Claude Opus 5 vs. Claude Sonnet 5: Choosing Which to Use

Anthropic hasn't published a direct Opus 5 vs. Sonnet 5 benchmark. Each launch post compares its model to its own predecessor and to Opus 4.8. So the numbers below tell part of the story. The rest is the same judgment call you'd make with any Opus vs. Sonnet pairing: how much ceiling do you actually need, and what are you willing to pay for it.

What Is Claude Opus 5?

Opus 5 is Anthropic's top-tier model, released July 24, 2026, priced at $5 per million input tokens and $25 per million output tokens. Anthropic calls it state-of-the-art on Frontier-Bench and GDPval-AA, and close to the larger Fable 5 model at half the price. It's the default model on Claude Max and the strongest model on Claude Pro.

Its standout trait is self-verification: checking its own work, building test harnesses when no data feed exists, pushing back on bad instructions instead of complying.

What Is Claude Sonnet 5?

Sonnet 5 is Anthropic's mid-tier model, released June 30, 2026. Anthropic describes it as its most agentic Sonnet yet, with performance close to Opus 4.8 at a lower price. It's the default model on Free and Pro plans, and available on Max, Team, Enterprise, and the API.

Pricing: $2 per million input tokens and $10 per million output tokens through August 31, 2026, then $3/$15 standard.

The Tier Difference, in Practical Terms

This is the part the release posts don't spell out, because it's just how Anthropic's model lineup works generally, not specific to this pair.

Opus is the ceiling model

You reach for it when being wrong is expensive, when a task runs long and autonomous with no one checking in partway through, or when the problem is genuinely novel rather than a variation on something the model has seen a thousand times. Deep multi-file refactors, ambiguous research questions, long agentic runs where errors compound — that's Opus territory. You're paying for the model to catch its own mistakes before they become your problem.

Sonnet is the workhorse

It's built to be fast and cheap enough to run constantly: chat assistants, high-volume classification, iterative coding where you're in the loop reviewing every few steps anyway, latency-sensitive applications. The judgment ceiling is lower, but for a large share of real work, you never bump into it. Most teams' day-to-day usage should skew Sonnet, with Opus called in selectively for the tasks that actually need it.

Sonnet 5 specifically is Anthropic's attempt to push that ceiling higher than usual for a mid-tier model — hence "most agentic Sonnet yet" and the claim that it approaches Opus 4.8 on some tasks. That's the context for reading the benchmark numbers below: Sonnet 5 isn't just cheap, it's cheap and closer to Opus-tier than earlier Sonnets were.

What the Release Numbers Actually Show

Price

The clearest, least ambiguous difference. Sonnet 5 runs $2/$10 (through Aug 31) or $3/$15 (standard) against Opus 5's flat $5/$25 — Sonnet 5 costs 40–60% of Opus 5 depending on the pricing window.

Alignment 

The one place the two release posts name each other directly. Anthropic's automated audit found Opus 5 "adheres to Claude's Constitution better than Opus 4.8, Sonnet 5, or Fable 5" — a real, stated result, not an inference. Opus 5 scores 2.3 on that audit, its safest score yet. Sonnet 5 improved on its own predecessor (lower hallucination, lower sycophancy, better resistance to prompt-injection hijacks) but Anthropic notes it still runs a higher misaligned-behavior rate than Opus 4.8.

Cybersecurity

Both models were kept off deliberate cyber training, but they land in different places.

Opus 5 comes close to Mythos 5 at finding vulnerabilities, even though it lags on turning them into exploits. Sonnet 5 sits further back: Anthropic says outright that Sonnet 5 has "substantially poorer cyber capabilities than Opus 4.8 and Mythos 5," and on a Firefox exploit-development eval it never produced a full working exploit (0.0% success, only a slight edge over Sonnet 4.6 on partial attempts).

If your use case touches anything cyber-adjacent, this gap is real and it favors Opus 5 clearly.

Coding and knowledge work

Here the two posts use different benchmark sets, so there's no clean scoreline.

Opus 5 more than doubles Opus 4.8 on Frontier-Bench v0.1 and lands within 0.5% of Fable 5 on CursorBench 3.2 at half the cost. Sonnet 5 is described as approaching Opus 4.8 on BrowseComp and OSWorld-Verified at high effort.

Read together with the tier logic above: Sonnet 5 gets close on the specific tasks Anthropic chose to highlight, but Opus 5's wins look larger and broader. Treat this one as directional, not measured.

When to Choose Which

Choose Opus 5 if 

the task is high-stakes, long-running, or genuinely novel; you're doing life sciences, chemistry, or complex multi-step agentic work; you need the most aligned model available; anything cybersecurity-adjacent is in scope.

Choose Sonnet 5 if

you're running high volume, latency matters, or the task is well-scoped enough that you don't need Opus's judgment ceiling; you're on Free or Pro where it's already the default; cost per token is the binding constraint.

Neither, if

you need cybersecurity work with reduced guardrails. Opus 5 falls back to Opus 4.8 automatically, and Sonnet 5 sits behind Opus 4.8 outright.

Final Thoughts

Most of the deciding factor here is the same one that decides any Opus-vs-Sonnet question: does this task need the ceiling, or just needs to get done reliably at scale? Sonnet 5 pushes that ceiling higher than past Sonnets have, which is the real news in its launch. But where Anthropic did put a number on the two models together — the alignment audit — Opus 5 came out ahead, and the cybersecurity story points the same direction. Default to Sonnet 5 for volume, reach for Opus 5 when the task can't afford a wrong answer.