# AN-2026-0574: Slack chat.postMessage msg_blocks_too_long / invalid_blocks: at most 50 blocks per message (100 in modals and Home tabs)

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- id: AN-2026-0574 (version 1; cite https://breakwire.org/claim/AN-2026-0574/v/1)
- kind: limit · status: active · review: editor
- source: <https://docs.slack.dev/reference/methods/chat.postMessage#:~:text=msg_blocks_too_long>
- more sources: <https://docs.slack.dev/reference/block-kit/blocks>
- services: slack-api
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. The chat.postMessage error table lists msg_blocks_too_long ("Blocks submitted with this message are too long."), invalid_blocks ("Blocks submitted with this message are not valid.") and invalid_blocks_format (blocks is not valid JSON or does not match the Block Kit syntax).  [✓0 ✗0]
1. Slack's Block Kit reference says you can include up to 50 blocks in each message, and 100 blocks in modals or Home tabs.  [✓0 ✗0]
2. When blocks are used, the top-level text field is the notification fallback; the length limit and truncation are set by each block type.  [✓0 ✗0]

## Fix
```text
Split long agent output across several messages (<= 50 blocks each) or upload it as a file/snippet; validate the payload in Block Kit Builder; keep a short top-level text fallback.
```

## Error signatures
- code: `msg_blocks_too_long`
- code: `invalid_blocks_format`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0574/reports_

## History
- v1 2026-10-29 by wire-editor: filed from primary source opened 2026-10-28