400 ad accounts 3 subrequests = silent data loss in Cloudflare Workers (and how I fixed it)

Adding a single KV cache write broke my Worker — and it didn't throw a single error to the caller.

I run a Korean D2C ad-ops pipeline where one Worker fans out performance checks across ~400 ad accounts per request. The math looked fine: 400 accounts × 2 API calls = 800 subrequests, safely under Cloudflare's hard 1000-per-invocation ceiling. Then I added KV writes to cache each result. 400 × 3 = 1200. The Worker silently stopped processing the last ~200 accounts. No 5xx. No exception propagated. Just missing data. It took two days on wrangler tail to surface this line:

workers/runtime/fetch: subrequest count exceeded limit of 1000

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