Claude Sonnet 5 vs Claude Opus 4.8: Which to Use in 2026

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.

How do Claude Sonnet 5 and Opus 4.8 compare on benchmarks?

Independent benchmark org Artificial Analysis evaluates both. On Intelligence Index v4.3.2 (October 2026), Sonnet 5 at max effort scores 38.16 against Claude Opus 4.8 (Max) at 41.79 — a 3.6-point gap in the flagship's favour. Against the model you are most likely upgrading from, Claude Sonnet 4.6 (Max) scores 30.06, so Sonnet 5 is a real 8-point gain. One note on reading these: AA rebased this index from v4.1.1 to v4.3.2, which moves every score on the board because the benchmark basket changed, not the models. There is no conversion factor, so a Sonnet 5 figure of 53 from earlier coverage is a v4.1.1 number and is not comparable to anything here.

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 60% below Opus 4.8: $2/$10 per million against $5/$25. That $2/$10 rate is permanent — Anthropic announced it as introductory pricing through 31 August 2026 with a step up to $3/$15 scheduled for 1 September, then cancelled the increase. Its pricing page now states the $2/$10 rate “is now the standard price”.

Yet on Artificial Analysis's Intelligence Index v4.3.2, Sonnet 5 at max effort cost $5.09 per completed task against Opus 4.8 (Max) at $4.08 — about 25% more expensive per task, despite charging 60% less per token. 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 index rebase from v4.1.1 to v4.3.2 moved both absolute numbers, but it did not change the direction of the result — if anything the gap widened, from about 15% to about 25%.

The practical takeaway: you cannot compare these two models on per-token price alone. Note that the effort dial moves this a lot — at high rather than max, Sonnet 5 costs $1.79 per index task for 31.66 points, so you are trading roughly 6.5 points for two-thirds of the bill. 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.

Building with these models? Hire engineers who already have.

Choosing a model is the easy part — wiring it into a production agent, controlling token spend, and shipping reliable tool-use pipelines is where teams get stuck. Hire vetted remote developers through Codersera to extend your team with engineers who build agentic systems on Claude and other frontier models. Faster hiring, lower risk, and a risk-free trial to confirm technical fit.

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 60% below Opus 4.8 ($2/$10 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 came out higher: $5.09 against Opus 4.8's $4.08 on Artificial Analysis's Intelligence Index v4.3.2 at max effort, about 25% more.

What is Claude Sonnet 5's pricing?

$2 per million input tokens and $10 per million output tokens, permanently. That began as introductory pricing through 31 August 2026, but Anthropic cancelled the scheduled increase to $3/$15 and the pricing page now lists $2/$10 as standard. Opus 4.8 is $5/$25 per million. Both models are now listed as legacy; the current equivalents are Claude Sonnet 5.5 (also $2/$10) and Claude Opus 5.5 ($4/$20).

Should I use Opus 4.8 or a newer Opus?

A newer one, if you are starting now. Anthropic lists Opus 4.8 under “Legacy models (still available)” alongside Opus 5. The current model is Claude Opus 5.5 at $4/$20, which scores 57.62 on Artificial Analysis's Intelligence Index v4.3.2 against Opus 4.8 (Max) at 41.79. Budget note: Opus 5.5 is far more verbose, so its measured cost per index task is $5.98 — higher than Opus 4.8's $4.08 and marginally higher than Opus 5's $5.86, even though it charges less per token than either.

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?

They tie at their top effort tiers on Artificial Analysis's Intelligence Index v4.3.2 — Sonnet 5 (Max) 38.16, GPT-5.5 (Xhigh) 38.36 — but GPT-5.5 leads by about 5 points at high effort, and AA measured it as cheaper per completed task at every tier despite its $5/$30 pricing. See our full Claude Sonnet 5 vs GPT-5.5 comparison.