feat: add server variable input for API Explorer via custom JS

[fern-api]

What

Adds a vanilla JS script that injects a dedicated Space name input field into the API Explorer ("Try It") panel for endpoints that use OpenAPI server variables.

Why

Addresses the server variable discoverability issue described in fern-api/fern#14493. Currently, users must double-click the base URL to edit server variables — this is not discoverable and leads to confusion.

How it works

1. A MutationObserver watches for .playground-endpoint-baseurl elements in the API Explorer panel.
2. When a URL with a known placeholder (e.g. {Your_Space_Name}, YOUR_SPACE) is detected, a labeled input field is injected into the explorer form.
3. As the user types their space name:
   • The script writes the resolved URL to Fern's Jotai atomWithStorage localStorage keys (selected-environment-url and per-API variants)
   • Dispatches synthetic StorageEvents so React re-renders the URL bar and code samples
   • A computed URL preview is shown below the input
   • The value persists across page reloads via localStorage
4. Works for both REST API (https://{space_name}.signalwire.com) and Compatibility API (https://{space_name}.signalwire.com/api/laml/2010-04-01) endpoints.

Changes

v2 (6c32ca2)

• Fixed duplicate injection — uses document.getElementById() check instead of data attribute, preventing multiple sections from appearing
• Fixed URL not updating — replaced unreliable double-click-to-edit approach with direct localStorage writes to Fern's Jotai atomWithStorage keys + synthetic StorageEvent dispatch, which updates both the URL bar and code samples
• Debounced MutationObserver to avoid rapid re-processing
• Idempotent CSS injection via element ID check

v1 (74cdc76)

• Initial implementation with MutationObserver + DOM injection

Files changed

• fern/server-variable-input.js — Custom JS script (434 lines)
• fern/docs.yml — Added js: section to load the script with afterInteractive strategy

Related

• FR - OpenAPI Server Variable Input Filed fern-api/fern#14493

Requested by: Fern Support

[fern-api]

Requested by: Devon White
Slack thread: View conversation

[github-actions]

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