Anthropic shipped Claude Sonnet 5 on June 30, 2026, and it landed with an unusual twist: a mid-tier model that scores near the flagship on agentic work, priced below it, yet in practice can cost more per task. If you are deciding between Sonnet 5 and Claude Opus 4.8 for your engineering workflows, the sticker price is the wrong place to start. This guide breaks down where each model wins, what the benchmarks actually say, and how to route work between them by cost, speed, and difficulty.
New to Sonnet 5? Start with our Claude Sonnet 5 launch guide for the full feature rundown, then come back here for the head-to-head decision. If you are also weighing OpenAI's flagship, see Claude Sonnet 5 vs GPT-5.5.
What is the difference between Claude Sonnet 5 and Claude Opus 4.8?
Both are Anthropic models with a 1-million-token context window, and both are strong. The difference is positioning:
- Claude Sonnet 5 is the agentic mid-tier workhorse. It is tuned to take many small steps: call tools, run loops, retry, and grind through long-running automation. On agentic knowledge-work benchmarks it sits just ahead of Opus 4.8.
- Claude Opus 4.8 is Anthropic's flagship. It is the stronger model on heavy reasoning and deep knowledge work — the frontier-physics, hard-math, dense-analysis end of the spectrum — where raw thinking matters more than tool orchestration.
In other words, Sonnet 5 is built to do a lot; Opus 4.8 is built to think hard. That framing predicts almost every routing decision below.
| Claude Sonnet 5 | Claude Opus 4.8 | |
|---|---|---|
| Positioning | Agentic mid-tier workhorse | Reasoning flagship |
| AA Intelligence Index (max effort) | 53 (+6 vs Sonnet 4.6) | ~2-3 points higher on heavy reasoning |
| Best at | Agentic knowledge work (AA-Briefcase, GDPval-AA) — just ahead of Opus 4.8 | Heavy reasoning & deep knowledge (e.g. CritPt frontier physics) |
| Input price / M tokens | $2 intro → $3 standard (Sep 1, 2026) | $5 |
| Output price / M tokens | $10 intro → $15 standard | $25 |
| Real cost per task* | ~$2.29 (works harder) | ~15% lower than Sonnet 5 |
| Token behavior | ~40% more output tokens, ~3x agentic turns vs Sonnet 4.6 | More economical per completed task |
| Context window | 1M tokens | 1M tokens |
| Availability | Generally available | Generally available (Fable 5 sits above, limited availability) |
*Per-task cost measured by Artificial Analysis across its Intelligence Index evaluation, early July 2026.
How do Claude Sonnet 5 and Opus 4.8 compare on benchmarks?
Independent benchmark org Artificial Analysis evaluated Sonnet 5 ahead of release. At maximum reasoning effort, Sonnet 5 scores 53 on the Artificial Analysis Intelligence Index — a 6-point jump over Sonnet 4.6, and enough to place it fifth overall at launch. That is genuinely close to the flagship tier.
But "close overall" hides a split:
- On agentic knowledge work — the AA-Briefcase and GDPval-AA style benchmarks that reward tool use and multi-step task completion — Sonnet 5 edges ahead of Opus 4.8, trailing only Fable 5 (which is a premium, limited-availability model, not something most teams can build on yet).
- On heavy reasoning — dense, frontier problems like CritPt physics (where even the best models land around 17%) — Opus 4.8 remains stronger. This is the regime where extra raw reasoning depth outweighs the ability to take more steps.
So if your workload looks like "an agent doing real work across tools and files," Sonnet 5 is at or above the flagship. If it looks like "one very hard question that needs the deepest possible single answer," Opus 4.8 pulls ahead.
Why does Claude Sonnet 5 cost more per task than Opus 4.8?
This is the counterintuitive part, and it is the single most important thing to understand before you pick.
Sonnet 5's per-token price is roughly half of Opus 4.8: $2/$10 per million (input/output) during the introductory window through August 31, 2026, then $3/$15 after — the same rate as Sonnet 4.6. Opus 4.8 is $5/$25.
Yet Artificial Analysis measured Sonnet 5 at about $2.29 per task on the Intelligence Index — roughly 15% more expensive per task than Opus 4.8, despite the lower sticker price. The reason: Sonnet 5 "works harder." At max effort it burns about 40% more output tokens per task than Sonnet 4.6, and on agent-heavy benchmarks it runs close to 3x as many agentic turns to reach an answer. More tokens and more loops at a lower rate can still add up to a higher bill.
