When Claude Opus 5.5 Earns Its Price Over Claude Sonnet 5.5
When Claude Opus 5.5 Earns Its Price Over Claude Sonnet 5.5
Here are three kinds of work where the Fronset benchmark scored Claude Sonnet 5.5 and Claude Opus 5.5 on the same task. On two, the scores are tied and Claude Sonnet 5.5 costs far less. On the third, Claude Opus 5.5 is clearly better, and it is the only one of the three where the extra spend buys quality.
How to tell a tie from a win
Each score in the benchmark carries an interval. When two models’ intervals overlap, the benchmark cannot say which one is better, and price or serving mode should decide. Only a gap that clears both intervals is a real difference in quality.
This matters because the usual instinct for important work is to send it to the bigger model. That instinct costs money every time the bigger model’s lead sits inside the interval.
The three comparisons
Scores are out of 10, with their intervals. Prices are as listed in the benchmark.
| Kind of work | Claude Sonnet 5.5 | Claude Opus 5.5 | Reading |
|---|---|---|---|
| Judging whether a claim stays consistent | 8.69 (±0.26) at $0.03 per sync run [^1] | 8.6 (±0.24) at $0.36 [^1] | Tied |
| Writing image-generation prompts | 8.8 (±0.14) at $0.01 [^2] | 8.93 (±0.14) at $0.03 [^2] | Tied |
| Writing section prompts | 8.17 (±0.46) at $0.05 per sync run [^3] | 9.11 (±0.22) at $0.10 [^3] | Claude Opus 5.5 ahead |
On claim consistency, the two models are tied and Claude Sonnet 5.5 costs a twelfth as much [^1]. That is the strongest value case in the set: paying for Claude Opus 5.5 here buys nothing measurable.
On image-generation prompts, the benchmark itself says Claude Sonnet 5.5, Claude Opus 5.5, GLM-5.3 and Thinking Machines Inkling cannot be ordered [^2]. Claude Sonnet 5.5 costs a third of what Claude Opus 5.5 does on this work [^2].
A smaller model is worth a look here too. Claude Haiku 5.5 scores 8.55 (±0.19) at $0.0007 on the same task, and its interval overlaps Claude Sonnet 5.5’s [^2]. The benchmark notes that some prices on this task are repriced at current rates rather than measured, so small cross-provider price gaps are indicative only [^2].
Section prompts are where the result flips. Claude Opus 5.5’s lead clears both intervals [^3], at twice the price. Claude Sonnet 5.5’s interval on this work is also the widest among its reported scores, so its result is less certain as well as lower.
What to do with it
- Route claim-consistency judging to Claude Sonnet 5.5 instead of Claude Opus 5.5, after checking its answer rate (see below).
- For image-generation prompts, start with Claude Sonnet 5.5 and test Claude Haiku 5.5 against your own quality bar.
- Pay for Claude Opus 5.5 on section prompts when the quality bar is high.
- On work not covered here, compare intervals before assuming the bigger model wins. A missing comparison is not evidence either way.
The caveat that matters: answer rate
A score covers only the calls that came back, so it has to be read next to the answer rate [^4][^5]. On claim consistency, Claude Opus 5.5’s score comes at a 19% answer rate [^1]. The benchmark gives no answer rate for Claude Sonnet 5.5 on that work, so its coverage there is unknown.
A tie on quality therefore says nothing about how often each model returns an answer. If your pipeline needs a result on nearly every call, measure Claude Sonnet 5.5’s answer rate on your own traffic before switching. Read the other way, Claude Opus 5.5’s low answer rate is one more reason not to treat it as the safe default for that work.
The live benchmark at https://fronset.ai/benchmark/ shows each task’s scores, intervals and prices. Set the quality bar your workload needs, compare the tiers that clear it, and check batch against sync pricing before committing a route.