Claude Sonnet 5 pushes agent capability into the default tier

TL;DR

Anthropic released Claude Sonnet 5 on June 30, calling it its most agentic Sonnet yet — a mid-tier model whose higher-effort runs can match Opus 4.8 on some agent tasks, at introductory pricing of $2/$10 per million tokens (in/out) through August 31. It’s now the default model on Free and Pro plans, which means near-premium agent behavior just became the thing most Claude users get without asking. Watch the tokenizer footnote before comparing rate cards.

What happened

Anthropic shipped Sonnet 5 across all plans, Claude Code, and its API (claude-sonnet-5), positioning it as a substantial upgrade over Sonnet 4.6 in reasoning, tool use, coding, computer use, and knowledge work. The company’s framing: agentic gains had recently concentrated in its Opus-class models, and “Sonnet 5 narrows the gap: its performance is close to that of Opus 4.8, but at lower prices.”

The cost-performance data is the interesting part. On the agentic-search benchmark BrowseComp and the computer-use benchmark OSWorld-Verified, Anthropic’s published curves show Sonnet 5 strictly above Sonnet 4.6 at every effort level, with a much wider range of cost-performance options than Opus 4.8 — and higher-effort Sonnet 5 matching Opus on some tasks. Opus 4.8 runs $5/$25 per million tokens; Sonnet 5 lands at $2/$10 through August 31, then $3/$15. Anthropic also raised rate limits across Chat, Cowork, Claude Code, and the platform to absorb higher-effort token usage.

Early-access customer quotes back the agentic claim with specifics: Cursor’s co-founder says agents “stay on plan” and ship multi-step changes; one Rust engineer describes Sonnet 5 unprompted writing a reproducing test, fixing the bug, then stashing the fix to confirm the bug returned; insurance and legal teams report end-to-end task completion where previous models stalled midway.

On safety, Anthropic reports lower rates of hallucination, sycophancy, and overall misaligned behavior than Sonnet 4.6, and better resistance to prompt injection — though somewhat higher misalignment rates than Opus 4.8 and Mythos Preview on its automated behavioral audit. Notably, the company says it did not deliberately train Sonnet 5 on cybersecurity: on an exploit-development evaluation built with Mozilla against Firefox vulnerabilities, Sonnet 5 never produced a working exploit (0.0%, same as Sonnet 4.6), though its partial-success rate ticked up. It ships with real-time cyber safeguards on by default — the Opus 4.7/4.8 tier of safeguards, less strict than those on Fable 5.

Why it matters

The default model tier is where agent adoption actually scales. If teams can get near-Opus behavior from the mid-tier model — and dial effort up only when a task warrants it — they can reserve frontier capacity for the moments that require it, and the economics of running agents all day change materially. Making that model the free-tier default also means Anthropic’s broadest user base is now testing agentic behavior whether they planned to or not.

The caveat is tokenization: Sonnet 5 uses an updated tokenizer, and Anthropic says the same input can map to roughly 1.0–1.35× as many tokens depending on content type — the introductory pricing is explicitly set to make the transition “roughly cost-neutral.” Model price comparisons still need workload measurements, not a glance at the rate card.

The fine print

The benchmark story has a wrinkle: Anthropic issued a same-day correction to its BrowseComp chart after the original used a simpler methodology that understated Sonnet 5’s performance; the updated chart matches the system card’s setup (a 10M-token budget with compaction and programmatic tool calling). Sonnet 4.6 comparison scores were also restated for two evaluations due to grader and methodology changes. All numbers here are Anthropic’s own, detailed in the Sonnet 5 System Card.

The model menu now includes an effort dial, a tokenizer footnote, and temporary pricing. Buying intelligence increasingly resembles booking an airline seat.