Claude Sonnet 5 vs Claude Opus 4.8: The Complete Comparison

The Verdict

Claude Sonnet 5 is the first Sonnet that makes Opus look optional for most workloads. Opus 4.8 is still the stronger model. But the gap is now small enough that the price difference becomes the deciding factor.

On June 30, 2026, Anthropic released Claude Sonnet 5, its most agentic Sonnet yet. One month earlier it shipped Claude Opus 4.8, the strongest Opus to date. Both share the same 1M context window, the same 128K output cap, the same January 2026 training cutoff. The question that matters: when does the cheaper model actually lose?

The answer: less often than before. Sonnet 5 wins Terminal-Bench 2.1, ties on knowledge work and multidisciplinary reasoning with tools, and stays within a few points everywhere else. Opus 4.8 keeps a real edge on deep coding, olympiad math, and computer use. Here is every difference that decides which one to run.

At a Glance

The spec sheets are nearly identical. Same context, same output cap, same training cutoff, same cloud partners. The differences are price (Sonnet is 40-60% cheaper), speed (Sonnet is faster), and the handful of benchmark gaps documented below.

Benchmark Head-to-Head

Across 11 shared benchmarks from Anthropic's system cards, Sonnet 5 wins one outright (Terminal-Bench 2.1), ties one within half a point (HLE with tools), and trails on nine. But the margins tell the story: 7 of those 9 deficits are under 6 points. The gap only opens wide on olympiad math (USAMO: 17.2 points).

Knowledge work tells a separate story. On GDPval-AA v2 (the Artificial Analysis professional-task benchmark), Sonnet 5 scores 1618 Elo to Opus 4.8's 1603, statistically tied and both trailing only Fable 5. On Real-World Finance v2, the two are again indistinguishable: Elo 1219 vs 1222. On AA-Briefcase, Sonnet 5 leads 1393 to 1352, running longer trajectories (183 turns vs 55) to get there.

Where Sonnet 5 Wins

Sonnet 5 is not just "cheaper Opus." There are tasks where it is genuinely the better model.

• Terminal-Bench 2.1: 80.4 vs 74.6. A 5.8-point lead on complex command-line workflows. The system card credits mini-SWE-agent as the harness, and notes that Terminus-2 produces 2.7x more timeouts at xhigh effort. Sonnet 5 handles terminal-heavy coding better than any Opus released to date.
• Knowledge work parity. GDPval-AA v2 (Elo 1618 vs 1603), Real-World Finance v2 (1219 vs 1222), and AA-Briefcase (1393 vs 1352). On the professional tasks that mirror how companies actually use these models, the two are interchangeable.
• BrowseComp: comparable cost-accuracy.At 84.7% (single agent), Sonnet 5 matches Opus 4.8's accuracy at a given task cost on agentic web search. The system card states they are "comparable."
• Speed.Anthropic labels Sonnet 5 "Fast" and Opus 4.8 "Moderate." For latency-sensitive production workloads (customer-facing agents, real-time research), speed is a feature, not a nice-to-have.
• Prompt injection robustness. Sonnet 5 shows 0.1% attack success on coding (Opus: 0.41%), ties on computer use (0.07% each), and dominates browser use (0.93% vs 31.5% for Opus without safeguards).

Where Opus 4.8 Still Leads

Opus 4.8 earns its premium on the hardest tasks. The pattern is consistent: as difficulty and task horizon increase, the gap widens.

The standout is USAMO 2026: Opus 4.8 scores 96.7% on olympiad proof-based math where Sonnet 5 manages 79.5%. This is a 17-point gap, the widest in the comparison, and reflects a genuine tier difference in deep mathematical reasoning. On SWE-bench Pro, which uses large multi-file diffs from actively maintained repositories, Opus holds a 6-point lead that tracks with its advantage on longer, messier engineering work.

One caveat worth noting: Sonnet 5 uses a different tokenizer than Opus 4.8. The same input can map to roughly 1.0 to 1.35x more tokens on Sonnet 5, depending on content type. Anthropic set the introductory pricing to make the transition cost-neutral, but at standard pricing this tokenizer difference slightly narrows the effective price gap.

Pricing: Same Curve, Different Price Tags

This is the structural change. Previous Sonnets fell well short of Opus, creating two separate price-performance tiers. Sonnet 5 and Opus 4.8 now sit on a single continuous cost-performance curve. At any effort level, Sonnet 5 offers lower cost and Opus 4.8 offers higher accuracy, with the ranges overlapping.

The practical math: an output-heavy agent workload that costs $1,000/day on Opus 4.8 lands near $400/day on Sonnet 5 at standard pricing, or $400/day at intro rates. For teams running hundreds of agents in parallel, this is the difference between viable and prohibitive. Both models support up to 90% cost savings with prompt caching and 50% with batch processing.

Which Model for Which Workload

Choose Claude Sonnet 5 for high-volume production agents, customer-facing chatbots, content generation at scale, real-time research, and any workload where latency and cost per token are primary constraints. It handles multi-step coding, tool use, and knowledge work at a level that required Opus a month ago. For the majority of API workloads, Sonnet 5 is the correct default.

Choose Claude Opus 4.8for maximum accuracy on hard coding (multi-file repository work, long-horizon SWE), deep mathematical reasoning, computer-use browser agents where 83% is materially better than 81%, or any task where you are already hitting Sonnet 5's ceiling and need the extra points. Opus 4.8's fast mode ($10/$50 at 2.5x speed) is also an option when you need Opus quality with reduced latency.

The middle path: use both. Anthropic's own blog shows that tuning the effort level across both models creates a single curve. Route hard tasks to Opus at high effort, routine tasks to Sonnet at low effort, and adjust the split until the bill and the quality meet your requirements.

Compare both live on the LLM Stats comparison page, or see each model's full benchmark profile for Claude Sonnet 5 and Claude Opus 4.8.