LaunchDarkly

Lightweight, dependency-free, in-memory LaunchDarkly API fake for testing code that uses the real launchdarkly-api / @launchdarkly/node-server-sdk clients (and the language-agnostic LaunchDarkly REST API). Includes a minimal SDK eval endpoint.

Default port: 4816

Quick start

import { LaunchdarklyServer } from "./services/launchdarkly/src/server.js";

const server = new LaunchdarklyServer(4816);
await server.start();
// ... run your app/tests ...
await server.stop();

Drive the REST API with the Authorization: <api-key> header (no scheme prefix, matching LaunchDarkly):

const res = await fetch("http://127.0.0.1:4816/api/v2/flags/default", {
  headers: { Authorization: "api-parlel" },
});
const { items } = await res.json();

State is in-memory and ephemeral.

Implemented operations

REST routes require an Authorization header (any non-empty value). The SDK eval endpoint is unauthenticated, matching the streaming/polling SDK contract.

Projects & flags

GET /api/v2/projects — list projects ({ items, _links }).
GET /api/v2/flags/:projectKey — list feature flags ({ items, _links }).
POST /api/v2/flags/:projectKey — create a flag (201). Flag shape { key, name, kind, variations: [...], environments: {...} }. 409 on duplicate key.
GET /api/v2/flags/:projectKey/:featureFlagKey — retrieve a flag.
PATCH /api/v2/flags/:projectKey/:featureFlagKey — update a flag. Supports RFC6902 patch (replace /name, /description) and semantic instructions (turnFlagOn / turnFlagOff per environmentKey).
DELETE /api/v2/flags/:projectKey/:featureFlagKey — delete a flag (204).

SDK evaluation

GET /sdk/eval/:envKey/users/:base64user — evaluate all flags for a base64-encoded user. Returns { <flagKey>: { value, variation, version, trackEvents } }.

Service & inspection operations (parlel extensions)

GET / — service metadata.
GET /health — health check ({ status: "ok" }).
POST /__parlel/reset — reset all state (re-seeds the default project + flag).
GET /__parlel/flags — list flags grouped by project.
OPTIONS * — CORS preflight (204).

Access via MCP / preview URL

In a parlel pool, the service is reachable at its preview URL (host/port shown by the pool); point the API base URL / SDK base URI at it. Through the parlel MCP server, the projects/flags/eval routes are exposed as a tool surface so an AI agent can create and toggle flags and read evaluations.

Surface coverage

This emulator faithfully replicates the API surface most application code and agents exercise. Anything below the supported lines is either an intentional design choice for a fast, zero-cost local emulator (✓ By design) or a candidate for a future release (⟳ Roadmap) — never a silent inaccuracy.

Legend: ✅ fully supported · ◐ accepted (stored, not strictly enforced) · ✓ by design · ⟳ on the roadmap.

Feature	Status
Projects list	✅ Supported
Flags CRUD (list/create/get/patch/delete)	✅ Supported
Semantic patch (`turnFlagOn`/`turnFlagOff`) + RFC6902 subset	✅ Supported
SDK `/sdk/eval` flag evaluation	✅ Supported
Targeting rules / prerequisites / segments evaluation	⟳ Roadmap — env `on` toggle drives value
Streaming (SSE) flag updates	⟳ Roadmap — polling eval only
Experiments / Metrics / Audit log	⟳ Roadmap
API-key validity / role enforcement	✓ By design — Intentional for a local, zero-cost test emulator

Manifest

See services/launchdarkly/manifest.json:

name: launchdarkly, image: parlel/launchdarkly:1.0
port: 4816, protocol: http, healthcheck: /health, startup ≈ 100ms
env: LAUNCHDARKLY_API_KEY, LAUNCHDARKLY_SDK_KEY, LAUNCHDARKLY_HOST

Configuration — `test.env`

Copy these into your test.env (used by the bridge sidecar flow). Tokens are Parlel's seeded test credentials — any non-empty value is accepted by the emulator, so you rarely need to change them. Swap in real credentials only when pointing at the live service in prod.env.

LAUNCHDARKLY_API_KEY=api-parlel
LAUNCHDARKLY_SDK_KEY=sdk-parlel
LAUNCHDARKLY_HOST=http://parlel-bridge:4816
LAUNCHDARKLY_BASE_URL=http://parlel-bridge:4816