The DM thread-anchor contract was resolved only in send(). _send_media() —
backing send_image, send_image_file, send_voice, send_video and send_document
— passed reply_to straight to the frame and never touched metadata, so
attachments egressing through the same connector-side Slack sender got both
failure shapes this branch set out to remove:
flat mode → reply_to survives, the image threads UNDER the user's DM
message (the original reported symptom)
thread mode → no metadata.thread_id, and threadTs() never reads reply_to,
so the image lands in the home channel instead of the
per-message thread
Both are reachable: gateway/run.py delivers agent artifacts through
send_voice/send_document.
Extract the three steps that must always happen together (mode gate, mirrored
reply_to_message_id strip, metadata promotion) into
_apply_slack_thread_anchor and route BOTH lanes through it, so text and media
cannot drift again. The media lane copies caller metadata rather than mutating
it — these helpers are called in loops with a shared mapping.
Also fold send_typing/stop_typing's duplicated status-anchor blocks into
_with_status_thread_anchor. They had already drifted (stop_typing omitted the
platform check) and the clear must target the thread the heartbeat set or the
status line sticks until Slack's own timeout.
Tests: media lane pinned in both modes plus the channel and
no-caller-mutation cases; verified as real by reverting the fix and watching
them fail.
Ben Barclay
33833e232e