feature Flags

License

Apache-2.0