Sonnet 5 vs Opus 4.8
Two Claude models built on the same chassis — identical 1M-token context, the same tokenizer, the same adaptive-thinking dial. What actually separates them is one number: price per capability point. Here is that number, laid out against Anthropic's own scores.
Start with what does NOT differ: Sonnet 5 and Opus 4.8 share a 1M-token context window, the same tokenizer generation, the same January 2026 cutoff and the same closed, hosted-only delivery. The decision is a single trade — capability margin against a 2.5x price multiple. On the nine head-to-head benchmarks both System Cards report, Opus 4.8 is ahead on six, but read the size of those wins, not just the count: they run 2.2 to 6.6 points (SWE-bench Verified 88.6 vs 85.2, SWE-bench Pro 69.2 vs 63.2, Humanity's Last Exam no-tools 49.8 vs 43.2), Sonnet 5 takes Terminal-Bench 2.1 outright (80.4 vs 74.6), and BrowseComp plus HLE-with-tools are effectively level. Opus 4.8 lists at $5 / $25 per million tokens; Sonnet 5 at $2 / $10 introductory, $3 / $15 from September 1, 2026 — so you are paying roughly two-and-a-half times as much for a lead that is usually low single digits. That framing answers the page: meter by Sonnet 5, and spend the Opus premium only where a few benchmark points change the outcome of the work.
Benchmark head-to-head
Nine benchmarks appear on both cards. The Sonnet 5 column is from the Claude Sonnet 5 System Card, the Opus 4.8 column from the Claude Opus 4.8 System Card; both were run under the same standard configuration (adaptive thinking at max effort, averaged over five trials) and carry an identical GPT-5.5 reference column across the two cards on every shared row. The winner cell is bold, and the margin is spelled out — because the margin, not the checkmark, is what you are deciding about.
| Benchmark | Sonnet 5 | Opus 4.8 | Margin |
|---|---|---|---|
| SWE-bench Verifiedcoding | 85.2% | 88.6% | Opus +3.4 |
| SWE-bench Prohard, real repos | 63.2% | 69.2% | Opus +6.0 |
| SWE-bench Multilingual9 languages | 78.3% | 84.4% | Opus +6.1 |
| Terminal-Bench 2.1agentic terminal | 80.4% | 74.6% | Sonnet +5.8 |
| OSWorld-Verifiedcomputer use | 81.2% | 83.4% | Opus +2.2 |
| BrowseCompagentic search | 84.7% | 84.3% | ≈ even (Sonnet +0.4) |
| Humanity's Last Examno tools | 43.2% | 49.8% | Opus +6.6 |
| Humanity's Last Examwith tools | 57.4% | 57.9% | ≈ even (Opus +0.5) |
| AutomationBenchworkflow automation | 13.5% | 15.5% | Opus +2.0 |
This is two card runs read side by side, not one controlled bench. The alignment holds because both cards use the same standard configuration and share an identical GPT-5.5 reference column, but treat it as directional. Terminal-Bench 2.1 is the row to discount: Opus 4.8's 74.6 was recorded at high rather than max effort and through a different harness than Sonnet 5's figure, and Anthropic flags the benchmark as latency-sensitive, so Sonnet 5's lead there is partly configuration rather than a clean capability gap.
Two figures that might be expected here are left off on purpose. GDPval-AA is reported on different versions across the two cards (Opus 4.8 on v1, Sonnet 5 on v2), which are not comparable, and GPQA is not published in the Sonnet 5 card. We show the rows we can stand behind and drop the rest rather than pair mismatched numbers.
LLM Index Score
Separately from Anthropic's own head-to-head, both models carry an LLM Index Score — our single composite built from the LiveBench raw task set, weighted and unmodified. It is a different lens (a broad public benchmark rather than the vendor's own suite) and lands in the same place:
A 3.6-point composite gap, the same direction and roughly the same size as the vendor deltas above. Two independent measurements agreeing on a small lead is more useful than either alone — and neither is large enough to settle the decision by itself. See how the score is built.
