Anthropic released Claude Sonnet 5 on June 30, 2026, claiming performance close to Opus 4.8 at lower prices, along with a system card detailing its safeguards. This release offers developers a more cost-effective alternative near the top-tier Opus performance, potentially accelerating adoption of Anthropic's models in applications requiring high intelligence. Sonnet 5 features a 1 million token context window, 128,000 output tokens, no longer supports sampling parameters like temperature, and uses a new tokenizer that increases token counts by about 30% for English, effectively raising prices despite unchanged per-token rates.
Background
Anthropic classifies its models into tiers: Sonnet (mid-range), Opus (high-end), and Mythos (specialized for cybersecurity, not publicly released). The system card explains that Sonnet 5's safeguards are similar to Opus 4.7/4.8 because it is less capable at cyber tasks than Mythos 5. The new tokenizer standardizes tokenization across models, leading to higher token counts for most languages except Simplified Chinese.