WebSocket duplicates seed context in response.create.input -> 400 Duplicate item found (Esc undo/retry)

What version of Codex CLI is running?

codex-cli 0.116.0

What subscription do you have?

N/A (API key only)

Which model were you using?

gpt-5.4

What platform is your computer?

Darwin 24.6.0 arm64 arm

What terminal emulator and version are you using (if applicable)?

WezTerm 20251201-075747-d3b0fdad

What issue are you seeing?

When using the Responses WebSocket transport (supports_websockets=true), Codex CLI sometimes sends duplicated items within a single response.create input.

The server rejects the request with:
400 invalid_request_error: Duplicate item found with id rs_... (param: input)

This often happens after pressing Esc to undo/back out input and then sending another message.

What steps can reproduce the bug?

1. Configure a model provider with wire_api=responses and supports_websockets=true.
2. 2. Run codex and send a message.
3. 3. Press Esc to undo/back out input, then type a new message and send.
4. 4. Observe the request fails with 400 Duplicate item found.

WS capture evidence (mitmproxy): In the WebSocket response.create frame, input contains duplicated seed context in the same request. Example: inputLen=13; the seed developer message starting with <permissions instructions> repeats 6x and the contextual user seed message repeats 6x before the real user message.

Workaround: supports_websockets=false (HTTP/SSE) avoids duplication and succeeds.

What is the expected behavior?

Seed context should be injected at most once per request. A user turn should not contain duplicated seed messages in response.create.input, and should not fail with 400.

Additional information

Example mitmproxy flow IDs:

• WS failure with duplicated seed input + 400: 98651cf3-186d-4c30-9ac9-a8a99508e719
• - HTTP success (no duplication): 7f165578-3571-4735-af54-0e08c22c0b5a

I can attach full WS frames if needed.