The chassis, side by side
| Sonnet 5 | Opus 4.8 | |
|---|---|---|
| Released | June 30, 2026 | May 28, 2026 |
| API model ID | claude-sonnet-5 | claude-opus-4-8 |
| Price (input / output) | $2 / $10 intro → $3 / $15 (Sep 1, 2026) | $5 / $25 per MTok |
| Context / max output | 1M in / 128K out | 1M in / 128K out |
| Comparative latency | Fast | Moderate |
| Fast mode | No (Opus-tier only) | Yes ($10 / $50 per MTok) |
| Adaptive thinking | Yes (effort defaults to high) | Yes (effort defaults to high) |
| Knowledge cutoff | Jan 2026 | Jan 2026 |
| Availability | Claude API, Bedrock, Google Cloud, Microsoft Foundry — plus the default on Free/Pro and in Claude Code | Claude API, Amazon Bedrock, Claude Platform on AWS, Google Cloud, Microsoft Foundry |
| Self-host | No — closed, hosted only | No — closed, hosted only |
What the gap costs
Because the two share a tokenizer generation, a token count on one is a token count on the other — so the price gap maps straight onto the same workload with no conversion. Take a steady agentic-coding load of 10M input and 2M output tokens a day:
| Sonnet 5 | Opus 4.8 | |
|---|---|---|
| Per day (10M in / 2M out) | $40 intro · $60 standard | $100 |
| Per 30-day month | $1,200 intro · $1,800 standard | $3,000 |
| Vs Opus 4.8 each month | −$1,800 intro · −$1,200 standard | — |
Introductory Sonnet 5 pricing ($2 / $10) runs through August 31, 2026; standard ($3 / $15) applies from September 1. Prompt caching bills cache reads at 0.1x input on both models, so a prefix-heavy workload shrinks each column by the same proportion and leaves the ratio between them unchanged.
Which should you pick?
Default to Sonnet 5 when…
- The workload is high-volume — assistants, agents, batch pipelines — where a 2.5x per-token gap compounds far faster than a few benchmark points repay it.
- Latency is part of the product: Sonnet 5 is the faster of the two by default, and Anthropic positions it as its best balance of speed and intelligence.
- You are inside the introductory window (through Aug 31, 2026) and want the $2 / $10 rate before it steps up to $3 / $15.
- The task tolerates a low-single-digit capability gap — which, on this data, is most of them.
Escalate to Opus 4.8 when…
- A few benchmark points genuinely change the outcome — hard multi-file refactors, long-horizon agents, work where a wrong answer is expensive to catch.
- The job leans on exactly where Opus's margin is largest: SWE-bench Pro, multilingual coding, and tool-free reasoning (HLE), each a 6-point lead.
- You need fast mode — the premium high-speed tier that only the Opus line offers.
- Per-token cost is a rounding error against the value of the task, so the ceiling is worth more than the saving.
Before you commit
- Terminal-Bench 2.1 is Sonnet 5's only outright win and also the least clean row — Opus 4.8 ran it at high (not max) effort through a different harness, and the benchmark is latency-sensitive. Weight it lightly.
- Sonnet 5's introductory $2 / $10 rate ends August 31, 2026 and rises to $3 / $15 on September 1, which narrows — but does not close — the price gap with Opus 4.8.
- Fast mode (premium-priced, markedly faster output) exists only on the Opus tier; Sonnet 5 has no equivalent, so latency-critical work that also wants top capability has one path.
- Both are closed and hosted-only — no downloadable weights on either, so neither is on the table if you need to self-host, run offline, or freeze a version.
The full read
Most model comparisons open by asking which one is better. That is the wrong first question here, because the honest answer — Opus 4.8, narrowly, on most benchmarks — hides how little rides on it. Sonnet 5 and Opus 4.8 are not rival architectures; they are two points on one line. They carry the same 1M-token context window, the same 128K output ceiling, the same January 2026 knowledge cutoff, the same tokenizer generation, and the same adaptive-thinking control that defaults to high effort. Strip away price and you are choosing between two models that behave more alike than any spec sheet admits.
So the useful question is not who wins but by how much, and against what. Opus 4.8 leads six of the nine shared benchmarks, but the leads are modest: 3.4 points on SWE-bench Verified, 6.0 on SWE-bench Pro, 6.6 on the tool-free slice of Humanity's Last Exam, and low single digits elsewhere. Sonnet 5 wins Terminal-Bench 2.1, the softest row on the board, and draws level on agentic search and tool-assisted reasoning. Our own LLM Index Score — a separate, LiveBench-based composite — puts the same gap at 3.6 points. Two independent measurements landing on a small, consistent Opus lead is a stronger signal than either on its own, and neither is large enough to make Sonnet 5 the wrong tool for a task it can do.
