Feature request: allow a separate model for context compaction

What variant of Codex are you using?

CLI

What feature would you like to see?

Add a dedicated configuration option to let Codex use a different model for context compaction than the main conversation model.

Today compaction appears to use the same model path / session model that is handling the main thread. That makes compaction expensive and, depending on the selected model, can also reduce reliability or throughput for long-running sessions.

A separate compaction model would make it possible to keep a strong main model for normal work while using a smaller / cheaper / faster model for summarization-only compaction work.

Examples:

• main thread: gpt-5-codex
• compaction: gpt-5-codex-mini

or

• main thread: high-capability planning / execution model
• compaction: lightweight summarization model

Potential config shapes:

model = "gpt-5-codex"
compaction_model = "gpt-5-codex-mini"

or

[compaction]
model = "gpt-5-codex-mini"

Nice-to-have related controls:

• optional reasoning_effort override for compaction
• explicit fallback to main model if the compaction model is unavailable
• visibility in /status or logs showing which model handled the most recent compaction

Why this would help:

• reduces cost for long-lived sessions
• can make compaction faster
• lets users tune quality vs cost separately for summarization work
• matches other requests for per-role / per-phase model selection in Codex

Additional information

I searched for an existing issue specifically about using a separate model for compaction and did not find one. The closest related issues I found were:

• #4106 (Control over auto-compaction parameters)
• #5558 (Separate model selection for Plan vs Execute modes)
• #11795 (Allow configuring subagent model and reasoning_effort in config)

Those are adjacent, but this request is narrower and specifically about the compaction path.