Evaluation API Overview

The Evaluation API is the service your SDKs communicate with to fetch flag configurations, evaluate flags, stream real-time updates, and report analytics events. Use this reference if you are building a custom SDK for a language we don’t yet support.

Base URL

https://eval.featureflip.io/v1/sdk

All endpoint paths below are relative to this base URL.

Authentication

Every request must include an SDK key in the Authorization header:

Authorization: Bearer <your-sdk-key>

SDK keys are scoped to a single environment within a project. You can create and manage them in the Featureflip dashboard under Project Settings > Environments > SDK Keys.

An invalid or missing key returns 401 Unauthorized:

{
  "error": "Invalid SDK key"
}

Request and response format

Content type — Send request bodies as JSON with Content-Type: application/json.
Successful responses — Return 200 OK or 202 Accepted with a JSON body (or an SSE stream).
Error responses — Return a JSON body with an error field.

Status codes

Code	Meaning
`200`	Request succeeded.
`202`	Accepted for async processing (events).
`401`	SDK key is missing, expired, or invalid.
`404`	Flag not found.
`429`	Rate limit exceeded.

Rate limiting

All endpoints are rate-limited to 1,000 requests per minute per SDK key. Exceeding this limit returns 429 Too Many Requests.

Endpoints at a glance

Method	Path	Description
`GET`	`/flags`	Fetch all flag configurations
`GET`	`/flags/{key}`	Fetch a single flag configuration
`POST`	`/evaluate`	Evaluate a flag for a user context
`POST`	`/events`	Record analytics events
`GET`	`/stream`	Subscribe to real-time flag updates (SSE)

See the individual endpoint pages for request/response details and data type definitions:

Flags — fetch flag configurations and data types
Evaluate — evaluate a flag for a user context
Events — record analytics events
Streaming — subscribe to real-time updates via SSE