Claude Opus 5.5 vs Sonnet 5.5: which one to use

TL;DR Sonnet 5.5 (released 28 September 2026) is within about two points of Opus 5.5 on most of Anthropic’s published benchmarks and even ahead on Terminal-Bench 4.0, at half the list price ($2/$10 vs $4/$20). Use Sonnet 5.5 for well-scoped everyday coding and office work; keep Opus 5.5 for long, open-ended tasks that need sustained judgment. On apimodels.app Opus 5.5 is $2.40/$12 and Sonnet 5.5 is $1.20/$6, both 40% below list.

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 more affordable 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.

Benchmarks: Anthropic’s published numbers

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, and on chart reading without tools (Chartography) about three. The widest gap in Anthropic’s table is FrontierCode, which checks whether a change could be merged without human edits: 54.4% against 46.2% — a hint of where the bigger model still earns its price, in 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% (Max effort)	largest gap
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
Humanity’s Last Exam (tools)	67.7%	64.5%
Chartography (no tools)	64.4%	61.6%	chart reading

Price: official list and apimodels.app

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 costs $1.20 / $6 with cache reads at $0.12 and writes at $1.50, also 40% below list, which puts it under Sonnet 5 ($1.60 / $8) 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	$1.20 / $6	$1.60 / $8
Released	22 Sep 2026	28 Sep 2026	earlier

Which one to use

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 FrontierCode 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 list price, but Anthropic reports 5.5 is over 30% faster and up to 30% less per task, with Terminal-Bench going from 10.3% to 70.6%. On apimodels.app 5.5 is also lower per token ($1.20 / $6 against $1.60 / $8), so if you are on Sonnet 5 for agentic coding, switching is a one-string change.

Inicio rápido

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-5-5"]:
    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])

Preguntas frecuentes

Is Sonnet 5.5 better than Opus 5.5?

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 FrontierCode clearly ahead (54.4% vs 46.2%). Anthropic positions Sonnet 5.5 for well-scoped tasks and Opus 5.5 for complex judgment.

How much more affordable is Sonnet 5.5 than Opus 5.5?

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 $1.20 / $6 and Sonnet 5 is $1.60 / $8.

Can I call Sonnet 5.5 on apimodels.app?

Yes, since 6 October 2026. Use model claude-sonnet-5-5 on /v1/messages or the OpenAI-compatible /v1/chat/completions, at $1.20 / $6 per million tokens with cache reads at $0.12 and writes at $1.50, 40% below list. Opus 5.5 and Sonnet 5 are on the same key.

What about Claude Haiku 5.5?

Released on 7 October 2026. Haiku 5.5 is the high-volume tier of the 5.5 family: $0.10 / $0.50 per 1M tokens on Anthropic's list and $0.09 / $0.45 on apimodels.app, with every rate x5 once a prompt passes 100K tokens. Anthropic's own advice is to keep Sonnet 5.5 or Opus 5.5 for complex agentic coding — on Terminal-Bench 4.0 Haiku 5.5 scores 39.2% against Sonnet 5.5's 70.6%.

What changed from Sonnet 5 to Sonnet 5.5?

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%.

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