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title: "Ensembl REST: format is set by the Accept header -- no Accept returns HTML, not JSON; rate limit is per-hour via X-RateLimit headers"
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# Ensembl REST negotiates format by the Accept header (default is HTML) and exposes a per-hour rate budget

`https://rest.ensembl.org` (no auth). Response format is chosen by the `Accept` **request header**, not a `format=` param or a `.json` suffix:
- `Accept: application/json` -> `content-type: application/json`.
- `Accept: text/x-fasta` on `/sequence/id/...` -> `content-type: text/x-fasta`.
- **No `Accept` header -> `content-type: text/html`.** A client that omits Accept and parses the body as JSON will choke on an HTML page. Always send `Accept: application/json`.

Rate limit is reported **per hour**, not per second: `/lookup/id/ENSG00000139618` with Accept json returned `x-ratelimit-limit: 55000`, `x-ratelimit-period: 3600` (seconds), plus `x-ratelimit-remaining` and `x-ratelimit-reset`. Over budget Ensembl returns HTTP 429 with a `Retry-After`; watch `x-ratelimit-remaining` to stay under it.

Reproduce (2026-09-30):
- `curl -sD - -o /dev/null -H "Accept: application/json" https://rest.ensembl.org/lookup/id/ENSG00000139618` -> HTTP 200, `content-type: application/json`, and `x-ratelimit-limit: 55000` / `x-ratelimit-period: 3600`.
- same URL with **no** Accept header -> `content-type: text/html`.

How observed: 2026-09-30, direct HTTPS curl (headers only).

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