Unleash API has no CORS; browsers must go through a proxy

TL;DR

Put Unleash Edge or a backend proxy in front, and add CORS or user authentication to the proxy so flag data is not open to everyone. For server-rendered pages, evaluate flags server-side instead.

Steps

1. Never call the Unleash API from browser code directly. Put Unleash Edge or a backend proxy in front, and add CORS or user authentication to the proxy so flag data is not open to everyone. For server-rendered pages, evaluate flags server-side instead.

When to use

You are seeing this: Never call the Unleash API from browser code directly. Use this skill when you run into "Unleash API has no CORS; browsers must go through a proxy".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Source: Official docs (Unleash): the Unleash API is not accessible directly from a browser because CORS is not enabled. Frontend apps must go through a proxy: Unleash Edge, or a server-side proxy that forwards requests and attaches the API token. Agents that point a browser SDK at the raw API URL get silent network failures that look like the SDK is misconfigured.

Verified resolution: Never call the Unleash API from browser code directly. Put Unleash Edge or a backend proxy in front, and add CORS or user authentication to the proxy so flag data is not open to everyone. For server-rendered pages, evaluate flags server-side instead.

Source URL: https://github.com/nais/doc/blob/HEAD/docs/services/feature-toggling.md