Claude Opus 5.5 and Sonnet 5.5 are not simply a best model and a budget model. Anthropic launched Opus first as a more efficient model for difficult, sustained work, then introduced Sonnet as a fast, lower-cost partner for well-scoped work. Sonnet is close enough on some benchmarks to make the choice interesting. The useful question is which one finishes your task reliably, quickly, and at an acceptable total cost.
Start with Sonnet 5.5 for routine coding, bug fixes, repeatable support and research steps, and polished documents or slides. Escalate to Opus 5.5 for ambiguous investigations, codebase-wide changes, architecture, difficult synthesis, or work where one wrong assumption makes the output unusable. Test both on the same acceptance criteria before changing a production default.
Watch Both Official Launch Videos
Introducing Claude Opus 5.5
Anthropic's Opus 5.5 video was published 22 September 2026. The official release page is the source of record for its pricing, evaluation setup, safeguards, and access.
Introducing Claude Sonnet 5.5
Anthropic's Sonnet 5.5 video was published 28 September 2026, alongside the Sonnet release page. These are product announcements, not independent head-to-head tests.
Opus 5.5 vs Sonnet 5.5 at a Glance
| Question | Opus 5.5 | Sonnet 5.5 |
|---|---|---|
| Best starting role | Complex, open-ended work requiring sustained judgment | Well-scoped everyday work and fast iteration |
| Typical examples | Large migrations, architecture, hard investigations, multi-source analysis | Bug fixes, routine agent steps, documents, slides, spreadsheets, design iteration |
| API input / output per 1M tokens | $4 / $20 | $2 / $10 |
| Cache read / write per 1M tokens | $0.20 / $5 | $0.20 / $2.50 |
| Vendor-reported speed gain | More than 30% faster output than Opus 5 | More than 30% faster output than Sonnet 5 |
| Claude Platform model ID | claude-opus-5-5 | claude-sonnet-5-5 |
These are Anthropic's Opus and Sonnet standard API prices and claims as published in September 2026. The speed gains compare each model with its own predecessor; they are not a direct claim that Sonnet is 30% faster than Opus.
The Token Rate Is Only the Starting Price
Anthropic says Opus 5.5 costs about 40% less per typical task than Opus 5 at default settings, even though its input and output token rates are only 20% lower. Its explanation is fewer tokens per task plus cheaper cache reads. Anthropic says Sonnet 5.5 costs up to 30% less per task than Sonnet 5 despite unchanged input and output rates, again because it generally uses fewer tokens. These are vendor measurements on particular workloads, not guaranteed savings for every prompt.
For the new pair, Sonnet's uncached input and output rates are half Opus's, but both list cache reads at $0.20 per million tokens. A heavily cached agent will therefore not see a simple two-to-one invoice difference. Higher effort, extra tool calls, retries, and human corrections can move the total further. Opus also offers a fast mode at $8 input and $40 output per million tokens, with up to 2.5 times the standard serving speed; treat that as a separate price tier.
Why One Benchmark Cannot Pick the Winner
Anthropic's Sonnet launch table reports Sonnet 5.5 at 70.6% on Terminal-Bench 4.0 versus 66.4% for Opus 5.5. That is a notable result, but it does not settle all agentic coding. The same table reports Opus ahead on FrontierCode (54.4% versus Sonnet's 46.2% at Max effort), CursorBench (57.8% versus 55.5%), and GDPval-AA knowledge work (1846 versus 1844). Some scores use different effort settings and safeguards, and Anthropic cautions that benchmark margins are an imperfect guide to actual work.
Anthropic still describes Opus as clearly stronger at complex, open-ended work requiring sustained judgment. Sonnet's strong scores mean it deserves a real trial on tasks that once required Opus, not that it should automatically replace Opus everywhere. The closeness of two GDPval points is especially poor evidence for declaring a practical winner on a specific company's work.
A Practical Routing Rule
- Default to Sonnet when the request is bounded, the input is clear, and success can be checked: fix a known bug, transform a document, draft slides from a template, classify an inbox item, or implement a small approved design.
- Escalate to Opus when the model must discover the problem, reconcile conflicting evidence, plan across repositories, or make difficult tradeoffs that are hard to specify in advance.
- Use both deliberately for mixed projects: Opus for the first architecture or investigation pass; Sonnet for scoped implementation and repeated revisions; a human for approval. The handoff must carry requirements and decisions, not just a vague summary.
- Do not equate benchmark safety with permission. Anthropic describes new safeguards and stronger behavioral audits for both releases. Keep normal review, access limits, tests, and source checking for any agent touching code, customer data, or external systems.
Availability, Limits, and One API Trap
Anthropic says both models are available across Claude products and the Claude Platform, as well as AWS, Google Cloud, and Microsoft Azure. With Opus 5.5, it also announced higher five-hour usage limits for Pro, Max, Team, and seat-based Enterprise plans. This is a plan-limit increase, not unlimited use. Anthropic said Haiku 5.5 would join the family in the following weeks; that is a forward-looking announcement, not a current availability claim.
Developers should not swap claude-sonnet-5 for claude-sonnet-5-5 without testing request parameters. Anthropic's migration guide says Sonnet 5.5 uses adaptive thinking by default, rejects thinking.type: disabled, and offers between_tools as the lowest-thinking alternative. It also advises parsing response blocks by type and re-running effort and cost evaluations. For an existing integration, that migration check matters as much as the launch benchmark.
Run a Five-Task Test Before Switching
- Choose five real tasks: one routine code fix, one ambiguous bug, one document or slide, one data-heavy question, and one longer agent workflow.
- Give both models the same inputs, tools, acceptance tests, and relevant context. Start at the effort settings you would actually use, then test one higher setting only where the first pass fails.
- Record completed-task rate, elapsed time, input and output tokens, cache use, tool errors, retries, and reviewer minutes.
- Inspect scope creep and factual support. A polished answer that changes unrelated code or invents a source is a failure.
- Route repeatable wins to Sonnet, reserve Opus for tasks where its extra judgment changes the outcome, and revisit the rule as pricing or models change.
Official Sources and Credits
- Anthropic: Introducing Claude Opus 5.5 (primary video, published 22 September 2026).
- Anthropic: Introducing Claude Sonnet 5.5 (supplemental video, published 28 September 2026).
- Opus 5.5 release page and system card.
- Sonnet 5.5 release page and system card.
- Claude Platform: migrating to Sonnet 5.5.
Primary video published 22 September 2026; article checked 29 September 2026. Product prices, limits, and model behavior can change. Benchmark figures are Anthropic-reported unless otherwise stated.