Opus 5.5 vs Sonnet 5
Choose a model by task difficulty and total cost.
Claude Opus 5.5 vs Sonnet 5 is a workload decision. Sonnet 5 is the lower-cost starting point for well-scoped coding. Opus 5.5 is worth evaluating when a task needs sustained investigation, difficult architectural choices or fewer costly corrections. Paying more is useful only if the completed result improves enough to justify it.
Checked September 28, 2026. This comparison uses Anthropic's current specifications, pricing and release documentation. We did not run an independent head-to-head benchmark or latency test. Recommendations are evaluation starting points; cost examples hold token counts constant and are not measured task bills.
Opus 5.5 and Sonnet 5 at a glance
| Decision point | Sonnet 5 | Opus 5.5 |
|---|---|---|
| Direct API model ID | claude-sonnet-5 | claude-opus-5-5 |
| Context / standard maximum output | 1M / 128K tokens | 1M / 128K tokens |
| Inputs / output | Text and images / text | Text and images / text |
| Default effort | high | medium |
| Thinking | Adaptive | Adaptive, always on |
| Anthropic latency category | Fast | Moderate |
| Uncached input / output per 1M tokens | $2 / $10 | $4 / $20 |
| Cache read per 1M tokens | $0.20 | $0.20 |
| Starting role | Clear, frequent, testable work | Difficult, open-ended, high-consequence work |
Sources: model specifications and API pricing. Prices are standard global direct API rates in USD; tools, regional modifiers and other service modes are separate.
Both models offer the same headline context and standard output capacity. Opus 5.5 does not buy a larger advertised window here. Its potential value is how it uses the evidence in that window, which your own tasks need to establish.
When to start with Sonnet 5
Use Sonnet as the baseline for a scoped feature, a reproduced bug, test generation around an existing interface, routine refactoring or a focused code review. These tasks often have clear acceptance checks, so you can tell whether the lower-cost route is sufficient.
Sonnet's advantage is especially relevant at volume. Small differences per request accumulate across agent steps, retries and daily usage. Keep the task bounded, provide the required files and run verification before escalating the model.
This is a workflow recommendation, not a claim that Sonnet always completes these tasks correctly. Neither model compensates for missing requirements or an unavailable test environment.
When Opus 5.5 is worth testing
Try Opus when a plausible patch keeps missing the root cause, a change crosses service boundaries, a migration must preserve subtle behavior, or a wrong decision could create substantial repair work. Give it the same evidence and acceptance criteria as Sonnet.
Anthropic positions Opus 5.5 for long-running agentic coding and knowledge work. Its launch material reports gains over earlier models, but a result against Opus 5 does not establish a specific gain over Sonnet 5. The Opus 5.5 review explains the published evaluations and their conditions.
Once the architecture or diagnosis is settled, routine implementation can return to the lower-cost route. Use clear task boundaries and carry forward verified findings rather than silently switching models in the middle of an unresolved conversation.
API pricing: not every workload costs twice as much
Opus 5.5's uncached input and output rates are twice Sonnet 5's, but their cache-read rate is the same. Five-minute cache writes cost $2.50 for Sonnet and $5 for Opus per million tokens; one-hour writes cost $4 and $8.
| Identical token workload | Sonnet 5 | Opus 5.5 |
|---|---|---|
| 100K uncached input + 20K billed output | $0.40 | $0.80 |
| 100K uncached + 800K cache-hit input + 20K billed output | $0.56 | $0.96 |
The second row is 0.1 × $2 + 0.8 × $0.20 + 0.02 × $10 = $0.56 for Sonnet, and 0.1 × $4 + 0.8 × $0.20 + 0.02 × $20 = $0.96 for Opus. It excludes the earlier cache write and assumes reported cache hits. The token quantities include billed thinking output, not just visible answer text.
These examples exclude tool fees, retries and platform-specific adjustments. An agent can change its token consumption when the model or effort changes. Measure total billed cost divided by accepted tasks, and record human correction time separately.
OmniaKey is a separate gateway route. Its current Opus 5.5 and Sonnet 5 pages own the available route and quote; the direct Anthropic table is not a gateway invoice. Claude subscriptions are also separate from metered API charges.
Speed and effort: compare the complete task
Anthropic labels Sonnet's latency “Fast” and Opus 5.5's “Moderate.” Those are vendor categories, not an independently measured tokens-per-second ratio. A faster answer can still lead to a longer task if it needs more retries or corrections.
Begin with each model's documented default: Sonnet at high, Opus 5.5 at medium. That compares normal starting configurations, not equal compute budgets. If you also test a shared named effort, report it separately; the same label does not guarantee the same token budget or work performed.
Keep prompts, repository state, tools and acceptance tests fixed. Record time to first useful output, total elapsed time, retries, billed tokens and acceptance. Run multiple representative tasks rather than turning one favorable output into a universal ranking.
Check the API contract before switching
Opus 5.5 requires adaptive thinking. Disabled thinking or manually budgeted thinking returns an error; forced tool_choice modes any and tool are unsupported. Its thinking blocks also have conversation-preservation rules, and progress text between tool calls can appear in thinking blocks.
Do not copy a working Sonnet request, change only the model name and assume every setting remains valid. Read the Opus 5.5 migration guide, test tool loops and verify the selected route's behavior. For choosing a model within the CLI, use the Claude Code model guide.
Frequently asked questions
Is Opus 5.5 always better than Sonnet 5 for coding?
No universal result follows from the documentation. Opus is positioned for harder work; Sonnet can be sufficient and cheaper for tasks it passes reliably. Compare your acceptance results, total cost and correction time.
Is Sonnet 5 half the price?
Its standard direct uncached input and output rates are half Opus 5.5's. Cache reads cost the same, and full task bills depend on token mix, thinking, retries and tools.
Do I get more context with Opus 5.5?
Both list a 1M context window and 128K standard maximum output. These are capacity limits, not included or free usage. Other API modes have separate limits and conditions.
Should I replace the older Opus 5 comparison?
Use this article for 5.5. The Opus 5 vs Sonnet 5 comparison remains useful for the older model. Keep the version explicit when comparing results and prices.