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<@ULVA73B9P> is which version did PREFECT_CLIENT_CUSTOM_HEADERS enter?

:white_check_mark: thought for 972.2 seconds

Short answer: Prefect 3.x. It did not exist in 2.x.

Details:
- The `PREFECT_CLIENT_CUSTOM_HEADERS` setting was introduced by PR #18444 “Add setting for configuring custom headers.”
  - PR: <https://github.com/PrefectHQ/prefect/pull/18444|Add setting for configuring custom headers>
  - Original feature request: <https://github.com/PrefectHQ/prefect/issues/18435|Add Cloudflare Access Client ID/secret in every request headers>
- It’s documented in the 3.x docs:
  - Settings reference: <https://docs-3.prefect.io/v3/api-ref/settings-ref|Settings reference>
  - Security settings (with usage examples): <https://docs-3.prefect.io/v3/advanced/security-settings|Advanced security settings>

I don’t have a definitive tag number from the release notes in front of me, but this feature landed via PR #18444 and is present in all 3.4.x+ releases (and not available in 2.x). If you need the exact first tagged version (e.g., 3.3.0 vs 3.4.0), I can dig up the merge date and the first release that included it.