Auto 1M context mode for supported models to balance speed/cost vs large-context reliability

[zemaj]

What variant of Codex are you using?

CLI

What feature would you like to see?

Add a first-class auto 1M context mode for supported models, so Codex can use the larger context window only when it is actually needed.

This is related to #13653.

Problem

Right now, using 1M context is basically all-or-nothing.

If I manually configure a model for a 1M context window, then every turn pays the tradeoff:

• potentially slower responses
• higher usage cost
• more aggressive large-context behavior than many turns actually need

But if I do not configure 1M context, then long-running or context-heavy turns can fail or compact too early.

What I want is not “always 1M” and not “never 1M”.
I want:

• normal/faster/cheaper behavior most of the time
• 1M context only when the upcoming turn actually needs it

Proposed UX

Introduce a higher-level config like:

context_mode = "disabled" | "1m" | "auto"

Suggested behavior:

• disabled
  • use the normal context tier / existing behavior
• 1m
  • always use the 1M context tier on supported models
• auto
  • default to normal behavior for most turns
  • evaluate each new idle user turn before sending it
  • if the upcoming turn appears small enough, stay on the normal path
  • if the upcoming turn is large enough, switch into 1M-style behavior and compact only when needed

Why this matters

The main value of auto is balancing:

• speed
• token usage cost
• reliability on long or complex follow-up turns

In other words:

• most turns should stay fast and cheap
• only the turns that really need large context should pay for large context

Relation to #13653

I think #13653 is still useful for explicit context-tier selection.

But even with explicit tier selection, there is still a missing UX:

• users often do not want to choose “always 1M”
• they want Codex to decide when large context is necessary

So I think these are complementary:

• Support explicit context-length selection for models with multiple context tiers #13653: explicit context-length selection
• this issue: an automatic mode that avoids the all-or-nothing tradeoff

Suggested implementation direction

A reasonable implementation might:

• only enable 1M behavior on models that actually support it
• estimate the next-turn in-context size, not lifetime token totals
• skip any 1M/compaction logic below a safe lower bound
• compact immediately near the top of the effective 1M window
• in the middle band, prefer preserving recent context for continuation/follow-up turns
• keep low-level knobs like model_context_window and model_auto_compact_token_limit as advanced overrides, not the main UX

UI ideas

Surface context mode and status in /context, for example:

• 1M Context
• 1M Auto
• Checking context...
• Compacting...

Additional information

The main thing I’m trying to solve is the current all-or-nothing tradeoff:

• use 1M when necessary
• avoid it most of the time to improve speed and usage cost

[zemaj]

Wrote a patch for this here: https://github.com/zemaj/codex/tree/context-mode