## Why
Thread settings can change while a turn is running, and delegated review or
compaction steps can select a different model. Each request needs a consistent
set of settings resolved against the model that will execute it.
## What changed
- Capture immutable resolved settings for each model step, including model
metadata, reasoning options, service tier, approvals, and personality.
- Keep in-flight steps on their captured settings while applying thread updates
to subsequent work.
- Resolve inherited reasoning summaries and filter service tiers against the
selected model and feature configuration for review and compaction steps.
## Testing
Added coverage for settings updates during paused turns, model changes,
previous-model compaction, review-model defaults, and service-tier filtering.
GitOrigin-RevId: fca4b3b305379f1bf26a5cac75ee0dd1747ac1f6
## Why
Token-budget sessions need a final opportunity to preserve important state before the current context is compacted.
## What changed
- When `auto_compact_fallback_prompt` is configured, reserve `auto_compact_fallback_buffer_tokens` beyond the base auto-compaction limit.
- At the base limit, inject the fallback prompt once as a developer message while keeping the normal tool surface available. Continue reporting zero base-window tokens during this reserve.
- Roll over when the reserve or full model context is exhausted, and skip the fallback when a new context was already requested.
## Testing
Added coverage for using the fallback reserve, rolling over after it is exhausted, and bypassing the fallback for an explicit `new_context` request.
GitOrigin-RevId: 32983ecadc5cb4927e15ecd7b1e896cb52a9ad3b
## Why
With `model_auto_compact_token_limit_scope = "body_after_prefix"`, the
persistent prefix should not count against the active body window.
`get_context_remaining` and the token-budget reminder should report the
same usable body-after-prefix window that auto-compaction uses, rather
than the total token count since the session began.
This is stacked on #29664 so the mechanical move from `turn.rs` is
isolated from the behavior fix.
## What
- Extends `ContextWindowTokenStatus` with `context_remaining_tokens`.
- Updates `get_context_remaining` to use the shared context-window
accounting.
- Adds integration coverage for body-after-prefix reminder timing and
`get_context_remaining` output.
## Testing
- `just test -p codex-core body_after_prefix_window`
- `just test -p codex-core auto_compact_body_after_prefix`
- `just fix -p codex-core`
## Why
This PR keeps the mechanical helper extraction separate from the
behavior change in #29665. The follow-up needs the token-window
accounting from `turn.rs` in another call path, but reviewing that is
much easier when the helper extraction is separate from the semantic
change.
## What
- Adds `session/context_window.rs` with `ContextWindowTokenStatus`.
- Moves the existing auto-compaction token-status calculation out of
`session/turn.rs`.
- Replaces the duplicated inline remaining-token calculation in
`turn.rs` with `tokens_until_compaction()`.
This PR is intended to be behavior-preserving. The
`get_context_remaining` behavior change is stacked separately in #29665.
## Testing
- `just test -p codex-core auto_compact_body_after_prefix`
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/29664).
* #29665
* __->__ #29664
joeflorencio-openai