docs: document send-optional-defaults playground setting

[devin-ai-integration]

Overview

Fern's API Explorer previously dropped the declared defaults of optional header, query, and path parameters, so readers had to type those values by hand on every request. A new opt-in send-optional-defaults playground setting in docs.yml makes the Explorer pre-fill and send those defaults, so readers can run realistic requests out of the box. This matters for APIs that expect an optional header (for example a mock-response selector) on every call. It defaults to false, so existing docs sites are unaffected.

Implements docs for: feat(docs): accept send-optional-defaults playground setting in docs.yml (fern-api/fern#17206) and feat(docs): honor default on optional playground params via send-optional-defaults (fern-api/fern-platform#13331).

Changes

• fern/products/docs/pages/api-references/customize-api-ref.mdx: added send-optional-defaults to the intro of Setting API Explorer options, a new Sending defaults for optional parameters section (behavior, default, where it can be set), and an entry under the playground object in the configuration options reference.
• fern/products/docs/pages/api-references/api-explorer.mdx: cross-reference from Autopopulate with examples to the new section.
• fern/products/docs/pages/changelog/2026-07-31.mdx: Docs changelog entry (api-reference, docs.yml).

Preview: https://fern-preview-devin-1785517710-send-optional-defaults.docs.buildwithfern.com/learn/docs/api-references/customize-api-reference-layout#sending-defaults-for-optional-parameters

Verified with pre-commit run (Vale: 0 errors; remaining warning/suggestion are on pre-existing lines) and fern check --warnings (0 errors).

Link to Devin session: https://app.devin.ai/sessions/3a9c2221c0e8428786e6f8d20211b09e

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🌿 Preview your docs: https://fern-preview-devin-1785517710-send-optional-defaults.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/api-references/api-explorer
• learn/docs/api-references/customize-api-reference-layout
• learn/docs/changelog/2026/7/31

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