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## 1.21.1

**`(fix):`** Fix `additional_headers` in a request's options being unable to override a global header that was
set at client construction. Every default header was protected from `additional_headers`, so a
caller who configured a global header on the client (e.g. `X-Greenhouse-Id`) could not vary it per
request — the per-request value was silently dropped. The API's non-literal global headers are
now overridable per request, while SDK metadata (`User-Agent`, `X-Fern-*`), auth headers, and
literal headers stay protected as before.