Summary

Cron jobs and heartbeats currently have no way to target a specific Slack thread. When a cron job is created from a Slack thread, delivery uses lastThreadId from session history — which points to whichever thread was last active, not necessarily the intended one. Heartbeat has no thread targeting at all.

This is the same class of problem as #19365 and #19366 (Telegram topic targeting), but for Slack threads.

Current behavior

• Cron: delivery.to accepts a Slack channel ID (e.g. C12345) but has no syntax to specify a thread_ts. Delivery falls back to lastThreadId from session history, which may be a different thread.
• Heartbeat: heartbeat.to accepts a channel ID but cannot target a specific thread.

Proposed solution

Add a thread targeting syntax for Slack, similar to how #19367 added :topic:NNN for Telegram. For example:

C12345:thread:1234567890.123456

This would be parsed in resolveSessionDeliveryTarget() with a Slack-specific guard (similar to the Telegram isTelegramContext guard added in #19367).

Config examples:

Cron:

{
  "delivery": {
    "channel": "slack",
    "to": "C12345:thread:1234567890.123456"
  }
}

Heartbeat:

{
  "agents": {
    "defaults": {
      "heartbeat": {
        "target": "slack",
        "to": "C12345:thread:1234567890.123456"
      }
    }
  }
}

Related

• [Bug]: Cron jobs deliver to wrong chat — ignores Telegram topic thread routing #19365 — Cron delivers to wrong Telegram topic (fixed in fix(telegram): cron and heartbeat messages land in wrong chat instead of target topic #19367)
• [Feature]: Support :topic: syntax in heartbeat.to for Telegram topic delivery #19366 — Heartbeat Telegram topic support (fixed in fix(telegram): cron and heartbeat messages land in wrong chat instead of target topic #19367)
• fix(telegram): cron and heartbeat messages land in wrong chat instead of target topic #19367 — PR that fixes Telegram topic targeting; same approach could be extended for Slack

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