Claude Opus 5.5 vs Claude Sonnet 5.5
Opus 5.5 leads 7 of 8 benchmarks and excels at complex reasoning; Sonnet 5.5 wins the top agentic coding test, responds faster and costs half as much per token.
Both models share a 1M-token context window and a June 2026 knowledge cutoff, but differ sharply on price, speed and thinking behavior. This Claude model comparison breaks down the official specifications, benchmarks and practical trade-offs so you can pick the right one for coding, agents or everyday API work.
Last updated 2026-09-30 · All data sourced from Anthropic's official documentation
At a Glance: Which Model Should You Use?
Choose Claude Opus 5.5 when…
You run long-running agents, complex multi-step reasoning or high-stakes code review where a small quality edge matters more than latency or cost.
Choose Claude Sonnet 5.5 when…
You need fast iteration on routine coding, high-volume batch processing, latency-sensitive APIs or everyday tasks where the 2x cost saving adds up.
Opus 5.5 vs Sonnet 5.5: Key Specifications
API identifiers, pricing, context limits, thinking behavior and more — sourced directly from Anthropic's documentation.
| Spec | Claude Opus 5.5 | Claude Sonnet 5.5 |
|---|---|---|
| API model ID | claude-opus-5-5 | claude-sonnet-5-5 |
| Released | Sep 22, 2026 | Sep 28, 2026 |
| Tagline | For long-running agentic coding and knowledge work | The best combination of speed and intelligence |
| Latency | Moderate | Fast |
| Input price (per MTok) | $4 | $2 |
| Output price (per MTok) | $20 | $10 |
| Context window | 1M tokens | 1M tokens |
| Max output | 128K tokens (300K on Batch API) | 128K tokens (300K on Batch API) |
| Thinking | Adaptive, always on, cannot be disabled | Adaptive; can be tuned with between_tools (no up-front thinking at effort <= high) |
| Default effort | medium | high |
| Knowledge cutoff | Jun 2026 | Jun 2026 |
| Input modalities | Text + images | Text + images |
| Retirement | Not sooner than Sep 22, 2027 | Not sooner than Sep 28, 2027 |
Pricing: Sonnet 5.5 Costs Half as Much per Token
At list price, Sonnet 5.5 is exactly half of Opus 5.5: $2 / $10 vs $4 / $20 per million tokens (MTok) for input / output. A typical 100K-input, 20K-output request costs $0.40 on Sonnet and $0.80 on Opus.
| Pricing Tier | Claude Opus 5.5 | Claude Sonnet 5.5 |
|---|---|---|
| Standard API (per MTok) | $4 / $20 | $2 / $10 |
| Batch API (50% off) | $2 / $10 | $1 / $5 |
| Cache write (5 min) | $5 | $2.50 |
| Cache write (1 hour) | $8 | $4 |
| Cache read | $0.20 (0.05x base) | $0.20 (0.1x base) |
Both models share the same $0.20 per MTok cache-read price. That is 5% of Opus's base input cost vs 10% of Sonnet's — so heavy prompt caching significantly narrows the effective cost gap.
Opus 5.5 also offers a fast mode (research preview, API only) at $8 / $40 per MTok, up to 2.5x speed. Sonnet 5.5 does not have a fast mode — it is already the faster model by default.
Benchmark Results: Opus 5.5 Leads 7 of 8
Official scores from Anthropic. Effort level varies by benchmark, so results are not always directly comparable across rows — check the notes column.
| Benchmark | Claude Opus 5.5 | Claude Sonnet 5.5 | Note |
|---|---|---|---|
| Terminal-Bench 4.0 | 66.4% (Xhigh) | 70.6% (Max) | Agentic coding; Opus std err ±2.6 |
| FrontierCode 1.1 Main | 54.4% | 46.2% (Max) / 52.1% (Xhigh) | Sonnet lower at Max due to timeouts |
| CursorBench 4.0 | 57.8% | 55.5% | — |
| GDPval-AA v2.1 (Elo) | 1846 | 1844 | Essentially tied |
| AA-Briefcase v1.1 (Elo) | 1822 | 1811 | — |
| Humanity's Last Exam | 67.7% | 64.5% | With tools |
| OSWorld 2.1 | 81.8% | 80.1% | Partial scoring |
| Chartography | 64.4% | 61.6% | No tools |
Takeaway: Opus 5.5 edges ahead on 7 of 8 benchmarks by narrow margins (≤ 3.2 pp). Sonnet 5.5 takes Terminal-Bench 4.0 — the most widely cited agentic coding benchmark.
Independent Evaluations & Third-Party Data
Scores from independent platforms. Numbers may differ from Anthropic's tables due to different evaluation setups and effort settings.
- Artificial Analysis Intelligence Index v4.3.2: Sonnet 5.5 = 56, Opus 5.5 = 58. Per-task costs vary by effort level. artificialanalysis.ai
- CodeRabbit code review evaluation (Sep 28, 2026): Sonnet caught 6/13 hard cases (46.2%) vs Opus 8/13 (61.5%); actionable precision 41.2% vs 66.7%. Sonnet recommended for routine PR review, Opus for high-risk changes. Full review
- LMArena — community-driven head-to-head model arena. lmarena.ai
- OpenRouter — live pricing and availability across providers: Opus 5.5 / Sonnet 5.5
- Vals AI — reproducible, open-weight model evaluation. vals.ai
Sonnet 5.5 vs Opus 5.5: Which to Choose by Use Case
Not every task needs the most powerful model. Use this table to match your workload to the model that gives you the best balance of quality, speed and cost.
