Slack Cloud Agent fails to start (invalid_blocks / “agent failed to start”); Web Cloud Agents work fine

Support Bug Reports
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1

The bug appear

Cloud Agent (GitHub, Slack, Web, Linear)

Describe the Bug

Hi,

I’m a beginner with development and I’m stuck on the Slack Cloud Agent setup.
Cursor Web works fine for me, but I can’t start agents from Slack, so I’d really appreciate some help.

Where does the bug appear?
Cloud Agent via Slack integration

Describe the Bug
Slack integration connects and CursorCursor settings works, but starting or listing Cloud Agents from Slack fails. Cursor Web Cloud Agents work normally on the same account/repo.

Update:

About 5 minutes before the failure, Slack Cloud Agents were working normally for me (an agent actually started and ran a task).

Then I invited both Cursor and Claude into the same Slack channel and mentioned both in one message.
Right after that, Cursor started returning:
The agent failed to start. Please try again.

A follow-up message mentioning only Cursor also failed with the same error.
Claude still responds normally in that channel.

So this may be related to mentioning Cursor together with another bot, or inviting both bots into the same channel.

Steps to Reproduce

1.Connect Slack from Cursor Integrations / reinstall the Cursor Slack app
2.In Slack Cursor run Cursor settings and set a repository
3.Model tried both cursor-grok-4.5-high-fast and claude-sonnet-5-high
4.Base branch: develop / repository deCursorault
5.Run Cursor list my agents
6.Run a simple a@Cursorent prompt, e.g. Cursor agent summariz@Cursor README in one line Cursor in [repo-name], …
7.Also tried: Unlink Slack, reconnect, Reset to Team Settings, reinstall Sl@Cursorck app

Expected Behavior

1.Cursor list my agents returns the agent list
2.A simple prompt starts a Cloud Agent in Slack

Screenshots / Screen Recordings

スクリーンショット 2026-08-09 12.46.49.png
スクリーンショット 2026-08-09 12.46.49.png491×255 14.3 KB

Operating System

MacOS

Version Information

IDE: Cursor
Version: 3.14.7

For AI issues: which model did you use?

Model name: cursor-grok-4.5-high-fast and claude-sonnet-5-high (both tried for Slack Cloud Agent)

Additional Information

This is a Slack Cloud Cursorgent issue, not a@Cursor IDE chat issue.
Cursor settings works.
Cursor list my agents returns invalid_blocks.
Starting an agent from Slack returns: The agent failed to start. Please try again.
Cursor Web Cloud Agents work normally.
Tried unlink/reconnect/reinstall Slack app.
On some failure messages, View request ID is not shown.

Does this stop you from using Cursor

Yes - Cursor is unusable

Request ID
serverGenReqId_45594bd7-f3c5-43b5-8b20-16008659c504
(from an earlier failure; some later failures did not expose View request ID)

2

Hi there!

We detected that this may be a bug report, so we’ve moved your post to the Bug Reports category.

To help us investigate and fix this faster, could you edit your original post to include the details from the template below?

Bug Report Template - Click to expand

Where does the bug appear (feature/product)?

  • Editor, Tab & Chat (autocomplete, Composer, in-editor agent)
  • Terminal & commands
  • Models, pricing & API keys (availability, Auto/Max, BYOK/Bedrock)
  • MCP & tools
  • Cloud Agents & Automations (cursor.com/agents, scheduled/event)
  • BugBot & Code Review
  • Cursor CLI
  • Cursor Mobile
  • Remote (SSH / Dev Containers / WSL)
  • Account, billing & login
  • Something else…

Describe the Bug
A clear and concise description of what the bug is.

Steps to Reproduce
How can you reproduce this bug? We have a much better chance at fixing issues if we can reproduce them!

  1. …
  2. …
  3. …

Expected Behavior
What is meant to happen here that isn’t working correctly?

Screenshots / Screen Recordings
If applicable, attach images or videos (.jpg, .png, .gif, .mp4, .mov)

Operating System

  • Windows 10/11
  • MacOS
  • Linux

Version Information

  • For Cursor IDE: Menu → About Cursor → Copy
  • For Cursor CLI: Run agent about in your terminal
IDE:
Version: 2.xx.x
VSCode Version: 1.105.1
Commit: ......

CLI:
CLI Version 2026.01.17-d239e66

For AI issues: which model did you use?
Model name (e.g., Sonnet 4, Tab…)

For AI issues: add Request ID with privacy disabled
Request ID: f9a7046a-279b-47e5-ab48-6e8dc12daba1
For Background Agent issues, also post the ID: bc-…

Additional Information
Add any other context about the problem here.

Does this stop you from using Cursor?

  • Yes - Cursor is unusable
  • Sometimes - I can sometimes use Cursor
  • No - Cursor works, but with this issue

The more details you provide, the easier it is for us to reproduce and fix the issue. Thanks!

6

Update / Resolved!

I found the root cause on my side.

My repo had a .cursor/environment.json file, and the ports field was written incorrectly as a bare number array, like:

“ports”: [8080]

Cloud Agents expect ports to be objects, like:

“ports”: [
{
“name”: “web”,
“port”: 8080
}
]

Because of that, Cloud Agent startup failed with:
Failed to parse environment.json
Expected object, received number

After I fixed the ports schema and merged it, Slack Cloud Agents started working again for me.

If anyone else hits “The agent failed to start” / invalid_blocks while Web agents sometimes work, check .cursor/environment.json and make sure ports uses objects, not bare numbers.

Thanks!