Sonnet 5 is now the cheaper model. That is not the same as the cheaper answer.

At the GPT-5.6 launch the story was simple. Terra was the cheap capable one and OpenAI was undercutting Anthropic on price.

Check the two pricing pages today and that has quietly inverted.

Identical on input. Sonnet 5 is 17% cheaper on output. Terra is two index points ahead.

So the pairing has become close, in the way that makes people waste an afternoon on a spreadsheet and still pick wrong.

Why the price comparison flipped

Neither side announced it as a response to the other.

OpenAI cut the whole GPT-5.6 line. Terra went from $2.50 / $15 to $2 / $12. Sol and Luna came down too, Luna hardest, from $1 / $6 to $0.20 / $1.20.

Anthropic did something quieter that mattered more. It cancelled a price rise. Sonnet 5 launched at $2 / $10 described as introductory pricing that would end on 31 August 2026 and step up to $3 / $15. That step never happened, and the docs now say plainly that the increase will not occur. Sonnet 5 sits at $2 / $10 permanently.

Had that rise gone through, Terra at $2 / $12 would today be the cheaper model on both axes and this article would not exist. One cancelled price change decided the entire comparison.

The number that actually decides the bill

A 17% output discount sounds decisive until you look at how many tokens each model spends getting to the end.

Artificial Analysis found Sonnet 5, at maximum effort, generating around 300 million tokens during its evaluation. That was roughly five times the comparison median. Claude Code users have reported the same shape independently: cheap per token, but on large jobs it takes more steps, consumes more context, and can finish more expensive than a stronger model would have been.

Put those two facts next to each other and the arithmetic stops being about rates. A model 17% cheaper per token that takes twice as many steps costs you more. The one that is 20% dearer and finishes in a single pass is the bargain.

This is the same argument I made about Luna against Sonnet 5, where the price gap is tenfold and still does not decide it, and about GPT-6 Astra, where OpenAI’s entire cost claim rests on step count rather than rate.

It cuts both ways here. I am not telling you Terra wins on cost per task. I am telling you that neither of us knows until it runs on your work, and that the two-dollar row in the price table is not evidence.

The tiebreaker nobody puts in the comparison

Here is the part that decides it in practice, and it has nothing to do with either model.

Sonnet 5 is the default inside Claude Code. Terra lives in ChatGPT and Codex. If you already run one of those harnesses, the model is not really the unit you are choosing. You are choosing the tool, the prompts you have tuned, the permissions you have configured, and whatever scaffolding you built around it.

Moving that costs more than either price gap recovers, and not in licence fees. The cost is the fortnight where everything is slightly worse while you rebuild the parts you forgot you had.

What would change my mind

Three things, and I will update this piece when any of them is published.

The next Artificial Analysis round on v4.1.1, with both models measured on the same scale. Two points on the old scale may be one point or four on the new one.

A published cost-per-completed-task comparison for these two specifically. The 300 million token figure is the only step-count evidence in this article and it comes from one evaluation of one model.

Any further move on price. This entire comparison turned on a rise that was cancelled rather than on anything either model does, which tells you how thin the margin is.

Questions people asked

Is GPT-5.6 Terra better than Claude Sonnet 5?

Modestly, on capability: 55 against 53 on the index reported at the GPT-5.6 launch. On price the order reverses, since both cost $2 per million input tokens but Sonnet 5 outputs at $10 against Terra’s $12. Two points for a 20% higher output rate is close enough that the harness you already use usually decides it.

Which is cheaper?

Sonnet 5, on list price. That only settles the bill if both take a similar number of steps, and Sonnet 5 has the worse record there: around 300 million tokens on one benchmark, five times the median. On long runs the cheaper rate can produce the larger invoice.

Should I switch?

Not on these numbers alone. The gap is smaller than the cost of changing harness, re-tuning prompts and rebuilding tooling. Switch when you are moving ecosystem anyway, or when you have measured your own cost per finished task and Terra wins it.

What is Terra actually for?

It is the balanced tier, covering analysis, research, document work and moderately hard coding, with Sol kept as an escalation rather than a default. If your work is professional output that rarely needs frontier reasoning, that is the tier it was designed for.

If you want this applied to your own stack rather than to a price table, the contact page is the fastest route to me.