Claude Opus 5.5

• - 🏢 Anthropic's flagship Opus-tier model, succeeding Claude Opus 5.
• - 📏 1M-token context window with 128K max output tokens.
• - 🧠 Adaptive thinking always on; effort is the main control lever.
• - 🔧 Built for long-running agentic coding, tool loops and computer use.
• - ⚡ Anthropic reports ~40% lower running cost than Opus 5 on typical workloads.
• - 👁️ Multimodal inputs, function calling and web search supported.
• - 🆕 Breaking changes: thinking can't be disabled, forced tool use errors.
• - 📚 Knowledge work gains: documents, analysis, long-running collaboration.

Anthropic

Anthropic PBC is an American artificial intelligence company headquartered in San Francisco. Structured as a public benefit corporation, the lab develops large language models under the Claude name, with a research emphasis on building reliable, steerable,…

Claude Opus 5.5 is Anthropic's top-tier Claude model, released in 2026 as the direct successor to Claude Opus 5, which itself was a step change over Claude Opus 4.8. Anthropic positions it for long-running agentic coding and knowledge work, with a one-million-token context window, up to 128K output tokens per response (and, on the batch API with a beta header, up to 300K), plus vision input, tool calling and web search.

The clearest generational change is efficiency. Anthropic states that Opus 5.5 costs about 40% less to run than Opus 5 on typical workloads, and that on Terminal-Bench 4.0 — a multi-step command-line task benchmark — Opus 5.5 at default effort surpasses Opus 5 at maximum effort for roughly a fifth of the cost, per the company's own reported results.

Behaviourally, thinking is now always adaptive and cannot be switched off, making the effort setting the primary lever for trading depth against latency and cost. Migrating from Opus 5 involves four breaking changes: thinking cannot be disabled, forced tool use returns an error, thinking blocks are bound to a specific model and conversation, and the older computer-use tool version is no longer accepted on the Claude API and Google Cloud.

Within the wider lineup it sits above the Sonnet tier, including Claude Sonnet 5, alongside Claude Fable 5.1 and the earlier low-latency Claude Opus 5 Fast.