## TL;DR

Upgrade Flipt past the v2 beta builds: preflight (OPTIONS) requests failing despite CORS being enabled was a bug in the v2 betas, fixed in the stable v2 release. If you are on a beta, upgrading is the fix.

## The problem

With `cors: enabled: true` in the Flipt YAML config, GET requests work but the OPTIONS preflight request does not supply the same CORS settings, so flipt-web (or any browser client) fails.

## Fix it

1. Check your Flipt version.
   Success check: you are on a v2 beta build, confirming the bug applies.

2. Upgrade to the stable v2 release or newer.
   Success check: the installed version is past the beta line.

3. Re-run the browser flow that failed.
   Success check: the OPTIONS preflight returns the CORS headers and the request succeeds.

## When this applies

- Flipt v2 beta with `cors: enabled: true` in config.
- GET works, OPTIONS preflight fails.

## When it does not apply

- Flipt v1; the bug is v2-beta-specific.
- CORS failures caused by your own reverse proxy stripping headers.
- Non-browser clients (no preflight involved).

## Compatibility

- Flipt v2 stable+ (fixed). Affected: v2 beta builds.

## Variant phrasings

- Flipt preflight OPTIONS fail CORS set
- flipt-web CORS preflight failure
- Flipt v2 beta CORS bug

## Why it happens

The v2 beta server did not apply the CORS middleware to OPTIONS requests, only to the main routes. The stable release applies it uniformly.

## Edge cases

- If it persists on stable v2, check for a proxy in front of Flipt stripping the headers.
- Source: https://github.com/flipt-io/flipt/issues/4620

---

## Vectle citations

- Exact version: <https://vectle.com/skills/skl_PiqUYCeqlhPcxAtrKGrEmg/versions/skv_eDvc29OszVF0kaua8ypZqw>
- Canonical HTML skill page: <https://vectle.com/skills/skl_PiqUYCeqlhPcxAtrKGrEmg>
- Current Markdown URL: <https://vectle.com/skills/skl_PiqUYCeqlhPcxAtrKGrEmg/markdown>

## Selected public attribution and original notices/references

```json
{
  "author": "issueatlas",
  "client": null,
  "notices_and_references": [
    "https://github.com/flipt-io/flipt/issues/4620"
  ]
}
```