Is Claude Sonnet 5 or GPT-5.6 Terra cheaper?

GPT-5.6 Terra is cheaper on all three of the workload shapes tested. On a RAG workload of 12k input and 700 output tokens at 50,000 requests per month with 60% caching, Claude Sonnet 5 costs $1,173 per month and GPT-5.6 Terra costs $972.00. Headline rates are $2/$10 versus $2/$12 per million input/output tokens.

Claude Sonnet 5 vs GPT-5.6 Terra

GPT-5.6 Terra is the cheaper of the two across every workload shape tested below. Headline rates are $2 in / $10 out for Claude Sonnet 5 versus $2 in / $12 out for GPT-5.6 Terra, per million tokens. Note that sticker price understates the gap: Claude Sonnet 5 tokenizes the same text into roughly 30% more tokens, which the figures below account for.

Real monthly cost, three workloads

Workload	Claude Sonnet 5	GPT-5.6 Terra	Cheaper
Support chatbot 1.5k in / 400 out · 100k req/mo · 30% cached	$804.70	$699.00	GPT-5.6 Terra (1.2× cheaper)
RAG / doc Q&A 12k in / 700 out · 50k req/mo · 60% cached	$1,173	$972.00	GPT-5.6 Terra (1.2× cheaper)
Coding agent 25k in / 2.5k out · 20k req/mo · 80% cached	$1,014	$880.00	GPT-5.6 Terra (1.2× cheaper)

Costs include prompt caching at the stated hit rate and per-model tokenizer correction. Adjust for your own traffic →

Specifications side by side

	Claude Sonnet 5	GPT-5.6 Terra
Provider	Anthropic	OpenAI
Input / M tokens	$2	$2
Output / M tokens	$10	$12
Cached input	$0.2	$0.2
Context window	1000K	1050K
Batch discount	−50%	−50%
Tokenizer factor	×1.3	×1
Min prefix to cache	1024	—
Announced retirement	2027-06-30+	—

Which should you pick?

On cost alone, GPT-5.6 Terra wins every workload shape above.

Caching behaviour differs. Claude Sonnet 5 requires a prefix of at least 1,024 tokens before caching activates. Below those thresholds cache_control is ignored silently, with no error — which can quietly erase the saving you were counting on.

Lifecycle matters as much as price here. Claude Sonnet 5 has an announced shutdown date of 2027-06-30 or later. A model that is marginally cheaper but retires within your planning horizon is rarely the better choice.

Full detail: Claude Sonnet 5 · GPT-5.6 Terra

All figures verified 2026-08-18 against official provider documentation. Fields a provider does not publish are shown as “—” rather than estimated.