Switching from Sol to the Codex Model

This is the migration this whole site's headline comparison is built around, and it's worth working through concretely rather than just reading the spec sheet — what actually changes about how you work once you've made the switch, not just what the numbers say on paper.

The immediate, visible changes

Per-token price drops meaningfully on both input and output — the first thing you'll notice in any cost tracking you have in place. Context window shrinks, which matters for exactly the workloads that were leaning on Sol's larger window: a session accumulating a lot of file content or long conversation history will hit the smaller ceiling sooner than it would have on Sol. Knowledge cutoff moves backward by several months, worth checking against your actual task if it depends on very recent libraries, APIs or events.

The control you gain that you didn't have before

The Codex-optimized model publishes explicit reasoning-effort levels — a control Sol's own page doesn't state at all. If your workflow would benefit from tuning how much the model reasons before answering — faster, lighter answers for straightforward edits, deeper reasoning for a genuinely tricky problem — this migration is the only way to get that control on this site's current roster, not a nice-to-have that happens to come bundled with the price drop.

What doesn't change, and why that matters

Rate limits stay in the same published tier group, so this migration costs nothing in throughput — a detail worth stating plainly because it removes one variable from the decision entirely. Maximum output length is published as identical between the two as well, so response-length ceilings aren't part of this tradeoff either. What you're actually trading is narrower than "everything about the model changes": it's specifically context headroom and recency against price and reasoning control.

The one thing that goes from known to unknown

Sol publishes the long-context repricing rule explicitly. The Codex-optimized model's own page doesn't state it either way — recorded honestly here as not published, not assumed to inherit Sol's rule and not assumed absent either. If your prior workflow depended on understanding exactly where that threshold sat to budget large requests, this migration removes that certainty; you're working with a smaller window in the first place, which naturally reduces how often you'd approach a long-context threshold regardless of whether one applies, but the rule itself is genuinely unstated for this model.

Checking your actual session sizes before committing

The single most useful thing to do before this migration, rather than after, is measure how close your typical sessions actually get to the smaller model's window using the context-window planner. A session that comfortably fits today under Sol's larger window may not fit as comfortably — or at all — once you've switched, and finding that out before switching, rather than mid-task after the migration, is a much better position to discover it from.

Making the switch itself

/model changes the active model for a single running session — the right tool for trying the migration on one task before committing broadly. For a persistent default across every new session, the CLI's config file is where that setting belongs; see the config-file reference for the exact key rather than typing /model at the start of every session indefinitely.

Reversing the decision if it doesn't work out

Nothing about this migration is permanent or hard to undo — switching back to Sol, whether mid-session with /model or by reverting a config default, costs nothing beyond the switch itself. Treating this as a real trial worth testing on actual work, rather than a one-way decision to agonize over, is the more useful way to approach it: run a few real sessions on the Codex-optimized model, see whether the smaller window and older knowledge actually bite in practice, and decide from real evidence rather than from the spec sheet alone.

A concrete way to run the trial

Rather than switching every session going forward and hoping for the best, pick a specific, recurring task — a category of bug fix, a specific kind of refactor you do often — and run it on the Codex-optimized model exclusively for a couple of weeks while keeping everything else on the default. That gives you a real, comparable sample of how the smaller window and reasoning-effort control actually perform against the specific work you do most, rather than a single anecdotal session that may not be representative of your broader usage.

What a favorable outcome actually looks like

A successful migration for a given workflow looks like sessions that complete in about the same number of turns, without the model needing more back-and-forth to compensate for a smaller window than it had before, and a total cost that's meaningfully lower once you account for the whole session, not just the sticker price per token. If sessions are taking noticeably more turns or hitting the smaller context ceiling regularly, that's a real signal the window difference matters for this specific workflow, and staying on the larger-window default for it is a legitimate, informed decision — not a failure to successfully migrate, just evidence this particular task doesn't fit the tradeoff being offered.

Mixing both models within one workflow

There's no rule requiring an all-or-nothing choice — a team might reasonably run most day-to-day interactive work on the default for its larger window and headroom, while routing a specific, well-scoped, high-volume automated task (a CI check, a batch review pass) to the Codex-optimized model specifically for its lower price and reasoning-effort control. Deciding per workflow, rather than per account, is a perfectly reasonable way to capture the benefit of each without forcing a single global choice that's imperfect for at least some of what you do.