Is Claude Sonnet 5.5 as good as Opus 5.5?

Close on most published benchmarks, not on open-ended work. As of September 28, 2026, Sonnet 5.5 scores 1844 vs 1846 on GDPval-AA v2.1, 80.1% vs 81.8% on OSWorld 2.1, 55.5% vs 57.8% on CursorBench 4.0, and 52.1% vs 54.4% on FrontierCode 1.1 — and it beats Opus 5.5 on Terminal-Bench 4.0 (70.6% vs 66.4% at xhigh). Anthropic says that in its own and external testing Opus 5.5 remains clearly stronger at complex, open-ended tasks requiring sustained judgment.

How much cheaper is Sonnet 5.5 than Opus 5.5?

Exactly half on list price: $2 vs $4 per million input tokens, $10 vs $20 output, $2.50 vs $5 cache writes; cache reads are $0.20 on both. On the reference 30K-input / 5K-output agent step that is $0.11 for Sonnet 5.5 versus $0.22 for Opus 5.5. Per task the gap can be wider or narrower because Sonnet 5.5 at low or medium effort emits far fewer thinking tokens than Opus 5.5, while at max effort Anthropic says the two land at similar cost with comparable results.

When should I use Opus 5.5 instead of Sonnet 5.5?

Use Opus 5.5 for multi-hour autonomous work, ambiguous specs, architecture and research tasks where judgment matters more than speed, and for the highest Humanity's Last Exam-style reasoning (67.7% vs 64.5%). Use Sonnet 5.5 for well-scoped coding tasks, bug fixes, documents, slides, spreadsheets, UI polish and latency-sensitive chat — it is 30%+ faster than Sonnet 5 and often within two points of Opus 5.5 at half the price.

Can I mix Sonnet 5.5 and Opus 5.5 in one agent?

Yes, with two caveats as of September 28, 2026. First, Sonnet 5.5 cannot read thinking blocks produced by Opus 5, Opus 5.5, Fable or Mythos models — the API silently drops them (200 response, not billed), so a handoff loses the reasoning trail. Second, the advisor tool lets a Sonnet 5.5 executor consult Opus 5.5, Opus 5, Sonnet 5.5, Fable 5/5.1 or Mythos 5/5.1 as advisor, with advice returned encrypted. The common pattern is Sonnet 5.5 as the worker and Opus 5.5 as planner or reviewer.

Quick Answer

Claude Sonnet 5.5 vs Opus 5.5: Which to Use (Sep 2026)

Published: September 29, 2026

The short answer

Claude Sonnet 5.5 (September 28, 2026) is half the price of Claude Opus 5.5 (September 22, 2026) and lands within two points of it on most of Anthropic’s published benchmarks, so it is the default for well-scoped coding, documents and chat; Opus 5.5 is still the model for open-ended, multi-hour work that needs sustained judgment. Sonnet 5.5 costs $2/$10 per million tokens versus $4/$20, generates 30%+ faster than Sonnet 5, and actually beats Opus 5.5 on Terminal-Bench 4.0 (70.6% vs 66.4%). Anthropic’s own line is that Opus 5.5 “remains clearly stronger at complex, open-ended work.”

Side by side

	Claude Sonnet 5.5	Claude Opus 5.5
Released	Sep 28, 2026	Sep 22, 2026
Input / output (per MTok)	$2 / $10	$4 / $20
Cache read / 5m write / 1h write	$0.20 / $2.50 / —	$0.20 / $5 / $8
Fast mode	—	$8 / $40 (API only)
Context / max output	1M / 128K	1M / 128K
Knowledge cutoff	June 2026	June 2026
Default effort (API)	High	Medium
Default effort (Claude Code, apps)	Medium	Medium
Effort levels	5 (low → max)	5 (low → max)
Cyber safeguards	Yes (falls back to Sonnet 5)	Yes
AA Intelligence Index	Not yet scored	58 (current index)
Terminal-Bench 4.0	70.6%	66.4% (xhigh)
FrontierCode 1.1 (Main)	52.1% (xhigh)	54.4%
CursorBench 4.0	55.5%	57.8%
GDPval-AA v2.1	1844	1846
AA-Briefcase v1.1	1811	1822
Humanity’s Last Exam (tools)	64.5%	67.7%
OSWorld 2.1 (partial)	80.1%	81.8%
Chartography (no tools)	61.6%	64.4%
Reference 30K-in / 5K-out step	$0.11	$0.22

Benchmarks are Anthropic’s, published September 28, 2026. Opus 5.5 specs and its AA score are from the Opus 5.5 launch.

