Claude Opus 5.5 - Anthropic's latest flagship model

What is Claude Opus 5.5

Claude Opus 5.5 is the first flagship model in the Claude 5.5 series released by Anthropic, with performance benchmarked against its ownClaude Fable 5.1. The model leads on coding and knowledge work benchmarks such as Terminal-Bench 4.0 (66.4%) and GDPval-AA v2.1 (1846 Elo)GPT-6 Astra; input and output prices are reduced by 20%, cache reads by 60%, total cost for typical tasks is 40% lower, and output is more than 30% faster. At the same time, communication and expression are greatly improved, safety audits achieve the best results in history, and for the first time Opus is equipped with Fable-level safety guardrails.

Main features of Claude Opus 5.5

• Agentic coding: Scored 66.4% on Terminal-Bench 4.0, can complete migration of 680,000 lines of code in one day, and audit a 200,000-line codebase in 3 hours.
• Large-scale code refactoring: Skilled at codebase-level migration and auditing, such as rewriting HAProxy from C to Rust, 2.5 hours faster than Fable 5.1 and 51% lower cost.
• Knowledge work and research: Reaches 1846 Elo on GDPval-AA v2.1, can achieve zero fabrication of numbers when writing financial reports and building financial models, and will also proactively discover errors in evaluation instructions.
• Computer operation: Scored 81.8% on OSWorld 2.0, can autonomously complete complex cross-application computer operation tasks.
• Efficient communication and writing: Has learned to put conclusions first and reduce jargon, making output more in line with human reading habits and greatly improving the long-term collaboration experience.
• Automated business workflows: Scored 40.0% on AutomationBench, can independently complete cross-system business process automation (Zapier evaluation).
• Mathematics and reasoning: Reaches 67.7% on Humanity’s Last Exam (with tools), with the strongest multidisciplinary reasoning ability currently available.
• Enterprise-grade security protection: Action-by-action review before execution, open-source sandbox, vulnerability scanning before code merge; prompt injection resistance is tied for best with Fable 5.1.

Technical principles of Claude Opus 5.5

• Infrastructure: Based on a large Transformer backbone, parameter count not disclosed; the core mechanism is Adaptive Thinking, which dynamically allocates reasoning compute according to task difficulty. Users can control depth through effort levels, and the medium level can match the performance of the previous highest level.
• Training method: Continues and reinforces the Constitutional AI / RLAIF paradigm: tightening RL training environment screening, improving alignment rewards, and automatically generating diverse safety training scenarios, reducing from the source the misaligned behavior of bending the rules to achieve goals.
• Efficiency optimization: Through trajectory-level reinforcement learning, rewards completing the task rather than a complete process, enabling the model to achieve results with fewer steps and tokens; the tradeoff is that under max effort, thinking tokens increase by 60%, exchanging more internal computation for fewer external actions.
• Runtime protection: Three-layer architecture: classifiers screen in real time before each action is executed; an open-source sandbox isolates the runtime environment; independent vulnerability review before code merge, achieving the double insurance of model-layer alignment plus system-layer interception.
• Capability-guardrail linkage: When tasks such as cyberattacks or biological risks are detected, requests are automatically transparently downgraded to specialized models (such as cybersecurity fallback to Opus 4.8); in high-risk domains, access is granted with certification to review agencies, tightened by default and opened on demand.

How to use Claude Opus 5.5

• Direct use on web/App: Log in toClaudeor Claude desktop/mobile app, subscribe to a Pro, Max, Team, or Enterprise plan to select Opus 5.5 in the model selector.
• API calls: Create an API Key in the Claude Platform console, fill in the model ID as claude-opus-5-5 to call it via the Anthropic SDK or REST interface, billed at $4 input / $20 output per million tokens.
• Using it in Claude Code: Install Claude Code then select Opus 5.5 as the model in the terminal or VS Code extension, suited for programming and agent tasks.
• Choose a thinking level: Control reasoning depth via the effort parameter (low/med/high/xhigh/max); the default medium level is enough for everyday tasks, and you can max it out for complex tasks to avoid meaningless token consumption.
• Enable Fast mode as needed: For latency-sensitive scenarios (such as interactive programming), you can enable Fast mode in Claude Code and Platform for a 2.5x speed boost, at double the price of $8/$40.
• Access via cloud platforms: Enterprise users can call it directly on AWS Bedrock, Google Cloud Vertex AI, or Microsoft Azure Foundry without building their own access layer.
• Apply for special permissions: Institutions involved in life sciences R&D can apply for the Life Sciences Verification Program, and cybersecurity practitioners can apply for the Cyber Verification Program, unlocking full capabilities in the corresponding domains after approval.

Core advantages of Claude Opus 5.5

• A generational lead in programming ability: Terminal-Bench 4.0 reaches 66.4%, leading GPT-6 Astra by 8.5 percentage points, and can complete the migration of 680,000 lines of code in one day.
• Cost drops significantly: Input and output prices drop by 20%, cache reads by 60%, and the total cost of typical tasks is 40% lower than Opus 5.
• Faster: Output speed increases by more than 30%, and Fast mode can reach 2.5x speed.
• A qualitative change in communication experience: Conclusions come first, with less jargon and insider talk, curing the "verbosity problem" that Opus 5 was most criticized for, making long-term collaboration more efficient.
• Reliable for knowledge work: GDPval-AA v2.1 reaches 1846 Elo, fabricates zero financial figures, and can even proactively discover hidden errors in evaluation instructions.
• Best safety record: The highest score in automated behavior audit history, 85% fewer jailbreak attempts than the previous generation, and tied with Fable 5.1 as the strongest against prompt injection.
• Comprehensive enterprise-grade protection: Pre-action review, open-source sandboxing, and vulnerability scanning before code merge, supporting unattended long-running operation.
• Crushing rivals on value for money: The default level can match GPT-6 Astra's highest level, at only one-fifth to two-fifths of its cost per individual task.

Project address of Claude Opus 5.5

• Project official website：https://www.anthropic.com/claude-opus-5-5

Comparison of Claude Opus 5.5 with similar competitors

Application scenarios of Claude Opus 5.5

• Large-scale code migration and refactoring: Complete migration of 680k lines of code in one day, rewrite HAProxy from C to Rust in 9.5 hours, suitable for legacy system modernization.
• Codebase audit and security review: Audit and fix a 200k-line codebase in 3 hours, catch 72% of known bugs even at low thinking level (measured by Deloitte), suitable for enterprise-grade code quality inspection.
• Financial analysis and investment research: Can write financial reports based on web information with zero fabricated data, build M&A financial models, and in Walleye Capital's evaluation can also proactively discover hidden errors in instructions.
• Knowledge work and report writing: Covers real work tasks across 44 occupations, with conclusions presented first and clear expression, suitable for long-document scenarios such as consulting, law, and research.
• Automated business processes and agent operations: Can perform cross-system tasks unattended for long periods in a sandbox, suitable for enterprise back-office process automation.