TL;DR: Use Claude Opus 5.5 for complex work that needs careful judgment, and Claude Sonnet 5.5 for well-scoped everyday tasks like fixing bugs and creating polished documents. Sonnet 5.5 runs 30%+ faster and costs up to 30% less per task at $2/$10 per million tokens, while nearly matching Opus 5.5 on several benchmarks. Both share a 1 million token context window.
Anthropic released Claude Sonnet 5.5 on September 28, 2026, six days after Opus 5.5. Here is how the two compare and when to use each.
Sonnet 5.5 vs Opus 5.5 at a Glance
| Feature | Sonnet 5.5 | Opus 5.5 |
|---|---|---|
| Released | Sept 28, 2026 | Sept 22, 2026 |
| Input / output price | $2 / $10 per M tokens | $4 / $20 per M tokens |
| Speed | 30%+ faster than Sonnet 5 | 30%+ faster than Opus 5 |
| Context window | 1M tokens | 1M tokens |
| Max output | 128K tokens | 128K tokens |
| Best for | Everyday tasks, bug fixes, docs | Complex work, careful judgment |
When to Use Sonnet 5.5
Sonnet 5.5 is the faster, lower-cost complement to Opus 5.5. It is strongest at well-scoped everyday tasks, fixing bugs, and creating polished documents, slides and spreadsheets. It also has a strong eye for design. On Terminal-Bench 4.0, an agentic coding test, it scores notably higher than Sonnet 5.
When to Use Opus 5.5
Opus 5.5 is built for complex work requiring careful judgment – the kind of multi-step reasoning where sustaining context across difficult problems matters. If a task is ambiguous or high-stakes, Opus is the safer pick.
The Cost Angle
For most people the cheaper model may be the more important release. Sonnet 5.5 holds Sonnet 5’s price while running faster, and on some agentic coding tests the cheaper model even edges out Opus 5.5. For high-volume or budget-sensitive work, start with Sonnet 5.5 and escalate to Opus only when a task genuinely needs it.
The Practical Rule
Default to Sonnet 5.5. Reach for Opus 5.5 when the work is complex, ambiguous, or needs sustained judgment. This keeps costs down without sacrificing quality on the tasks that matter.
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