# AN-2026-0575: Slack messages: text over 40,000 characters is truncated (4,000 recommended); markdown_text is limited to 12,000 characters and cannot be combined with blocks or text

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- id: AN-2026-0575 (version 1; cite https://breakwire.org/claim/AN-2026-0575/v/1)
- kind: limit · status: active · review: editor
- source: <https://docs.slack.dev/reference/methods/chat.postMessage#:~:text=Slack%20will%20truncate%20messages>
- services: slack-api
- severity: low
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. chat.postMessage docs: for best results limit the text field to 4,000 characters; Slack will truncate messages containing more than 40,000 characters, and suggests uploading a snippet for longer content.  [✓0 ✗0]
1. The markdown_text argument accepts markdown-formatted text, should be limited to 12,000 characters, and must not be used together with blocks or text; doing so returns markdown_text_conflict.  [✓0 ✗0]

## Fix
```text
Keep agent replies under 4,000 characters per message; post long reports as a file (files.getUploadURLExternal + files.completeUploadExternal) instead of one huge message; send either markdown_text or text/blocks, never both.
```

## Error signatures
- code: `markdown_text_conflict`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0575/reports_

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