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api-cors-preflight

workers/api-cors-preflight/README.md

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api-cors-preflight

Cloudflare Worker bound to api.worldmonitor.app/*. Owns CORS at the edge: short-circuits OPTIONS preflights (without forwarding to Vercel) and stamps matching CORS headers onto every non-OPTIONS response on the way back to the browser.

Why this exists separately from api/_cors.js

Three CORS surfaces sit in front of every browser request to api.worldmonitor.app:

  1. Cloudflare Worker (this directory) — sees the request first; the preflight response the browser actually checks comes from here.
  2. Vercel edge function api/_cors.js#getCorsHeaders — runs per-request for non-OPTIONS, and supplies CORS headers that the Worker then overrides with its own copy on the way out.
  3. vercel.json — no longer pins static /api/* CORS headers (removed in PR #3923 because the wildcard ACAO: * was incompatible with credentialed requests).

When the app switched to credentials: 'include' (HttpOnly cookies, PR #3913), the Worker's preflight response was missing Access-Control-Allow-Credentials: true. Repo-side fixes (PR #3923) could not close the outage because the preflight never reaches Vercel. Moving the Worker source in-repo means future CORS changes:

  • Show up in git log / git blame / code review / greptile.
  • Get unit-tested in this directory (index.test.mjs).
  • Get smoke-tested against live prod (tests/cors-preflight-live.test.mjs).
  • Deploy from CI on merge (.github/workflows/deploy-worker.yml).

Deploy

From CI (preferred)

Merge to main → .github/workflows/deploy-worker.yml runs wrangler deploy automatically when workers/api-cors-preflight/** changes. Requires repo secrets:

  • CLOUDFLARE_API_TOKEN — token with Workers Scripts:Edit + Workers Routes:Edit for the worldmonitor.app zone.
  • CLOUDFLARE_ACCOUNT_ID — the CF account that owns the Worker.

From your laptop (fallback)

sh
cd workers/api-cors-preflight
npm install
export CLOUDFLARE_API_TOKEN=...
export CLOUDFLARE_ACCOUNT_ID=...
npm run deploy

Tests

sh
# Unit tests against the Worker module directly (fast, deterministic).
cd workers/api-cors-preflight && npm test

# Live smoke test against prod. Gated by env var so it doesn't run in PR gates
# (false positives during deploys).
LIVE_SMOKE=1 tsx --test tests/cors-preflight-live.test.mjs

Keep in sync

The Worker's allowlist + Allow-Headers list must be a superset of what api/_cors.js#getCorsHeaders returns. If the Worker rejects an origin that the function would accept, the browser sees a mismatched origin echo and CORS rejects the request. Drift between the two is the load-bearing trap this package exists to make visible. Update both files together.

~/.claude/skills/worldmonitor-architecture-gotchas/reference/cloudflare-worker-overrides-vercel-cors-for-preflight.md captures the full post-mortem of the 2026-05-27 CORS outage that motivated pulling the Worker into the repo. Read it before touching this Worker.