Claude Sonnet 4.5: The Best Coding Model in the World

Anthropic has just released Claude Sonnet 4.5, and it’s making bold claims: “the best coding model in the world” and “the strongest model for building complex agents.” After diving into the announcement and benchmarks, these claims are backed by impressive results that push the boundaries of what AI can do for software development.

What Makes Sonnet 4.5 Special?

Claude Sonnet 4.5 represents a significant leap forward in three key areas:

  1. Real-world software engineering: State-of-the-art coding capabilities
  2. Computer use and agentic tasks: Dramatic improvements in autonomous operation
  3. Extended reasoning: Ability to maintain focus for 30+ hours on complex, multi-step tasks

All of this comes at the same price point as Claude Sonnet 4: $3 per million input tokens and $15 per million output tokens.

Benchmark Performance: The Numbers Speak

Software Engineering Excellence

The most impressive metric is Sonnet 4.5’s performance on SWE-bench Verified, achieving 77.2% accuracy. This benchmark tests real-world software engineering tasks—the kind of work developers do every day. This isn’t about toy problems; it’s about solving actual GitHub issues in real codebases.

Agentic Task Performance

On OSWorld (a benchmark measuring autonomous computer use), Sonnet 4.5 scores 61.4%—a massive jump from Sonnet 4’s 42.2% just four months ago. This represents nearly a 50% relative improvement in the model’s ability to operate autonomously and handle complex, multi-step workflows.

Broad Improvements Across Domains

Beyond coding, Sonnet 4.5 shows enhanced performance across:

  • Mathematical reasoning
  • Domain-specific evaluations in finance, law, medicine, and STEM
  • Complex problem-solving requiring extended focus

The ability to maintain concentration for 30+ hours on intricate tasks sets a new standard for AI persistence and reliability.

Enhanced Claude Code Experience

The release comes with significant upgrades to Claude Code, the terminal-based coding assistant:

Checkpoints and Rollback

New checkpoint functionality allows you to:

  • Save progress at any point during long coding sessions
  • Roll back to previous states if something goes wrong
  • Experiment with confidence knowing you can easily revert changes

Improved Interface

  • Refreshed terminal interface for better readability
  • Native VS Code extension for seamless IDE integration
  • Enhanced code execution and file creation capabilities

Claude Agent SDK

The infrastructure powering Claude Code is now available to developers through the Claude Agent SDK. This enables you to build your own long-running agents with the same complexity-handling capabilities that power Claude Code.

API Improvements for Agent Builders

Developers building on the Claude API get new tools designed for extended agent operations:

  • Context editing feature: Efficiently manage and modify context during long-running tasks
  • Memory tool: Enable agents to maintain state and recall information across interactions

These features make it practical to build agents that can work autonomously for hours or even days on complex projects.

Real-World Impact: Customer Results

The proof is in the production deployments. Companies using Sonnet 4.5 are reporting significant improvements:

  • 44% reduction in vulnerability intake time for security teams
  • 0% error rate on code editing tasks (compared to 9% with previous models)
  • 18% increase in planning performance for complex workflows

These aren’t marginal gains—they represent step-change improvements in productivity and reliability.

Safety and Alignment: A New Standard

Perhaps most impressive is that Sonnet 4.5 achieves these performance gains while becoming Anthropic’s most aligned frontier model to date:

  • Reduced sycophancy (excessive agreeableness)
  • Lower rates of deception and power-seeking behaviors
  • Enhanced defenses against prompt injection attacks
  • Released under AI Safety Level 3 (ASL-3) protections

This demonstrates that safety and capability are not trade-offs—you can have both.

Availability and Access

Claude Sonnet 4.5 is available immediately through:

  • Claude API: Use model ID claude-sonnet-4-5
  • Claude apps: Web and mobile interfaces
  • Claude Code: Terminal-based coding assistant

The consistent pricing with Sonnet 4 means you can upgrade to the more capable model without budget concerns.

What This Means for Developers

Sonnet 4.5 represents a new tier of AI capability for software development:

For individual developers: More reliable code generation, better understanding of complex codebases, and an AI pair programmer that can work alongside you for extended sessions.

For teams: Automation of routine tasks, faster code reviews, and agentic systems that can handle multi-hour workflows autonomously.

For enterprises: Production-ready AI with strong safety guarantees, reduced error rates, and measurable productivity improvements.

The Agentic Future

The emphasis on “building complex agents” in this release signals where AI development tools are heading. It’s not just about autocomplete or answering questions—it’s about AI systems that can:

  • Execute multi-step workflows autonomously
  • Maintain context across hours or days
  • Make decisions and course-correct independently
  • Integrate with your existing tools and processes

Sonnet 4.5’s ability to stay focused for 30+ hours makes this vision practical. You can deploy an agent to work on a complex refactoring, security audit, or feature implementation and trust it to see the task through to completion.

Comparing to Alternatives

While other AI labs have released strong coding models, Sonnet 4.5’s combination of factors is unique:

  • SWE-bench Verified leadership demonstrates real-world coding superiority
  • Same pricing as the previous generation makes it a no-brainer upgrade
  • Safety-first approach provides confidence for production deployments
  • Agentic capabilities enable use cases beyond traditional code completion

The 30+ hour sustained focus capability is particularly noteworthy—most AI models struggle to maintain coherence and effectiveness over extended sessions.

Getting Started

If you’re already using Claude API or Claude Code, upgrading is straightforward:

# API example
from anthropic import Anthropic

client = Anthropic()
message = client.messages.create(
    model="claude-sonnet-4-5",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "Review this code for security issues..."}
    ]
)

For Claude Code users, the latest version automatically uses Sonnet 4.5 when you select the sonnet model.

The Bigger Picture

Claude Sonnet 4.5 isn’t just an incremental update—it’s a statement about where AI coding assistants are heading. The combination of:

  • State-of-the-art coding performance
  • Extended reasoning capabilities
  • Strong safety and alignment
  • Accessible pricing
  • Production-ready reliability

…creates a new baseline for what developers should expect from AI assistance.

As software engineering becomes increasingly collaborative between humans and AI, having models that can reliably handle complex, multi-hour tasks autonomously changes what’s possible. Sonnet 4.5 makes this future accessible today.

Learn More

Whether you’re building the next generation of agentic systems or just want better code completion, Claude Sonnet 4.5 represents a significant step forward in AI-assisted development. The best coding model in the world? The benchmarks and customer results make a compelling case.