Claude Opus 5.5 vs Sonnet 5.5: Which Should You Use?

Claude Sonnet 5.5 is the model to reach for by default: it matches or beats Opus 5.5 on several agentic coding benchmarks at roughly half the price. Claude Opus 5.5 is still the better choice for long, open-ended work — deep research, codebase-wide migrations, multi-step audits — where sustained judgment matters more than raw speed or cost. Both launched within days of each other in late September 2026, and Anthropic's own benchmark table, not marketing copy, is the clearest way to see where each one actually wins.

What changed: Sonnet 5.5 and Opus 5.5 at a glance

Anthropic shipped Claude Opus 5.5 on September 22, 2026, cutting its list price from the previous Opus 5 generation, then followed on September 28, 2026 with Claude Sonnet 5.5, a mid-tier model that keeps Sonnet 5's exact sticker price while posting large benchmark gains, most visibly a jump on Terminal-Bench 4.0 from 10.3% (Sonnet 5) to 70.6%. The two releases landed close enough together that they're now evaluated as a pair: Opus 5.5 is the strongest model Anthropic sells, and Sonnet 5.5 is the model built to get most of the way there for less than half the money.

That framing — "near-Opus capability, Sonnet pricing" — is Anthropic's own pitch, and it holds up on some benchmarks and not others. The rest of this piece works through the actual numbers from Anthropic's launch pages and pricing documentation, not the marketing summary.

Both models belong to the same "5.5" refresh of Anthropic's lineup. A third tier, Claude Haiku 5.5, is described on Anthropic's pricing documentation as coming in the following weeks, aimed at high-volume, cost-sensitive use cases where even Sonnet 5.5's reasoning is more than a task needs. Until that ships, Sonnet 5.5 and Opus 5.5 are the two frontier options developers and subscribers are choosing between.

Pricing compared: Sonnet 5.5 vs Opus 5.5 per million tokens

Both models are priced identically to their immediate predecessors in base input and output rate — Anthropic didn't change sticker prices at launch for either tier — but Opus 5.5's cache-read price did drop sharply from Opus 5's $0.50 to $0.20 per million tokens, matching the cut Sonnet got a generation earlier. These figures come from Anthropic's own model pricing documentation, which also publishes the consumer-facing summary at claude.com/pricing.

The ratio is consistent and simple to remember: Opus 5.5 costs exactly twice Sonnet 5.5 on every line of the pricing table. That makes the "is it worth it" question mostly about whether the benchmark gap, where there is one, justifies paying 2x. For high-volume, well-scoped work, Anthropic also has Claude Haiku 5.5 as a cheaper third tier, aimed at applications where neither Sonnet's nor Opus's extra reasoning is needed at all.

Benchmark scores compared

Anthropic's own launch page for Sonnet 5.5 publishes a head-to-head table against Opus 5.5 (plus Sonnet 5 and a competitor model for reference). The picture is mixed rather than one model simply beating the other across the board:

So the "Sonnet matches Opus" claim is real on exactly one of these eight benchmarks — Terminal-Bench, which measures agentic command-line and terminal tasks — and GDPval-AA is close enough to call a tie. Everywhere else, including knowledge work, computer use, and the hardest reasoning tests, Opus 5.5 keeps a clear lead, typically by two to nine points. Anthropic's footnotes also flag a pre-release bug that may have slightly understated Sonnet 5.5's scores on GDPval-AA and AA-Briefcase, so those two gaps could narrow slightly once corrected figures are published.

Where Sonnet 5.5 wins: agentic terminal and coding work

Terminal-Bench 4.0 is the one benchmark where Sonnet 5.5 doesn't just close the gap with Opus, it overtakes it — 70.6% versus 66.4%. That benchmark specifically scores agentic terminal-and-shell tasks: the kind of multi-step command-line work that shows up in CI pipelines, dev-ops scripting, and "fix this failing build" loops. Anthropic also notes Sonnet 5.5 at its lowest "effort" setting already beats Sonnet 5's best score on CursorBench, which is the practical case for using Sonnet 5.5 as a default coding assistant: it's fast, cheap, and strong specifically at the bounded, well-defined tasks that make up most day-to-day engineering work.

Where Opus 5.5 still leads: complex, open-ended reasoning

Opus 5.5 keeps its lead everywhere the task is less bounded. On Humanity's Last Exam, OSWorld (computer-use), Chartography, and FrontierCode, Opus 5.5 outscores Sonnet 5.5 by margins ranging from roughly two to nine points. Anthropic's own description of Opus 5.5 is explicit about this positioning, stating that it's "particularly good at long and sprawling jobs like codebase-wide migrations and audits" — work that requires holding a lot of context and making judgment calls over many steps, rather than executing a single well-defined task quickly.

