Transcript.CustomSegment and the .custom case on Transcript.Segment were in the Xcode-beta 27 builds. WWDC26 session 339 covers them as the way a model package extends the protocol for new modalities and server side tool output.
In the Xcode 27 GM they are gone along with the matching action on the executor generation channel.
Is there any additional information as to what happened to custom segments, a replacement, etc? I don't see any mentions of this change in any release notes, though perhaps I'm looking in the wrong place.