Is Claude Opus 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 Opus 5 costs $2,931 per month and GPT-5.6 Terra costs $972.00. Headline rates are $5/$25 versus $2/$12 per million input/output tokens.

Claude Opus 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 $5 in / $25 out for Claude Opus 5 versus $2 in / $12 out for GPT-5.6 Terra, per million tokens. Note that sticker price understates the gap: Claude Opus 5 tokenizes the same text into roughly 30% more tokens, which the figures below account for.

Real monthly cost, three workloads

Workload	Claude Opus 5	GPT-5.6 Terra	Cheaper
Support chatbot 1.5k in / 400 out · 100k req/mo · 30% cached	$2,012	$699.00	GPT-5.6 Terra (2.9× cheaper)
RAG / doc Q&A 12k in / 700 out · 50k req/mo · 60% cached	$2,931	$972.00	GPT-5.6 Terra (3.0× cheaper)
Coding agent 25k in / 2.5k out · 20k req/mo · 80% cached	$2,535	$880.00	GPT-5.6 Terra (2.9× 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 Opus 5	GPT-5.6 Terra
Provider	Anthropic	OpenAI
Input / M tokens	$5	$2
Output / M tokens	$25	$12
Cached input	$0.5	$0.2
Context window	1000K	1050K
Batch discount	−50%	−50%
Tokenizer factor	×1.3	×1
Min prefix to cache	512	—
Announced retirement	2027-07-24+	—

Which should you pick?

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

Caching behaviour differs. Claude Opus 5 requires a prefix of at least 512 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 Opus 5 has an announced shutdown date of 2027-07-24 or later. A model that is marginally cheaper but retires within your planning horizon is rarely the better choice.

Full detail: Claude Opus 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.