Execution failed: CAPIError: 400 {"error":{"code":"bad_request","type":"websocket_error","message":"Duplicate item found with id fc_call_ikBrgmi06fcJeO6uIpJJEQbJ. Remove duplicate items from your input and try again."}}

[lgphp]

Describe the bug

This started happening suddenly in the evening with the same Copilot CLI version and the same workflow.

Important detail:

• Plain chat still works.
• The failure only happens on the turn after a tool/function call.
• During the day, the same flow worked normally.
• In the evening, follow-up turns after tool calls started failing with:
  "Duplicate item found with id fc_call_..."

This suggests a server-side regression or rollout affecting tool-call session continuation, rather than a local client version issue.

Affected version

GitHub Copilot CLI 1.0.55. Run 'copilot update' to check for updates.

Steps to reproduce the behavior

No response

Expected behavior

No response

Additional context

No response

[salmanhasni]

+1

[Yajur-Grover]

+1 - after starting a new copilot session, the issue did not re-occur.

[mir-am]

I have the same issue with GPT 5.5 and GPT 5.4.

[lsromanls]

+1 — same underlying failure here.

Observed on Copilot CLI 1.0.54 before auto-update to 1.0.55.
Model in my logs: gpt-5.4 xhigh.
Failure is the same websocket_error / duplicate tool-call item pattern:

Duplicate item found with id fc_call_...

Important detail from my logs:

• context usage was only 25.5% (69430 / 272000), so this does not look like a context-window issue
• the same fc_call_* ID was reused across retries
• stack points into getCompletionWithTools / runAgenticLoop
• plain instructions/context size is small; repo instructions file is concise

This looks consistent with a session/tool-call continuation defect rather than user error.

Seems to be working fine in v1.0.52 (you have to use --no-auto-update switch)