state_conflict
What happened
A chat moves through states, and an action is accepted only in the state it belongs to. When the chat is in another state, because something else changed it, because the text you were confirming changed, or because another turn is still running, the action is refused with 409 and nothing is changed. The reply names the chat's state in state.
What to do
Refresh the chat to see where it is, then repeat the action. If another turn is still running, wait for it to finish.
Example
json
{
"error": "This chat is not ready for that action right now.",
"code": "state_conflict",
"state": "in_flight_elsewhere"
}