One week after macOS 27.2 shipped, Apple seeded the first macOS 27.3 developer beta (build 26C5035r) on November 3, 2026, and opened the public beta two days later. It is a more substantial update than its predecessor, led by a long-requested Safari redesign.
Safari: Vertical Tabs and Adaptive Dark Mode
The most visible change is in Safari. Vertical tabs finally arrive as a sidebar layout, years after Edge and Arc made them popular. Adaptive Dark Mode re-renders websites that have no dark theme of their own, and Siri AI can now manage the browser: ask it to clear the cache, group the tabs you have open on a topic, or close everything you haven't touched this week.
Markdown in Quick Look
Press the space bar on a .md file in Finder and macOS 27.3 renders the formatting instead of showing raw syntax. A small change, but one that developers and note-takers have requested for a decade.
Live Activities Come to the Menu Bar
Ongoing tasks such as a playing track in Music, a running timer in Clock or an active screen recording now appear as compact indicators in the menu bar, mirroring Live Activities on iPhone. Third-party apps can adopt the same API.
Everything Else in Beta 1
- A new Pro Light screen saver.
- Microsoft Graph support for Exchange accounts in Mail, Calendar, Contacts, Notes and Reminders. In beta 1 you have to remove and re-add the account to switch protocols.
- New developer tools for connecting third-party app content and actions to Siri AI, using Markdown-based instructions.
- Groundwork for safety checks on public iCloud links: content shared with “anyone with the link” will be scanned for harmful material. Those items are not end-to-end encrypted.
- Siri AI in Traditional Chinese, Norwegian, Turkish and Vietnamese is listed as “coming soon”; several reports tie it to 27.3, but Apple has not confirmed the version.
- Not confirmed: automatic password changes in Passwords, which had been rumored for this release.
Release Timing and Known Issues
Beta 1 is build 26C5035r. On Apple's recent cadence (27.2 took six weeks from first beta to release), expect macOS 27.3 to ship in mid-December 2026. Early testers report the usual first-beta rough edges, and Xcode 27.3 beta has simulator boot crashes. Don't install this on a Mac you depend on; if you must try it, read our downgrade guide first, because leaving a beta means erasing the drive.
How to Get It
Enrol at System Settings → General → Software Update → Beta Updates. The public beta is available to anyone with an Apple Account; the developer beta needs a (free) developer account. Both are the same build this round.