[Feature]: Show agent-to-agent and thread-to-thread messages as compact expandable rows #23110

Follow-up requested by maintainers

This is the remaining agent-to-agent and thread-to-thread part of #22413, filed separately as requested in the closing comment.

#22638 addressed automation messages. The closing comment explicitly identifies compact rows for messages between agents and threads as still unresolved.

Problem

I use T3 threads that receive messages from agents working in other threads. Those messages are largely un-useful for me to read and their chat bubbles make it difficult to find the last message I personally wrote and remember what I was doing in the receiving thread before other agents started posting.

This concerns messages sent between full T3 threads. Even when a long message is truncated behind “Show full message,” its preview still takes up substantial space.

Requested behavior

Show incoming agent-to-agent and thread-to-thread messages as compact entries by default:

• A single-line row with the sending thread or agent identified and a short preview.
• Expand the row to read the complete original message, then collapse it again.
• Keep messages personally written by the user in their ordinary presentation.
• Apply the compact presentation consistently on web, desktop, and mobile.

The messages should remain available to the receiving agent in full; this request concerns their presentation in the conversation.

Related

• Collapse automation and agent-to-agent messages more aggressively by default #22413: original report and maintainer request for this focused follow-up.
• feat: show automation messages as compact timeline rows #22638: merged compact presentation for automation messages.
• Discussion #20243: related proposal for compact incoming agent reports and source-thread navigation.

Prepared with GPT-6.1 Sol through the Codex harness in T3 Code, based on the reporting user's experience.