Claude Sonnet 5.5: Speed, Cost & Security

Claude Sonnet 5.5, released by Anthropic on September 28, 2026, is presented as a faster and more capable successor to the legacy Sonnet 5 while retaining the same $2 per million input-token and $10 per million output-token pricing, context window, and output limit. Anthropic reports output speeds more than 30% faster, up to 30% lower task costs through reduced token and tool use, a lower cacheable-prompt threshold, and benchmark results approaching or sometimes exceeding Opus 5.5, although many performance figures remain vendor-reported and require independent validation. The release adds frontier-style cyber controls, including visible fallback to Sonnet 5 for certain higher-risk cyber and competing-model-development requests, reasoning-extraction blocks, and account-bound thinking blocks intended to limit distillation. It also reports improved prompt-injection resistance, but emphasizes that organizations remain responsible for access controls, tool permissions, data protection, monitoring, and governance. Migration is not fully drop-in because forced tool selection and non-default sampling parameters are rejected, conversations must generally remain append-only to preserve thinking, and streaming behavior differs. The guide recommends Sonnet 5.5 for high-volume production agents and terminal-oriented coding work, Opus 5.5 for the most difficult review or architecture tasks, and evaluation of GPT-6 Sol as another similarly priced vendor option.

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