Webhooks reject embed-only payloads from Discord and Slack compatible apps with error 400

Summary

When attempting to use webhooks via many popular self hosted apps, I've found that embed only may not be supported on the webhook. For example, within seer sending a test payload I receive the following error in the apps logs: Using the discord webhook integration:

2026-02-23T16:08:46.438Z [debug][Notifications]: Sending Discord notification {"type":"TEST_NOTIFICATION","subject":"Test Notification"}
2026-02-23T16:08:46.559Z [error][Notifications]: Error sending Discord notification {"type":"TEST_NOTIFICATION","subject":"Test Notification","errorMessage":"Request failed with status code 400","response":{"code":"INVALID_FORM_BODY","message":"Invalid form body.","errors":[{"path":"message_data","message":"Invalid message data.","code":"INVALID_MESSAGE_DATA"}]}}

Using the slack webhook integration:

2026-02-23T16:10:39.644Z [debug][Notifications]: Sending Slack notification {"type":"TEST_NOTIFICATION","subject":"Test Notification"}
2026-02-23T16:10:39.796Z [error][Notifications]: Error sending Slack notification {"type":"TEST_NOTIFICATION","subject":"Test Notification","errorMessage":"Request failed with status code 400","response":{"code":"CANNOT_SEND_EMPTY_MESSAGE","message":"You can't send an empty message."}}

Zerobyte results in a generic 400 error:

16:15:12 error Failed to send notification: failed to send notification to generic webhook: server returned unexpected response status code: 400 Bad Request

16:15:12 error ...../api/v1/notifications/destinations/6/test: Failed *** send test notification: failed to send notification to generic webhook: server returned unexpected response status code: 400 Bad Request

Pretty sure Zerobyte uses Shoutrrr for sending out webhooks in most of its processes. Odd that the Unraid System notification works fine when using the discord notification - it shows up as an embed only which is intended. I originally thought that may have been the problem.

Steps to reproduce

Add webook under discord, slack, or generic webhook into Seer. Then test. Add webook under discord, slack, or generic webhook into Zerobyte. Then test.

Content-Type: application/json X-Forgejo-Delivery: dafe99be-4bf4-4621-8485-3f2dd78fa6f1 X-Forgejo-Event: action_run_success X-Forgejo-Event-Type: action_run_success { "username": "Forgejo", "embeds": [ { "title": "feat: add twemoji country flags Action Succeeded in hq/web main", "description": "", "url": "https://code.throwdown.live/hq/web/actions/runs/60", "color": 1754624, "footer": {}, "author": { "name": "Vexcited", "url": "https://code.throwdown.live/Vexcited", "icon_url": "https://code.throwdown.live/avatars/cf9eb2f14e5408329576912748877a077e8c3445b7de697f976cb4ac632ad065" } } ] }

X-Infrastructure: web-nyc-3-api X-Request-Id: 12dd61095b5be7505545fa8ac1217cab {"code":"INVALID_FORM_BODY","message":"Invalid form body.","errors":[{"path":"message_data","message":"Invalid message data.","code":"INVALID_MESSAGE_DATA"}]}

3 comments

RexSystem 1 vote 1d ago

This was closed in a bulk cleanup before Fluxer V2 without being checked or fixed. It may work now, so it is waiting for someone to confirm whether the bug still happens.

RexSystem 1 vote 7mo ago originally by @Scout339 on GitHub

im having a similar issue with my own selfhosted app I made... Works with other test webhook websites, but doesnt work with Fluxer's webhooks as of now image

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RexSystem 1 vote 7mo ago originally by @vexcited on GitHub

Happens the same here with Forgejo. When looking at the details, Forgejo sends a POST request to the Webhook URL with the following body...

...and Fluxer replies with a 400 error,

Not sure which part of this controller is messing up everything... https://github.com/fluxerapp/fluxer/blob/77a6897180750801cc020262c94878736ff79799/packages/api/src/webhook/WebhookController.tsx#L353-L387