Mobile: subsequent agent replies in a thread attach to the original post

[potseeslc]

Summary

In the Buzz mobile app, an agent's first reply in a thread works, but subsequent in-thread calls to the agent do not reply to the newer message. The agent reply is attached to the original top-level chat post instead.

Steps to reproduce

1. In the mobile app, create a channel message and mention an agent.
2. Let the agent reply in the resulting thread.
3. Add another reply in that same thread and mention/call the agent again.
4. Observe where the agent's response is attached.

Actual behavior

The subsequent agent response replies to the first/top-level chat post rather than the latest in-thread message.

Expected behavior

The agent response should use the current in-thread message as its reply target, preserving the conversation's nesting/context.

Impact

Follow-up conversations with agents on mobile lose their correct reply context, making threaded exchanges difficult to follow.

[MPIsaac-Per]

This is not mobile-specific, and it looks intentional rather than a bug. Worth deciding explicitly, because the current behaviour and the expectation in this issue are both defensible.

Reproduced on desktop against a self-hosted relay (ghcr.io/block/buzz:sha-25e7864, buzz-acp from the 0.4.26 .deb, claude-agent-acp 0.62.0):

1. Post A at channel top level.
2. Reply B in A's thread, mentioning the agent.
3. Agent's reply carries exactly one e tag:

["e", "<A>", "", "reply"]

It points at the thread root, not at B, and there is no root marker. So the reply is a sibling of the message that mentioned the agent.

That is what resolve_reply_anchor is written to do (crates/buzz-acp/src/queue.rs:1201):

/// Returns `Some(id)` only for human-facing turns:
///   - in a thread → the thread ROOT, keeping the reply flat at layer 1
///   - top-level   → the triggering event id, which becomes the new thread root

So the anchoring is deliberate, aimed at stopping replies nesting a level deeper each exchange, and it applies to every client rather than just mobile.

The tension is real in both directions. #2748 argues the opposite for DMs, that replies anchor too deep and should anchor to root, with #2766 and #2778 open against it. Meanwhile flat-at-layer-1 loses which message was answered once more than one exchange is happening in a single thread, which is what this issue is describing.

One shape that satisfies both: keep root anchoring when the triggering event is the root or its direct child, and anchor to the triggering event beyond that depth. Threads stay shallow in the common case without discarding the reply target in longer exchanges.

Happy to put up a PR if maintainers decide which behaviour they want.