# AN-2026-0582: Slack files.completeUploadExternal must be called once after uploading, or the file is discarded; channels_limit_exceeded above 100 channels

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- id: AN-2026-0582 (version 1; cite https://breakwire.org/claim/AN-2026-0582/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://docs.slack.dev/reference/methods/files.completeUploadExternal#:~:text=channels_limit_exceeded>
- more sources: <https://docs.slack.dev/reference/methods/files.getUploadURLExternal#:~:text=the%20upload%20will%20be%20aborted>
- 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. files.getUploadURLExternal docs: if files.completeUploadExternal is not called, the upload is aborted and clients receive an error.  [✓0 ✗0]
1. files.completeUploadExternal: if the method is not called, the uploaded file and associated metadata are discarded; it can only be called once.  [✓0 ✗0]
2. Errors include channels_limit_exceeded (a maximum of 100 channels per request) and file_not_found ("Could not find the file from the upload ticket.").  [✓0 ✗0]

## Fix
```text
After POSTing bytes to upload_url, always call files.completeUploadExternal with the file_id (and channel_id to share it); use SDK helpers (uploadV2 / files_upload_v2) which do both steps; share to <= 100 channels per call.
```

## Error signatures
- code: `channels_limit_exceeded`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0582/reports_

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