The Careful One Got Cheap: Claude Sonnet 5.5 vs Opus 5.5 and GPT-6 Sol

Anthropic released Claude Sonnet 5.5 at 11:03 AM PT on September 28. Six days earlier I ran Opus 5.5 against GPT-6 Sol on six real changes from a production monorepo. Opus was the careful one: zero regressions in 20 runs. Sol cost 3.5x less and broke a passing test five times.

Sonnet 5.5 has exactly Sol’s per-token price. So the question is simple: can you get Opus’s care at Sol’s price?

The Sticker

Anthropic’s developer guide splits the work cleanly. Sonnet 5.5 is for “well-scoped everyday tasks”, Opus 5.5 for “complex work requiring careful judgment”.

For the hardest long-horizon work, an Opus model is the better choice.

— Anthropic, Sonnet 5.5 prompting guide

Anthropic’s own table is less clear about it. Sonnet 5.5 beats Opus 5.5 on Terminal-Bench 4.0 (70.6% against 66.4%) and trails it on every other row, by up to 8 points. The system card explains part of that win. Safeguards sent 1.2% of Sonnet’s Terminal-Bench requests to a fallback model, affecting 1.5% of trials. For Opus the figure was 10% of trials. The stated standard error is 2.5 points.

The Same Six Changes

I used the same bench as last week: six merged changes, each replayed from the commit before it on a clean snapshot with no git history, and graded by the tests the original fix shipped with. Same ticket-style prompts, same global rules file, medium effort, 20 runs. Every Sonnet run was served by Sonnet 5.5. None fell back.

Per change, clean runs and mean cost per run:

• It keeps what works. On Change A, Sol rewrote an existing user-facing message in all four runs and dropped the instruction it carried. Sonnet kept it in all four, like Opus. It also wrote smaller diffs than either: about 184 added lines on A, against about 240 for Sol and 460 to 495 for Opus.
• One change is almost the whole gap. On Change D, all three Sonnet runs failed the same six tests: the edge cases where the input should be ignored or left alone. Sonnet did add the safety check. It handled the edges differently from the original fix, and it did so the same way every time. On a different codebase, a different change would sit in that slot.
• Coverage is slightly behind. Sonnet changed 82% of the files the original fix changed. On Change B, one screen that no test covers needed an update: Opus fixed it in three of four runs, Sonnet in one, Sol in none. On Change C, all three models skipped the same part of the codebase.
• The price gap is real. Opus cost 2.8x to 4.7x as much as Sonnet per change, 3.9x overall. Sonnet came in under Sol overall, although Sol’s figure has no cache-write charge and Sonnet’s does.

Only Cheap at Medium

The cheap-model story depends on the effort setting. Artificial Analysis runs both models at max effort. Sonnet 5.5 writes 410M output tokens across its index, against 260M for Opus 5.5, and costs $7.60 per task, against $5.98. At max, the cheaper model is the more expensive one.

Anthropic’s own FrontierCode table says more thinking does not buy more score. Sonnet scores 52.1% at xhigh and 46.2% at max, because at max it more often ran a code-review skill and timed out or made out-of-scope edits. Simon Willison’s max-effort pelican “thought for 128,000 tokens” and ran out before it produced an SVG. At xhigh it took 41 seconds and about 6 cents.

Claude Code runs Sonnet 5.5 at medium by default. The API defaults to high. Check which one you are paying for.

Update: Every Effort Level

After publishing, I reran all three models at low, high, xhigh and max on the same six changes: 20 runs per model per level. The Opus and Sol medium figures come from a same-day rerun. No run timed out, fell back or hit a rate limit. The Codex logs confirm Sol’s effort level. For the Claude runs the CLI accepted the flag, and output tokens rose at every step, but the requests don’t record the setting.

• Sonnet broke nothing at any level. Zero regressions in 100 runs.
• Only Sonnet at max beat medium. It closed the Change D blind spot (three clean runs of three) for 13 clean runs, at 35x the medium bill.
• At max, Sonnet costs more than Opus. $415 against $307 on this bench, which matches Artificial Analysis.
• My bench disagrees with the FrontierCode footnote. Sonnet at max scored higher than at xhigh here, not lower.

I wrote up the full effort curve, including what more effort did to Opus, separately.

Was Opus Nerfed?

On launch day a friend said Opus 5.5 “seems token hungry today”. A few Reddit posts say the same about Max plan limits. About as many say usage is fine, and I found nothing from Anthropic about a change.

My own Claude Code transcripts don’t show it. Across 888 Opus 5.5 turns after the launch, the median output per turn was 368 tokens, against 355 earlier that day. Context per turn was flat at about 275k tokens. That measures the model, not the meter. One Reddit user thinks cache reads now count against the Max quota. If that is true, tokens would stay flat while limits drain faster. I can’t confirm it from my data, and neither can anyone else yet.

OpenAI showed the same week what a disclosed change looks like. Thibault Sottiaux of the Codex team announced that the reopened $200 Pro plan will “net out at half the dollar in API spend compared to the old Pro $200 plan”. In exchange, OpenAI committed to “not reintroducing the 5h limit”. If Anthropic has changed how Max counts Opus, it has not said so.

What This Doesn’t Show

• Three or four runs per change. Enough to see a habit repeat, not tight rates. Sonnet’s gap to Opus is one change.
• One codebase, my prompts, a newer CLI. The Opus and Sol runs used an earlier Claude Code version than the Sonnet runs.
• List prices, not my bill. All of it ran on subscriptions.

What I’m Changing

• Sonnet 5.5 at medium replaces Sol where tests and review are strong. Same price, and no regressions in 100 runs across five effort levels.
• Medium, not high or xhigh, for Sonnet. Neither added a clean run.
• Opus 5.5 stays for thin tests and unattended runs. Its edge over Sonnet is small, and it is the edge-case handling Sonnet missed on Change D.
• Max on Sonnet only when a clean run is worth 35x. It was the best result on my bench, and it cost more than Opus at max.

Last week the choice was care or price. This week the care costs the same as the cheap option, as long as you leave the effort on medium.