Where Sonnet 5.5 is “good enough”

The two-point pattern is the story. On GDPval-AA (real deliverables across 44 occupations and nine industries) Sonnet 5.5 is at 1844 versus 1846. On computer use it is 80.1% versus 81.8%. On CursorBench, built from real Cursor sessions, it is 55.5% versus 57.8%. Those gaps are inside the noise of most production evals, and they come at half the token price plus a faster response.

Anthropic’s cost-vs-accuracy charts add a second argument: at Low or Medium effort, Sonnet 5.5 beats Sonnet 5’s best score for about a tenth of the cost per task, and on FrontierCode at High effort it scores 10 points above Sonnet 5 at roughly one fifteenth of the cost. If your workload was fine on Sonnet 5, Sonnet 5.5 is a free upgrade with a lower bill; if you were paying for Opus because Sonnet 5 fell short, re-run the eval before renewing that decision.

The Terminal-Bench inversion (70.6% vs 66.4%) is worth a careful read. Terminal-Bench 4.0 rewards multi-step command-line execution with tight scope, exactly the “well-scoped” work Anthropic aims Sonnet 5.5 at, and Sonnet 5.5’s habit of batching tool calls reduces step count. It does not mean Sonnet 5.5 is the better coding model overall — FrontierCode and CursorBench still go to Opus.

Where Opus 5.5 still earns 2x

Open-ended, long-horizon work. Anthropic’s external testers and its own team found Opus 5.5 “clearly stronger” when the task needs sustained judgment: ambiguous specs, refactors across large systems, research synthesis. The Opus 5.5 migration notes cover its always-on thinking and default-medium effort.
Hard reasoning. Humanity’s Last Exam with tools: 67.7% vs 64.5%.
Max effort behaviour. Sonnet 5.5’s FrontierCode score drops from 52.1% at xhigh to 46.2% at max because it starts running multi-agent code review that overruns scope. Opus 5.5 at max effort emits roughly 119K output tokens per Artificial Analysis task, which is expensive but does not show that failure mode. If you need “throw everything at it,” Opus.
An independent score. Opus 5.5 has an Artificial Analysis Intelligence Index of 58 (Fable 5.1 and GPT-6 Astra sit at 53). Sonnet 5.5 had no independent score as of September 29, 2026, so every number above is vendor-reported.

Cost per task, not per token

List price says 2x. Real spend depends on effort and tokens per task:

At low/medium effort, Sonnet 5.5 is far cheaper than 2x: fewer thinking tokens and fewer tool calls. Balyasny measured ~121K tokens per answer on Sonnet 5.5 versus 497K on Sonnet 5; Base44 needed 3.6 build iterations versus 7.7 on Opus 5.
At xhigh/max, Anthropic says Sonnet 5.5 “can perform comparably at a similar cost” to Opus 5.5 — the price advantage evaporates because Sonnet thinks longer to get there.
Cache-heavy agents see identical $0.20 cache reads on both, so the 2x gap applies only to uncached input and to output.

Practical rule: run Sonnet 5.5 at medium first. Escalate to Opus 5.5 only for the tasks that fail, not by default. See how to choose an LLM reasoning effort level.

Mixing both in one system

Two API details decide the architecture. Sonnet 5.5 reads thinking blocks from Sonnet 5, Opus 4.8, Haiku 4.5 and earlier, but not from Opus 5, Opus 5.5, Fable or Mythos; the API drops those blocks without an error. So a conversation that starts on Opus 5.5 and switches to Sonnet 5.5 loses its reasoning trail mid-stream. The supported pattern is the advisor tool: a Sonnet 5.5 executor can call Opus 5.5 (or Opus 5, Sonnet 5.5, Fable 5/5.1, Mythos 5/5.1) for advice, returned encrypted. Opus 4.8 and earlier are no longer accepted as advisors. Worker-Sonnet, reviewer-Opus is the cheapest configuration that keeps Opus judgment in the loop.

Decision rule

Bug fixes, scoped features, tests, CLI tasks: Sonnet 5.5, medium effort.
Docs, slides, spreadsheets, UI polish: Sonnet 5.5 — this is what Anthropic built it for.
Chat products, latency-sensitive: Sonnet 5.5, low or medium; it is Anthropic’s fastest Sonnet.
Multi-hour autonomous runs, ambiguous specs, architecture: Opus 5.5.
Anything that needs an independent benchmark to justify: Opus 5.5, until Sonnet 5.5 is scored.
Security research: both carry cyber safeguards; apply to the Cyber Verification Program.

Last verified: September 29, 2026. Prices are standard API list rates; see PRICING for the full table.