Claude Opus 5.5

Overview. Claude Opus 5.5 is Anthropic's flagship hybrid reasoning model, launched September 22, 2026, as the first entry in the Claude 5.5 family. It runs adaptive thinking at all times, works with a 1M-token context window, and accepts text and image input for text output. Developers reach for it when a task demands deep, multi-step reasoning rather than a fast, shallow answer.

Key strengths. Anthropic built Opus 5.5 specifically for reasoning-heavy work: legal document analysis, financial modelling, scientific research synthesis, and complex software engineering are the use cases the model is positioned around. Because adaptive thinking is always on and can be tuned through configurable effort levels (low, medium, high, max), it can trade latency for depth on demand rather than requiring a separate reasoning mode to be switched on. Its 1M-token context window also makes it a strong fit for reasoning over very large documents or codebases in a single pass.

When to use it. Use Claude Opus 5.5 for interactive workloads where reasoning quality matters more than raw response speed — contract or case-law analysis, financial models with many interdependent assumptions, scientific literature synthesis, or intricate multi-file coding tasks. Its large context window suits jobs that need the full document, transcript, or repository in view at once. It's a strong choice when a task is flagged as reasoning-intensive rather than a simple, high-volume completion.

Limitations. Adaptive thinking cannot be turned off in Opus 5.5, which is a breaking change from Opus 5 and means every request carries some reasoning overhead — this is not the model to pick for latency-sensitive, low-complexity calls. Standard output is capped at 128K tokens (300K is available only through the Batches API), the model has no audio input or output path, and its training data reaches through June 2026. Existing agent code that forces a specific tool via tool_choice will also need updating, since that pattern now returns an error.

Sovereignty note. Claude Opus 5.5 is built and operated by Anthropic and delivered exclusively via API through Anthropic and its authorised cloud partners — there is no self-hosted or open-weight option. No EU-residency guarantee or CLOUD Act carve-out is documented for this model, so it should be treated as a standard commercial API dependency rather than a Sovereign deployment.