Claude Sonnet 5.5 vs Opus 5.5: a practical routing rule for AI workflows
Sonnet 5.5 is faster and cheaper than Opus 5.5 while getting close on some work. Here?s a practical way to route routine tasks, coding and harder judgment-heavy work.
Anthropic released Claude Sonnet 5.5 on September 28 as a faster, lower-cost complement to Opus 5.5. The useful question for teams building AI workflows is not which model sits higher in the lineup. It is which steps need Opus-level judgment, and which can run on Sonnet without paying the higher unit cost every time.
What changed
Anthropic says Sonnet 5.5 generates output more than 30% faster than Sonnet 5 and can cost up to 30% less per task, even though its standard token prices are unchanged from Sonnet 5. The company attributes the per-task savings to the model needing fewer tokens to complete work.
Standard API pricing per 1M tokens
The price gap is simple; task economics are not
At standard API rates, Sonnet 5.5 costs half as much as Opus 5.5 for fresh input and output tokens, while both list the same $0.20 cache-read price. That makes Sonnet an obvious candidate for high-volume steps, but token price alone is not enough: a model that needs more retries, tool calls or human correction can erase the apparent saving. The right unit to measure is cost per successfully completed task.
A practical routing rule for workflows
Route by task shape, not model prestige
| Start with Sonnet 5.5 when? | Escalate to Opus 5.5 when? |
|---|---|
| The task is well-scoped and repeatable | The task is ambiguous, open-ended or judgment-heavy |
| You are processing many similar items | A mistake has unusually high downstream cost |
| Fast iteration matters, such as bug fixes or document drafts | The work needs sustained reasoning across many competing constraints |
| You can validate the result automatically or cheaply | Quality is difficult to verify after the fact |
Anthropic positions Sonnet 5.5 for well-scoped everyday tasks, bug fixing and polished documents, while describing Opus 5.5 as the stronger choice for complex work requiring careful judgment. That suggests a two-tier workflow: make Sonnet the default for bounded steps, then escalate only the cases that cross a complexity or confidence threshold.
Benchmarks are useful, but do not turn them into a routing policy
Anthropic reports that Sonnet 5.5 gets close to Opus 5.5 on several evaluations, but it also says Opus remains clearly stronger on complex, open-ended work requiring sustained judgment. Benchmark results are evidence about capability, not proof that one model will be cheaper or better on your own workflow.
How to test the routing decision
- Pick one repeated workflow with a clear success criterion.
- Run the same representative task set on Sonnet 5.5 and Opus 5.5.
- Track successful completions, retries, human corrections, latency and total token cost.
- Use Sonnet as the default only where quality stays inside your acceptable range.
- Define an escalation rule for low-confidence, high-risk or unusually complex cases instead of routing everything to Opus.
Sonnet 5.5 makes a strong default candidate for bounded, high-volume AI work because its standard input and output prices are half of Opus 5.5?s and Anthropic reports faster execution than Sonnet 5. Keep Opus as the escalation tier for work where extra judgment is worth the premium, and validate the split with cost per successful task rather than token price alone.
Sources & useful resources
- Anthropic: Claude Sonnet 5.5— Primary announcement, Sep. 28, 2026
- Anthropic: Claude Opus— Official Opus 5.5 pricing and positioning