Frequent websocket_error: Duplicate item found (fc_call_* replay from session-state)

[drapetomaniac]

Summary

I am repeatedly hitting this failure in Copilot CLI:

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

This appears to be session-state replay/corruption of prior fc_call_* items. Once it starts in a session, subsequent prompts in that same session fail quickly with the same duplicate-id error.

Frequency (from local ~/.copilot/session-state/*/events.jsonl)

• Sessions affected: 10
• Duplicate-item error events: 29
• Time window observed: 2026-05-28 ~18:46Z to ~19:43Z
• Worst session: 3604526b-3b17-4790-b5f5-9d4359783706 with 15 events in ~4.5 minutes

Additional affected sessions:

• 893105c9-9726-497f-a54c-c384bd5b51b3 (3)
• 6257b848-6652-450c-b949-b8d0477d3b56 (2)
• 63d34bda-bddd-4246-aca5-5258e94d88dd (2)
• ce4e9dd9-bca8-4181-ac70-7c398be8e204 (2)
• plus 5 more sessions with single occurrences

Repro pattern

1. Normal interaction with tool calls.
2. A turn fails with duplicate fc_call_* id.
3. Retrying in the same session keeps failing with duplicate-id style websocket 400s.
4. Only workaround is deleting the bad session directory or resetting session-state.

Impact

This happens frequently enough to materially disrupt use; repeated manual session resets are not productive.

Environment

• Copilot CLI version: 1.0.54
• OS: macOS (Darwin)

Please advise on mitigation and whether a client-side de-dup guard can be added before sending websocket payload items.

[collinbarrett]

possible duplicate of #3558

[oughnic]

Also hitting this on Windows (both Copilot CLI desktop app and the GitHub.com web agent). The error surface differs slightly from the OP (macOS CLI) — it shows as a top-level UI error rather than a session-state log entry:

Copilot Error
Copilot has encountered an unexpected error: [runtime:unclassified] Execution failed:
CAPIError: 400 Duplicate item found with id fc_call_fY9X7T78A5zCwzKmdhjgE2CU.
Remove duplicate items from your input and try again.
(Request ID: CC46:2C22C3:162F936:17D4280:6A189DA5)

Key differences from OP:

• Reproduces on Windows Copilot CLI desktop
• Also reproduces in GitHub.com Copilot agent (server-side / cloud) — so the duplication is not purely client-side session-state
• The fact that it hits both the local CLI and the web agent suggests a server-side session reconstruction issue, not just a local replay bug

Request ID for GitHub tracing: CC46:2C22C3:162F936:17D4280:6A189DA5

[oughnic]

Update: local session-state clearing does not resolve the issue.

After deleting all 19 local session-state directories for the affected repo (~/.copilot/session-state/*), the error immediately recurs on a brand-new session with a fresh duplicate ID:

CAPIError: 400 Duplicate item found with id fc_call_hGo6TjNXMlfBrJvaSGQCuUbQ.
(Request ID: 07C0:329977:510E68:57FDB6:6A18A31A)

The previous error used fc_call_fY9X7T78A5zCwzKmdhjgE2CU — so this is not a replay of corrupted local state. The service is generating duplicate tool-call IDs on each new session from scratch.

This conclusively points to a server-side issue in how the Copilot agent constructs its tool-call message payloads, independent of any client session history.