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0008 — Slack: `chat.update` in place is primary; threads are secondary; one colour attachment

Status: Accepted · 2026-08-07

Context

Section titled “Context”

Three verified facts drive this.

  1. Rate limits. chat.postMessage is a “Special” tier — roughly one message per second per channel. chat.update is Tier 3 (50+/min) and is not per-channel limited. Updating is both nicer for humans and ~50× cheaper.
  2. notification_reason (Alertmanager ≥ 0.32.0) tells us why a group was re-sent. "repeat interval elapsed" means nothing changed, this is a nag. Stock Alertmanager and Grafana Alerting both repost on it. That is the loudest, cheapest noise source in the category.
  3. Colour. Slack: “the color parameter currently does not have a block alternative.” Attachments are legacy but not deprecated, and are the only way to get a colour bar.

Also: the header block is plain_text only — no bold, no links — so a header costs the deep link for nothing. Sentry, Grafana OnCall and Alertmanager all use a section for the title.

Decision

Section titled “Decision”
  • chat.update on the existing root is the primary mechanism. Thread replies are the exception, gated by a per-channel verbosity setting and by the notification_reason → Reason → mode table in SPEC §H.6.
  • repeat interval elapsed → update only. Never post a new message.
  • Exactly one attachment wraps all blocks and carries color. Colour encodes STATE (Grafana OnCall’s verified palette: firing #a30200, acked #daa038, silenced #dddddd, resolved #2eb886); severity encodes as a leading emoji. The colour then always answers the question a human scanning a channel is actually asking: do I need to act?
  • Title is a section with a bold mrkdwn link. The header block is not used. The Block Kit alert block is not used at all (it is modals-only, despite the name).
  • On every state change the previous value is rendered struck through (~Firing~ → Resolved), so a reader who saw the card an hour ago can tell what changed at zero block cost.
  • The top-level text is a complete sentence, written deliberately: it is the push notification, the sidebar preview, the search snippet, and the only thing screen readers read.
  • oto never reads Slack back to reconstruct its own state. (channel_id, ts) in our DB is the memory of Slack. ts is stored as TEXT, never a float.

Consequences

Section titled “Consequences”
  • A flapping or repeating alert costs one Tier-3 update instead of a per-channel-limited post. A rendered_hash check skips no-op updates entirely.
  • The channel stays scannable: state is visible without opening a thread.
  • We accept the legacy-attachment risk (Slack warns content “may be wrapped, truncated, or hidden behind a ‘show more’”). Mitigated by keeping the card to ~7 blocks. No sunset date exists as of the research date.
  • Renderers must be pure functions with checked-in golden files and a CI Block Kit validator, because a broken layout is invalid_blocks — a config_invalid dead delivery.

Alternatives rejected

Section titled “Alternatives rejected”
  • Post a new message per lifecycle fact (stock Alertmanager’s behaviour): the noise oto exists to eliminate, and 50× more expensive.
  • No attachment, emoji only (Sentry’s approach): viable at the highest tier of the market and avoids the legacy dependency, but forfeits the peripheral-vision cue in a wall-of-alerts channel.
  • response_url for long-lived updates: 30 minutes and 5 uses. It is for the optimistic response to a click, nothing more.
  • Reading conversations.history for crash recovery: non-Marketplace distributed apps are throttled to 1 req/min with 15-object pages. Never depend on reading Slack.