preview_open rejects about:blank as "Invalid preview URL (parse: https:)"#25639

What happened

preview_open({ url: "about:blank" }) fails with:

Invalid preview URL (parse: https:; input length 11)

Agents call it to get a blank tab before navigating, or to check that the browser works, and give up on browser work when it fails. In the fleet this ended two lead rounds early (https://git.cafe/pingdotgg/t3code/issues/25379 r1, https://git.cafe/pingdotgg/t3code/issues/24171 r26). Leads reported it 8 times between 2026-10-21 and 2026-10-25.

Cause

normalizePreviewUrl adds https:// to any input without ://. about:blank becomes https://about:blank, new URL rejects the non-numeric port, and the error reports parse with protocol https:. So the message names neither the real input nor the real problem. The address-bar path (resolveAddressBarInput) lists about: as a known non-web scheme, but preview_open goes through normalizePreviewUrl directly (apps/server/src/preview/ServerBrowser.ts:1408).

Expected

Either about:blank opens a blank tab, or it's rejected as unsupported-protocol: about: with a hint like "omit url to open a blank tab". In both cases a scheme without // isn't turned into https://<scheme>:<rest>.

Where

packages/shared/src/preview.ts:183-210, normalizePreviewUrl (origin/main fe507c5a1e), line 189.

Related: https://git.cafe/pingdotgg/t3code/issues/22402 (validation errors don't say what's wrong).