That distinction mirrors how Anthropic has positioned Opus against other Claude tiers before — see how Opus 5.5 compares against Mistral Large 4 on the same kind of open-ended reasoning tasks. The pattern holds: Opus is the model Anthropic reaches for when it wants to claim the top reasoning score, and Sonnet is the model it reaches for when it wants to claim the best price-to-performance ratio.

The effort-level fairness caveat

One detail that complicates a clean "Sonnet vs Opus" comparison: several of these scores were run at different effort levels, a setting (low, medium, high, xhigh, max) that trades inference cost and latency for more thorough reasoning on each response. Anthropic's benchmark footnotes show Opus 5.5's Terminal-Bench score of 66.4% was run at "Xhigh" effort, while Claude Code and the consumer apps default new sessions to "Medium" for both models — meaning the head-to-head table isn't strictly apples-to-apples at default settings. Separately, Sonnet 5.5's FrontierCode score of 46.2% was recorded at "Max" effort and was actually lower than its score at a different setting, which Anthropic flags as unusual. The Claude Platform's documentation on effort levels confirms that higher settings generally cost more and run slower in exchange for more careful reasoning, so a real-world comparison run at matched effort settings could shift some of these gaps in either direction.

Cost at scale: what the price gap actually means

Because Opus 5.5 is priced at exactly double Sonnet 5.5 on every line item, the cost math is simple to run for any workload. A task that costs $1 in Sonnet 5.5 tokens costs $2 in Opus 5.5 tokens, every time, regardless of whether the task is a quick bug fix or a multi-hour audit. For teams running agentic coding loops or high-volume document processing, that 2x multiplier compounds fast: a workload that burns through 500 million output tokens a month costs $5,000 on Sonnet 5.5 versus $10,000 on Opus 5.5. Anthropic's own pricing documentation also lists batch processing at a further 50% discount on both models ($1/$5 per million tokens for Sonnet 5.5, $2/$10 for Opus 5.5), which is worth checking for any workload that doesn't need a synchronous response.

Prompt caching adds a second lever. Both models charge roughly 0.05x their base input price for a cache hit — $0.10 per million tokens on Sonnet 5.5, $0.20 on Opus 5.5 — against base input rates of $2 and $4. For workloads that repeatedly reuse the same long system prompt, tool definitions, or document context (a common pattern in agentic coding and retrieval-heavy pipelines), caching can cut effective input costs by well over 90% on either model, which narrows the practical cost gap between the two tiers for prompt-heavy, output-light tasks even though the sticker prices stay 2x apart.

Which model should you use?

Based on Anthropic's own benchmark table and its stated positioning, the practical breakdown is:

• Use Sonnet 5.5 for day-to-day coding, bug fixes, well-scoped agentic tasks, drafting documents, slides, and spreadsheets, and any high-volume workload where the 2x price difference adds up fast.
• Use Opus 5.5 for long, multi-step work that needs sustained judgment: large codebase migrations, deep research tasks, complex audits, or anything scored heavily on reasoning benchmarks like Humanity's Last Exam.
• Use a lower effort setting on Sonnet 5.5 when cost matters more than marginal quality — Anthropic notes Sonnet 5.5 at low effort already beats Sonnet 5's best recorded scores on some benchmarks, so it isn't a straight "use max effort" decision either.

Anthropic's own framing is useful here too: the company describes Sonnet 5.5 as complementing Opus 5.5 "best at lower effort settings, where it costs less per task," rather than positioning it as an outright Opus replacement.

Availability: where to access each model

Both models are available through the same channels: the Claude apps (web, desktop, mobile), Claude Code, the first-party API, and the three major clouds — Amazon Web Services, Google Cloud, and Microsoft Azure — so switching between them in an existing integration is typically a one-line model-ID change rather than a platform migration. Anthropic's product overview covers how each access path differs for subscribers versus API customers. The API identifier for Sonnet 5.5 is claude-sonnet-5-5; Opus 5.5 follows the equivalent claude-opus-5-5 naming pattern on its own launch page. Anthropic also states zero data retention is available for both models for customers who need it, which matters for regulated industries evaluating either tier for production use. That same infrastructure buildout is part of why Anthropic's business has drawn scrutiny well beyond model benchmarks — see Pandromeda's coverage of Anthropic's IPO filing and valuation for the financial side of that story.

What to do now

If you're building on the Claude API, the lowest-risk approach is to default new projects to Sonnet 5.5 and only route specific tasks to Opus 5.5 when they clearly need deep, multi-step reasoning — large refactors, research synthesis, or anything that currently requires several follow-up prompts to get right. If you're already paying for Opus-tier usage on tasks that look more like day-to-day coding or document drafting, Sonnet 5.5's benchmark table gives a concrete, Anthropic-sourced case for testing a downgrade and comparing output quality against the roughly 50% cost saving. Either way, treat the headline "near-Opus parity" claim as true for agentic terminal work specifically, not as a blanket statement about the two models — the full benchmark table above makes clear Opus 5.5 still leads on most other measures Anthropic publishes.

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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