Messaging

Posting, in-place updates, formatting, and rate limits for Slack's core messaging API

What This Section Covers

chat.postMessage and chat.update are the two methods behind almost everything a bot puts into a channel — a one-off notification, a threaded reply, or a pinned status board a cron job refreshes in place. Getting them right is mostly about three things: building the message body correctly (Block Kit vs. plain mrkdwn), knowing what chat.update can and cannot do, and staying inside the rate limits both methods enforce.

In This Section

• Posting and Block Kit — chat.postMessage essentials: channel resolution, thread_ts threading, blocks + text fallback, the common block types, and the unfurl flags

• Updating in Place — chat.update's absolute read-only guarantee, the pinned-dashboard pattern, and Table block limits

• Formatting — mrkdwn vs. Block Kit rich_text, escaping, mentions, links, date tokens, and when plain mrkdwn beats blocks

• Rate Limits — the Tier 1–4 model, Retry-After handling, the per-channel message-posting limit, and the 2025–2026 rate-limit changes for non-Marketplace apps

Two Methods, One Constraint

Both methods write to the same surface (a channel message) but under different constraints: chat.postMessage creates a new message and can be called by any app with the right scope; chat.update can only ever touch a message the calling app itself posted — see Updating in Place for why that makes it the strongest read-only guarantee Slack offers a bot-writable surface. If the goal is a live-refreshing status board rather than a stream of notifications, chat.update on a pinned message is the pattern to reach for first.