Growing number of active sessions start failing with persistent CAPI errors

After a couple interactions in a session, Github Copilot App starts to generate errors like
Execution failed: CAPIError: 400 The resource you requested was not found. (Request ID: 00000-a81acc34-0aa3-4947-9ac9-5cb965bb84b9)
The errors persist until I switch to a new session or tentatively restart the Github Copilot App.
I am using Github Copilot App using my Microsoft employee account so I have a Copilot license.

Start by reproducing the failure in the GitHub Copilot App after a couple of interactions in one session, recording the CAPI 400 error and request ID. Compare behavior after switching sessions or restarting the app; done means the persistent errors no longer occur during continued use.

Written by the indexing model from the issue text.