Claude Opus 5.5 vs Sonnet 5.5: Price, Benchmarks, Which to Use

Sonnet 5.5 is the one most developers should open first: it is half the price of Opus 5.5 per million tokens and, on Anthropic's own agentic-coding benchmark, it actually scores higher. Opus 5.5 still earns its premium on the hardest, longest-running jobs — multihour autonomous agents, large-scale refactors, and computer-use workflows — where its extra reasoning depth shows up in the numbers. The short version: start with Sonnet 5.5, and reach for Opus 5.5 when a task is too big, too open-ended, or too high-stakes for it.

A week apart, and both already in the API

Anthropic shipped the Claude 5.5 generation in two steps rather than one. Claude Opus 5.5 arrived first, on September 22, 2026, pitched as the new top-end model for long-running agentic coding and knowledge work. Claude Sonnet 5.5 followed six days later, on September 28, described by Anthropic as "a faster, lower-cost complement to Claude Opus 5.5" built for well-scoped everyday tasks, bug fixes, and polished documents, slides, and spreadsheets.

That framing matters for how to read everything that follows. These are not two tiers of the same capability with Opus simply "bigger." Anthropic tuned them for different jobs, and on at least one high-profile benchmark the cheaper model wins outright. Readers who want the full rollout story for either release can see Opus 5.5's full benchmark rundown or Sonnet 5.5's release details — this piece focuses on how the two stack up against each other.

Price per million tokens

The pricing gap is the cleanest way to understand Anthropic's intent for these two models. Sonnet 5.5 costs exactly half of Opus 5.5 on both input and output tokens, and roughly half on cache writes too.

For comparison, Opus 5.5 itself is a price cut from its predecessor: Anthropic dropped input/output pricing from Opus 5's $5 / $25 to $4 / $20, and cut cache-read pricing by 60%. Sonnet 5.5 held its list price exactly level with Sonnet 5's introductory $2 / $10 rate, but Anthropic says it now does roughly 30% more work per dollar because it finishes comparable tasks faster and in fewer tokens. Full details, including batch-API and per-platform rates, are on Anthropic's pricing page.

Context window and output limits are identical

One number that will not help you choose between these two models: context. Both Opus 5.5 and Sonnet 5.5 ship with a 1-million-token context window and a 128K-token maximum output on the synchronous Messages API (300K on the Message Batches API with the relevant beta header), according to Anthropic's model comparison table. Neither model has a context-window advantage over the other — the differentiation is entirely in price, speed, and the effort/reasoning profile each model defaults to.

That default effort setting is worth noting. Opus 5.5 ships with adaptive thinking always on but defaults to medium effort, while Sonnet 5.5 defaults to high. Anthropic's guidance is that tuning the effort parameter within a single model is often a better lever than switching models entirely — so before paying Opus prices, it can be worth testing Sonnet 5.5 at a higher effort setting first.

Benchmarks head-to-head

Anthropic published a direct comparison between the two models alongside the Sonnet 5.5 launch, and it is more competitive than the price gap suggests. Sonnet 5.5 does not simply trail Opus 5.5 at a discount — on agentic command-line work it wins outright.

Two things stand out. First, the generational jump from Sonnet 5 to Sonnet 5.5 is enormous across every one of these benchmarks — Anthropic's own figures show Sonnet 5.5 going from 10.3% to 70.6% on Terminal-Bench 4.0 alone, which the company has called the largest single-generation jump it has published on any benchmark this year. Second, Opus 5.5 and Sonnet 5.5 are far closer to each other than either is to last generation's Sonnet. On knowledge-work evaluations like GDPval-AA and AA-Briefcase, the two models are separated by only a point or two.

Opus 5.5 still leads on reasoning-heavy and multimodal evaluations — Humanity's Last Exam, FrontierCode, CursorBench, OSWorld, and Chartography all favor Opus by a modest but consistent margin. That pattern lines up with Anthropic's positioning: Opus 5.5 is tuned for depth and long-horizon autonomy, Sonnet 5.5 for fast, well-scoped execution.

Why the cheaper model wins on Terminal-Bench

The Terminal-Bench result is the single most useful data point in this comparison, because it inverts the usual "pay more, get more" assumption. Anthropic says Sonnet 5.5 at its default (medium) effort "far exceeds Sonnet 5's best score for less than a tenth of the cost per task" — and in doing so, it also edges out the more expensive Opus 5.5. Terminal-Bench measures agentic command-line coding: multi-step shell tasks that reward speed, tight tool loops, and not overthinking a well-defined problem, rather than open-ended research or planning. That is exactly the profile Anthropic designed Sonnet 5.5 for, and it shows: a faster, cheaper model that iterates in tighter loops can out-execute a deeper-reasoning one on tasks that do not need the extra depth.

