Slack error duplicate_channel_not_found: client_msg_id points at an invalid channel

This error belongs to Slack's idempotent-send machinery. chat.postMessage accepts a client_msg_id — a unique ID you attach so a retried request can be recognized as the same logical message instead of posting twice. duplicate_channel_not_found is documented as "Channel associated with client_msg_id was invalid": Slack matched your client_msg_id to a previous send attempt, but the channel recorded for that attempt does not line up with a resolvable channel for this one.

In practice it means your retry is inconsistent with the original: the same client_msg_id was reused with a different or now-invalid channel. That happens when IDs are generated from insufficiently unique inputs (a hash of the text, a non-unique job ID) so two genuinely different messages collide, or when a retry pipeline rewrites the destination between attempts while carrying the old ID forward.

Treat the client_msg_id as bound to one (channel, message) pair forever. If the destination legitimately changed, mint a new ID; if the collision was accidental, fix the generator to produce true uniqueness (UUIDs) per logical message.

{"ok": false, "error": "duplicate_channel_not_found"}

Make idempotency keys immutable properties of the message record, created once when the message is enqueued and never derived from mutable fields — hashing the text or reusing a job ID across destinations is how collisions are born. Log the client_msg_id with every send alongside the destination channel so collisions and mismatches are visible in one grep, and cover the retry path with a test that replays the same logical message twice.