| Scenario | Recommended | Why |
|---|---|---|
| Routine coding tasks | Sonnet 5.5 | Faster responses, half the cost — more than enough quality for everyday edits and features |
| Long-running agentic workflows | Opus 5.5 | Higher scores on 7 of 8 benchmarks; the small accuracy edge compounds over many-step chains |
| Code review (routine PRs) | Sonnet 5.5 | CodeRabbit data shows comparable catch rates on standard PRs at lower cost |
| Code review (high-risk changes) | Opus 5.5 | 61.5% hard-case detection vs 46.2% and higher actionable precision (CodeRabbit eval) |
| Document / slide generation | Sonnet 5.5 | Speed and cost matter more than marginal reasoning gains for content generation |
| High-volume batch processing | Sonnet 5.5 | $1 / $5 batch pricing (half of Opus) keeps large-scale jobs affordable |
| Latency-sensitive applications | Sonnet 5.5 | Listed as fast latency; Opus is moderate — and Opus fast mode costs 4x Sonnet |
| Complex multi-step reasoning | Opus 5.5 | Leads Humanity's Last Exam (67.7% vs 64.5%) and GDPval-AA; best for hard problems |
Switching Between Opus 5.5 and Sonnet 5.5: What to Know
Thinking blocks are not portable. Opus 5.5 thinking is only readable by Fable 5.1 and Mythos 5.1; no other model reads Sonnet 5.5 thinking blocks either. Switching mid-conversation silently drops earlier reasoning — the request still succeeds, and dropped blocks are not billed.
Thinking control works differently. Opus 5.5 thinking is always on and cannot be disabled. Sonnet 5.5 supports between_tools, which skips up-front thinking at effort ≤ high — useful for tool-heavy pipelines where latency matters.
No forced tool use on either model. Both return a 400 error when tool_choice is set to "any" or a specific tool name.
Identical context and output limits. Both support 1M-token context and 128K output (300K via Batch API). No long-context pricing premium applies.
The Bottom Line
For most developers, Sonnet 5.5 is the smarter default: it matches or nearly matches Opus on most benchmarks, leads Terminal-Bench 4.0 for agentic coding, responds faster and costs half as much per token. Reserve Opus 5.5 for workloads where a small accuracy edge justifies the premium — long-running agents, complex multi-file refactors and high-stakes code review. Both models share the same 1M-token context window, 128K output limit and platform availability, so switching between them as needs change is straightforward.
Opus 5.5 vs Sonnet 5.5: FAQ
What are the differences between Claude Opus 5.5 and Sonnet 5.5?
Both share a 1M-token context window, 128K max output, June 2026 knowledge cutoff and the same platform availability. The key differences are price ($4/$20 vs $2/$10 per MTok), speed (Opus is moderate, Sonnet is fast), thinking behavior (Opus always-on vs Sonnet tunable) and default effort level (medium vs high). On benchmarks, Opus leads 7 of 8 by narrow margins while Sonnet leads Terminal-Bench 4.0 for agentic coding.
Is Claude Opus 5.5 better than Sonnet 5.5?
Opus 5.5 leads 7 of 8 Anthropic benchmarks by narrow margins (the widest gap is 3.2 pp on Humanity's Last Exam). Sonnet 5.5 leads Terminal-Bench 4.0, the most widely cited agentic coding benchmark. Opus is the stronger pick for long-running agents, complex reasoning and high-stakes code review. Sonnet delivers comparable quality at half the per-token cost with faster latency. The right choice depends on whether the small accuracy edge or the cost and speed advantage matters more for your workload.
How much cheaper is Sonnet 5.5 than Opus 5.5?
Exactly half per token: $2 / $10 (Sonnet) vs $4 / $20 (Opus) per million input / output tokens. A typical 100K-input, 20K-output request costs $0.40 on Sonnet and $0.80 on Opus. Keep in mind that per-token price does not always equal per-task cost — at max effort Sonnet can produce more output tokens, which narrows the gap.
Can I use Claude Opus 5.5 for free?
No. As of September 2026, Opus 5.5 is not available on the free plan at claude.ai. It requires a Pro, Max, Team or Enterprise subscription (with raised rate limits on each). Sonnet 5.5 is the default model for both the free and Pro plans.
Do Opus 5.5 and Sonnet 5.5 have the same context window?
Yes. Both support a 1-million-token context window and up to 128K output tokens (300K via the Batch API with the beta header). No long-context pricing premium applies to either model.
Can I switch between Opus 5.5 and Sonnet 5.5 mid-conversation?
Yes, but thinking blocks are not portable. Opus 5.5 thinking is only readable by Fable 5.1 and Mythos 5.1; no other model reads Sonnet 5.5 thinking blocks. Switching mid-conversation silently drops earlier reasoning (the request still succeeds and dropped blocks are not billed).
Which model is better for coding?
Both are strong. Sonnet 5.5 leads Terminal-Bench 4.0 (70.6% vs 66.4%), the most popular agentic coding benchmark; Opus 5.5 leads FrontierCode 1.1 (54.4% vs 52.1% at xhigh) and CursorBench 4.0 (57.8% vs 55.5%). For routine coding — quick edits, feature scaffolding, test generation — Sonnet is likely the better value. For complex multi-file refactors, large migrations or high-stakes code review where catching edge cases matters, Opus may be worth the premium.
What is Opus 5.5 fast mode?
A research-preview option (API only) that doubles the token price to $8 / $40 per million tokens in exchange for up to 2.5x faster output, according to Anthropic. Sonnet 5.5 does not have a fast mode — it is already the faster model by default.
When were Opus 5.5 and Sonnet 5.5 released?
Opus 5.5 launched on September 22, 2026; Sonnet 5.5 followed on September 28, 2026. Both share a knowledge cutoff of June 2026 and guaranteed availability through at least September 2027.