It is also a reminder that "benchmarks favor the bigger model" is not a safe assumption to carry forward model generation after model generation. Anyone picking a model based on name recognition alone — defaulting to Opus because it is positioned as the flagship — may be both overpaying and, on this specific class of task, getting a slightly worse result.

Which model to use for what

Anthropic's own model-selection guidance lines up closely with the benchmark data above. The company's matrix for its current lineup puts Sonnet 5.5 as the default starting point for most day-to-day engineering and knowledge work, reserving Opus 5.5 for jobs that are long, open-ended, or high-stakes enough to justify the premium.

In practice, that means most teams should default to Sonnet 5.5 and only escalate to Opus 5.5 when evaluations show a specific capability gap — exactly the "start efficiency-first, upgrade only if necessary" workflow Anthropic recommends in its own documentation. A multi-model setup that routes bulk work to Sonnet 5.5 and escalates the hardest 10–20% of tasks to Opus 5.5 will typically beat running everything on Opus by cost without giving up much on quality.

Speed, latency, and the effort dial

Beyond raw benchmark scores, the two models feel different to build with. Anthropic lists Sonnet 5.5's comparative latency as "fast" against Opus 5.5's "moderate," and both support an effort parameter that trades intelligence for latency and cost without switching models outright. Opus 5.5 and Opus 5 also support an optional fast mode — a research preview that delivers up to 2.5x higher output speed at roughly double the standard price ($8 input / $40 output per million tokens) — for latency-sensitive applications that still need Opus-level reasoning. Sonnet 5.5 does not currently list a fast-mode option, largely because at its default settings it is already the faster of the two models.

How to access each model

Both models are generally available through the same channels: the Claude API (model IDs claude-opus-5-5 and claude-sonnet-5-5), Amazon Bedrock, Google Cloud Vertex AI, and Microsoft Foundry, as well as through Claude.ai for consumer and Team/Enterprise plans. Anthropic's committed retirement dates — not sooner than September 22, 2027 for Opus 5.5, and not sooner than September 28, 2027 for Sonnet 5.5 — give developers roughly a year of guaranteed availability to plan migrations around. Teams on Claude Code or other agentic tooling that built workflows around Claude's Agent Skills framework can point those skills at either model ID without changing the skill definitions themselves.

Safety and guardrails

Anthropic rolled out similar safety work alongside both releases. Opus 5.5 launched with what the company describes as its best scores to date on an internal automated behavioral audit, with reduced rates of boundary circumvention and stronger resistance to prompt-injection attempts. Sonnet 5.5 is the first Sonnet-class model to ship with cybersecurity safeguards comparable to Opus 5.5's, alongside the same biological-risk safeguards carried over from Sonnet 5 and new classifiers intended to prevent reasoning-extraction attacks. Neither announcement discloses specific red-team pass rates beyond these qualitative claims, so organizations with strict compliance requirements should still run their own evaluations rather than relying on the headline safety language alone.

What's next

Anthropic has already said more is coming: Haiku 5.5 was flagged as a near-term follow-up when Opus 5.5 shipped, which would complete the refresh of the entire current lineup at the 5.5 generation. Until that lands, Haiku 4.5 remains the budget option beneath Sonnet 5.5, priced at $1 / $5 per million tokens with a smaller 200K-token context window. Anyone evaluating the top of the lineup should also keep Claude Fable 5.1 in view — Anthropic's documentation positions it as the model to reach for only after Opus 5.5 at its highest effort settings still falls short on the most demanding reasoning or longest-horizon agentic work.

Bottom line

For nearly everything that is not a genuinely long-running or open-ended agentic job, Sonnet 5.5 is both the cheaper and, on at least one headline benchmark, the better-performing choice. Opus 5.5's premium pricing buys real gains on reasoning-heavy, multimodal, and computer-use work, but it is a smaller gap than the price difference implies. The practical move Anthropic itself recommends — start with Sonnet 5.5, measure against your own evaluations, and escalate to Opus 5.5 only where the numbers justify it — is likely to be the right call for most teams building on the Claude 5.5 generation today.

Written byTheo Park

Theo Park runs the AI desk at Pandromeda. He follows model launches from the frontier labs and the open-weight community, tracks the assistants and developer tools built on them, and explains what each release changes on pricing, capability and safety. His reporting leans on primary sources: model cards, technical reports, API documentation and the companies' own announcements.

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