Anthropic shipped Opus 5.5 on 22 September 2026 and Sonnet 5.5 six days later, so the question everyone is asking is whether the cheaper model is now good enough. Anthropic’s own framing is direct: Sonnet 5.5 is the faster, lower-cost complement for well-scoped everyday tasks, and Opus 5.5 is for complex work that needs careful judgment. The numbers below come from the two launch pages, not from third-party estimates.
Both are Claude 5.5 models with the same API shape, so switching is a one-string change. The practical decision is about cost per finished task, not the sticker price: Anthropic reports Sonnet 5.5 finishes tasks with fewer tokens and tool calls than Sonnet 5, and Opus 5.5 at 40% lower cost than Opus 5 on typical workloads.
On agentic coding the two are close and trade places: Sonnet 5.5 leads Terminal-Bench 4.0, Opus 5.5 leads FrontierCode and CursorBench. On knowledge work (GDPval-AA) they are two Elo points apart. The one row with a real gap is Chartography, where Opus 5.5 is far ahead — a hint of where the bigger model still earns its price: dense, multi-step reading and judgment rather than well-specified edits.
| Benchmark | Opus 5.5 | Sonnet 5.5 | Note |
|---|---|---|---|
| Terminal-Bench 4.0 | 66.4% | 70.6% | Sonnet 5 was 10.3% |
| FrontierCode 1.1 | 54.4% | 46.2% (high) | |
| CursorBench 4.0 | 57.8% | 55.5% | |
| GDPval-AA v2.1 | 1846 Elo | 1844 Elo | knowledge work |
| OSWorld 2.1 | 81.8% | 80.1% | computer use |
| Chartography | 89.0% | 61.6% | largest gap |
Sonnet 5.5 kept Sonnet 5’s price exactly, so the Sonnet line is half of Opus on both input and output. On apimodels.app Opus 5.5 costs $2.40 / $12 per million tokens, 40% below Anthropic’s list, billed per token with failed calls free. Sonnet 5.5 is not on our catalog yet; Sonnet 5 is available at $1.60 / $8, 20% below list, on the same key and endpoint.
| Per 1M tokens | Opus 5.5 | Sonnet 5.5 | Sonnet 5 |
|---|---|---|---|
| Anthropic list, input / output | $4 / $20 | $2 / $10 | $2 / $10 |
| Cache read / write (list) | $0.20 / $5 | $0.20 / $2.50 | — |
| apimodels.app | $2.40 / $12 | not yet listed | $1.60 / $8 |
| Released | 22 Sep 2026 | 28 Sep 2026 | earlier |
Pick Sonnet 5.5 when the task is well specified: fixing a known bug, writing tests for existing code, drafting docs, filling spreadsheets, running a terminal workflow you can describe step by step. Its Terminal-Bench lead and 30% faster output make it the better default for high-volume agent loops.
Pick Opus 5.5 when the task is open-ended or the cost of a wrong call is high: architecture changes across a large codebase, ambiguous requirements, long research, reading dense charts and documents. Anthropic’s own guidance is that Sonnet 5.5 stays clearly weaker on complex work that needs sustained judgment, and the Chartography gap agrees. Note that Opus 5.5 always thinks (adaptive thinking cannot be turned off); steer depth with effort, default medium.
Sonnet 5 versus 5.5: same price, but Anthropic reports 5.5 is over 30% faster and up to 30% cheaper per task, with Terminal-Bench going from 10.3% to 70.6%. If you are on Sonnet 5 for agentic coding, 5.5 is the upgrade to plan for.
cURL
curl https://api.apimodels.app/v1/messages \
-H "x-api-key: $APIMODELS_API_KEY" \
-H "anthropic-version: 2023-06-01" \
-H "Content-Type: application/json" \
-d '{
"model": "claude-opus-5-5",
"max_tokens": 16000,
"messages": [{"role": "user", "content": "Review this function for race conditions: ..."}]
}'Python
from openai import OpenAI
client = OpenAI(base_url="https://api.apimodels.app/v1", api_key="YOUR_APIMODELS_KEY")
# Same prompt, two models, one key: compare them on your own task before you commit.
for model in ["claude-opus-5-5", "claude-sonnet-5"]:
r = client.chat.completions.create(
model=model,
messages=[{"role": "user", "content": "Refactor this module and explain the risky parts: ..."}],
)
print(model, r.usage.prompt_tokens, r.usage.completion_tokens)
print(r.choices[0].message.content[:400])On some benchmarks, yes: Anthropic reports 70.6% vs 66.4% on Terminal-Bench 4.0. On most others Opus 5.5 is slightly ahead (CursorBench 57.8% vs 55.5%, GDPval-AA 1846 vs 1844) and on Chartography far ahead (89.0% vs 61.6%). Anthropic positions Sonnet 5.5 for well-scoped tasks and Opus 5.5 for complex judgment.
Half at list price: $2 / $10 per million tokens against $4 / $20. On apimodels.app Opus 5.5 is $2.40 / $12; Sonnet 5.5 is not listed yet and Sonnet 5 is $1.60 / $8.
Not yet. Opus 5.5 and Sonnet 5 are available now on the same key through /v1/messages or the OpenAI-compatible /v1/chat/completions. This page will be updated with the price the day Sonnet 5.5 is added.
Announced but not released. Anthropic said on 22 September 2026 that Haiku 5.5 would follow Opus 5.5 in the coming weeks, with no date, price or benchmarks yet. Until it ships, Sonnet 5.5 is the cheapest Claude 5.5 model.
Price is unchanged. Anthropic reports over 30% faster output, up to 30% lower cost per task through fewer tokens and tool calls, and Terminal-Bench 4.0 rising from 10.3% to 70.6%.