The practical takeaway: you cannot compare these two models on per-token price alone. If your task is short and bounded, Sonnet 5's cheap tokens win easily. If your task is open-ended and lets the model loop as much as it wants, budget for Sonnet 5 to spend — and cap its effort/turns if cost predictability matters.
When should you use Claude Sonnet 5?
Reach for Sonnet 5 when the work is agentic and high-volume:
- Coding agents and IDE assistants — multi-file edits, running tests, iterating on failures across a repo.
- Tool-heavy automation — pipelines that call APIs, query databases, and chain steps.
- Long-running background jobs — where the 1M-token context lets it hold a large codebase or document set in view.
- Cost-sensitive high-throughput tasks that are bounded (short prompts, capped output) so the cheap per-token rate actually lands as a cheap bill.
Sonnet 5 is the default for most day-to-day engineering automation. It is fast, flexible, and near-flagship on exactly the kind of work agents do.
When should you use Claude Opus 4.8?
Reach for Opus 4.8 when the work is hard and reasoning-bound:
- Frontier reasoning — research-grade math, science, and analysis where one wrong step invalidates the answer.
- Deep knowledge work — dense synthesis, legal or financial analysis, architecture decisions with many interacting constraints.
- Single high-stakes answers where you want the strongest possible one-shot response rather than a lot of iteration.
- Predictable per-task cost on hard problems — because Opus 4.8 reaches its answer with fewer loops, it can be the cheaper and stronger choice on genuinely difficult tasks.
The verdict: which Claude model should you pick?
There is no single winner — that is the point of a two-tier lineup. Route by workload:
- Default to Sonnet 5 for agents, coding, tool use, and high-volume automation. It is fast, flexible, and at or above the flagship on agentic benchmarks.
- Escalate to Opus 4.8 for the hardest reasoning, deepest knowledge work, and single high-stakes answers — where it is both stronger and, per completed task, often cheaper.
- Watch effort settings. Sonnet 5's cost advantage evaporates if you let it loop freely. Cap turns and output on open-ended tasks, or the cheap-per-token model becomes the expensive-per-task one.
Most teams will run both: Sonnet 5 as the everyday agent, Opus 4.8 on call for the problems that actually need a flagship. The smart move is a router that sends bounded, tool-heavy work to Sonnet 5 and escalates hard reasoning to Opus 4.8.
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Frequently asked questions
Is Claude Sonnet 5 better than Opus 4.8?
It depends on the task. On agentic knowledge work, Sonnet 5 sits just ahead of Opus 4.8. On heavy reasoning and deep knowledge work, Opus 4.8 is stronger. Neither is strictly "better" — they target different workloads.
Why is Claude Sonnet 5 cheaper per token but more expensive per task?
Sonnet 5's per-token price is about half of Opus 4.8 ($2/$10 introductory vs $5/$25). But it generates roughly 40% more output tokens and runs up to 3x more agentic turns per task, so its measured cost per task (~$2.29) came out about 15% higher than Opus 4.8 in Artificial Analysis testing.
What is Claude Sonnet 5's pricing?
Introductory pricing is $2 per million input tokens and $10 per million output tokens through August 31, 2026, then it moves to the standard $3/$15 — the same rate as Sonnet 4.6. Opus 4.8 is $5/$25 per million.
Do Claude Sonnet 5 and Opus 4.8 have the same context window?
Yes. Both offer a 1-million-token context window, so both can hold large codebases or document sets in a single request.
Should I use Sonnet 5 or Opus 4.8 for coding?
For most coding agents — multi-file edits, running tests, iterating on failures — Sonnet 5 is the default, since it is fast, cheap on bounded tasks, and at or above flagship level on agentic work. Escalate to Opus 4.8 for architecture decisions or unusually hard algorithmic problems that reward deeper reasoning.
How does Claude Sonnet 5 compare to GPT-5.5?
Sonnet 5 is the agentic mid-tier workhorse; GPT-5.5 is OpenAI's flagship, scoring 2-3 points higher at maximum reasoning effort but costing more. See our full Claude Sonnet 5 vs GPT-5.5 comparison.