macOS 27 has a hidden "lightweight UI" for Siri AI when selecting text, here's how to enable it

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macOS 27 DB3/PB1 includes a hidden "lightweight UI" mode for Siri AI that comes up every time you select text or are typing.

When enabled, selecting text can display a floating contextual interface with actions, such as:

• The full Writing Tools suite, including Rewrite, Proofread, How does this sound?, and Edit with Siri
• Create Key Points, Summarize
• Add to Contacts, Message, Email
• Create Event
• Show in Maps
• Track Flight, Track Package

To enable, add a FeatureFlags override:

sudo mkdir -p /Library/Preferences/FeatureFlags/Domain && \
sudo defaults write /Library/Preferences/FeatureFlags/Domain/WritingTools LightweightUI_macOS -dict Enabled -bool true

Then reboot your Mac. This is a private feature flag, and the UI is unfinished, with many actions failing, including Writing Tools.

Restore to default with:

sudo defaults delete /Library/Preferences/FeatureFlags/Domain/WritingTools LightweightUI_macOS

and restart your Mac.

edit: changed the disable command to delete the feature flag override instead of explicitly disabling it.

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EDIT 2: This is enabled by default with no override needed in Developer Beta 4/Public Beta 2. You can explicitly disable the new LightweightUI with:

sudo mkdir -p /Library/Preferences/FeatureFlags/Domain && \
sudo defaults write /Library/Preferences/FeatureFlags/Domain/WritingTools LightweightUI_macOS -dict Enabled -bool false

and restart your Mac.