Then set that margin next to the bill. Opus 4.8 costs $5 per million input tokens and $25 per million output; Sonnet 5 runs $2 / $10 through the end of August 2026 and $3 / $15 after — roughly a third to a half of Opus depending on the date. On a realistic agentic day that is $40 against $100, or $1,200 against $3,000 a month, for a capability difference that is usually a few points. There is no long-context surcharge on either model and both cache reads at a tenth of the input rate, so the ratio holds across whatever prompt shape you run.
The practical rule falls straight out of the arithmetic: meter by Sonnet 5 and escalate to Opus 4.8 by exception. Route the volume — the assistants, the retrieval, the routine agent turns — through the cheaper model, and reserve Opus for the specific work where its largest margins live: hard real-repo coding, multilingual codebases, and reasoning without tools. That is a workload-routing decision, not a loyalty one, and it changes as your traffic mix changes.
Two operational notes the benchmarks are silent on. First, only the Opus tier has fast mode, the premium high-throughput option, so a task that needs both top capability and low latency has a single home. Second, none of these numbers speak to rate limits, regional availability, or how each model behaves under sustained load — the things that most often decide whether a model is actually usable for you. The scores narrow the field; your own traffic settles it. These figures are Anthropic's reported data and our public-benchmark composite, not our hands-on runs, so read them as a well-sourced starting point and confirm on the workload you actually have.
Full index pages
FAQ
Is the Opus 4.8 lead over Sonnet 5 big enough to matter?
Usually not. Opus 4.8 is ahead on six of nine shared benchmarks, but the margins run 2.2 to 6.6 points, and our independent LLM Index Score composite shows the same 3.6-point gap. For a task that tolerates a few points of variance — most of them — Sonnet 5 is the right call at roughly 2.5x less. The margin only becomes decisive on the hardest coding and reasoning work, where Opus's leads are widest.
For coding, should I use Sonnet 5 or Opus 4.8?
It depends on how hard the coding is. On everyday and mid-complexity work the two are close — SWE-bench Verified is 88.6 for Opus 4.8 against 85.2 for Sonnet 5 — so Sonnet 5's lower price wins. Opus 4.8 pulls further ahead on the difficult end: SWE-bench Pro (real repos) 69.2 vs 63.2 and SWE-bench Multilingual 84.4 vs 78.3, both roughly 6-point gaps. Route routine coding to Sonnet 5 and escalate the hard, multi-file, cross-language jobs to Opus 4.8.
When is Opus 4.8 actually worth the extra cost?
When a few benchmark points change the result of the work rather than just the scoreboard — long-horizon agents, expensive-to-catch errors, the hardest reasoning and refactoring — or when you need fast mode, which only the Opus tier offers. At $5 / $25 against Sonnet 5's $2 / $10, you are paying about two-and-a-half times more, so the premium is justified when per-token cost is small next to what the task is worth, and rarely on high-volume traffic.
Are Sonnet 5 and Opus 4.8 the same underneath?
They share a great deal. Both have a 1M-token context window, up to 128K output tokens, a January 2026 knowledge cutoff, the same tokenizer generation, and the same adaptive-thinking dial that defaults to high effort — and both are closed, hosted-only Claude models. Because the tokenizer is shared, a token count on one is a token count on the other, which is why the price comparison maps cleanly onto any workload.
Why aren't GPQA or GDPval in this comparison?
Because we can't pair them honestly. GPQA isn't published in the Claude Sonnet 5 System Card, and GDPval-AA is reported on different versions across the two cards (Opus 4.8 on v1, Sonnet 5 on v2), which aren't directly comparable. Rather than place mismatched or missing numbers side by side, we show only the nine benchmarks both cards report under the same configuration.
Figures from the Claude Opus 4.8 and Claude Sonnet 5 System Cards and Anthropic's published pricing. Last verified 2026-07-24. Vendor-reported and public-benchmark data, not our own hands-on runs.