Cowork fails for all prompts with API Error: 400 Invalid 'tools': array too long (144 tools, max 128)

Question

Hello,

I am trying to use the new Cowork feature in Microsoft Copilot and am unable to start any session due to the following error:

API Error: 400 Invalid 'tools': array too long. Expected an array with maximum length 128, but got an array with length 144 instead.

Environment:

• Microsoft 365 work account
• Cowork feature in Microsoft Copilot
• Error occurs immediately after submitting a prompt

I initially encountered the problem while attempting to begin a software development project, so I suspected the prompt might be too large or complex.

To eliminate prompt-related causes, I repeated the test with the following minimal prompt:

Generate PHP "Hello World" script

The same error occurred immediately.

Observations:

• The error is identical regardless of prompt length or content.
• The reported tool count is always 144.
• The reported maximum remains 128.
• The request appears to fail before prompt processing begins.
• This suggests the issue may be related to the Cowork tool/connector inventory being passed to the backend rather than the prompt itself.

Questions:

1. Is this a known issue with Cowork?
2. Is there currently a limit of 128 registered tools/connectors available to a Cowork session?
3. Does this indicate a tenant configuration problem, or is it a service-side defect?
4. Is there a way for end users to see which tools/connectors Cowork is attempting to load?
5. Are there any recommended workarounds while this issue is being investigated?

I have attached screenshots demonstrating:

• Failure with a complex project prompt.
• Failure with a minimal "Hello World" prompt.

Since both fail with the exact same error, I believe the issue is unrelated to prompt content.

Any guidance would be appreciated.

Thank you.

Answer 1

Switch the model from "Auto" to GPT 5.5, that should solve it.