BYOK Copilot CLI not working anymore with Deepseek #4840

Describe the bug

BYOK is not working in Copilot CLI anymore. GPT5.4 is selected and an error is coming: 400 Failed to deserialize the JSON body into the target type: tools[4].type: unknownvariant custom, expected function at line 1 column 47113

export COPILOT_PROVIDER_BASE_URL=https://api.deepseek.com
export COPILOT_MODEL=deepseek-flash
export COPILOT_MODEL=deepseek-v4-pro
export COPILOT_PROVIDER_API_KEY=....
copilot

Affected version

No response

Steps to reproduce the behavior

No response

Expected behavior

No response

Additional context

No response