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

Anthropic released Claude Sonnet 5.5 on September 28, 2026, six days after Opus 5.5, and the interesting question is no longer whether a mid-tier model can beat its predecessor. It is how little now separates it from the flagship above it. Sonnet 5.5 keeps Sonnet 5's per-token prices, half of what Opus 5.5 charges, while Anthropic reports output more than 30% faster and per-task costs up to 30% lower than Sonnet 5.

• Pricing: $2 per million input tokens and $10 per million output, unchanged from Sonnet 5 and half of Opus 5.5's rates
• Efficiency: Anthropic reports 30%+ faster output and up to 30% lower cost per task than Sonnet 5
• Independent score: Artificial Analysis measured 56 on its Intelligence Index at max effort, second only to Opus 5.5 at 58
• Specs: Model ID claude-sonnet-5-5, a 1M-token context window, 128K max output and a June 2026 knowledge cutoff

How Does Claude Sonnet 5.5 Compare to Opus 5.5?

Anthropic's own evaluations show Sonnet 5.5 leaving its predecessor far behind and closing most of the gap to Opus 5.5:

The Opus 5.5 Terminal-Bench score comes from Xhigh effort, its highest-scoring setting.

The independent picture points the same way. Artificial Analysis, running its own harness, recorded an 18-point Intelligence Index gain over Sonnet 5 and measured 64% on Terminal-Bench 4.0, slightly ahead of the 60% it recorded for Opus 5.5 and GPT-6 Astra. On its GDPval-AA knowledge-work test, Sonnet 5.5 scored 1,844 Elo to Opus 5.5's 1,846.

Still, Anthropic says Opus 5.5 remains clearly stronger at complex, open-ended work that demands sustained judgment, and Artificial Analysis found Sonnet 5.5 behind on factual knowledge and scientific reasoning.

Why Effort Settings Decide What Sonnet 5.5 Costs

Because per-token prices did not change, the savings come from efficiency: the model reaches answers in fewer tokens. Anthropic reports that on several benchmarks, Sonnet 5.5 at Low or Medium effort beats Sonnet 5's best score for about a tenth of the cost per task. Among early testers, Slack measured roughly 14% fewer output tokens, and Zendesk says support tickets were processed 20% faster.

The dial cuts both ways. At max effort, Sonnet 5.5 used about 193,000 output tokens per Artificial Analysis index task, the most the firm has measured, lifting cost per task about 50% above Sonnet 5's; the firm rated High effort the most cost-competitive setting. Treat effort as a budget decision: Claude Code and the Claude apps default to Medium, the API to High, and Anthropic advises re-testing effort levels rather than reusing Sonnet 5 settings.

What Developers Need to Change Before Switching

Moving from Sonnet 5 starts with a model ID swap, but Anthropic lists five breaking changes. Three are the most likely to affect existing code:

• Turning thinking off with disabled now returns an error; use between_tools instead, at High effort or below
• Forced tool use is not supported; Anthropic recommends the default auto tool choice with strict tool use
• Thinking blocks are tied to the model and the conversation, so keep histories append-only

Sonnet 5.5 is live on the Claude API, Amazon Bedrock, Google Cloud, Microsoft Foundry and Claude Platform on AWS, with zero data retention available and retirement no sooner than September 28, 2027.

Stronger Safeguards Reach the Sonnet Tier

Because its cybersecurity capabilities are comparable to Opus 5's, Sonnet 5.5 is the first Sonnet to launch with the cyber safeguards Anthropic built for its most capable models. Routine bug fixing is unaffected, while higher-risk security requests visibly fall back to Sonnet 5, something Artificial Analysis saw in about 0.1% of its test tasks. Sonnet 5.5 is also the first Sonnet with classifiers that block reasoning extraction, a defense against large-scale distillation.

When Is Claude Haiku 5.5 Coming?

Haiku 5.5 is the remaining piece. Anthropic says the model, built for high-volume and cost-sensitive applications, will join the Claude 5.5 family in the coming weeks, and Reuters reports it is due soon. No pricing or specifications are public yet. Until then, Anthropic's documentation lists Claude Haiku 4.5 as the lineup's fastest model, at $1 and $5 per million tokens with a 200K-token context window. The open questions are whether Haiku 5.5 inherits the 1M-token context and effort controls its siblings share, and where Anthropic sets its price.

How the Claude 5.5 Lineup Divides the Work

Opus 5.5 remains the pick for long, open-ended work where judgment compounds; our Claude Opus 5.5 pricing and benchmarks breakdown covers that tier. Sonnet 5.5 becomes the sensible default for well-scoped coding, documents and spreadsheets at half the per-token price, three months after Claude Sonnet 5 brought near-flagship power to a lower price. Haiku 5.5 completes the set when it lands, and our AI model coverage will track it.

Sources: Anthropic: Introducing Claude Sonnet 5.5 — September 28, 2026; Claude Platform Docs: What's new in Claude Sonnet 5.5 — September 28, 2026; Artificial Analysis: Claude Sonnet 5.5 reaches #2 on the Intelligence Index — September 28, 2026; Reuters via Yahoo News — September 28, 2026; TechCrunch — September 28, 2026. The benchmark table and early-tester figures come from Anthropic's announcement; Intelligence Index and token-usage results are Artificial Analysis's own measurements.