Cowork Not Delivered Message

We had the same issue for weeks, very frustrating, and a fix was found. Below are the resolution details:

Resolution – Cowork “Not delivered / no response” issue

We’ve resolved this in our tenant and wanted to share the outcome.

Root cause

The issue was network security (Netskope) blocking Cowork’s runtime endpoints.

Symptoms observed:

• “Not delivered. Retry?”
• No response when submitting prompts
• Often fails on the second message
• Copilot Chat and other agents worked normally

This occurs because Cowork uses a different backend runtime path from standard Copilot Chat.

Fix

We resolved this by allowlisting the required Cowork endpoints.

Microsoft documentation

• Cowork network endpoints:
  https://support.microsoft.com/en-us/topic/cowork-network-endpoints-48dd7f76-1322-4c63-945b-d781632f501a

From Microsoft guidance, the required endpoints include:

m365.cloud.microsoft.com 

login.microsoftonline.com 

graph.microsoft.com 

*.gateway.prod.island.powerapps.com:443

The critical endpoint is:

*.gateway.prod.island.powerapps.com

Microsoft notes:

• This endpoint is used for Cowork routing and runtime
• Routing is dynamic by user and region
• Do not restrict to specific subdomains https://support.microsoft.com/en-us/topic/cowork-network-endpoints-48dd7f76-1322-4c63-945b-d781632f501a

Important behaviour observed

Once the endpoint was allowlisted:

• All previously failed and pending messages immediately appeared and processed in Cowork

This indicates:

• Requests were reaching the service
• Execution was blocked at the runtime layer
• Cowork queued and completed the tasks once connectivity was restored

Supporting references

• Community thread showing “Not delivered. Retry?” and second‑message failure pattern:
  Cowork Not Delivered Message (TechCommunity)
• Microsoft Learn guidance acknowledging cases where Cowork can be unresponsive or stuck:
  https://learn.microsoft.com/en-us/microsoft-365/copilot/cowork/best-practices

Recommendation

If Cowork is not working but Copilot Chat is:

• Review proxy / CASB / firewall configuration (for example, Netskope)
• Ensure this endpoint is allowed or bypassed:

*.gateway.prod.island.powerapps.com

• Validate that no SSL inspection or Conditional Access policy is interfering with this path

Notes

Cowork is currently a Frontier (preview) feature, and Microsoft notes:

• Behaviour may vary across users
• Rollout is region-based
• Backend routing is dynamic
  https://support.microsoft.com/en-us/topic/get-started-with-cowork-frontier-60be4cea-b913-471b-a0d8-f9ab73c6b0be

Summary

This is not a client, cache, or reinstall issue.

It is a network path dependency specific to Cowork, and can fail independently of the rest of Copilot.