gpt-5.6 on the Codex backend answers a large turn with a server-side
`compaction` checkpoint and no message. The checkpoint rides the
`codex_reasoning_items` sidecar, so the interim assistant message looks
"replayable" and `interim_replayable` suppresses the continuation nudge.
But replayable is not the same as different. A checkpoint carries no
answer and no new instruction, and a replayed checkpoint makes
`prune_pre_checkpoint_items` drop every pre-checkpoint item. Measured on
a real 262-message session: the wire collapses from 489 items to 12 —
all 186 `function_call` / `function_call_output` pairs deleted — and
ends on an empty assistant turn. The model has nothing to answer, so it
returns another empty response; the next attempt sends the same bytes
(the provider's prefix cache reports 99-100% on the repeats) and returns
the same nothing. Three attempts later the turn dies with "Codex
response remained incomplete after 3 continuation attempts" and the
whole turn's work is lost.
Keep the first continuation bare — the model often just needs another
turn, and nudging immediately would cut multi-phase work short. Once
that bare retry has also come back incomplete, it is proven not to work
for this turn, so every remaining attempt carries the nudge.
james47kjv
80764b6d39