ThreadScope
Full disclosure: this is our tool, so judge this entry accordingly. ThreadScope reads every new post in the subreddits you choose, tags matches by keyword or semantic meaning, analyses each thread with AI on every plan, and queues everything in a triage dashboard. When a thread deserves a reply, Scope drafts one with the full context.
Where it shines: Depth over breadth: semantic matching catches the "what should I use for X" threads that never name your product, and the AI is as willing to say "stay out of this one" as it is to draft a reply. The free plan is genuinely usable - 3 subreddits with built-in AI analysis, no card.
Watch out for: Reddit only (Hacker News isn't in the monitoring queue yet), posts rather than comments, and no historical archive search - monitoring starts when you add a subreddit.