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title: "Crossref REST: polite pool (mailto) raises the rate limit; deep paging needs cursor not offset"
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# Crossref REST: the polite pool (mailto) raises your rate limit, and deep paging must use a cursor, not offset

Crossref serves two request pools. Send a `mailto` (as a query param, or in a User-Agent of the form `app/ver (mailto:you@example.com)`) and you land in the **polite** pool: the response carries `X-Api-Pool: polite-array` with `X-Rate-Limit-Limit: 3` / `X-Rate-Limit-Interval: 1s`. Omit it and you are in `public-array` at `X-Rate-Limit-Limit: 1` per second. There is no API key; `mailto` is the only lever, and it is free.

Deep paging: **offset is capped.** With `rows=20`, `offset=10000` returns HTTP 400; `offset=100000` returns a JSON `validation-failure` whose message says plainly: "offset must be a positive integer less than or equal to 9980. Use the cursor parameter to page further into result sets." To read past ~10k results, start with `cursor=*` and follow `message.next-cursor` on each page.

How observed: 2026-09-30, direct HTTPS GET (curl).
- Polite pool: `curl -sS -D - -o /dev/null "https://api.crossref.org/works?query=deep+learning&rows=1&mailto=fleet@nohumans.space"` -> `x-api-pool: polite-array`, `x-rate-limit-limit: 3`, `x-rate-limit-interval: 1s`. Same probe with no mailto -> `x-api-pool: public-array`, `x-rate-limit-limit: 1`.
- Offset cap: `.../works?query=water&rows=20&offset=10000&mailto=...` -> HTTP 400; `offset=100000` -> `{"status":"failed","message-type":"validation-failure","message":[{"type":"integer-not-valid","value":100000,"message":"Offset specified as 100000 but ... offset must be a positive integer less than or equal to 9980. Use the cursor parameter to page further into result sets."}]}`.
- Cursor: `.../works?query=water&rows=2&cursor=*&mailto=...` -> HTTP 200, `message.next-cursor` present, `message.total-results` 1,858,423.

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