Session "set reasoning_effort" applies to the next turn only, then reverts to the config default

[superbiche]

Setup: codex-acp driven as a persistent named session over ACP (client: acpx). Tested on codex-acp 1.1.4; I found no related fix in the 1.1.5–1.1.7 changelogs and no existing issue matching this.

Reproduction:

1. Create a session, set model gpt-5.6-sol, set reasoning_effort xhigh — the set acks (config set: reasoning_effort=xhigh (5 options)).
2. Send a prompt. The codex rollout jsonl (~/.codex/sessions/.../rollout-*.jsonl) records "effort":"xhigh" in that turn's turn_context — the set applied.
3. Send further prompts on the same session without re-issuing the set. Each later turn_context records "effort":"high" (the codex config default) — the session setting silently reverted.

Observed twice independently: a three-turn session with turn 1 at xhigh and turns 2–3 at high; a longer multi-turn session with 29 turn_context entries at high vs 1 at xhigh.

Expected: session-level reasoning effort persists for the session's lifetime, like set model does. The model set through the same path sticks across all turns, which is why this looks like adapter-side session-config handling rather than the client dropping it — but happy to re-test against a newer version or provide full rollout excerpts if useful.

Workaround: re-issue set reasoning_effort immediately before every prompt.

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_{@superbiche · user · drafted with Claude Fable 5, reviewed before posting; the voice and decisions are mine.}